Particulate materials, such as suspensions or granular matter, are the most commonly used materials in industry after water. However, explaining their rheological properties remains a challenge. These systems are complicated by the presence of disorder as well as by structural and dynamical heterogeneities, often on multiple length scales. At high densities, such a granular fluid undergoes a jamming or glass transition where the dynamics stop. In recent years there has been a considerable effort to characterize this transition, and it has been realized that dynamical heterogeneities play a key role. However, there is no consensus concerning what causes such heterogeneities. This project will develop a novel method to measure the micro-scale elasticity of amorphous materials. This approach will be used both experimentally and numerically to characterize the disorder and heterogeneities of amorphous solids, and to investigate the jamming transition by which they are formed. The method consists of introducing probe particles of controlled shapes and sizes. Thermal noise causes the probe particles to rotate on a time scale governed by the elasticity of their local environment, and by their shape and size. Measuring the rotational dynamics of the probe by means of confocal microscopy, light scattering, or numerically in simulations will give access to local elastic properties. The range of time scales and length scales can be tuned by controlling the shape and size of the probes. This method will be employed in colloidal suspensions, both experimentally and numerically, to measure the evolution of elasticity and its spatial heterogeneities as the concentration of colloids is increased, and to test fundamental theories of the glass transition.
This project will create an experimental method to probe the microscopic properties of disordered granular materials, the most commonly used materials in industry after water. This method will address questions of fundamental and practical importance in the fields of particle flow, biophysics, soil mechanics, and material science. Insights gained from this study will help improve the design of glassy materials and advance our understanding of clogging or jamming, which are of important for multi-phase flows relevant to the oil industry and potentially for the lethal vaso-occlusive eventt?clogging?occurring in sickle cell disease. In addition to these applications, the subject matter of this project lends itself to educational and community outreach activities.
