A new view of new mechanical memories in glassy matter

About This Grant

Non-technical Abstract: The uneven surface of a sandbox shows that soft solids like sand, soil, or skin cream hold memories of their history. Their microscopic structure also holds memories, making their properties hard to control. In this project, researchers will deform a solid made of tightly packed oil drops, and a two-dimensional solid made of beads, while monitoring how the drops or beads rearrange. These tests will reveal where and how memory is stored. Results will help predict and control the strength of materials like soil, grain, and foam. These kinds of materials are vital to infrastructure, and industries from agriculture and food to personal care products. The research will also identify ways to design and use advanced materials that adapt to their environments. The project will train graduate and undergraduate students. A K-12 teacher will participate in this research and will develop a curriculum on materials and research methods. Teacher recruiting will include rural school districts in Central Pennsylvania. Technical Abstract: Many kinds of matter are challenging to describe because they do not relax to equilibrium; their properties depend on the past. This is true of amorphous solids, including glass, sand, soil, and mayonnaise, which share common challenges for predicting and controlling mechanical behavior. The disordered arrangement of particles in these materials is metastable, and it changes each time it is deformed. Prior work has shown a simple way that 2D samples encode and recall the amplitudes of oscillatory shear, but this picture omits some of amorphous solids’ most distinctive and challenging features. In this project, experiments will study a 2D solid made of repulsive colloids at an oil-water interface, and a 3D concentrated emulsion. While varying the amplitude and frequency of shear, tests will record the locations of particle rearrangements via particle tracking in 2D, and light scattering in 3D, measuring the number, size, hysteresis, and dynamics of individual plastic events. Major questions include how mechanical preparation alters the response to shear, what information is recoverable and what is erased, and how memory and plasticity differ from 2D to 3D. Some experiments will target unusual memories made possible when one rearrangement inhibits another, providing data about interactions that reflect the glassy character of these systems. The result will be a new experimental picture of these materials’ mechanical properties, collective dynamics, and history dependence from the microscopic to bulk scale; evidence that certain kinds of memory are generic signatures of glassiness; and design motifs for metamaterials. The project will also yield methods for studying memory in glassy matter and tracking relaxations in bulk emulsions, and improvements to widely used particle tracking software. 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.