Floor- or Ceiling-Sliding for Chemically Active, Gyrotactic, Sedimenting Janus Particles

Chemically
active particles achieve motility without external forces
and torques (“self-propulsion”) due to catalytic chemical
reactions at their surfaces, which change the chemical composition
of the surrounding solution (called “chemical field”)
and induce hydrodynamic flow of the solution. By coupling the distortions
of these fields back to its motion, a chemically active particle experiences
an effective interaction with confining surfaces. This coupling can
lead to a rich behavior, such as the occurrence of wall-bound steady
states of “sliding”. Most active particles are density
mismatched with the solution and, thus, tend to sediment. Moreover,
the often employed Janus spheres, which consist of an inert core material
decorated with a cap-like, thin layer of a catalyst, are gyrotactic
(i.e., “bottom-heavy”). Whether or not they may exhibit
sliding states at horizontal walls depends on the interplay between
the active motion and the gravity-driven sedimentation and alignment,
such as the gyrotactic tendency to align the axis along the gravity
direction being overcome by a competing, activity-driven alignment
with a different orientation. It is therefore important to understand
and quantify the influence of these gravity-induced effects on the
behavior of model chemically active particles moving in the vicinity
of walls. For model gyrotactic, self-phoretic Janus particles, here
we study theoretically the occurrence of sliding states at horizontal
planar walls that are either below (“floor”) or above
(“ceiling”) the particle. We construct “state
diagrams” characterizing the occurrence of such states as a
function of the sedimentation velocity and of the gyrotactic response
of the particle, as well as of the phoretic mobility of the particle.
We show that in certain cases sliding states may emerge simultaneously at both the ceiling and the floor, while the larger part of the
experimentally relevant parameter space corresponds to particles that
would exhibit sliding states only either at the floor or at the ceilingor
there are no sliding states at all. These predictions are critically
compared with the results of previous experimental studies, as well
as with our dedicated experiments carried out with Pt-coated, polystyrene-core,
or silica-core Janus spheres immersed in aqueous hydrogen peroxide
solutions.