posted on 2021-01-27, 14:38authored byGaëlle Rondepierre, Franz De Soete, Nicolas Passade-Boupat, François Lequeux, Laurence Talini, Laurent Limat, Emilie Verneuil
We
report on the contact line dynamics of a triple-phase system
silica/oil/water. When oil advances onto silica within a water film
squeezed between oil and silica, a rim forms in water and recedes
at constant velocity. We evidence a sharp (three orders of magnitude)
decrease of the contact line velocity upon the addition of cationic
surfactants above a threshold concentration, which is slightly smaller
than the critical micellar concentration. We show that, with or without
surfactant, and within the range of small capillary numbers investigated,
the contact line dynamics can be described by a friction term that
does not reduce to pure hydrodynamical effects. In addition, we derive
a model that successfully accounts for the selected contact line velocity
of the rim. We further demonstrate the strong increase of the friction
coefficient with surfactant bulk concentration results from the strongly
nonlinear adsorption isotherm of surfactants on silica. From the variations
of the friction coefficient and spreading parameter with surface concentration,
we suggest a picture in which the part of the adsorbed surfactants
that are strongly bound to the silica interface is trapped under the
oil droplet and is responsible for the large increase in line friction.