Chemokinesis-Driven Accumulation of Active Colloids in Low-Mobility Regions of Fuel Gradients

TL;DR

This study demonstrates that active colloids can accumulate in low-mobility regions of fuel gradients through chemokinesis, a non-directional motility response, offering a new approach for targeted colloid manipulation.

Contribution

It reveals chemokinesis as a mechanism for colloid accumulation in low-mobility zones, supported by simulations, theory, and experiments, distinct from chemotaxis.

Findings

Colloids accumulate in low-speed, salt-rich regions due to chemokinesis.

Simulation, theoretical model, and experiments are in agreement.

Chemokinesis enables targeted accumulation without directional sensing.

Abstract

Many motile cells exhibit migratory behaviors, such as chemotaxis (motion up or down a chemical gradient) or chemokinesis (when speed depends on concentration), which enable them to carry out vital functions including immune response, egg fertilization, and predator evasion. These have inspired researchers to develop self-propelled colloidal analogues to biological microswimmers, known as active colloids, that perform similar feats. Here, we study the behavior of half-platinum half-gold (Pt/Au) self-propelled rods in antiparallel gradients of hydrogen peroxide fuel and salt (which tends to slow the rods). Brownian Dynamics simulations, a Fokker-Planck theoretical model, and experiments demonstrate that the rods accumulate in low-speed (salt-rich, peroxide-poor) regions not because of chemotaxis, but because of chemokinesis. Chemokinesis is distinct from chemotaxis in that no directional sensing or reorientation capabilities are required. The agreement between simulations, model, and experiments bolsters the role of chemokinesis in this system. This work suggests a novel strategy of exploiting chemokinesis to effect accumulation of motile colloids in desired areas.