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Capillarity and dynamic wetting
KTH, School of Engineering Sciences (SCI), Mechanics, Fluid Physics. (Gustav Amberg)
2012 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

In this thesis capillary dominated two–phase flow is studied by means of nu- merical simulations and experiments. The theoretical basis for the simulations consists of a phase field model, which is derived from the system’s thermody- namics, and coupled with the Navier Stokes equations. Two types of interfacial flow are investigated, droplet dynamics in a bifurcating channel and sponta- neous capillary driven spreading of drops.

Microfluidic and biomedical applications often rely on a precise control of droplets as they traverse through complicated networks of bifurcating channels. Three–dimensional simulations of droplet dynamics in a bifurcating channel are performed for a set of parameters, to describe their influence on the resulting droplet dynamics. Two distinct flow regimes are identified as the droplet in- teracts with the tip of the channel junction, namely, droplet splitting and non- splitting. A flow map based on droplet size and Capillary number is proposed to predict whether the droplet splits or not in such a geometry.

A commonly occurring flow is the dynamic wetting of a dry solid substrate. Both experiments and numerical simulations of the spreading of a drop are presented here. A direct comparison of the two identifies a new parameter in the phase field model that is required to accurately predict the experimental spreading behavior. This parameter μf [P a · s], is interpreted as a friction factor at the moving contact line. Comparison of simulations and experiments for different liquids and surface wetting properties enabled a measurement of the contact line friction factor for a wide parameter space. Values for the contact line friction factor from phase field theory are reported here for the first time.

To identify the physical mechanism that governs the droplet spreading, the different contributions to the flow are measured from the simulations. An im- portant part of the dissipation may arise from a friction related to the motion of the contact line itself, and this is found to be dominating both inertia and viscous friction adjacent to the contact line. A scaling law based on the con- tact line friction factor collapses the experimental data, whereas a conventional inertial or viscous scaling fails to rationalize the experimental observation, supporting the numerical finding.

Place, publisher, year, edition, pages
Stockholm: KTH Royal Institute of Technology, 2012. , xi, 50 p.
Series
Trita-MEK, ISSN 0348-467X ; 2012:01
National Category
Fluid Mechanics and Acoustics Computational Mathematics
Identifiers
URN: urn:nbn:se:kth:diva-91329ISBN: 978-91-7501-282-7 (print)OAI: oai:DiVA.org:kth-91329DiVA: diva2:509587
Public defence
2012-03-23, Salongen KTHB, Osquars Backe 25, KTH, Stockholm, 10:00 (English)
Opponent
Supervisors
Funder
Swedish e‐Science Research Center
Note

QC 20120313

Available from: 2012-03-13 Created: 2012-03-13 Last updated: 2013-04-09Bibliographically approved
List of papers
1. Droplet dynamics in a bifurcating channel
Open this publication in new window or tab >>Droplet dynamics in a bifurcating channel
2010 (English)In: International Journal of Multiphase Flow, ISSN 0301-9322, E-ISSN 1879-3533, Vol. 36, no 5, 397-405 p.Article in journal (Refereed) Published
Abstract [en]

In the present paper we present a phenomenological description of droplet dynamics in a bifurcating channel that is based on three-dimensional numerical experiments using the Phase Field theory. Droplet dynamics is investigated in a junction, which has symmetric outflow conditions in its daughter branches. We identify two different flow regimes as the droplets interact with the tip of the bifurcation, splitting and non-splitting. A distinct criterion for the flow regime transition is found based on the initial droplet volume and the Capillary (Ca) number. The Rayleigh Plateau instability is identified as a driving mechanism for the droplet breakup close to the threshold between the splitting and non-splitting regime.

Keyword
Droplet dynamics, Cahn-Hilliard, Bifurcating channels, Wetting, Rayleigh-Plateau instability, Passive breakup, networks, fluid, flows, tubes, simulations, deposition, transport, system, model, time
National Category
Fluid Mechanics and Acoustics
Identifiers
urn:nbn:se:kth:diva-19394 (URN)10.1016/j.ijmultiphaseflow.2010.01.002 (DOI)000276584400005 ()2-s2.0-77549084272 (Scopus ID)
Note
QC 20100525Available from: 2010-08-05 Created: 2010-08-05 Last updated: 2017-12-12Bibliographically approved
2. Modeling of dynamic wetting far from equilibrium
Open this publication in new window or tab >>Modeling of dynamic wetting far from equilibrium
2009 (English)In: Physics of fluids, ISSN 1070-6631, E-ISSN 1089-7666, Vol. 21, no 12Article in journal (Refereed) Published
Abstract [en]

In this paper we present simulations of dynamic wetting far from equilibrium based on phase field theory. In direct simulations of recent experiments [J. C. Bird, S. Mandre, and H. A. Stone, Phys. Rev. Lett. 100, 234501 (2008)], we show that in order to correctly capture the dynamics of rapid wetting, it is crucial to account for nonequilibrium at the contact line, where the gas, liquid, and solid meet. A term in the boundary condition at the solid surface that naturally arises in the phase field theory is interpreted as allowing for the establishment of a local structure in the immediate vicinity of the contact line. A direct qualitative and quantitative match with experimental data of spontaneously wetting liquid droplets is shown.

