Apparent Slip in Presence of Surfactant and Remobilisation: It has been recently shown that traces of surfactants in liquid accumulate at the triple contact lines on downstream edges of menisci. This generates adverse Marangoni stresses large enough to immobilise menisci and thus eliminate lubrication from SHs [7–13], [P14]. Notably, there is long-standing literature on the surfactant-induced retardation of
gas bubbles rising in liquids per our work [P33] and references therein. Of particular interest here is the ability to remobilize menisci. Indeed, our experiments for rising bubbles [P33], and others for settling drops [14], show drag increases initially with surfactant concentration, which then decreases back down to that of a clean particle at sufficiently large concentrations due to near-uniform (albeit relatively large) concentration of surfactants. Remobilisation and removal of wake structures behind surfactant-immobilized rising gas bubbles has been shown theoretically by us ([P33], [P34], P35]). For surfactant concentrations above the critical micelle concentration (CMC) the models must be modified as we have done in related problems, e.g., [P30]. The proposed investigation of remobilization in SH microchannels is novel and will positively impact the feasibility and future of SH microchannel-based technologies. The coupling of the surfactant and velocity fields at the meniscus is a challenging multiphysics and multiscale problem; the proposing team has significant experience in such problems via analysis, computations and experiments.