Fabrication and characterization of bimetallic silica-based and 3D-printed active colloidal cubes

TL;DR

This study develops and characterizes bimetallic silica-based and 3D-printed active colloidal cubes with catalytic propulsion, demonstrating how material, size, and shape influence their movement and orientation in fluid environments.

Contribution

It introduces a versatile fabrication method for active colloidal cubes using silica and 3D printing, exploring their propulsion mechanisms and the effects of material and size on their behavior.

Findings

Gold layer thickness minimally affects propulsion speed.

Increased active force can counteract gravitational torque.

Speed scales with particle size due to drag effects.

Abstract

Simulations on self-propelling active cubes reveal interesting behaviors at both the individual and the collective level, emphasizing the importance of developing experimental analogs that allow to test these theoretical predictions. The majority of experimental realizations of active colloidal cubes rely on light actuation and or magnetic fields to have a persistent active mechanism, and lack material versatility. Here we propose a system of active bimetallic cubes whose propulsion mechanism is based on a catalytic reaction and study their behavior. We realize such a system from synthetic silica cuboids and 3D printed micro cubes, followed by the deposition of gold and platinum layers on their surface. We characterize the colloids dynamics for different thicknesses of the gold layer at low and high hydrogen peroxide concentrations. We show that the thickness of the base gold layer has only a minor effect on the self propulsion speed and in addition induces a gravitational torque which leads to particles with a velocity director pointing out of the plane thus effectively suppressing propulsion. We find that a higher active force can remedy the effects of torque, resulting in particle orientations that are favorable for in plane propulsion. Finally, we use 3D printing to compare our results to cubes made from a different material, size and roundness, and demonstrate that the speed scaling with increasing particle size originates from the size-dependent drag. Our experiments extend fabrication of active cubes to different materials and propulsion mechanisms and highlight that the design of active particles with anisotropic shapes requires consideration of the interplay between the shape and activity to achieve favorable sedimentation and efficient in plane propulsion.