Abstract

Colloidal motions in complex environments are ubiquitous in fundamental physics and biology. These situations invariably involve the intricate coupling between confined fluid flows, soft boundaries, surface forces and fluctuations. In the present study, such a coupling is investigated using a combination of holographic microscopy and advanced statistical inference. Specifically, the Brownian motion of soft micrometric oil droplets near rigid walls is quantitatively analyzed. All the key statistical observables are reconstructed with high precision, allowing for nanoscale resolution of local mobilities and femtonewton inference of conservative and nonconservative forces. The analysis reveals the existence of an intriguing, transient but large, soft-Brownian effect. The latter might be of importance for nanophysical and microbiological transport, target finding, or chemical reactions in crowded environments — and hence the whole life machinery.