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  • 1. Agarwal, Sahil
    et al.
    Del Sordo, Fabio
    Stockholm University, Nordic Institute for Theoretical Physics (Nordita). Yale University, USA.
    Wettlaufer, John S.
    Stockholm University, Nordic Institute for Theoretical Physics (Nordita). Yale University, USA; University of Oxford, UK.
    EXOPLANETARY DETECTION BY MULTIFRACTAL SPECTRAL ANALYSIS2017In: Astronomical Journal, ISSN 0004-6256, E-ISSN 1538-3881, Vol. 153, no 1, article id 12Article in journal (Refereed)
    Abstract [en]

    Owing to technological advances, the number of exoplanets discovered has risen dramatically in the last few years. However, when trying to observe Earth analogs, it is often difficult to test the veracity of detection. We have developed a new approach to the analysis of exoplanetary spectral observations based on temporal multifractality, which identifies timescales that characterize planetary orbital motion around the host star and those that arise from stellar features such as spots. Without fitting stellar models to spectral data, we show how the planetary signal can be robustly detected from noisy data using noise amplitude as a source of information. For observation of transiting planets, combining this method with simple geometry allows us to relate the timescales obtained to primary and secondary eclipse of the exoplanets. Making use of data obtained with ground-based and space-based observations we have tested our approach on HD 189733b. Moreover, we have investigated the use of this technique in measuring planetary orbital motion via Doppler shift detection. Finally, we have analyzed synthetic spectra obtained using the SOAP 2.0 tool, which simulates a stellar spectrum and the influence of the presence of a planet or a spot on that spectrum over one orbital period. We have demonstrated that, so long as the signal-to-noise-ratio >= 75, our approach reconstructs the planetary orbital period, as well as the rotation period of a spot on the stellar surface.

  • 2. Agarwal, Sahil
    et al.
    Wettlaufer, John S.
    Stockholm University, Nordic Institute for Theoretical Physics (Nordita). Yale University, USA; University of Oxford, UK.
    Fluctuations in Arctic sea-ice extent: comparing observations and climate models2018In: Philosophical Transactions. Series A: Mathematical, physical, and engineering science, ISSN 1364-503X, E-ISSN 1471-2962, Vol. 376, no 2129, article id 20170332Article in journal (Refereed)
    Abstract [en]

    The fluctuation statistics of the observed sea-ice extent during the satellite era are compared with model output from CMIP5 models using a multifractal time series method. The two robust features of the observations are that on annual to biannual time scales the ice extent exhibits white noise structure, and there is a decadal scale trend associated with the decay of the ice cover. It is shown that (i) there is a large inter-model variability in the time scales extracted from the models, (ii) none of the models exhibits the decadal time scales found in the satellite observations, (iii) five of the 21 models examined exhibit the observed white noise structure, and (iv) the multi-model ensemble mean exhibits neither the observed white noise structure nor the observed decadal trend. It is proposed that the observed fluctuation statistics produced by this method serve as an appropriate test bed for modelling studies. This article is part of the theme issue 'Modelling of sea-ice phenomena'.

  • 3. Agarwal, Sahil
    et al.
    Wettlaufer, John S.
    Stockholm University, Nordic Institute for Theoretical Physics (Nordita). Yale University, USA; University of Oxford, UK.
    Maximal stochastic transport in the Lorenz equations2016In: Physics Letters A, ISSN 0375-9601, E-ISSN 1873-2429, Vol. 380, no 1-2, p. 142-146Article in journal (Refereed)
    Abstract [en]

    We calculate the stochastic upper bounds for the Lorenz equations using an extension of the background method. In analogy with Rayleigh-Benard convection the upper bounds are for heat transport versus Rayleigh number. As might be expected, the stochastic upper bounds are larger than the deterministic counterpart of Souza and Doering [1], but their variation with noise amplitude exhibits interesting behavior. Below the transition to chaotic dynamics the upper bounds increase monotonically with noise amplitude. However, in the chaotic regime this monotonicity depends on the number of realizations in the ensemble; at a particular Rayleigh number the bound may increase or decrease with noise amplitude. The origin of this behavior is the coupling between the noise and unstable periodic orbits, the degree of which depends on the degree to which the ensemble represents the ergodic set. This is confirmed by examining the close returns plots of the full solutions to the stochastic equations and the numerical convergence of the noise correlations. The numerical convergence of both the ensemble and time averages of the noise correlations is sufficiently slow that it is the limiting aspect of the realization of these bounds. Finally, we note that the full solutions of the stochastic equations demonstrate that the effect of noise is equivalent to the effect of chaos.

  • 4. Agarwal, Sahil
    et al.
    Wettlaufer, John S.
    Stockholm University, Nordic Institute for Theoretical Physics (Nordita). Yale University, USA; University of Oxford, United Kingdom.
    The Statistical Properties of Sea Ice Velocity Fields2017In: Journal of Climate, ISSN 0894-8755, E-ISSN 1520-0442, Vol. 30, no 13, p. 4873-4881Article in journal (Refereed)
    Abstract [en]

    By arguing that the surface pressure field over the Arctic Ocean can be treated as an isotropic, stationary, homogeneous, Gaussian random field, Thorndike estimated a number of covariance functions from two years of data (1979 and 1980). Given the active interest in changes of general circulation quantities and indices in the polar regions during the recent few decades, the spatial correlations in sea ice velocity fields are of particular interest. It is thus natural to ask, How persistent are these correlations?'' To this end, a multifractal stochastic treatment is developed to analyze observed Arctic sea ice velocity fields from satellites and buoys for the period 1978-2015. Since it was previously found that the Arctic equivalent ice extent (EIE) has a white noise structure on annual to biannual time scales, the connection between EIE and ice motion is assessed. The long-term stationarity of the spatial correlation structure of the velocity fields and the robustness of their white noise structure on multiple time scales is demonstrated; these factors (i) combine to explain the white noise characteristics of the EIE on annual to biannual time scales and (ii) explain why the fluctuations in the ice velocity are proportional to fluctuations in the geostrophic winds on time scales of days to months. Moreover, it is shown that the statistical structure of these two quantities is commensurate from days to years, which may be related to the increasing prevalence of free drift in the ice pack.

  • 5. Bar-Dolev, Maya
    et al.
    Celik, Yeliz
    Wettlaufer, John S.
    Stockholm University, Nordic Institute for Theoretical Physics (Nordita).
    Davies, Peter L.
    Braslavsky, Ido
    New insights into ice growth and melting modifications by antifreeze proteins2012In: Journal of the Royal Society Interface, ISSN 1742-5689, E-ISSN 1742-5662, Vol. 9, no 77, p. 3249-3259Article in journal (Refereed)
    Abstract [en]

    Antifreeze proteins (AFPs) evolved in many organisms, allowing them to survive in cold climates by controlling ice crystal growth. The specific interactions of AFPs with ice determine their potential applications in agriculture, food preservation and medicine. AFPs control the shapes of ice crystals in a manner characteristic of the particular AFP type. Moderately active AFPs cause the formation of elongated bipyramidal crystals, often with seemingly defined facets, while hyperactive AFPs produce more varied crystal shapes. These different morphologies are generally considered to be growth shapes. In a series of bright light and fluorescent microscopy observations of ice crystals in solutions containing different AFPs, we show that crystal shaping also occurs during melting. In particular, the characteristic ice shapes observed in solutions of most hyperactive AFPs are formed during melting. We relate these findings to the affinities of the hyperactive AFPs for the basal plane of ice. Our results demonstrate the relation between basal plane affinity and hyperactivity and show a clear difference in the ice-shaping mechanisms of most moderate and hyperactive AFPs. This study provides key aspects associated with the identification of hyperactive AFPs.

