Physics Colloquium
Friday, January 29th, 2004 4 P.M.

Itai Cohen

Harvard University

Droplet formation and manipulation in microfluidic devices

Packing constraints play a crucial role in determining the structures formed by a colloidal suspension in thermodynamic equilibrium. However, many technological applications of colloidal suspensions entail application of large strains which drive the suspension out of equilibrium
and significantly modify its structure. In such flows, the complex interplay between shear induced stresses and particle packing which leads to structure formation is very poorly understood. To investigate this interplay, we have built a shear cell which can be loaded onto a confocal
microscope thus allowing us to image the 3-D microstructure of a dense colloidal suspension when it is subjected to an imposed oscillatory strain. In this talk, I will describe the dramatic restructuring of extremely dense colloidal suspensions subjected to shear and confined to a
narrow gap. I will show that confinement forces the suspension to adopt structures that include striking gaps in the particle packing which nevertheless allow the particles to pack more efficiently than those observed in bulk. I will then present a model that accounts for our observations by elucidating the interplay between shear stress, particle packing and geometric confinement that leads to these ordered but highly non-equilibrium structures.