Observation of Brownian elastohydrodynamic forces acting on confined soft colloids

Confined motions of soft particles are ubiquitous in microbiology. Examples at the micro- and nano-scales include blood cells flowing in vessels, or confined diffusion of synaptic neurotransmitters. These situations bring about viscous flows coupled to charged and soft confining entities, in presence of thermal fluctuations. Fluctuation-free scenarios have already proven the emergence of novel softness-induced near-contact forces, but the inclusion of fluctuations - crucial at microscopic scales - is yet to be explored.

We combine holographic microscopy and statistical inference [1] to characterize the three-dimensional motion of a single, free, colloid diffusing in a viscous fluid near a charged rigid wall, with nanometric precision. A rigid polystyrene colloid already behaves differently than in bulk scenarios, with statistics of displacements deviating from Gaussian statistics [2]. Also, femtonewton-like surface forces are extracted from the trajectories.

The innovative case of a soft micro-droplet is even more puzzling. While, at equilibrium, it behaves similarly as its rigid counterpart, at short time scales, we observe the emergence of novel transient piconewton-like inertia-less lift forces, stemming from a thermally-induced visco-capillary coupling [3]. We now focus on the impact of such couplings on the mobility of confined colloids, as these effects could tune transient migration strategies at microscopic scales, target finding or chemical reactions in crowded environments.