Sedimentation of self-propelled Janus colloids: polarization and pressure

TL;DR

This paper investigates sedimentation behavior of active Janus colloids through experiments and theory, revealing polarization phenomena and comparing different pressure definitions in dilute active systems.

Contribution

It provides experimental validation of polarization in sedimenting active colloids and compares multiple pressure concepts with theoretical predictions.

Findings

Polarization emerges outside the effective equilibrium regime.

Experimental polarization distribution matches theoretical predictions without fitting.

Different pressure definitions are compared and discussed.

Abstract

We study experimentally-using Janus colloids-and theoretically-using Active Brownian Particles- the sedimentation of dilute active colloids. We first confirm the existence of an exponential density profile. We show experimentally the emergence of a polarized steady state outside the effective equilibrium regime, i.e. when v_s is not much smaller than the propulsion speed. The experimental distribution of polarization is very well described by the theoretical prediction with no fitting parameter. We then discuss and compare three different definitions of pressure for sedimenting particles: the weight of particles above a given height, the flux of momentum and active impulse, and the force density measured by pressure gauges.