We research the soft matter physics that underlies squishy materials and living creatures. These disparate systems are united by a focus on nonequilibrium systems. After all, We live in a nonequilibrium world. Everyday, the sun rises, increasing the local temperature, and then sets, decreasing the temperature. Vapor condenses overnight, only to evaporate during the day. Snow melts in the afternoon sun, only to freeze again under the moon. We too are nonequilibrium systems. We consume food, which is broken down to energy and subsequently used or stored. Despite their ubiquity, however, generation of an understanding of nonequilibrium systems represents a major challenge for physicists.
If you look at mold growing on a piece of food, it may appear static, but its appearance conceals a world of activity. Though the cells lack motility, beneath the surface, cells are growing and dying, actively changing the mechanical make-up of this cellular solid. Reproducing allows cells to claim a larger territory, and killing other cells allows them to carve out space of their own. Studies of active matter typically focus on constituent activity via mobility, for example, in flocks of birds or swarms of fire ants; however, in these common microbial systems, activity arises by changing the constituent number, i.e., adding or removing parts of the system. These life and death events abruptly modify local pressure and stress networks, and create very different structures with very different resultant materials properties. Understanding activity via changing constituent number is critical to understanding cell-based solids like biofilms and tissues, in which cells reproduce and die.
We are also interested in the physics of non-living systems, like colloids. Colloids are especially well-suited for the study of nonequilibrium phenomena. The traditional colloidal suspension consists of solid particles (e.g., polystyrene), typically 1 nm to 100 μm in size, suspended in a fluid (e.g., water). Ink and paint are common examples of such colloids. However, colloids are not limited to solids suspended in liquids. Colloids include emulsions, which are fluids suspended in fluids (e.g., milk), aerosols, which are solid particles suspended in gases (e.g., dust), and foams, which are gases suspended in liquids (e.g., styrofoam). Micron-sized colloids are especially useful as they are small enough to experience Brownian motion, but large enough to be easily observed via optical microscopy. Some previous research efforts are explored below.
The Coffee-Ring Effect
If you have spilled a drop of coffee or tea and left it to dry, then you might have observed that the stain left behind is not uniform, but ring-shaped. Specifically, the stains are darker near the edges than in the middle (Fig. 1.1). While a stray drop of coffee may seem to be of trivial importance, it is actually rich with nonequilibrium physics. The so-called coffee-ring effect is the product of the interplay between fluid dynamics, surface tension, evaporation, diffusion, capillarity, and more. Understanding the coffee-ring effect requires understanding these complex parameters in a farfrom-equilibrium setting. Briefly, the edges of a drop easily become pinned and cannot recede towards the middle of a drop, i.e., the diameter of a pinned drop cannot decrease. However, the edges of a drop are thinner than the middle; thus, water flows from the middle of the drop to the edge of the drop to replenish what has evaporated away.
The Glass Transition
If you ask the layman to describe glass, they’ll think of windows and cups, and most likely they will tell you two things: it’s transparent and it’s hard. The transparency of window glass is understood as a result of an electronic band gap, but the reasons for why glass is hard is an active field of study. While ordered systems undergo a sharp phase transition from liquid to crystal, the transition from liquid to glass is defined more arbitrarily. As the glass transition is approached, particle dynamics dramatically slow down. This is evident from the particle relaxation time (τ ), i.e., the average time it takes a typical particle in the glass to be displaced by its diameter. However, τ does not change discontinuously with respect to volume fraction or temperature and a threshold is arbitrarily selected to define the glass transition point. Compounding this mystery is the rather surprising number of common physical features observed across a broad spectrum of jammed or dynamically arrested systems including colloidal suspensions, granular media, metallic glasses, and polymer glasses. The fact that systems with such different microscopic constituents behave qualitatively similarly has led to a search for unifying explanations.