Active Colloids in Harmonic Optical Potentials

TL;DR

This study investigates the behavior of active colloids in harmonic optical traps through experiments, theory, and simulations, revealing how activity influences particle distribution and confinement effects.

Contribution

It introduces a method to analyze active colloids in optical traps without optical torque, highlighting the impact of activity on distribution shapes under different confinement strengths.

Findings

Active colloids follow Gaussian distributions in strong traps.

Weaker traps produce non-Gaussian, bimodal distributions.

Optical confinement can be achieved without optical torque.

Abstract

We study the motion of active patchy colloids in an optical trap using experiments, theory and numerical simulations. To achieve isotropic and harmonic confinement, we prototype microparticles with a nearly uniform refractive index and verify that, in the absence of activity, the confined motion is identical to that of optically homogeneous Brownian particles. If the activity is turned on by means of vertical AC fields, the density distributions are described by Boltzmann-like statistics (Gaussian with effective temperature) only for strongly confining traps, whereas weaker potentials give rise to non-Gaussian distributions with a bimodal shape. Our results showcase a simple way to study active soft matter in optical potential landscapes eliminating the optical torque.