Physics Colloquium - Friday, February 2nd, 2007, 4:00 P.M.

E300 Math/Science Center; Refreshments at 3:30 P.M. in Room E200

Ivan Smalyukh
University of Illinois

Probing and Controlling Order in Soft Matter: From Confined Liquid Crystals to Aligned DNA

Future technologies and devices, such as flexible displays and all-optical computers, require materials with well-controlled periodic structures in the nanometer and micrometer ranges, as well as with properties such as optical activity and anisotropy, negative refraction and mechanical flexibility. Soft materials with the varying degrees of orientational and/or positional order can meet all of these requirements. Being relatively fragile forms of matter, they respond to low electric and magnetic fields, tiny temperature variations, and can be manipulated by light. This extraordinary sensitivity to external fields is ideal for the emerging applications, which, however, critically depend on finding the means to precisely control the molecullar and colloidal self-organization as well as on the knowledge of underpinning physical mechanisms. This lecture will demonstrate the non-contact control and three-dimentional imaging of the self-organized structures, even in the materials with homogeneous chemical composition but with spatially-varying molecular orientations. Optical tweezing is employed to measure the long range elasticity-mediated forces between colloidal particles in anisotropic fluids and to characterize the structures and defects. Moreover, by using the laser beams of intensity exceeding the threshold for the molecular realignment, a variety of structures are first optically generated and then spatially translated, rotated, sorted, and organized into superstructures such as periodic arrays. To visualize the three-dimensional patterns of molecular orientations, the fluorescence confocal imaging with well-controlled polarization states of probing light was performed and the special dye molecules following orientations of the materials host molecules were used. The optical imaging and trapping techniques provide the insights into the phenomena associated with field-induced transitions in liquid crystals, colloidal interactions and self-organization, and formation of periodic pattern by the DNA biopolymers. The laser-assisted molecular and colloidal self-assembly into periodic dielectric structures is lighting the way to the new photonic and electro-optic applications.