Keyword
drops, flow simulation, wetting, contact-line, interface
Identifiers
urn:nbn:se:kth:diva-19088 (URN)10.1063/1.3275853 (DOI)000273216700001 ()2-s2.0-76249091583 (Scopus ID)
Note
QC 20100525Available from: 2010-08-05 Created: 2010-08-05 Last updated: 2017-12-12Bibliographically approved
3. Dissipation in rapid dynamic wetting
Open this publication in new window or tab >>Dissipation in rapid dynamic wetting
2011 (English)In: Journal of Fluid Mechanics, ISSN 0022-1120, E-ISSN 1469-7645, Vol. 682, 213-240 p.Article in journal (Refereed) Published
Abstract [en]

In this article, we present a modelling approach for rapid dynamic wetting based on the phase field theory. We show that in order to model this accurately, it is important to allow for a non-equilibrium wetting boundary condition. Using a condition of this type, we obtain a direct match with experimental results reported in the literature for rapid spreading of liquid droplets on dry surfaces. By extracting the dissipation of energy and the rate of change of kinetic energy in the flow simulation, we identify a new wetting regime during the rapid phase of spreading. This is characterized by the main dissipation to be due to a re-organization of molecules at the contact line, in a diffusive or active process. This regime serves as an addition to the other wetting regimes that have previously been reported in the literature.

Keyword
contact lines, drops, interfacial flows (free surface)
National Category
Physical Sciences
Identifiers
urn:nbn:se:kth:diva-41294 (URN)10.1017/jfm.2011.211 (DOI)000294775800010 ()2-s2.0-80052182393 (Scopus ID)
Funder
Swedish Research Council
Note
QC 20110928Available from: 2011-09-28 Created: 2011-09-26 Last updated: 2017-12-08Bibliographically approved
4. Contact line dissipation in short-time dynamic wetting
Open this publication in new window or tab >>Contact line dissipation in short-time dynamic wetting
2012 (English)In: Europhysics letters, ISSN 0295-5075, E-ISSN 1286-4854, Vol. 97, no 4Article in journal (Refereed) Published
Abstract [en]

Dynamic wetting of a solid surface is a process that is ubiquitous in Nature, and also of increasing technological importance. The underlying dissipative mechanisms are, however, still unclear. We present here short-time dynamic wetting experiments and numerical simulations, based on a phase field approach, of a droplet on a dry solid surface, where direct comparison of the two allows us to evaluate the different contributions from the numerics. We find that an important part of the dissipation may arise from a friction related to the motion of the contact line itself, and that this may be dominating both inertia and viscous friction in the flow adjacent to the contact line. A contact line friction factor appears in the theoretical formulation that can be distinguished and quantified, also in room temperature where other sources of dissipation are present. Water and glycerin-water mixtures on various surfaces have been investigated where we show the dependency of the friction factor on the nature of the surface, and the viscosity of the liquid.

National Category
Mechanical Engineering
Identifiers
urn:nbn:se:kth:diva-91343 (URN)10.1209/0295-5075/97/44004 (DOI)000300844100016 ()2-s2.0-84857572550 (Scopus ID)
Funder
Swedish e‐Science Research Center
Note

QC 20120313

Available from: 2012-03-13 Created: 2012-03-13 Last updated: 2017-12-07Bibliographically approved
5. Universality in dynamic wetting dominated by contact-line friction
Open this publication in new window or tab >>Universality in dynamic wetting dominated by contact-line friction
2012 (English)In: Physical Review E. Statistical, Nonlinear, and Soft Matter Physics, ISSN 1539-3755, E-ISSN 1550-2376, Vol. 85, no 4, 045302- p.Article in journal (Refereed) Published
Abstract [en]

We report experiments on the rapid contact-line motion present in the early stages of capillary-driven spreading of drops on dry solid substrates. The spreading data fail to follow a conventional viscous or inertial scaling. By integrating experiments and simulations, we quantify a contact-line friction mu(f) which is seen to limit the speed of the rapid dynamic wetting. A scaling based on this contact-line friction is shown to yield a universal curve for the evolution of the contact-line radius as a function of time, for a range of fluid viscosities, drop sizes, and surface wettabilities.

National Category
Mechanical Engineering
Identifiers
urn:nbn:se:kth:diva-91361 (URN)10.1103/PhysRevE.85.045302 (DOI)000302856300001 ()2-s2.0-84860528166 (Scopus ID)
Funder
Swedish Research CouncilSwedish e‐Science Research Center
Note

QC 20120522.  Updated from submitted to published.

Available from: 2012-03-13 Created: 2012-03-13 Last updated: 2017-12-07Bibliographically approved

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