Programming Tactic Behaviors of Active Colloids via Surface Charge.

Self-diffusiophoretic colloids move autonomously by creating chemical gradients. These self-generated gradients lead to fluid motion at the particle surface, also known as phoretic slip, which determines the direction of colloidal motion. Phoretic slip is often modeled as dependent on the local chemical gradient and a material-dependent property called phoretic mobility. Reversing the direction of propulsion would require fine-tuning of these parameters. In this work, we show that by changing the charge of the colloids which has an effect on the phoretic mobility, we can indeed achieve a reversal in the direction of propulsion of Janus micromotors. In addition, we report that by simply tuning the surface charges of colloids, we can achieve reversals in much more complex tactic behaviors such as chemotaxis and rheotaxis. This work demonstrates a method for creating programmable active colloids by subtly tuning surface properties, contributing to the understanding of active matter.