****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.
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.
The behavior of materials is dictated by the arrangement of the atoms or particles within them. For example, the stiffness of metal is determined by defects in the otherwise ordered, crystalline arrangement of the atoms. Unlike metals, however, many of the materials in our daily lives are constructed from particles or proteins that are individually much larger than atoms; they range from nanometers to microns in size. These particles may be arranged in ordered or disordered ways, which determines the material properties. For example, the visible iridescence from gem opals is a result of their being composed of microscopic spheres that are arranged in a crystalline pattern. Such materials are especially attractive for applications when the particles spontaneously arrange into a useful structure. The spontaneous process is known as self assembly and is described by fundamental principles of thermodynamics and statistical mechanics. The goal of this project is to understand the mechanisms of self assembly in materials composed of particles confined to surfaces. Specifically, we focus on particles on a rigid surface (flat or curved) and particles on a deformable surface such as a membrane. These model systems were chosen to expose fundamental mechanisms, explain the properties of natural materials, and to lead the way toward new technologies. Our studies of particles on rigid surfaces focused on self-assembly of crystalline arrays. We use micron-size spheres in water as model "atoms;" they obey the same physical laws as atoms do, but we can follow their motions in real time as they form crystals (in minutes, compared to billionths of a second for atoms). We found that in many cases, a super-cooled liquid does not crystallize by the classical nucleation process (forming small crystallites in a single step). Instead, small clusters of an intermediate liquid phase are formed, which grow and then crystallize. This two-step nucleation pathway can greatly increase the rate of crystallization. We then studied crystal formation by particles on the surface of a cylinder. We chose this shape partly for its resemblance to natural materials such as microtubules in cells or carbon nanotubes, and partly to isolate a particular question: how does the finite size affect crystal formation? Unlike flat surfaces, cylinders impose a new constraint: there must be an integer number of particles around the circumference. Using computer simulations, we found that this constraint leads to oblique crystals that are not seen on flat surfaces. We also found crystals with a "seam" (a pair of dislocation lines) that winds around the cylinder like a helix. We developed a straightforward model that accounts for our results and shows how to control the self assembly. Our results should help develop robust routes to protein crystallization, may lead to improved properties of nanocrystalline solids and new opportunities in optics or microelectromechanical sensors. To study self assembly on deformable surfaces, we probe proteins or nanoparticles on lipid bilayer membranes (as a model of cell membranes). Unlike rigid surfaces, membranes change their shape in response to the particles. As a macroscopic example, two bowling balls on a trampoline may roll toward one another because each one deforms the trampoline downward; in an analogous way, proteins can push or pull other proteins in a membrane. In cells, this shape-driven assembly may be essential for communication and for transport of cargo. In one set of experiments, we studied a protein (called BAR) that is involved in endocytosis; we found that mechanical tension in the membrane strongly enhances BAR binding, which allowed us to understand BAR’s shape sensitivity in a new way. We are following up these experiments to find how the binding and assembly of synthetic nanoparticles may be controlled in a similar way. In another step toward the complexity of a cell, we studied protein binding on a membrane containing three types of lipids, which separated into distinct domains. We found that the proteins accumulate at the boundaries between domains. These results show how we might mimic membrane biology to direct assembly of protein or particles to create new materials that could respond to chemical or physical stimuli to alter flow or mechanical properties or appearance, or to release encapsulated nutrients or drugs. This project provided training for undergraduate and graduate students in physics and related disciplines. The PI is one of four organizers of the UMass Summer School on Soft Solids and Complex Fluids. Each year, this School provided an intensive, week-long training for approx. 45 students, who came to UMass Amherst from locations around the U.S.. This project also provided support for in-depth training of three graduate students in soft-matter physics. The PI also contributed to undergraduate training in research through mentoring students in lab research, through teaching of a research-based seminar for freshmen, and through implementation of a new four-year program called Integrated Concentration in Science (iCons) at UMass Amherst.
