This soft condensed matter physics project deals with disordered solids, which include glassy materials, polymer gels, particle gels, and sand piles. Although the microscopic details differ tremendously, there is increasing evidence that these materials are examples of a more general class known as jammed solids. From a fundamental point of view, the great challenge of these materials is that they are far from thermal equilibrium and continuum elasticity might not apply at microscopic or mesoscopic length scales. This project will use optical methods to obtain direct and quantitative imaging of forces, their spatial correlations, and the topology of inter-particle connections in three dimensional samples. Results are to be compared to recent theories, simulations and measurements on surfaces or on two-dimensional materials. This focus is on confocal optical microscopy of monodisperse liquid droplets with fluorescent surfaces, a model system that allows imaging of forces in the interior of three-dimensional samples. By varying the sizes of the droplets used, the systems studied will be varied between those solidified by attractive inter-particle forces and subject to strong thermal fluctuations, to those solidified by gravitational stresses at, effectively, zero temperature. The project also investigates how these force maps are changed by external stresses or by point forces applied inside the material. The project provides in-depth training of students for research in applied or fundamental programs.
Disordered solids are very common in everyday life, with examples including window glass, yogurt, soot, and sand piles. Although the microscopic details of these examples differ tremendously - molecules in one case and millimeter-sized rough sand grains in another - there is compelling evidence that they may be understandable within a single theoretical framework. From a fundamental point of view, the great challenge of these materials is that they never approach thermal equilibrium and do not form ordered (crystalline) structures. Moreover, continuum elasticity theory might be incorrect at length scales comparable to several times the size of the basic particle. This work will provide the first experimental maps of forces, structure, and connectivity inside these materials. The major experimental challenge is, somehow, to peer inside a sand pile and measure forces between adjacent particles. Here this is accomplished using liquid droplets (instead of sand grains) whose deformation is quantified in three dimensions using optical microscopy. In addition to allowing investigation of a number of new questions, the measurements will provide the first tests of theoretical predictions and computer simulations, and will connect with earlier experiments on two-dimensional materials or on the surface of three-dimensional materials. The project also provides training for students at all levels in soft condensed matter physics and advanced research techniques suitable for academic or industrial research positions.
