Symmetrical Catalytically Active Colloids Collectively Induce Convective Flow

Although much attention
has focused on self-motile asymmetrical
catalytically active “Janus” colloids as a route to
enable new fluidic transport applications, the motion of symmetrical
catalytically active colloids is less investigated. This is despite
isotropically active colloids being more accessible and commonly used
as supports for heterogeneous catalysis. Here, we addressed this by
systematically investigating the motion of platinum-coated colloids
capable of isotropically decomposing hydrogen peroxide. We observed
the onset of collective convective flow as the colloidal volume fraction
increased above a threshold. The ballistic velocities induced by the
collective flow were quantified by particle tracking and were found
to increase with the volume fraction. We also determined the associated
increase in the Péclet number as evidence of the potential
to use convection as a simple method to enhance mass transport rates.
By determining the persistence lengths, we were able to correlate
the magnitude of convective flow with the overall catalytic activity
per unit volume. This suggests that the mechanism for the collective
flow is driven by chemical activity-induced local density differences.
Finally, we discussed these results in the context of potential new
fluidic applications and highlighted the role that activity-induced
convection may play in experiments designed to investigate self-motile
catalytic systems.