Research Highlight: "Colloids: A microscopic army"
2016 DFD Gallery of Fluid Motion entry
Recently, we have discovered a new kind of flow instability in a system of rotating colloids. When a particle rolls near a wall, roation leads to both translation and strong advective flows, even very far from the particle. When many particles are together in a dense group, these strong flows lead to collective effects.
We find that if a dense group of these particles begins to roll together, a shock structure will quickly develop, which then becomes unstable, and emits fingers of well-defined wavelength. This is a very different kind of instability than usually encounerted in low-Reynold's number flows: the instability wavelength is controlled not by driving torque or fluid viscosity, but a geometric parameter: the microroller’s distance above the container floor. In colloboration with Blaise Delmotte and Aleks Donev, we have used large-scale 3D simulaitons to help unstand the dynamics of this instaility. The fingering behavior is at its core truly a flow instability: the dynamics can be completely reporduced using hydrodynamic interactions alone.
When the instability further evolves, a new kind of structure emerges. Our 3D simulations indicate that when the rotating particles are pushed away from the floor, the fingertips can break off, forming self-sustained, persistant clusters, we call "critters". These stable clusters are held together only by hydrodynamic interactions. Critters are quite robust, and appear to be an attractor in the dynamics of this system.
Research Highlight: "Physicists gain insight into why materials break"
A material's response to forcing is ultimately controlled by its structure, although these connections are not always well understood, especially in soft materials. Fracture offers a unique way to probe a material; how something falls apart can reveal what was holding it together.
Solids can break in many ways, from slow, plastic deformation to rapid and catastrophic shattering. In colloboration with the Vitelli group, I studied the failure of meta-materials; structures that can designed to have tunable elastic properties. We showed that as these materials approach a rigidity transition, their failure behavior changes dramatically, from slow, diffuse failure to a rapid, running crack. Undering and controlling this failure behavior offers both a new tool for measuring soft material properties as well as a way to design materials with tunable, engineered failure properities.
These ideas also translate to a model 1D system: a perforated geometery, created by introducing holes into a solid sheet. Even in this simple model system, a dramatic transition in failure mode occurs as a function of geomtery: rapid failure via a running crack is completely replaced by random breaking. Thus, by choosing appropriate geometric parameters, the entire dynamics and structure of the failure process can be controlled.
Research Highlight: "Fluid dynamics: The air down there"
When a drop hits a dry surface, there are many possible outcomes, from complete rebound, to violent shattering, to smooth spreading. Splashing - or no splashing - is often desired in industrial applications, but surprisingly, the instability that causes a drop to splash is unknown. In fact, it was only recently discovered that the surrounding gas is a critical control parameter for splashing.
In order to better understand the influence of the ambient air, I developed a new technique to study splashing: ultrafast interference imaging. This technique combines high-speed imaging with reflected light interference, allowing direct imaging of any air layer beneath a spreading drop. This technique allowed us to measure, with high temporal and spatial resolution, exactly where the air was - and was not - at every stage of the splashing process. Surprisingly, we found that there is no air underneath a spreading drop at the time of sheet ejection, demonstrating that the air must act only on the leading edge of an encroaching drop to create a splash.