  • 6.
    Billström, Anders
    et al.
    Halmstad University, School of Business and Engineering (SET), Centre for Innovation, Entrepreneurship and Learning Research (CIEL), Knowledge Entrepreneurship and Enterprise Research (KEEN).
    Ulvenblad, Pia
    Halmstad University, School of Business and Engineering (SET), Centre for Innovation, Entrepreneurship and Learning Research (CIEL).
    Winborg, Joakim
    Halmstad University, School of Business and Engineering (SET), Centre for Innovation, Entrepreneurship and Learning Research (CIEL), Knowledge Entrepreneurship and Enterprise Research (KEEN).
    Lindholm Dahlstrand, Åsa
    Halmstad University, School of Business and Engineering (SET), Centre for Innovation, Entrepreneurship and Learning Research (CIEL), Knowledge Entrepreneurship and Enterprise Research (KEEN).
    To get them on board: The role of communication for attracting stakeholders to incubator businesses2011Conference paper (Refereed)
  • 7. Lee, Alpha A.
    et al.
    Vella, Dominic
    Wettlaufer, John S.
    Stockholm University, Nordic Institute for Theoretical Physics (Nordita). University of Oxford, UK; Yale University, USA .
    Fluctuation spectra and force generation in nonequilibrium systems2017In: Proceedings of the National Academy of Sciences of the United States of America, ISSN 0027-8424, E-ISSN 1091-6490, Vol. 114, no 35, p. 9255-9260Article in journal (Refereed)
    Abstract [en]

    Many biological systems are appropriately viewed as passive inclusions immersed in an active bath: from proteins on active membranes to microscopic swimmers confined by boundaries. The nonequilibrium forces exerted by the active bath on the inclusions or boundaries often regulate function, and such forces may also be exploited in artificial active materials. Nonetheless, the general phenomenology of these active forces remains elusive. We show that the fluctuation spectrum of the active medium, the partitioning of energy as a function of wavenumber, controls the phenomenology of force generation. We find that, for a narrow, unimodal spectrum, the force exerted by a nonequilibrium system on two embedded walls depends on the width and the position of the peak in the fluctuation spectrum, and oscillates between repulsion and attraction as a function of wall separation. We examine two apparently disparate examples: the Maritime Casimir effect and recent simulations of active Brownian particles. A key implication of our work is that important nonequilibrium interactions are encoded within the fluctuation spectrum. In this sense, the noise becomes the signal.

  • 8. MacMinn, Christopher W.
    et al.
    Dufresne, Eric R.
    Wettlaufer, John S.
    Stockholm University, Nordic Institute for Theoretical Physics (Nordita). Yale University, USA; University of Oxford, United Kingdom.
    Large Deformations of a Soft Porous Material2016In: Physical Review Applied, ISSN 2331-7019, Vol. 5, no 4, article id 044020Article in journal (Refereed)
    Abstract [en]

    Compressing a porous material will decrease the volume of the pore space, driving fluid out. Similarly, injecting fluid into a porous material can expand the pore space, distorting the solid skeleton. This poromechanical coupling has applications ranging from cell and tissue mechanics to geomechanics and hydrogeology. The classical theory of linear poroelasticity captures this coupling by combining Darcy's law with Terzaghi's effective stress and linear elasticity in a linearized kinematic framework. Linear poroelasticity is a good model for very small deformations, but it becomes increasingly inappropriate for moderate to large deformations, which are common in the context of phenomena such as swelling and damage, and for soft materials such as gels and tissues. The well-known theory of large-deformation poroelasticity combines Darcy's law with Terzaghi's effective stress and nonlinear elasticity in a rigorous kinematic framework. This theory has been used extensively in biomechanics to model large elastic deformations in soft tissues and in geomechanics to model large elastoplastic deformations in soils. Here, we first provide an overview and discussion of this theory with an emphasis on the physics of poromechanical coupling. We present the large-deformation theory in an Eulerian framework to minimize the mathematical complexity, and we show how this nonlinear theory simplifies to linear poroelasticity under the assumption of small strain. We then compare the predictions of linear poroelasticity with those of large-deformation poroelasticity in the context of two uniaxial model problems: fluid outflow driven by an applied mechanical load (the consolidation problem) and compression driven by a steady fluid throughflow. We explore the steady and dynamical errors associated with the linear model in both situations, as well as the impact of introducing a deformation-dependent permeability. We show that the error in linear poroelasticity is due primarily to kinematic nonlinearity and that this error (i) plays a surprisingly important role in the dynamics of the deformation and (ii) is amplified by nonlinear constitutive behavior, such as deformation-dependent permeability.

  • 9.
    Mancarella, Francesco
    et al.
    Stockholm University, Nordic Institute for Theoretical Physics (Nordita).
    Style, Robert W.
    Wettlaufer, John S.
    Stockholm University, Nordic Institute for Theoretical Physics (Nordita). Yale University, USA.
    Interfacial tension and a three-phase generalized self-consistent theory of non-dilute soft composite solids2016In: Soft Matter, ISSN 1744-683X, E-ISSN 1744-6848, Vol. 12, no 10, p. 2744-2750Article in journal (Refereed)
    Abstract [en]

    In the dilute limit Eshelby's inclusion theory captures the behavior of a wide range of systems and properties. However, because Eshelby's approach neglects interfacial stress, it breaks down in soft materials as the inclusion size approaches the elastocapillarity length L equivalent to gamma/E. Here, we use a three-phase generalized self-consistent method to calculate the elastic moduli of composites comprised of an isotropic, linear-elastic compliant solid hosting a spatially random monodisperse distribution of spherical liquid droplets. As opposed to similar approaches, we explicitly capture the liquid-solid interfacial stress when it is treated as an isotropic, strain-independent surface tension. Within this framework, the composite stiffness depends solely on the ratio of the elastocapillarity length L to the inclusion radius R. Independent of inclusion volume fraction, we find that the composite is stiffened by the inclusions whenever R < 3L/2. Over the same range of parameters, we compare our results with alternative approaches (dilute and Mori-Tanaka theories that include surface tension). Our framework can be easily extended to calculate the composite properties of more general soft materials where surface tension plays a role.

  • 10.
    Mancarella, Francesco
    et al.
    Stockholm University, Nordic Institute for Theoretical Physics (Nordita).
    Style, Robert W.
    Wettlaufer, John S.
    Stockholm University, Nordic Institute for Theoretical Physics (Nordita). University of Oxford, UK; Yale University, USA.
    Surface tension and the Mori-Tanaka theory of non-dilute soft composite solids2016In: Proceedings of the Royal Society. Mathematical, Physical and Engineering Sciences, ISSN 1364-5021, E-ISSN 1471-2946, Vol. 472, no 2189, article id 20150853Article in journal (Refereed)
    Abstract [en]

    Eshelby's theory is the foundation of composite mechanics, allowing calculation of the effective elastic moduli of composites from a knowledge of their microstructure. However, it ignores interfacial stress and only applies to very dilute composites-i.e. where any inclusions are widely spaced apart. Here, within the framework of the Mori-Tanaka multiphase approximation scheme, we extend Eshelby's theory to treat a composite with interfacial stress in the non-dilute limit. In particular, we calculate the elastic moduli of composites comprised of a compliant, elastic solid hosting a non-dilute distribution of identical liquid droplets. The composite stiffness depends strongly on the ratio of the droplet size, R, to an elastocapillary lengthscale, L. Interfacial tension substantially impacts the effective elastic moduli of the composite when R/L less than or similar to 100. When R < 3L/2 (R = 3L/2) liquid inclusions stiffen (cloak the far-field signature of) the solid.

  • 11.
    Mancarella, Francesco
    et al.
    Stockholm University, Nordic Institute for Theoretical Physics (Nordita).
    Wettlaufer, John S.
    Stockholm University, Nordic Institute for Theoretical Physics (Nordita). Yale University, USA; University of Oxford, UK.
    Surface tension and a self-consistent theory of soft composite solids with elastic inclusions2017In: Soft Matter, ISSN 1744-683X, E-ISSN 1744-6848, Vol. 13, no 5, p. 945-955Article in journal (Refereed)
    Abstract [en]

    The importance of surface tension effects is being recognized in the context of soft composite solids, where they are found to significantly affect the mechanical properties, such as the elastic response to an external stress. It has recently been discovered that Eshelby's inclusion theory breaks down when the inclusion size approaches the elastocapillary length L = gamma/E, where gamma is the inclusion/host surface tension and E is the host Young's modulus. Extending our recent results for liquid inclusions, here we model the elastic behavior of a non-dilute distribution of isotropic elastic spherical inclusions in a soft isotropic elastic matrix, subject to a prescribed infinitesimal far-field loading. Within our framework, the composite stiffness is uniquely determined by the elastocapillary length L, the spherical inclusion radius R, and the stiffness contrast parameter C, which is the ratio of the inclusion to the matrix stiffness. We compare the results with those from the case of liquid inclusions, and we derive an analytical expression for elastic cloaking of the composite by the inclusions. Remarkably, we find that the composite stiffness is influenced significantly by surface tension even for inclusions two orders of magnitude more stiff than the host matrix. Finally, we show how to simultaneously determine the surface tension and the inclusion stiffness using two independent constraints provided by global and local measurements.

