Imaging the Dynamics of Freezing and Melting with Colloids

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****NON-TECHNICAL ABSTRACT****The freezing and melting of crystals are fascinating and technologically important phenomena, yet are very difficult to study in the laboratory. Experiments that follow the motions of individual atoms as they form crystals can provide powerful insight, but major technical challenges prevent many of the needed measurements. This project will use microscopic spherical particles suspended in solution (colloidal particles) as ?model atoms,? with which to study crystallization and melting. These particles follow the same physical principles that govern atoms or molecules, but are much larger and slower than atoms and hence directly visible in optical microscopes. The experiments will offer new insight into the role of transient structures, which are not stable but which can nonetheless control the rate at which crystals grow or melt, or even arrest crystal growth. The research may lead to more efficient crystallization of proteins (a major step in determining their structure and function), and controlled crystallization of atoms in nanoparticles (potentially leading to novel materials). The project will provide training for graduate and undergraduate students and includes outreach to high-school students to expose them to the excitement and career opportunities in cutting-edge science. The project will also support a new series of visits and seminars by physicists working in industrial research, with the goals of creating opportunities for collaborative research and of exposing students to a range of career options.****TECHNICAL ABSTRACT****The phenomena of freezing and melting are familiar in everyday life and have been studied intensively for many years, but many important questions remain unanswered. Recent experiments have shown that micron-sized particles or droplets can be used as powerful experimental models, providing direct insight into these phenomena with single-particle resolution. This individual-investigator award supports experiments with colloidal spheres and liquid droplets whose interaction potentials can be tuned to induce crystallization, gelation, or melting. The particle motions will be quantitatively tracked in two or three dimensions using optical microscopy. Of particular interest is the role of thermodynamically metastable states, which have a major effect on the free-energy barriers, the rates, and on the final state in cases where the system is trapped out of equilibrium. The results should be helpful in applications such as crystallization of globular proteins or synthesis of inorganic nanocrystals. Graduate and undergraduate students will play key roles in the project and receive training in laboratory research. An outreach program will expose high-school students to the excitement and career potential of cutting-edge science. Reflecting the investigator?s combined interests in industrial applications of soft matter and in training graduate students, the project will also support a series of visits and seminars by physicists currently working in industry.