Complex Systems Seminar<br>Orienting Colloidal Objects by Programmed Sedimentation<br><b>Speaker: Tom Witten (University of Chicago)</b>

Speaker: Tom Witten (University of Chicago)

Colloidal particles in a liquid sink or rise under gravity; that's why we have to "shake well before using." Unless these particles are especially symmetric, they also rotate as they drift. The rotation at any moment is proportional to the force via an Onsager "twist matrix" that depends on its orientation. The equations of motion resemble the Euler equations for a freely rotating rigid body, though dissipative and chiral effects alter the motion markedly. Under constant forcing a particle generally attains a state of uniform rotation. In this talk we explore what happens when gravity changes direction in a programmed way. We describe two protocols that lead to complete alignment of an ensemble of identical particles: all have the same orientation and all rotate with the same phase. Such phase coherence resembles that of spins in a pulsed nuclear magnetic resonance setup. Increasingly, as we seek to manipulate microscale biological and man-made objects en masse, it will be useful to manipulate their orientation by a uniform dynamical protocol.