  • 12.
    Marath, Navaneeth K.
    et al.
    Stockholm University, Nordic Institute for Theoretical Physics (Nordita).
    Wettlaufer, John S.
    Stockholm University, Nordic Institute for Theoretical Physics (Nordita). Yale University, USA.
    Hydrodynamic interactions and the diffusivity of spheroidal particles2019In: Journal of Chemical Physics, ISSN 0021-9606, E-ISSN 1089-7690, Vol. 151, no 2, article id 024107Article in journal (Refereed)
    Abstract [en]

    It is intuitive that the diffusivity of an isolated particle differs from those in a monodisperse suspension, in which hydrodynamic interactions between the particles are operative. Batchelor [J. Fluid Mech. 74, 1-29 (1976) and J. Fluid Mech. 131, 155-175 (1983)] calculated how hydrodynamic interactions influenced the diffusivity of a dilute suspension of spherical particles, and Russel et al. [Colloidal Dispersions (Cambridge University Press, 1991)] and Brady [J. Fluid Mech. 272, 109-134 (1994)] treated nondilute (higher particle volume fraction) suspensions. Although most particles lack perfect sphericity, little is known about the effects of hydrodynamic interactions on the diffusivity of spheroidal particles, which are the simplest shapes that can be used to model anisotropic particles. Here, we calculate the effects of hydrodynamic interactions on the translational and rotational diffusivities of spheroidal particles of arbitrary aspect ratio in dilute monodisperse suspensions. We find that the translational and rotational diffusivities of prolate spheroids are more sensitive to eccentricity than for oblate spheroids. The origin of the hydrodynamic anisotropy is that found in the stresslet field for the induced-dipole interaction. However, in the dilute limit, the effects of anisotropy are at the level of a few percent. These effects have influence on a vast range of settings, from partially frozen colloidal suspensions to the dynamics of cytoplasm. 

  • 13.
    Moon, Woosok
    et al.
    Stockholm University, Faculty of Science, Department of Mathematics. Stockholm University, Nordic Institute for Theoretical Physics (Nordita).
    Agarwal, Sahil
    Wettlaufer, John S.
    Stockholm University, Nordic Institute for Theoretical Physics (Nordita). Yale University, USA; University of Oxford, United Kingdom.
    Intrinsic Pink-Noise Multidecadal Global Climate Dynamics Mode2018In: Physical Review Letters, ISSN 0031-9007, E-ISSN 1079-7114, Vol. 121, no 10, article id 108701Article in journal (Refereed)
    Abstract [en]

    Understanding multidecadal variability is an essential goal of climate dynamics. For example, the recent phenomenon referred to as the global warming hiatus may reflect a coupling to an intrinsic, preindustrial, multidecadal variability process. Here, using a multifractal time-series method, we demonstrate that 42 data sets of 79 proxies with global coverage exhibit pink-noise characteristics on multidecadal timescales. To quantify the persistence of this behavior, we examine high-resolution ice core and speleothem data to find pink noise in both pre- and postindustrial periods. We examine the spatial structure with an empirical orthogonal function analysis of the monthly averaged surface temperature from 1901 to 2012. The first mode clearly shows the distribution of ocean heat flux sinks located in the eastern Pacific and the Southern Ocean and has pink-noise characteristics on a multidecadal timescale. We hypothesize that this pink-noise multidecadal spatial mode may resonate with externally driven greenhouse gas forcing, driving large-scale climate processes.

  • 14. Moon, Woosok
    et al.
    Wettlaufer, John S.
    Stockholm University, Nordic Institute for Theoretical Physics (Nordita). Yale University, USA; University of Oxford, United Kingdom.
    A Stochastic Dynamical Model of Arctic Sea Ice2017In: Journal of Climate, ISSN 0894-8755, E-ISSN 1520-0442, Vol. 30, no 13, p. 5119-5140Article in journal (Refereed)
    Abstract [en]

    The noise forcing underlying the variability in the Arctic ice cover has a wide range of principally unknown origins. For this reason, the analytical and numerical solutions of a stochastic Arctic sea ice model are analyzed with both additive and multiplicative noise over a wide range of external heat fluxes Delta F-0, corresponding to greenhouse gas forcing. The stochastic variability fundamentally influences the nature of the deterministic steady-state solutions corresponding to perennial and seasonal ice and ice-free states. Thus, the results are particularly relevant for the interpretation of the state of the system as the ice cover thins with Delta F-0, allowing a thorough examination of the differing effects of additive versusmultiplicative noise. In the perennial ice regime, the principal stochastic moments are calculated and compared to those determined from a stochastic perturbation theory described previously. As Delta F-0 increases, the competing contributions to the variability of the destabilizing sea ice-albedo feedback and the stabilizing longwave radiative loss are examined in detail. At the end of summer the variability of the stochastic paths shows a clear maximum, which is due to the combination of the increasing influence of the albedo feedback and an associated memory effect,'' in which fluctuations accumulate from early spring to late summer. This is counterbalanced by the stabilization of the ice cover resulting from the longwave loss of energy from the ice surface, which is enhanced during winter, thereby focusing the stochastic paths and decreasing the variability. Finally, common examples in stochastic dynamics with multiplicative noise are discussed wherein the choice of the stochastic calculus (Ito or Stratonovich) is not necessarily determinable a priori from observations alone, which is why both calculi are treated on equal footing herein.

  • 15. Moon, Woosok
    et al.
    Wettlaufer, John S.
    Stockholm University, Nordic Institute for Theoretical Physics (Nordita). Yale University, USA; University of Oxford, UK.
    A unified nonlinear stochastic time series analysis for climate science2017In: Scientific Reports, ISSN 2045-2322, E-ISSN 2045-2322, Vol. 7, article id 44228Article in journal (Refereed)
    Abstract [en]

    Earth's orbit and axial tilt imprint a strong seasonal cycle on climatological data. Climate variability is typically viewed in terms of fluctuations in the seasonal cycle induced by higher frequency processes. We can interpret this as a competition between the orbitally enforced monthly stability and the fluctuations/noise induced by weather. Here we introduce a new time-series method that determines these contributions from monthly-averaged data. We find that the spatio-temporal distribution of the monthly stability and the magnitude of the noise reveal key fingerprints of several important climate phenomena, including the evolution of the Arctic sea ice cover, the El Nino Southern Oscillation (ENSO), the Atlantic Nino and the Indian Dipole Mode. In analogy with the classical destabilising influence of the ice-albedo feedback on summertime sea ice, we find that during some time interval of the season a destabilising process operates in all of these climate phenomena. The interaction between the destabilisation and the accumulation of noise, which we term the memory effect, underlies phase locking to the seasonal cycle and the statistical nature of seasonal predictability.

  • 16.
    Moon, Woosok
    et al.
    Stockholm University, Faculty of Science, Department of Mathematics. Stockholm University, Nordic Institute for Theoretical Physics (Nordita).
    Wettlaufer, John S.
    Stockholm University, Nordic Institute for Theoretical Physics (Nordita). Yale University, USA.
    Coupling functions in climate2019In: Philosophical Transactions. Series A: Mathematical, physical, and engineering science, ISSN 1364-503X, E-ISSN 1471-2962, Vol. 377, no 2160, article id 20190006Article in journal (Refereed)
    Abstract [en]

    We examine how coupling functions in the theory of dynamical systems provide a quantitative window into climate dynamics. Previously, we have shown that a one-dimensional periodic non-autonomous stochastic dynamical system can simulate the monthly statistics of surface air temperature data. Here, we expand this approach to two-dimensional dynamical systems to include interactions between two subsystems of the climate. The relevant coupling functions are constructed from the covariance of the data from the two sub-systems. We demonstrate the method on two tropical climate indices, the El-Nino-Southern Oscillation (ENSO) and the Indian Ocean Dipole (IOD), to interpret the mutual interactions between these two air-sea interaction phenomena in the Pacific and Indian Oceans. The coupling function reveals that the ENSO mainly controls the seasonal variability of the IOD during its mature phase. This demonstrates the plausibility of constructing a network model for the seasonal variability of climate systems based on such coupling functions. This article is part of the theme issue 'Coupling functions: dynamical interaction mechanisms in the physical, biological and social sciences'.

