Microscopic structure and collapse of depletion-induced gels in vesicle-polymer mixtures

We present the behavior of depletion-induced gels for vesicle-polymer mixtures when the ratio of the polymer radius of gyration to the mean vesicle radius is 0.09 and 0.27. As the polymer concentration increases, density gradients build up and an interface is developed between a highly turbid vesicle-rich phase and a polymer-rich phase. Increasing the polymer concentration further forms a gel (CP=0.3 and 0.1 wt% for Rg/a approximately 0.09 and 0.27, respectively), which subsequently collapses. This collapse is characterized by a slow initial rising for a finite delay time, a rapid collapse, and a slow final compaction to an equilibrium height. However, we observe a remarkably different polymer concentration dependence on the collapse rate. Unlike other colloidal gels, we find that the delay time for the vesicle collapse decreases with increasing polymer concentration. We show that this behavior can be accounted for by considering the permeability for solvent backflow, which is directly related to the characteristic pore area of the gel obtained using confocal microscopy.