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W. G. Pritchard Lab Seminar: 11:15 AM - 12:15 PM, 106 McAllister Bldg

	             **Monday September 10, 2007**
	
Electrohydrodynamics and epitrochoids: dynamic pattern formation of 
micron-scale objects near electrodes

William D. Ristenpart

School of Engineering & Applied Sciences
Harvard University


Abstract:

Developing techniques to manipulate small objects is crucial for designing 
microfluidic lab-on-a-chip devices for chemical synthesis and medical 
diagnostic applications. Here I present experimental and theoretical work 
on a technique based on electrohydrodynamic (EHD) flow. Application of an 
electric field induces an EHD flow around individual objects near an 
electrode. Nearby objects are mutually entrained in the respective flows, 
causing them to move toward one another. The superposition of flows around 
many objects often leads to the formation of striking patterns. For 
example, mixtures of rigid colloids with different polarizability arrange 
into morphologies including 'flowers', 'islands', and hexagonal or 
square superlattices. In contrast, suspensions composed of unilamellar 
vesicles of different sizes arrange into dynamic orbiting patterns, in 
which small vesicles orbit around larger vesicles in preferred 
orientations (i.e. 'bands'). This phenomenon is captured by a simple 
numerical model of point dipoles moving in a cellular flow, yielding a 
surprisingly rich phase diagram of trajectories including Cassini ovals 
and epitrochoids.   

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