  • 17. O'Rourke, J. G.
    et al.
    Riggs, A. J. E.
    Guertler, C. A.
    Miller, P. W.
    Padhi, C. M.
    Popelka, M. M.
    Wells, A. J.
    West, A. C.
    Zhong, J. -Q
    Wettlaufer, John S.
    Stockholm University, Nordic Institute for Theoretical Physics (Nordita).
    Mushy-layer dynamics in micro and hyper gravity2012In: Physics of fluids, ISSN 1070-6631, E-ISSN 1089-7666, Vol. 24, no 10, p. 103305-Article in journal (Refereed)
    Abstract [en]

    We describe the results of experiments on mushy layers grown from aqueous ammonium chloride solution in normal, micro, and hyper gravity environments. In the fully developed chimney state, the chimney plume dynamics differ strikingly when conditions change from micro to hyper gravity. In microgravity, we find fully arrested plume motion and suppressed convection. As gravity exceeds Earth conditions, we observe a host of phenomena, ranging from arched plumes that undergo forced Rayleigh-Taylor instabilities to in-phase multiple plume oscillatory behavior. For the same initial solute concentrations and fixed boundary cooling temperatures, we find that, in runs of over two hours, the averaged effects of microgravity and hypergravity result in suppressed growth of the mushy layers, a phenomenon caused by a net enhancement of convective heat and solute transport from the liquid to the mushy layers. These behaviors are placed in the context of the theory of convecting mushy layers as studied under normal laboratory conditions.

  • 18.
    Politis, Diamanto
    et al.
    Lund University, Sweden.
    Winborg, Joakim
    Halmstad University, School of Business and Engineering (SET), Centre for Innovation, Entrepreneurship and Learning Research (CIEL), Knowledge Entrepreneurship and Enterprise Research (KEEN).
    Lindholm Dahlstrand, Åsa
    Halmstad University, School of Business and Engineering (SET), Centre for Innovation, Entrepreneurship and Learning Research (CIEL), Knowledge Entrepreneurship and Enterprise Research (KEEN).
    Exploring the mindset of university entrepreneurs: Do they have a different resource logic?2008In: Frontiers of Entrepreneurship Research, ISSN 0740-7416, Vol. 28, no 4Article in journal (Refereed)
    Abstract [en]

    Principal Topic: Universities are today increasingly acknowledged as powerful drivers of innovation, job creation and economic growth. To promote and support university-based entrepreneurship there has been a tremendous increase in the supply of entrepreneurship courses and the creation of business incubators. As a result of this development there is an increasing group of entrepreneurs that have been educated or fostered in the university context, and who often continue to develop their new ventures in close interaction with the university. The principal research question we ask in this paper is whether university entrepreneurs have a different resource logic compared to entrepreneurs that start up their ventures independently of the university and its surrounding innovation system. Resource logic is in the paper defined as a set of ideas for how to secure and use resources in the start-up process, and we link this concept to three streams of research that can be related to the resource logic of entrepreneurs; effectual decision making, bootstrapping orientation; and personal networking.

    Method: The empirical study was designed as a questionnaire survey. The questionnaire was sent out to two groups of entrepreneurs resulting in responses from 182 university entrepreneurs and 209 non- university entrepreneurs. The hypotheses are tested using parametric and non-parametric tests in SPSS.

    Results and Implications: In line with our hypotheses the results suggest that university entrepreneurs to a larger extent have a mindset that favours both effectual reasoning and the use of bootstrapping. When it comes to use of network contacts the results were however contrary to our hypothesis. In sum, our findings add to our knowledge about the extent to which the close connection to the university has any significant influence on the resource acquisition behaviour of university entrepreneurs once they start an entrepreneurial career. The paper develops and uses the concept of “resource logic”. On the basis of this concept the paper provides general implications for our understanding of differences in the mindset of entrepreneurs in the start-up process.

  • 19.
    Politis, Diamanto
    et al.
    Halmstad University, School of Business, Engineering and Science, Centre for Innovation, Entrepreneurship and Learning Research (CIEL).
    Winborg, Joakim
    Halmstad University, School of Business, Engineering and Science, Centre for Innovation, Entrepreneurship and Learning Research (CIEL).
    Lindholm Dahlstrand, Åsa
    Halmstad University, School of Business, Engineering and Science, Centre for Innovation, Entrepreneurship and Learning Research (CIEL).
    Exploring the resource logic of student entrepreneurs2012In: International Small Business Journal, ISSN 0266-2426, E-ISSN 1741-2870, Vol. 30, no 6, p. 659-683Article in journal (Refereed)
    Abstract [en]

    This study responds to the call in recent research for comparative studies examining whether student entrepreneurs are different to other kinds of entrepreneurs. Based on institutional theory, the specific question we ask in this study is whether student entrepreneurs who start up their firms in close relation to the university have a different resource logic compared to entrepreneurs who start their firms outside the university context. We define resource logic as the individual's set of ideas for how to secure and use resources, and we link this concept to theories of effectual reasoning and bootstrapping to develop our argument. Moreover, we identify two different viewpoints about the effects of the university milieu on the resource logic of student entrepreneurs and we develop hypotheses to test the different viewpoints. The findings give overall support for the view that student entrepreneurs have developed a resource logic that favours both effectual reasoning and the use of bootstrapping methods.

  • 20.
    Pramanik, Satyajit
    et al.
    Stockholm University, Nordic Institute for Theoretical Physics (Nordita).
    Wettlaufer, John S.
    Yale University, USA; University of Oxford, UK.
    Confinement effects in premelting dynamics2017In: Physical review. E, ISSN 2470-0045, E-ISSN 2470-0053, Vol. 96, no 5, article id 052801Article in journal (Refereed)
    Abstract [en]

    We examine the effects of confinement on the dynamics of premelted films driven by thermomolecular pressure gradients. Our approach is to modify a well-studied setting in which the thermomolecular pressure gradient is driven by a temperature gradient parallel to an interfacially premelted elastic wall. The modification treats the increase in viscosity associated with the thinning of films, studied in a wide variety of materials, using a power law and we examine the consequent evolution of the confining elastic wall. We treat (1) a range of interactions that are known to underlie interfacial premelting and (2) a constant temperature gradient wherein the thermomolecular pressure gradient is a constant. The difference between the cases with and without the proximity effect arises in the volume flux of premelted liquid. The proximity effect increases the viscosity as the film thickness decreases thereby requiring the thermomolecular pressure driven flux to be accommodated at higher temperatures where the premelted film thickness is the largest. Implications for experiment and observations of frost heave are discussed.

  • 21.
    Pramanik, Satyajit
    et al.
    Stockholm University, Nordic Institute for Theoretical Physics (Nordita).
    Wettlaufer, John S.
    Stockholm University, Nordic Institute for Theoretical Physics (Nordita). Yale University, USA; University of Oxford, England.
    CONFINEMENT-INDUCED CONTROL OF SIMILARITY SOLUTIONS IN PREMELTING DYNAMICS AND OTHER THIN FILM PROBLEMS2019In: SIAM Journal on Applied Mathematics, ISSN 0036-1399, E-ISSN 1095-712X, Vol. 79, no 3, p. 938-958Article in journal (Refereed)
    Abstract [en]

    We study the combined effects of nonlocal elasticity and confinement-induced ordering on the dynamics of thermomolecular pressure gradient driven premelted films bound by an elastic membrane. The confinement-induced ordering is modeled using a film thickness dependent viscosity. When there is no confinement-induced ordering, we recover the similarity solution for the evolution of the elastic membrane, which exhibits an in finite sequence of oscillations. However, when the confinement-induced viscosity is comparable to the bulk viscosity, the numerical solutions of the full system reveal the conditions under which the oscillations and similarity solutions vanish. Implications of our results for general thermomechanical dynamics, frost heave observations, and cryogenic cell preservation are discussed. Finally, through its influence on the viscosity, the confinement effect implicitly introduces a new universal length scale into the volume flux. Thus, there are a host of thin film problems, from droplet breakup to wetting/dewetting dynamics, whose properties (similarity solutions, regularization, and compact support) will change under the action of the confinement effect. Therefore, our study suggests revisiting the mathematical structure and experimental implications of a wide range of problems within the framework of the confinement effect.

