Deformation and flow of highly polydisperse amorphous solids

Emory University, Atlanta GA

Investigators

Abstract

This is an experimental study of how disordered materials flow. The flow of materials like mud, cement, foam, and dirt in landslides is quite different from the flow of liquids. These materials have been studied before but their complexity makes it difficult to determine general principles about how these interesting substances flow. Some past studies used simple systems to try to mimic these complicated materials: a standard simple system is a liquid full of round particles with a specific size. It is clear that these simple systems fail to reproduce many of the interesting flow behaviors of the real materials. This work studies the flow of materials with intermediate complexity: liquids with particles with a wide variety of sizes in the same sample. Optical microscopy is used to examine the motion of the particles and see how they rearrange, for example, how small particles are forced to move around large particles. These observations will be used to develop basic ideas about the flow of other disordered materials, which will lead to better understanding of how to move, mix, and process complex materials of industrial relevance. Undergraduate and graduate students will be engaged in conducting the research. Additionally, the researchers regularly host field trips of visiting students (first grade through high school) to teach them about soft squishy materials and how scientists study these materials. The objective of this project is to understand how amorphous solids flow, in particular to connect particle-scale rearrangements to macroscopic flow profiles and rheological behavior. The samples are highly polydisperse emulsions, where the largest droplets are ten times the radius of the smallest droplets (or more). Flowing and sheared samples are studied with video and confocal microscopy. This enables the motion of individual droplets to be measured. When a large strain is applied, droplets rearrange irreversibly. These rearrangements can be quantified in terms of topological changes, magnitudes of displacements, and how non-affine the flow is locally. The project connects these details of droplet rearrangements to the size of individual droplets. More broadly, these details likely depend on the overall droplet size distribution. Additionally, the microscopic details must connect with macroscopic rheological properties. Complementary experiments can be done with solid particles, and simulations provide a third route to explore these questions. The overall motivation is to bridge the gap between prior model systems that studied mixtures of particles of fairly similar sizes, and complex real-world materials such as mud and cement. This will lead to new microscopic understanding of the shear of amorphous materials with large size polydispersity. At least one undergraduate and one graduate student will be involved with the research. The research team will also host field trips each year to demonstrate to visiting students that cutting edge science can be done with simple materials which they can literally get their hands on. This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.