Alignment, Rising, Sticking, and Phototaxis: Modulating the Behavior of Hematite Micropeanuts

Artificial active colloids have been an active area of research in the field of active matter and microrobotic systems. In particular, light driven semi-conductor particles have been shown to display interesting behaviors ranging from phototaxis (movement toward or away from a light source), rising from the substrate, inter-particle attraction, attraction to the substrate, or other phenomenon. However, these observations involve multiple different designs of particles in varying conditions, making it unclear how the experimental parameters, such as pH, peroxide concentration, and light intensity, affect the outcomes. In this work, a peanut-shaped hematite semi-conductor particle was shown to exhibit a rich range of behavior as a function of the experimental conditions. The particles show rising, sticking, phototaxis, and in-plane alignment of their long axes perpendicular to a magnetic field. A theoretical model accounting for gravity, van der Waals forces, electric double layer interactions with the glass surface, and self-diffusiophoresis is formulated to describe the system. Incorporating experimental data for the dependence of various properties on pH and ionic concentrations, the balance of competing effects in the model explains many of the observed behaviors, providing insight into the relevant physical phenomena and how different environmental conditions can lead to such a rich diversity of behavior.