# Fabrication and Characterization of Bimetallic Silica-Based and 3D-Printed Active Colloidal Cubes

## TL;DR

Researchers created bimetallic active colloidal cubes using silica and 3D printing, and studied how their shape and material affect propulsion and sedimentation.

## Contribution

A new fabrication method for active colloidal cubes using catalytic propulsion and 3D printing, enabling material versatility and shape control.

## Key findings

- The thickness of the gold layer has a minor effect on self-propulsion speed but induces a gravitational torque during sedimentation.
- Higher active force can counteract gravitational torque, allowing for in-plane propulsion with the metal cap on the side.
- Speed scaling with particle size is due to size-dependent drag, confirmed by comparing different materials and shapes via 3D printing.

## Abstract

Simulations on self-propelling
active cubes reveal interesting
behaviors at both the individual and the collective level, emphasizing
the importance of developing experimental analogues that allow testing
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
microcubes, 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 during sedimentation. For low activity,
this gravitational torque orients the particles such that their velocity
director is pointing out of the plane, thus effectively suppressing
propulsion. We find that a higher active force can remedy the effects
of torque, resulting in all possible particle orientations, including
one with the metal cap on the side, which is 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 the 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 shape and activity
to achieve favorable sedimentation and efficient in-plane propulsion.