(200c) Brownian Motion of Janus and Patchy Particle Swimmers

The ability to control and drive particle motion at the micro- and nanometer scale has fascinated researchers across the world for several decades and has recently received renewed attention due to the advent of Janus particle research. A Janus particle is a particle with two chemically or physically distinct halves. If one of the two halves is chosen such that it is reactive toward a solute molecule present in the solution surrounding the Janus particle, an asymmetric concentration gradient of product molecules is formed. The asymmetric concentration gradient results in an uneven body force experienced by the Janus particle leading to the directed, ballistic motion of the particle, a process also called diffusiophoresis. Diffusiophoretic velocities of a few microns per second have been reported. Unfortunately, the presence of Brownian rotation leads to a randomization of the motion at long time steps and prohibits the use of this motion for directional transport.

   Over the past few years, we have studied the behavior of patchy particles, i.e., particles with reactive material patches ranging from 11 to 50% of the particle surface. As expected, we find a reduced velocity for particles with smaller patches due to the less pronounced concentration gradient. We find that the velocity is particle radius independent when 2.4 and 5 μm particles are studied in good agreement with predictions by Golastanian. Surprisingly, we find that the velocity of the patchy particles does not decay as fast as the decrease in patch sizes would suggest, but rather that patchy particles exhibit an enhanced diffusiophoretic velocity. Careful analysis of the experimental data reveals that the patch asymmetry is the reason for the enhanced velocities and a potential explanation involving hindered Brownian rotation of these particles near solid surfaces will be presented that could potentially enable the directed motion of Janus and Patchy particles over much longer distances.