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Drops with insoluble surfactant squeezing through interparticle constrictions

Published online by Cambridge University Press:  10 September 2019

Jacob R. Gissinger ,
Alexander Z. Zinchenko Open the ORCID record for Alexander Z. Zinchenko [Opens in a new window]  and
Robert H. Davis Open the ORCID record for Robert H. Davis [Opens in a new window]
Jacob R. Gissinger
Affiliation:
Department of Chemical and Biological Engineering, University of Colorado, Boulder, CO 80309-0596, USA
Alexander Z. Zinchenko*
Affiliation:
Department of Chemical and Biological Engineering, University of Colorado, Boulder, CO 80309-0596, USA
Robert H. Davis*
Affiliation:
Department of Chemical and Biological Engineering, University of Colorado, Boulder, CO 80309-0596, USA
*
†Email addresses for correspondence: alexander.zinchenko@colorado.edu, robert.davis@colorado.edu
†Email addresses for correspondence: alexander.zinchenko@colorado.edu, robert.davis@colorado.edu
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Abstract

The interfacial behaviour of surfactant-laden drops squeezing through tight constrictions in a uniform far-field flow is modelled with respect to capillary number, drop-to-medium viscosity ratio and surfactant contamination. The surfactant is treated as insoluble and non-diffusive, and drop surface tension is related to surfactant concentration by a linear equation of state. The constriction is formed by three solid spheres held rigidly in space. A characteristic aspect of this confined and contaminated multiphase system is the rapid development of steep surfactant-concentration gradients during the onset of drop squeezing. The interplay between two physical effects of surfactant, namely the greater interface deformability due to decreased surface tension and interface immobilization due to Marangoni stresses, results in particularly rich drop-squeezing dynamics. A three-dimensional boundary-integral algorithm is used to describe drop hydrodynamics, and accurate treatment of close squeezing and trapped states is enabled by advanced singularity subtraction techniques. Surfactant transport and hydrodynamics are coupled via the surface convection equation (or convection–diffusion equation, if artificial diffusion is included), the interfacial stress balance and a solid-particle contribution based on the Hebeker representation. For extreme conditions, such as drop-to-medium viscosity ratios significantly less than unity, it is found that upwind-biased methods are the only stable approaches for modelling surfactant transport. Two distinct schemes, upwind finite volume and flow-biased least squares, are found to provide results in close agreement, indicating negligible numerical diffusion. Surfactant transport is enhanced by low drop-to-medium viscosity ratios, at which extremely sharp concentration gradients form during various stages of the squeezing process. The presence of surfactant, even at low degrees of contamination, significantly decreases the critical capillary number for droplet trapping, due to the accumulation of surfactant at the downwind pole of the drop and its subsequent elongation. Increasing the degree of contamination significantly affects surface mobility and further decreases the critical capillary number as well as drop squeezing times, up to a threshold above which the addition of surfactant negligibly affects squeezing dynamics.

JFM classification

Drops and Bubbles: Drops Low-Reynolds-number flows: Porous media Low-Reynolds-number flows: Boundary integral methods
Type
JFM Papers
Information
Journal of Fluid Mechanics , Volume 878 , 10 November 2019 , pp. 324 - 355
Copyright
© 2019 Cambridge University Press 

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