  • 22. Schollick, Julia M. H.
    et al.
    Style, Robert W.
    Curran, Arran
    Wettlaufer, John S.
    Stockholm University, Nordic Institute for Theoretical Physics (Nordita). University of Oxford, United Kingdom; Yale University, United States.
    Dufresne, Eric R.
    Warren, Patrick B.
    Velikov, Krassimir P.
    Dullens, Roel P. A.
    Aarts, Dirk G. A. L.
    Segregated Ice Growth in a Suspension of Colloidal Particles2016In: Journal of Physical Chemistry B, ISSN 1520-6106, E-ISSN 1520-5207, Vol. 120, no 16, p. 3941-3949Article in journal (Refereed)
    Abstract [en]

    We study the freezing of a dispersion of colloidal silica particles in water, focusing on the formation of segregated ice in the form of ice lenses. Local temperature measurements in combination with video microscopy give insight into the rich variety of factors that control ice lens formation. We observe the initiation of the lenses, their growth morphology, and their final thickness and spacing over a range of conditions, in particular the effect of the particle packing and the cooling rate. We find that increasing the particle density drastically reduces the thickness of lenses but has little effect on the lens spacing. Therefore, the fraction of segregated ice formed reduces. The effect of the cooling rate, which is the product of the temperature gradient and the pulling speed across the temperature gradient, depends on which parameter is varied. A larger temperature gradient causes ice lenses to be initiated more frequently, while a lower pulling speed allows for more time for ice lenses to grow: both increase the fraction of segregated ice. Surprisingly, we find that the growth rate of a lens does not depend on its undercooling. Finally, we have indications of pore ice in front of the warmest ice lens, which has important consequences for the interpretation of the measured trends. Our findings are relevant for ice segregation occurring in a wide range of situations, ranging from model lab experiments and theories to geological and industrial processes, like frost heave and frozen food production.

  • 23. Style, Robert W.
    et al.
    Boltyanskiy, Rostislav
    Che, Yonglu
    Wettlaufer, J. S.
    Stockholm University, Nordic Institute for Theoretical Physics (Nordita). Yale Univ, New Haven, CT 06520 USA.
    Wilen, Larry A.
    Dufresne, Eric R.
    Universal Deformation of Soft Substrates Near a Contact Line and the Direct Measurement of Solid Surface Stresses2013In: Physical Review Letters, ISSN 0031-9007, E-ISSN 1079-7114, Vol. 110, no 6, p. 066103-Article in journal (Refereed)
    Abstract [en]

    Droplets deform soft substrates near their contact lines. Using confocal microscopy, we measure the deformation of silicone gel substrates due to glycerol and fluorinated-oil droplets for a range of droplet radii and substrate thicknesses. For all droplets, the substrate deformation takes a universal shape close to the contact line that depends on liquid composition, but is independent of droplet size and substrate thickness. This shape is determined by a balance of interfacial tensions at the contact line and provides a novel method for direct determination of the surface stresses of soft substrates. Moreover, we measure the change in contact angle with droplet radius and show that Young's law fails for small droplets when their radii approach an elastocapillary length scale. For larger droplets the macroscopic contact angle is constant, consistent with Young's law. DOI: 10.1103/PhysRevLett.110.066103

  • 24. Style, Robert W.
    et al.
    Che, Yonglu
    Park, Su Ji
    Weon, Byung Mook
    Je, Jung Ho
    Hyland, Callen
    German, Guy K.
    Power, Michael P.
    Wilen, Larry A.
    Wettlaufer, John S.
    Stockholm University, Nordic Institute for Theoretical Physics (Nordita). Yale Univversity, USA.
    Dufresne, Eric R.
    Patterning droplets with durotaxis2013In: Proceedings of the National Academy of Sciences of the United States of America, ISSN 0027-8424, E-ISSN 1091-6490, Vol. 110, no 31, p. 12541-12544Article in journal (Refereed)
    Abstract [en]

    Numerous cell types have shown a remarkable ability to detect and move along gradients in stiffness of an underlying substrate-a process known as durotaxis. The mechanisms underlying durotaxis are still unresolved, but generally believed to involve active sensing and locomotion. Here, we show that simple liquid droplets also undergo durotaxis. By modulating substrate stiffness, we obtain fine control of droplet position on soft, flat substrates. Unlike other control mechanisms, droplet durotaxis works without imposing chemical, thermal, electrical, or topographical gradients. We show that droplet durotaxis can be used to create large-scale droplet patterns and is potentially useful for many applications, such as microfluidics, thermal control, and microfabrication.

  • 25. Toppaladoddi, S.
    et al.
    Wettlaufer, John S.
    Stockholm University, Nordic Institute for Theoretical Physics (Nordita). University of Oxford, UK; Yale University, USA.
    The combined effects of shear and buoyancy on phase boundary stability2019In: Journal of Fluid Mechanics, ISSN 0022-1120, E-ISSN 1469-7645, Vol. 868, p. 648-665Article in journal (Refereed)
    Abstract [en]

    We study the effects of externally imposed shear and buoyancy driven flows on the stability of a solid-liquid interface. A linear stability analysis of shear and buoyancy-driven flow of a melt over its solid phase shows that buoyancy is the only destabilizing factor and that the regime of shear flow here, by inhibiting vertical motions and hence the upward heat flux, stabilizes the system. It is also shown that all perturbations to the solid-liquid interface decay at a very modest shear flow strength. However, at much larger shear-flow strength, where flow instabilities coupled with buoyancy might enhance vertical motions, a re-entrant instability may arise.

  • 26. Toppaladoddi, Srikanth
    et al.
    Succi, Sauro
    Wettlaufer, John S.
    Stockholm University, Nordic Institute for Theoretical Physics (Nordita). Yale University, USA; University of Oxford, UK.
    Roughness as a Route to the Ultimate Regime of Thermal Convection2017In: Physical Review Letters, ISSN 0031-9007, E-ISSN 1079-7114, Vol. 118, no 7, article id 074503Article in journal (Refereed)
    Abstract [en]

    We use highly resolved numerical simulations to study turbulent Rayleigh-Benard convection in a cell with sinusoidally rough upper and lower surfaces in two dimensions for Pr = 1 and Ra = [4 x 10(6), 3 x 10(9)]. By varying the wavelength. at a fixed amplitude, we find an optimal wavelength lambda(opt) for which the Nusselt-Rayleigh scaling relation is (Nu - 1 proportional to Ra-0.483), maximizing the heat flux. This is consistent with the upper bound of Goluskin and Doering [J. Fluid Mech. 804, 370 (2016)] who prove that Nu can grow no faster than O(Ra-1/2) as Ra -> infinity, and thus with the concept that roughness facilitates the attainment of the so-called ultimate regime. Our data nearly achieve the largest growth rate permitted by the bound. When lambda << lambda(opt) and lambda >> lambda(opt), the planar case is recovered, demonstrating how controlling the wall geometry manipulates the interaction between the boundary layers and the core flow. Finally, for each Ra, we choose the maximum Nu among all., thus optimizing over all lambda, to find Nu(opt) - 1 = 0.01xRa(0.444).

