(323b) On the Stability and Stokesian Dynamics Simulations of Catalytically Active Colloidal Swimmers Near a Planar Wall

Mozaffari, A. - Presenter, City College of New York, Levich Institute of Physico-Chemical Hydrodynamics
Maldarelli, C. - Presenter, The City College of New York
Sharifi-Mood, N. - Presenter, University of Pennsylvania
Koplik, J. - Presenter, City College of the City University of New York

Diffusiophoretically self-propelled locomotors are a class of active colloids in which a particle autonomously swims through the liquid as a result of an unbalanced interaction with solute molecules present more on one side of the motor. This asymmetric solute distribution is maintained by a surface catalytic reaction which generates the solute on one side of the particle through the partial coating of the surface with catalytically active and inert faces.   Self-propelled locomotors have drawn a great deal of attention for their use in microscale mixing, as roving sensors and as transporters of molecular cargo and biological cells along small scale pathways. While most of the theoretical work on diffusiophoretic motion has studied the case of rectilinear propulsion through an infinite media,  applications find motors translating along landscapes with boundaries. For the simplest case of diffusiophoretic self-propulsion near a planar infinite wall with zero solute flux, and repulsive solute-colloid interactions, hydrodynamic solutions for Stokes flow have shown that that for large catalytically active areas pointed away from the wall, and for distances less than the particle radius, the particles can skim along the surface without rotation, or can become stationary. These regimes arise because the wall,  impenetrable to solute,  alters the solute gradient around the colloid by allowing build-up of solute in the gap between the wall and the colloid even when the active area faces away from the wall. The buildup changes the diffusiophoretic propulsion, and the interplay of this force with the hydrodynamic resistance of the wall allows for the skimming and stationary states.

In this presentation we first present a linear stability analysis for an active colloid at stationary and skimming states and show that the system is marginally stable at these states for high Peclet number (ratio of advection to diffusion, and proportional to the particle radius), we then investigate the effect of Brownian motion on the skimming and stationary regimes  through Stokesian dynamic simulations. We find critical values of the particle Peclet number above which deterministic skimming and stationary behavior are realized, and below which less predictable behavior is found in which the colloid can be repelled from or intersect with the wall.  Significant Brownian motion at low Peclet number can create unique regimes in which the particle ranges above and below a fixed distance from the surface.