CAREER: Nonlinear Waves and Fluctuations in Jammed Systems

Emory University, Atlanta GA

Investigators

Abstract

Nontechnical: As a society, we are very good at putting things in order: cars parked in designated rows in a garage, oranges piled neatly at the local grocer, or papers stacked in perfect bundles on an office desk. We do this, perhaps unconsciously, because ordered arrangement often saves the most space. Similarly, out of all of the solid forms of matter, physicists perhaps know the most about crystalline materials; the structural order makes them easier to conceptualize and to define mathematically. However, many solid materials can be rigid, yet have no well-defined order. These materials often behave in peculiar ways in response to external stimuli. Consider a pile of sand at the bottom of an hourglass. What seems stable enough can suddenly avalanche upon the addition of a few extra grains. Or even a snarled traffic jam: what determines the boundary between a flowing state and a rigid one? Our world is full of similar examples where systems exist in a region near marginal stability. This project aims to understand the ways in which a jammed, amorphous solid can vibrate and fluctuate. By using "model" systems such as colloidal dust suspended in a gaseous plasma, the principal investigator and research team can visualize the individual motion of particles which constitute the near-effortless structural rearrangements in amorphous systems. Not only do the research activities shed light on the individual molecular motions common in amorphous solids such as glasses, but they provide a connection between dynamics and disorder in a broad range of physical systems. In parallel to this research effort, the project includes the creation of an after-school science club at a local elementary school in Dekalb county, which hosts the 3rd largest school system and is the most diverse county in Georgia. Technical: This project focuses on the nature and origin of mechanical rigidity in amorphous solids, a subject which permeates many areas of condensed matter physics and materials science. Both experiments and simulations are used to investigate the dynamics of low-frequency vibrational modes and the role of geometric confinement in jammed and glassy systems, where small changes in temperature or density result in an enormous increase in the kinetic timescales of motion. The project addresses two major topics, the first of which is the origin of low-frequency vibrational modes in disordered solids. It has been known for 40 years that molecular glasses have excess vibrational modes at low frequencies. An ideal model system to study this phenomena is an under-damped ensemble of colloidal particles suspended in a plasma, commonly known as a "dusty plasma". This system is used to directly visualize and characterize the T = 0 vibrational modes in a way not possible in molecular glasses. The second project addresses fluctuation-induced forces in jammed systems. The jamming transition has many similarities to 2nd-order phase transitions, such as a diverging length scale near the critical point. By implementing geometric confinement to control the spectrum of force chains that can exist near jamming, in analogy to critical Casimir forces in thermal systems, the research team can quantify this length. The experiments use soft, slippery, polymer hydrogel particles whose size can be varied with salt concentration as a "sandbox" for studying the jamming transition in the absence of particle friction. In parallel to both research efforts, the project includes the creation of an after-school science club at a local elementary school in Dekalb county, which hosts the 3rd largest school system and is the most diverse county in Georgia.