  • 27. Toppaladoddi, Srikanth
    et al.
    Wettlaufer, John S.
    Stockholm University, Nordic Institute for Theoretical Physics (Nordita). Yale University, USA; University of Oxford, United Kingdom.
    Penetrative convection at high Rayleigh numbers2018In: Physical review fluids, ISSN 2469-990X, Vol. 3, no 4, article id 043501Article in journal (Refereed)
    Abstract [en]

    We study penetrative convection of a fluid confined between two horizontal plates, the temperatures of which are such that a temperature of maximum density lies between them. The range of Rayleigh numbers studied is Ra = [10(6),10(8)] and the Prandtl numbers are Pr = 1 and 11.6. An evolution equation for the growth of the convecting region is obtained through an integral energy balance. We identify a new nondimensional parameter, Lambda, which is the ratio of temperature difference between the stable and unstable regions of the flow; larger values of Lambda denote increased stability of the upper stable layer. We study the effects of Lambda on the flow field using well-resolved lattice Boltzmann simulations and show that the characteristics of the flow depend sensitively upon it. For the range Lambda = [0.01,4], we find that for a fixed Ra the Nusselt number, Nu, increases with decreasing Lambda. We also investigate the effects of Lambda on the vertical variation of convective heat flux and the Brunt-Vaisala frequency. Our results clearly indicate that in the limit Lambda -> 0 the problem reduces to that of the classical Rayleigh-Benard convection.

  • 28. Toppaladoddi, Srikanth
    et al.
    Wettlaufer, John S.
    Stockholm University, Nordic Institute for Theoretical Physics (Nordita). Yale University, USA; University of Oxford, UK.
    Statistical Mechanics and the Climatology of the Arctic Sea Ice Thickness Distribution2017In: Journal of statistical physics, ISSN 0022-4715, E-ISSN 1572-9613, Vol. 167, no 3-4, p. 683-702Article in journal (Refereed)
    Abstract [en]

    We study the seasonal changes in the thickness distribution of Arctic sea ice, g(h), under climate forcing. Our analytical and numerical approach is based on a Fokker-Planck equation for g(h) (Toppaladoddi and Wettlaufer in Phys Rev Lett 115(14): 148501, 2015), in which the thermodynamic growth rates are determined using observed climatology. In particular, the Fokker-Planck equation is coupled to the observationally consistent thermodynamic model of Eisenman and Wettlaufer (Proc Natl Acad Sci USA 106: 28-32, 2009). We find that due to the combined effects of thermodynamics and mechanics, g(h) spreads during winter and contracts during summer. This behavior is in agreement with recent satellite observations from CryoSat-2 (Kwok and Cunningham in Philos Trans R Soc A 373(2045): 20140157, 2015). Because g(h) is a probability density function, we quantify all of the key moments (e. g., mean thickness, fraction of thin/thick ice, mean albedo, relaxation time scales) as greenhouse-gas radiative forcing, Delta F-0, increases. The mean ice thickness decays exponentially with Delta F-0, but much slower than do solely thermodynamic models. This exhibits the crucial role that ice mechanics plays in maintaining the ice cover, by redistributing thin ice to thick ice-far more rapidly than can thermal growth alone.

  • 29. Toppaladoddi, Srikanth
    et al.
    Wettlaufer, John S.
    Stockholm University, Nordic Institute for Theoretical Physics (Nordita). Yale University, USA; University of Oxford, UK.
    Theory of the Sea Ice Thickness Distribution2015In: Physical Review Letters, ISSN 0031-9007, E-ISSN 1079-7114, Vol. 115, no 14, article id 148501Article in journal (Refereed)
    Abstract [en]

    We use concepts from statistical physics to transform the original evolution equation for the sea ice thickness distribution g(h) from Thorndike et al. into a Fokker-Planck-like conservation law. The steady solution is g(h) = N(q)h(q)e(-h/H), where q and H are expressible in terms of moments over the transition probabilities between thickness categories. The solution exhibits the functional form used in observational fits and shows that for h << 1, g(h) is controlled by both thermodynamics and mechanics, whereas for h >> 1 only mechanics controls g(h). Finally, we derive the underlying Langevin equation governing the dynamics of the ice thickness h, from which we predict the observed g(h). The genericity of our approach provides a framework for studying the geophysical-scale structure of the ice pack using methods of broad relevance in statistical mechanics.

  • 30.
    Ulvenblad, Pia
    et al.
    Halmstad University, School of Business, Engineering and Science, Centre for Innovation, Entrepreneurship and Learning Research (CIEL), Knowledge Entrepreneurship and Enterprise Research (KEEN).
    Berggren, Eva
    Halmstad University, School of Business, Engineering and Science, Centre for Innovation, Entrepreneurship and Learning Research (CIEL), Knowledge Entrepreneurship and Enterprise Research (KEEN).
    Winborg, Joakim
    Halmstad University, School of Business, Engineering and Science, Centre for Innovation, Entrepreneurship and Learning Research (CIEL), Knowledge Entrepreneurship and Enterprise Research (KEEN).
    The role of entrepreneurship education and start-up experience for handling communication and liability of newness2013In: International Journal of Entrepreneurial Behaviour & Research, ISSN 1355-2554, E-ISSN 1758-6534, Vol. 19, no 2, p. 187-209Article in journal (Refereed)
    Abstract [en]

    Purpose – The aim of this study is to test the assumption that ability to handle communication and iability of newness (LoN) is enhanced by academic entrepreneurship education and/or previous start-upexperience.

    Design/methodology/approach – The data collection includes a questionnaire with a total sample of 392 responding entrepreneurs in Sweden. Statistical analyses are made between entrepreneurs with academic entrepreneurship education respectively previous start-up experience. Findings – The findings show that entrepreneurs with experience from entrepreneurship education report more developed communicative skills in the dimensions of openness as well as adaptation, whereas the dimension of other-orientation is found to be learned by previous start-up experience. When it comes to perceived problems related to LoN the differences between the groups were not as strong as assumed. However, the differences observed imply that also for handling LoN the authors identify a combined effect of possessing start-up experience as well as experience from entrepreneurship education. Consequently, entrepreneurs with experience from both, show in total the most elaborated skills.

    Practical implications – One way to improve future entrepreneurship educations is to make students more aware of the mutual profit in a business agreement and how to communicate this in a marketing situation. Another suggestion is to include starting business as a course work.

    Originality/value – This study not only meets the call for actual outcome from entrepreneurship educations in terms of changed behaviour but also for interdisciplinary research in the entrepreneurship field in integrating leadership research with focus on communication.

  • 31.
    Ulvenblad, Pia
    et al.
    Halmstad University, School of Business and Engineering (SET), Centre for Innovation, Entrepreneurship and Learning Research (CIEL), Knowledge Entrepreneurship and Enterprise Research (KEEN).
    Blomkvist, Marita
    Halmstad University, School of Business and Engineering (SET), Centre for Innovation, Entrepreneurship and Learning Research (CIEL), Knowledge Entrepreneurship and Enterprise Research (KEEN).
    Winborg, Joakim
    Halmstad University, School of Business and Engineering (SET), Centre for Innovation, Entrepreneurship and Learning Research (CIEL), Knowledge Entrepreneurship and Enterprise Research (KEEN).
    Academic entrepreneurship - The structure of incubator management and best practice reported on Swedish business incubators’ web sites2010Conference paper (Other academic)
    Abstract [en]

    The aim of this paper is to describe the extent and content of information regarding incubator management structure, selection, support and network mediation on Swedish incubators websites and analyse connections between different incubator management structure and the content of incubator best practice. The data is based on information reported on 44 incubator websites in Sweden processed within the SPSS system. The findings show that incubators with more male representation in board and coach/advisor personnel report a more active part in business support including network mediation.

  • 32.
    Ulvenblad, Pia
    et al.
    Halmstad University, School of Business and Engineering (SET), Centre for Innovation, Entrepreneurship and Learning Research (CIEL), Knowledge Entrepreneurship and Enterprise Research (KEEN).
    Blomkvist, Marita
    Halmstad University, School of Business and Engineering (SET), Centre for Innovation, Entrepreneurship and Learning Research (CIEL), Knowledge Entrepreneurship and Enterprise Research (KEEN).
    Winborg, Joakim
    Halmstad University, School of Business and Engineering (SET), Centre for Innovation, Entrepreneurship and Learning Research (CIEL), Knowledge Entrepreneurship and Enterprise Research (KEEN).
    Academic entrepreneurship: the structure of incubator management and best practice reported on Swedish business incubators websites2011In: International Journal of Entrepreneurship and Small Business, ISSN 1476-1297, E-ISSN 1741-8054, Vol. 12, no 4, p. 445-458Article in journal (Refereed)
    Abstract [en]

    The aim of this paper is to describe the extent and content of information regarding incubator management structure, selection, support and network mediation on Swedish incubators websites and analyse connections between how different incubator management structure have an impact on the content of incubator best practice. The data is based on information reported on 44 incubator websites in Sweden processed within the SPSS system. The findings show that incubators with more male representation in board and coach/advisor personnel report a more active part in business support including network mediation.

