Grand Séminaire d'Institut

Vendredi 31 Mai 2024 à 11h00.

Brownian motion near soft interfaces

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Confined motions of soft particles are ubiquitous in microbiology. Examples at the micro- and nano- scales include blood cells flowing in vessels, antibody recognition, or confined diffusion of synaptic neuro- transmitters. 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.

Aiming at unravelling the link between confined viscous flow, softness and thermal, i.e. Brownian, motion, we combine holographic microscopy and statistical inference [1]. This state-of-the art technique allows for a robust, broadband, and non-fluorescent super-resolved characterization of the three-dimensional motion of a single, free, colloid diffusing in a viscous fluid near a charged rigid wall, with nanometric pre- cision. Already in the case of a micrometric rigid polystyrene bead, striking differences compared to bulk Brownian motion are measured and quantified. Namely, the statistics of displacements deviates from Gaus- sian distributions [2]. Also, femtonewton-like bulk and surface forces, resolved at the fundamental thermal noise limit, which include screened electrostatic repulsion and weight, are extracted from the trajectories.

The novel case of a deformable micro-sphere is even more puzzling. Specifically, we study the confined Brownian motion of single low-surface-tension viscous oil micro-droplets. While, at equilibrium, these soft droplets behave similarly as their rigid counterparts, at short time scales, we observe the emergence of novel transient piconewton-like inertia-less lift forces acting on the droplets, stemming from a thermally-induced visco-capillary coupling [3]. This type of forces could impact transient migration strategies, relevant to microbiological and nanophysical transport.

References:
[1] Lavaud, M., Salez, T., Louyer, Y., & Amarouchene, Y. (2021). Stochastic inference of surface-induced effects using Brownian motion. Physical Review Research, 3(3):L032011, July 2021.
[2] Alexandre, A., Lavaud, M., Fares, N., Millan, E., Louyer, Y., Salez, T., Amarouchene, Y., Guérin, T., & Dean, D. S. (2023). Non-Gaussian diffusion near surfaces. Physical Review Letters, 130(7), 077101.
[3] Fares, N., Lavaud, M., Zhang, Z., Jha, A., Amarouchene, Y., Salez, T. (2024). Observation of Brownian elastohydrodynamic forces acting on confined soft colloids. Submitted.
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