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Capillarity and wetting of non-Newtonian droplets
KTH, School of Engineering Sciences (SCI), Mechanics.ORCID iD: 0000-0002-5915-0789
2016 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Capillarity and dynamic wetting of non-Newtonian fluids are important in many natural and industrial processes, examples cover from a daily phenomenon as splashing of a cup of yogurt to advanced technologies such as additive manufacturing. The applicable non-Newtonian fluids are usually viscoelastic compounds of polymers and solvents. Previous experiments observed diverse interesting behaviors of a polymeric droplet on a wetted substrate or in a microfluidic device. However, our understanding of how viscoelasticity affects droplet dynamics remains very limited. This work intends to shed light on viscoelastic effect on two small scale processes, i.e., the motion of a wetting contact line and droplet splitting at a bifurcation tip.

 

Numerical simulation is employed to reveal detailed information such as elastic stresses and interfacial flow field. A numerical model is built, combining the phase field method, computational rheology techniques and computational fluid dynamics. The system is capable for calculation of realistic circumstances such as a droplet made of aqueous solution of polymers with moderate relaxation time, impacting a partially wetting surface in ambient air.

 

The work is divided into three flow cases. For the flow case of bifurcation tube, the evolution of the interface and droplet dynamics are compared between viscoelastic fluids and Newtonian fluids. The splitting or non-splitting behavior influenced by elastic stresses is analyzed. For the flow case of dynamic wetting, the flow field and rheological details such as effective viscosity and normal stress difference near a moving contact line are presented. The effects of shear-thinning and elasticity on droplet spreading and receding are analyzed, under inertial and inertialess circumstances. In the last part, droplet impact of both Newtonian and viscoelastic fluids are demonstrated. For Newtonian droplets, a phase diagram is drawn to visualize different impact regions for spreading, splashing and gas entrapment. For viscoelastic droplets, the viscoelastic effects on droplet deformation, spreading radius and contact line motion are revealed and discussed.

Place, publisher, year, edition, pages
KTH Royal Institute of Technology, 2016. , 50 p.
Keyword [en]
Dynamic wetting, contact line, diffusive interface, viscoelasticity, non-Newtonian, microfluidics, droplet impact, droplet spreading
National Category
Physical Sciences
Research subject
Engineering Mechanics
Identifiers
URN: urn:nbn:se:kth:diva-184146ISBN: 978-91-7595-921-4 (print)OAI: oai:DiVA.org:kth-184146DiVA: diva2:915139
Public defence
2016-04-22, Kollegiesalen, Brinellvägen 8, Stockholm, 10:15 (English)
Opponent
Supervisors
Note

QC 20160329

Available from: 2016-03-29 Created: 2016-03-28 Last updated: 2016-04-02Bibliographically approved
List of papers
1. Viscoelastic droplet dynamics in a y-shaped capillary channel
Open this publication in new window or tab >>Viscoelastic droplet dynamics in a y-shaped capillary channel
2016 (English)In: Physics of fluids, ISSN 1070-6631, E-ISSN 1089-7666, Vol. 28, no 3, 033103- p.Article in journal (Refereed) Published
Abstract [en]

Non-Newtonian droplet dynamics commonly exist in microfluidic systems. We report simulations of viscoelastic (VE) droplets traveling in a two dimensional capillary bifurcation channel. A numerical system combining phase field method, VE rheology, and Stokes flow equations is built. As a generic microfluidic two-phase problem, we study how a non-Newtonian droplet that approaches a channel bifurcation will behave. We identify conditions for when a droplet will either split into two or be directed into one of the branches. In particular, we study the importance of the non-Newtonian properties. Our results reveal two different non-Newtonian mechanisms that can enhance splitting and non-splitting of droplets with respect to Newtonian droplets, depending on the size of droplet and capillary number.