  • 33.
    Ulvenblad, Pia
    et al.
    Halmstad University, School of Business, Engineering and Science, Centre for Innovation, Entrepreneurship and Learning Research (CIEL), Knowledge Entrepreneurship and Enterprise Research (KEEN).
    Winborg, Joakim
    Halmstad University, School of Business, Engineering and Science, Centre for Innovation, Entrepreneurship and Learning Research (CIEL), Knowledge Entrepreneurship and Enterprise Research (KEEN).
    Billström, Anders
    Halmstad University, School of Business, Engineering and Science, Centre for Innovation, Entrepreneurship and Learning Research (CIEL), Knowledge Entrepreneurship and Enterprise Research (KEEN).
    Entrepreneurs’ perceptions of stakeholder importance and timing - A Pecking Order Stakeholder (POS) perspective of incubator businesses2012Conference paper (Refereed)
  • 34. Weady, Scott
    et al.
    Agarwal, Sahil
    Wilen, Larry
    Wettlaufer, John S.
    Stockholm University, Nordic Institute for Theoretical Physics (Nordita). Yale University, USA; University of Oxford, UK;.
    Circuit bounds on stochastic transport in the Lorenz equations2018In: Physics Letters A, ISSN 0375-9601, E-ISSN 1873-2429, Vol. 382, no 26, p. 1731-1737Article in journal (Refereed)
    Abstract [en]

    In turbulent Rayleigh-Benard convection one seeks the relationship between the heat transport, captured by the Nusselt number, and the temperature drop across the convecting layer, captured by the Rayleigh number. In experiments, one measures the Nusselt number for a given Rayleigh number, and the question of how close that value is to the maximal transport is a key prediction of variational fluid mechanics in the form of an upper bound. The Lorenz equations have traditionally been studied as a simplified model of turbulent Rayleigh-Benard convection, and hence it is natural to investigate their upper bounds, which has previously been done numerically and analytically, but they are not as easily accessible in an experimental context. Here we describe a specially built circuit that is the experimental analogue of the Lorenz equations and compare its output to the recently determined upper bounds of the stochastic Lorenz equations [1]. The circuit is substantially more efficient than computational solutions, and hence we can more easily examine the system. Because of offsets that appear naturally in the circuit, we are motivated to study unique bifurcation phenomena that arise as a result. Namely, for a given Rayleigh number, we find a reentrant behavior of the transport on noise amplitude and this varies with Rayleigh number passing from the homoclinic to the Hopf bifurcation.

  • 35. Wells, Andrew J.
    et al.
    Wettlaufer, J. S.
    Stockholm University, Nordic Institute for Theoretical Physics (Nordita).
    Orszag, Steven A.
    Nonlinear mushy-layer convection with chimneys: stability and optimal solute fluxes2013In: Journal of Fluid Mechanics, ISSN 0022-1120, E-ISSN 1469-7645, Vol. 716, p. 203-227Article in journal (Refereed)
    Abstract [en]

    We model buoyancy-driven convection with chimneys - channels of zero solid fraction - in a mushy layer formed during directional solidification of a binary alloy in two dimensions. A large suite of numerical simulations is combined with scaling analysis in order to study the parametric dependence of the flow. Stability boundaries are calculated for states of finite-amplitude convection with chimneys, which for a narrow domain can be interpreted in terms of a modified Rayleigh number criterion based on the domain width and mushy-layer permeability. For solidification in a wide domain with multiple chimneys, it has previously been hypothesized that the chimney spacing will adjust to optimize the rate of removal of potential energy from the system. For a wide variety of initial liquid concentration conditions, we consider the detailed flow structure in this optimal state and derive scaling laws for how the flow evolves as the strength of convection increases. For moderate mushy-layer Rayleigh numbers these flow properties support a solute flux that increases linearly with Rayleigh number. This behaviour does not persist indefinitely, however, with porosity-dependent flow saturation resulting in sublinear growth of the solute flux for sufficiently large Rayleigh numbers. Finally, we consider the influence of the porosity dependence of permeability, with a cubic function and a Carman-Kozeny permeability yielding qualitatively similar system dynamics and flow profiles for the optimal states.

  • 36.
    Wettlaufer, John S.
    Stockholm University, Nordic Institute for Theoretical Physics (Nordita). Yale University, USA; University of Oxford, UK.
    Surface phase transitions in ice: from fundamental interactions to applications2019In: Philosophical Transactions. Series A: Mathematical, physical, and engineering science, ISSN 1364-503X, E-ISSN 1471-2962, Vol. 377, no 2146, article id 20180261Article, review/survey (Refereed)
    Abstract [en]

    Interfaces divide all phases of matter and yet in most practical settings it is tempting to ignore their energies and the associated implications. There are many reasons for this, not the least of which is the introduction of a new pair of canonically conjugate variables-interfacial energy and its counterpart the surface area. A key set of questions surrounding the treatment of multiphase flows concerns how and when we must account for such effects. I begin this discussion with an abbreviated review of the basic theory of lower-dimensional phase transitions and describe a range of situations in which the bulk behaviour of a two-phase (and in some cases two-component) system is dominated by surface effects. Then I discuss a number of settings in which the bulk and surface behaviour can interact on equal footing. These can include the dynamic and thermodynamic behaviour of floating sea ice, the freezing and drying of colloidal suspensions (such as soil) and the mechanisms of protoplanetesimal formation by inter-particle collisions in accretion discs. This article is part of the theme issue 'The physics and chemistry of ice: scaffolding across scales, from the viability of life to the formation of planets'.

  • 37.
    Winborg, Joakim
    Halmstad University, School of Business and Engineering (SET), Centre for Innovation, Entrepreneurship and Learning Research (CIEL).
    Pengar är inte allt: sociala nätverk skapar viktiga resurser2004In: Lokal ekonomi för hållbar tillväxt, Stockholm: Verket för näringslivsutveckling (NUTEK) , 2004, p. 93-98Chapter in book (Other (popular science, discussion, etc.))
    Abstract [sv]

    Utbildningen av potentiella och etablerade företagare bör ge dessa kreativa finansieringsmöjligheter mer utrymme. Idag läggs mycket tid och energi på att lära företagare hur man skriver och presenterar en affärsplan riktad till externa finansiärer. Genom att från början utgå från företagets aktiviteter och de resursbehov dessa skapar kan alternativa lösningar identifieras innan extern finansiering blir aktuellt. Finansiärer som inriktar sig på nya och små företag har mycket att vinna på att beakta värdet av olika bootstrappingmetoder för ett företags utveckling. att beakta värdet av olika bootstrappingmetoder för ett företags utveckling. Finansiell bootstrapping innebär att företaget kommer åt och utnyttjar andras resurser utan att detta syns i företagets balansräkning. Detta innebär att ett före-resurser utan att detta syns i företagets balansräkning. Detta innebär att ett företag i många fall är starkare än vad som framgår av dess balansräkning. Vid värderingen av nya och små företag bör finansiärer om möjligt beakta före-Vid värderingen av nya och små företag bör finansiärer om möjligt beakta företagets position i olika nätverk av företag. Företagets nätverksposition är näm-tagets position i olika nätverk av företag. Företagets nätverksposition är nämligen mycket viktig för möjligheterna att anskaffa de resurser företaget behöver. ligen mycket viktig för möjligheterna att anskaffa de resurser företaget behöver. Företagets position i olika nätverk inverkar också på möjligheterna att dra till sig resurser på förmånliga villkor. Företag som återfinns i centrum av ett nätverk har, allt annat lika, bättre möjligheter att bli framgångsrika.

