A colloidal dance: synchronizing motion of brownian particles
Thomas A. Witten, University of Chicago
Watch a small flake of plastic gradually sink in a glass of water. The asymmetric flake pivots into a preferred orientation and then rotates steadily as it sinks. The reason for this rotational response to gravity is independent of length scale; thus it applies to microscopic colloidal objects as well. Analogous responses occur under other forces, eg from electric fields. The response is governed by a 3×3 matrix called the twist matrix that depends on the object’s shape and its mass distribution. The twist matrix controls the motion in much the same way as the (symmetric) inertia tensor controls the free tumbling motion of a body. However, the twist matrix gives a richer response because it is not symmetric. In this talk we explore ways to use this rotational response to organize a sample of objects so that they all rotate in unison, with the same orientation. This complete orientation can be achieved without using interaction between the particles; instead we exploit the transient pivoting response before the object settles into a fixed rotational axis. We demonstrate two types of programmed, time-dependent forcing that bring a set of randomly oriented particles into a common orientation. The first is a simple alternating rocking motion. The second is a rotating force analogous to the rotating magnetic field in an NMR device. Once oriented, the objects must respond identically to further forcing; they translate and rotate in unison.