Novel Studies of Topological Order and Pinning Effects in Colloidal Crystals

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w:\awards\awards96\*.doc 9804083 Ling This experimental research project focuses on topological order and pinning effects in colloidal crystals. Three classical problems are addressed: 1) commensurate and incommensurate phases and phase transitions, 2) stability of topological order of an elastic lattice in a random pinning potential, 3) effects of a Peierls-Nabarro potential on edge dislocations and melting kinetics. These problems are encountered in surface science, self- assembled nanostructure materials, low dimensional electron systems, disordered superconductors, micromechanics of solids, and fundamental studies of statistical mechanics. Colloidal suspensions are the media for the present study. Experiments employ digital video microscopy, in combination with advanced optical tweezers technology, to study the statics and dynamics of colloidal particles confined between two glass slides. On the glass slides, a mesoscopic structure which acts as a quenched periodic or random pinning potential for the colloidal particles will be made using lithographic techniques. An optical tweezer will be used to study the elastic and plastic responses of a colloidal crystal to a local distortion. The proposed research will benefit the undergraduate physics program, and will expose students to advanced microfabrication technology, optics, image analysis and condensed matter physics. This research program is interdisciplinary in nature and involves students, who receive excellent training in preparation for careers in industry, government laboratories or academia. %%% This experimental research project focuses on basic mechanical properties of two-dimensionsal elastic crystals. Two-dimensional crystals are ordered arrays, with some similarity to an array of marbles on a Chinese checker board, or a bedspring. The sort of question is what happens t o the overall array if one or a few of the objects are pushed; does the array smoothly distort or does it come apart. A related question is the response if, in the presence of a uniform external force, a few locations of the lattice are pinned down. Does the array continue to move, does it stop (pinned), or does it break up. The experiments in this work actually will be done on colloidal crystals, which are weakly bound ordered arrays of molecules. These crystals are relatively easy to deal with experimentally because the intermolecular forces are weak and the molecules are relatively large. The results on such a model system are expected to be applicable to many other situations. In the present work on colloidal crystals, three classical problems are addressed: 1) commensurate and incommensurate phases and phase transitions, 2) stability of topological order of an elastic lattice in a random pinning potential, 3) effects of a Peierls-Nabarro potential on edge dislocations and melting kinetics. These problems are encountered in surface science, self-assembled nanostructure materials, low dimensional electron systems, disordered superconductors, micromechanics of solids, and fundamental studies of statistical mechanics. Colloidal suspensions are the media for the present study. Experiments employ digital video microscopy, in combination with advanced optical tweezers technology, to study the statics and dynamics of colloidal particles confined between two glass slides. On the glass slides, a mesoscopic structure which acts as a quenched periodic or random pinning potential for the colloidal particles will be made using lithographic techniques. An optical tweezer will be used to study the elastic and plastic responses of a colloidal crystal to a local distortion. The proposed research will benefit the undergraduate physics program, and will expose stude nts to advanced microfabrication technology, optics, image analysis and condensed matter physics. This research program is interdisciplinary in nature and involves students, who receive excellent training in preparation for careers in industry, government laboratories or academia. ***