  • 38.
    Winborg, Joakim
    Halmstad University, School of Business and Engineering (SET), Centre for Innovation, Entrepreneurship and Learning Research (CIEL).
    Use of financial bootstrapping in new businesses: a question of last resort?2009In: Venture Capital: an International Journal of Entrepreneurial Finance, ISSN 1369-1066, E-ISSN 1464-5343, Vol. 11, no 1, p. 71-83Article in journal (Refereed)
    Abstract [en]

    This study examines motives for using financial bootstrapping in new businesses. First, it identifies and labels groups of new business founders based on their motives for using bootstrapping. Second, it examines the relation between variables referring to the founder and the business and the motives. The data were collected in a questionnaire sent by post to 120 new business founders in Swedish business incubators. The results show that 'lower costs' is the most important motive, followed by 'lack of capital', and, surprisingly, 'fun helping others and getting help from others'. On the basis of a cluster analysis three groups of founders were identified, based on differences in their motives for using bootstrapping. The groups were labeled cost-reducing bootstrappers, capital-constrained bootstrappers and risk-reducing bootstrappers. The relative experience of the founder is the most significant influence for using bootstrapping. As experience is gained the new business founder learns more about the advantages and motives for using bootstrapping. The resource acquisition behavior changes from initially focusing on reducing costs towards a proactive focus on reducing the risk in the business.

  • 39.
    Winborg, Joakim
    Halmstad University, School of Business and Engineering (SET), Centre for Innovation, Entrepreneurship and Learning Research (CIEL), Knowledge Entrepreneurship and Enterprise Research (KEEN).
    Why do they use financial bootstrapping?: a quantitative study of new business managers2008In: Entrepreneurship, Sustainable Growth and Performance: Frontiers in European Entrepreneurship Research / [ed] Hans Landström, Hans Crijns, Eddy Laveren & David Smallbone, Cheltenham: Edward Elgar Publishing, 2008, p. 77-90Chapter in book (Refereed)
  • 40.
    Winborg, Joakim
    et al.
    Halmstad University, School of Business, Engineering and Science, Centre for Innovation, Entrepreneurship and Learning Research (CIEL), Knowledge Entrepreneurship and Enterprise Research (KEEN).
    Landström, Hans
    Halmstad University, School of Business, Engineering and Science, Centre for Innovation, Entrepreneurship and Learning Research (CIEL).
    Financial bootstrapping in small businesses - A resource-based view on small business finance1997In: Frontiers of entrepreneurship research 1997: proceedings of the Seventeenth Annual Entrepreneurship Research Conference / [ed] Reynolds, PD; Bygrave, WD; Carter, NM; Davidsson, P; Gartner, WB; Mason, CM; McDougall, PP, Babson Park: Center for Entrepreneurial Studies, Babson College , 1997, p. 471-485Conference paper (Refereed)
    Abstract [en]

    It seems fair to argue that a major parr of the research in small business finance has been focused on the supply of capital, departuring from a rather narrow definition of finance referring mainly to "capital" as such. In our opinion research in small business finance has to originate from the small business manager's own logic, and the definition of finance has to be extended to include the different resources needed in the business. In line with this reasoning this study focuses on small business managers' use of measures in order to meet the need for resources without using external capital from institutional sources, called financial bootstrapping measures. The focus on resources needed makes us believe that the resource-based theory can be fruitful in order to help us understand small business finance. The research process was initiated with a number of exploratory interviews. On the basis of this empirical framework, together with a literature study, a questionnaire was constructed and sent to 900 small business managers in Sweden. From the explorative interviews a total of 32 different bootstrapping measures were identified. The bootstrapping measures were separated into two comprehensive groups of measures; (i) measures with the aim of reducing need for capital, and (ii) measures used in order to meet need for capital. The cluster analysis undertaken resulted in the identification of six clusters of bootstrappers, differing fundamentally from each other with respect to the use of bootstrapping measures. Further, independent variables discriminating between the six clusters were isolated in order to get a picture of the typical business in each cluster. On the basis of these pictures the six clusters were labelled: (1) delaying bootstrappers, (2) relationship oriented bootstrappers, (3) subsidy bootstrappers, (4) minimizing bootstrappers, (5) non-bootstrappers and finally, (6) the private owner financed bootstrappers. For future research and policy making we would like to emphasize the importance of broadening the focus when discussing small business finance, to include the small business manager's own logic encompassing the resource acquisition process as such, in order to better understand the way small business managers handle capital requirements.

  • 41.
    Winborg, Joakim
    et al.
    Halmstad University, School of Business, Engineering and Science, Centre for Innovation, Entrepreneurship and Learning Research (CIEL).
    Landström, Hans
    Halmstad University, School of Business and Engineering (SET).
    Financial bootstrapping in small businesses: Examining small business managers' resource acquisition behaviors2001In: Journal of Business Venturing, ISSN 0883-9026, E-ISSN 1873-2003, Vol. 16, no 3, p. 235-254Article in journal (Refereed)
    Abstract [en]

    In recent years, small businesses have received much attention from policy makers and researchers, in that these businesses are considered important for economic growth and job creation. At the same time small businesses are assumed to face major problems in securing long-term external finance, which is regarded as restraining their development and growth. Small business managers are assumed to use institutional finance as a means of meeting the need for resources, and as a consequence the major part of the research on small business finance has focused on constraints in the supply of institutional (market) finance. As we see it, most small business managers handle the need for resources using means other than external finance by applying different kinds of financial bootstrapping methods. Financial bootstrapping refers to the use of methods for meeting the need for resources without relying on long-term external finance from debt holders and/or new owners. However, these other means of resource acquisition have, with few exceptions, not been focused on within earlier research on small business finance. Against this background, the purpose of this study is to describe small business managers' use of different financial bootstrapping methods, and, more importantly, to develop concepts that can help Its better understand small business managers' financial bootstrapping behaviors. The research process was initiated with a number of unstructured interviews conducted with small business managers, accountants, consultants, bank officials, and researchers, in order to identify different financial bootstrapping possibilities. On the basis of the interviews and an earlier study on financial bootstrapping, resulting in the identification of 32 bootstrapping methods, a questionnaire was constructed and sent to 900 small business managers in Sweden. Given the limited knowledge within the area of financial bootstrapping, the study is based on explorative factor analysis and cluster analysis. From the cluster analysis six clusters of bootstrappers were identified differing from each other with respect to the bootstrapping methods used and the characteristics of the business. On the basis of this information the different clusters were labeled: (I) delaying bootstrappers; (2) relationship-oriented bootstrappers; (3) subsidy-oriented bootstrappers; (4) minimizing bootstrappers; (5) non-bootstrappers; and (6) private owner-financed bootstrappers. The groups of financial bootstrappers show differences in their orientation toward resource acquisition, representing different aspects of art internal mode of resource acquisition a social mode of resource acquisition, and a quasi-market mode of resource acquisition. We find that the delaying bootstrappers, private owner-financed bootstrappers, and minimizing bootstrappers all represent an internal mode of resource acquisition. The relationship-oriented bootstrappers follow a socially oriented mode of resource acquisition, whereas the subsidy-oriented bootstrappers apply quasi-market oriented resource acquisition. This study contributes to our empirical understanding by providing knowledge about the financial bootstrapping methods used in small businesses Furthermore, by developing concepts this study contributes to the conceptual development of our knowledge about financial bootstrapping. The implication of this study is that financial bootstrapping is a phenomenon which deserves more attention in future research on small business finance. At the same time, financial bootstrapping behavior is probably a more general phenomenon appearing in different contexts, such as R&D activities in large businesses, financing startups, etc. Finally, the study points out implications for small business managers, consultants, teachers, etc. Practitioners often tend to focus on market solutions to resource needs. This study shows, however, that this strong focus cart be questioned. Resources needed in small businesses can in many situations be secured using financial bootstrapping methods, referring to internally oriented and socially oriented resource acquisition strategies.

  • 42.
    Winborg, Joakim
    et al.
    Halmstad University, School of Business and Engineering (SET), Centre for Innovation, Entrepreneurship and Learning Research (CIEL), Knowledge Entrepreneurship and Enterprise Research (KEEN).
    Politis, Diamanto
    Halmstad University, School of Business and Engineering (SET), Centre for Innovation, Entrepreneurship and Learning Research (CIEL), Knowledge Entrepreneurship and Enterprise Research (KEEN).
    Can bootstrapping be learnt from experience?: the role of human capital for explaining bootstrapping orientation in new businesses2009Conference paper (Other academic)
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