Drop Squeezing through Interparticle Constrictions with Insoluble Surfactant

Despite the prevalence of surfactants in confined biological and subsurface settings, their
influence on drop dynamics under tight squeezing conditions remains largely uncharacterized.
Using a fully three-dimensional boundary-integral algorithm, the interfacial behavior of
surfactant-laden drops squeezing through tight constrictions in uniform far-field flow is
modeled under an extensive range of fluid and and surfactant properties. 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, due to the
interplay between drop hydrodynamics and Marangoni stresses. The presence of surfactant,
even at low degrees of contamination, is found to significantly decrease the critical capillary
number for droplet trapping, due to the accumulation of surfactant at the downwind pole of the
drop and its subsequent elongation. 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. Increasing the degree of contamination decreases drop squeezing
times, up to a maximum value above which the addition of surfactant negligibly affects
squeezing dynamics.