CPM Seminar

Growth and Deformation of Colloidal Crystals and Glasses

Peter Schall

Harvard University

Colloidal suspensions are of great scientific interest, motivated not only by their technological applications but also by their potential use as models to investigate the collective behavior of systems, in which Brownian motion produces an equilibrium state. The colloidal particles - several ten nanometers to micrometers in size - self-organize into structures (vapor, liquid, glass, crystal) similar to atoms in different phases of condensed matter. Since the particles can be observed optically in real time, colloids can be used to study the dynamics of these phases. Fast scanning confocal microscopes allow for locating the individual colloidal particles accurately in three-dimensionsional space and following their motion in time.

We use crystalline and amorphous colloidal suspensions as models to study the behavior of their atomic counterparts under applied stresses. We find that strained colloidal crystals exhibit dislocations, one-dimensional defects in the crystalline lattice, which show remarkable similarities to dislocations in atomic crystals. Using confocal microscopy, we are able to study the nucleation, motion and interaction of dislocations on the particle scale. Furthermore, we are able to follow dislocation propagation on a much larger length scale using a self-made laser diffraction microscope. This apparatus is analogous to the transmission electron microscope used to study dislocations in atomic crystals.

In the amorphous suspension, we are able to follow the motion of the individual particles during deformation and identify local events that are indicative of basic localized shear events of the amorphous state.

I will present a variety of experiments on colloidal crystals and glasses, such as the epitaxial crystal growth on a stretched substrate, shear experiments, and nanoindentation experiments using a conventional sewing needle as an indenter.