Force Maps, Aging, and Elasticity in Random, Non-Equilibrium Solids

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Technical Abstract:Optical microscopy will be used to measure structure, topology, forces and elastic constants inside disordered solids in three dimensions. The great scientific challenge of these materials is that they are both random and far from thermal equilibrium and thus not readily described by thermodynamics. The basic approach of this project is to measure the positions of particles in two or three dimensions, along with their thermal fluctuations or the distortions of their shape by contact forces. The particles used will range from micron-size neutrally buoyant colloidal spheres -- whose motions are thermal -- to 20-micrometers heavy droplets, which form static piles with zero effective temperature. Attractive interactions induced among the small particles lead to gelation and this process will also be studied in reverse. The data allow unusually explicit comparisons of forces, elastic constants, and material structure, which are essential to develop and test theoretical models. High-school, undergraduate, and graduate students participate in the research. The project also informs development of a course in soft matter physics, which will be shared with instructors at other institutions.Non-technical Abstract: Disordered solids are very common in everyday life, with examples including window glass, polymer gels, particle gels, and sand piles. Although the microscopic details of these examples differ tremendously -- molecules in one case and millimeter-sized sand grains in another -- there is increasing evidence that they are examples of a more general phenomenon known as "jamming". The goal of this project is to use advanced techniques of optical microscopy to measure structure, topology, forces, and elastic constants throughout these materials. One set of measurements focuses on the forces among droplets inside a concentrated emulsion, which serves as a useful model of a sandpile without static friction. A parallel set of measurements focuses on the formation and melting of random particle aggregates (gels). The results will contribute to a deep and broadly applicable understanding of elasticity and aging in disordered solids. A key component of this project is the participation of high-school, undergraduate, and graduate students, who will receive training in soft matter physics suitable for careers in industry, national labs, or academia.