Recent Dissertations: Kenneth Desmond (Physics) examines how jamming unfolds in soft materials.
Nov. 26, 2012 - Anyone who's been caught in rush hour gridlock knows what it means to get jammed - bottlenecks slow the flow of traffic, cars pile up, and open roads suddenly become parking lots. But for physicists like Emory PhD Kenneth Desmond, a "jammed system" is a technical term that can refer to far more than just congested roadways. In scientific parlance, jamming occurs when a liquid, granular, or otherwise flowing substance clogs up and slows down, becomes more rigid as its density increases, and possibly ceases to move entirely. Kenneth's dissertation, "Structure, Dynamics, and Forces of Jammed Systems," examines how jamming unfolds not in a situation like on a highway, where the elements at play (cars) ideally remain discrete and don't smash into each other and change form, but in an even more complicated state of affairs: in soft materials.
The Importance of Being Squishy
In our day-to-day life, we are surrounded by what physicists call soft materials - substances like toothpaste, cosmetics, paint, and mayonnaise that maintain their shape like a solid but will flow when stressed. "Many of the foods we eat are soft materials, and were tuned to be that way to improve their taste, and some of the protective gear worn in sports and other activities are soft materials that have been fine-tuned to optimize safety.”
Kenneth’s dissertation examines how soft materials become jammed – transition into solid-like states. “These materials,” Kenneth explains, “are composed of densely packed microscopic particles, and their solid-like behavior can be controlled by altering the particle concentration. By increasing the particle concentration, the system becomes more solid-like and, at a critical concentration, these materials undergo a jamming transition and maintain their shape in the absence of stress, like shaving cream in your hand or peanut butter in a jar." Specifically, Kenneth considered three scenarios - the jamming/glass transition in colloids (materials like toothpaste), the influence of boundaries on jamming transitions (like grains poured into a silo), and the jamming transition of frictionless emulsion droplets (materials like mayonnaise).
This visualization shows substance rearranging itself into a more compact, jammed structure as crowding take place. Kenneth produced this model using a technique called Delaunay Triangulation.
Observing jamming effects across multiple kinds of substances and at a sufficiently precise level required new experimental and observational techniques. Kenneth elaborates: "While materials like shaving cream, mayonnaise, peanut butter, toothpaste, and cosmetic products have obvious differences, their solid-like behavior seems to have a common origin, namely the crowding effect of microscopic particles. To understand this effect, one must be able to look inside these materials and visualize the particles and the forces between particles." Working in the lab of Emory Professor Eric Weeks, Kenneth developed a novel method, involving an experimental model system composed of densely packed, frictionless droplets and the use of confocal microscopy to image the motion and forces acting on them.
Analyzing his data, Kenneth discovered patterns holding true across the three scenarios he investigated. "I was able to verify for the first time certain predictions from theory and simulations on how the crowding effect contributes to the solid-like response of soft materials," he says. Kenneth's findings also suggest new possibilities for examining and using other soft materials." These findings demonstrate a universality," Kenneth notes, "Which means we can apply a single theory to explain and predict the mechanical properties of these seemingly different materials."
In the second scenario Kenneth modeled, a random packing of disks and spheres was funneled through finite-sized confined systems of both two and three dimensions. Kenneth found that the geometries of the confining walls generated a structure that was stronger the closer to the wall the material was.
Emory and Beyond: Flexing Mussels
Kenneth's research from stems a longstanding personal passion, which Emory offered him an ideal environment to pursue. "I've always enjoyed studying systems that I could see and touch, and I've often wondered how we can tune materials to be more useful in our daily lives," Kenneth says. "While I was an undergraduate at the Rochester of Institute of Technology, one the professors knew of Eric Weeks at Emory University and his work on soft materials. He suggested that I apply, and I came to Emory to work with Professor Weeks." Weeks’ guidance, along with the high-end technological resources available at Emory, gave Kenneth all he needed to explore the questions that drove him.
Kenneth is now a Postdoctoral Fellow in Mechanical Engineering at the Materials Research Laboratory at the University of California Santa Barbara. He is still focusing on squishy substances - only this they come in shells and are alive. "I am studying how marine mussels are able to adhere to wet surfaces," Kenneth explains. "My research may someday contribute to the development of new bio-inspired wet adhesives." Toothpaste, mayonnaise, and now shellfish - Kenneth certainly has a lot to think about while whenever he's jammed up in Southern California traffic.
This visualization shows how the packed complexity of a substance increases as the confines of its containers grow tighter.
Written by Patrick Blanchfield, PhD Candidate in Comparative Literature and a graduate assistant in the Laney Graduate School.