Place, publisher, year, edition, pages
American Institute of Physics (AIP), 2016
National Category
Natural Sciences
Research subject
Engineering Mechanics
Identifiers
urn:nbn:se:kth:diva-184141 (URN)10.1063/1.4943110 (DOI)2-s2.0-84960904107 (Scopus ID)
Note

QC 20160329

Available from: 2016-03-28 Created: 2016-03-28 Last updated: 2017-11-30Bibliographically approved
2. Dynamic wetting of viscoelastic droplets
Open this publication in new window or tab >>Dynamic wetting of viscoelastic droplets
2015 (English)In: Physical Review E. Statistical, Nonlinear, and Soft Matter Physics, ISSN 1539-3755, E-ISSN 1550-2376, Vol. 92, no 4, 043002Article in journal (Refereed) Published
Abstract [en]

We conduct numerical experiments on spreading of viscoelastic droplets on a flat surface. Our work considers a Giesekus fluid characterized by a shear-thinning viscosity and an Oldroyd-B fluid, which is close to a Boger fluid with constant viscosity. Our results qualitatively agree with experimental observations in that both shear thinning and elasticity enhances contact line motion, and that the contact line motion of the Boger fluid obeys the Tanner-Voinov-Hoffman relation. Excluding inertia, the spreading speed shows strong dependence on rheological properties, such as the viscosity ratio between the solvent and the polymer suspension, and the polymeric relaxation time. We also discuss how elasticity can affect contact line motion. The molecular migration theory proposed in the literature is not able to explain the agreement between our simulations and experimental results.

Place, publisher, year, edition, pages
[Wang, Yuli; Minh, Do-Quang; Amberg, Gustav] Royal Inst Technol, Dept Mech, Linne FLOW Ctr, S-10044 Stockholm, Sweden. [Wang, Yuli] Jiangsu Univ, Sch Energy & Power Engn, Zhenjiang 212013, Peoples R China.: , 2015
National Category
Fusion, Plasma and Space Physics
Identifiers
urn:nbn:se:kth:diva-176343 (URN)10.1103/PhysRevE.92.043002 (DOI)000362445200021 ()2-s2.0-84945179169 (Scopus ID)
Note

QC 20151109

Available from: 2015-11-09 Created: 2015-11-03 Last updated: 2017-12-01Bibliographically approved
3. Events and conditions in droplet impact: a phase field prediction
Open this publication in new window or tab >>Events and conditions in droplet impact: a phase field prediction
(English)In: International Journal of Multiphase Flow, ISSN 0301-9322, E-ISSN 1879-3533Article in journal (Refereed) Submitted
National Category
Physical Sciences
Research subject
Engineering Mechanics
Identifiers
urn:nbn:se:kth:diva-184142 (URN)
Note

QS 2016

Available from: 2016-03-28 Created: 2016-03-28 Last updated: 2017-11-30Bibliographically approved
4. Impact of viscoelastic droplets
Open this publication in new window or tab >>Impact of viscoelastic droplets
(English)In: Soft Matter, ISSN 1744-683X, E-ISSN 1744-6848Article in journal (Refereed) Submitted
Abstract [en]

We conduct numerical experiments on polymeric droplets hitting a at solid surface, inrealistic circumstances. The results present time-resolved non-Newtonian stresses actingin the droplet, which so far have not been measured experimentally. Comparing withthe simulation of the impact of a Newtonian droplet, the eects of viscoelasticity ondroplet behaviors such as splashing, the maximum spreading diameter and deformationare analyzed. With detailed information on the contact region, we demonstrate how thecontact line behaves according to the transition of uid from elasticity dominated toshear-thinning dominated when a droplet expands and contracts on the substrate. Thework is inspired by several experiments on similar cases and we discuss whether and howthe elasticity in an impinging droplet takes eect.

National Category
Physical Sciences
Research subject
Engineering Mechanics
Identifiers
urn:nbn:se:kth:diva-184143 (URN)
Note

QS 20160329

Available from: 2016-03-28 Created: 2016-03-28 Last updated: 2017-11-30Bibliographically approved

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