Mathieu Leocmach

Wall enhanced polarity makes active particles climb up

We study experimentally a sediment of self-propelled Brownian particles in contact with a vertical wall. Our system is a monolayer of micron-size gold-platinum Janus particles, which become active upon adding a solution of hydrogen peroxide due to self-phoretic propulsion mechanisms. Although the passive system does not wet the wall, we observe a particle-thick wetting layer that rises against gravity with increasing self-propulsion. Using Brownian dynamics simulations, we reproduce this phenomenon and disentangle the respective effects of activity-dependent adhesion that is specific to our system (i.e. phoretic or hydrodynamic) and of wall accumulation generic to any self-propelled particle system. We also find that wall alignment plays a complex role, either decreasing or increasing the height of the wetting layer depending on the absence or presence of wall adhesion. More precisely, we find that an aligning wall enhances polarity heterogeneities already present in the bulk. For instance, gravity-induced upward polarity in the dilute part of the bulk is enhanced at the wall, causing an upward flux at the wall. Such steady-state flux has no equivalent in a passive wetting layer.