My research interests span a broad range of subjects in soft-matter physics and biological physics. Here are some short blurbs about my current and past research projects (see my current research in PHYSICS TODAY magazine!):
Spatial Structure in Synthetic Cooperative Cell Growth
This fluorescent image shows growth of two distinct yeast (s. cerevisiae) strains on a dextrose-rich agar pad. The picture is about 1 mm wide,while the typical yeast cell is 4 microns wide. They are metabolically marked (ADH1 promoter) with red (DsRed) and yellow (YFP) fluorescent proteins in order to distinguish them. The red cells are genetically altered so that they can not produce the nucleotide adenine, but they over-produce the amino acid lysine. The yellowish green cells can not produce lysine, but they over-produce adenine. In order for each strain to survive, obligitory cooperation is required (W. Y. Shou, PNAS, 2007). In the picture to the right, bright "pods" of "yellow" and "red" cells develop due to the need for cooperation. If cooperation was not required for growth, then the distribution of the active (bright) cells would be more uniform and homogenous. This type of "synthetic ecology" gives us a quantitative method for studying the basic components of a cooperative biological system and the necessary features for a system-wide sustainable existence. I am currently developing a mathematical model that describes the dynamics of growth and metabolite release of these cells, and experimentally investigating the population dynamics and evolution of cooperation in this system. One fascinating aspect of this system is how "cheaters" (those who do not over-produce) affect the dynamics, and how the spatially structued growth (agar pad) is different than growth in a homogenous liquid culture. These issues are currently being explored.
From Bubbles to Droplets: a Bifurcation in Fluid Pinch-off
When a drop of fluid breaks up into two pieces, a singularity is formed at the moment of disconnection. This occurs for both bubbles and droplets, however, the mechanism driving the singularity in both cases is quite different. In my work with Prof. Peter Taborek at UCIrvine, we have developed a system to continuously vary the density ratio of the interior to exterior fluid in the bubble/droplet system. We use xenon gas in water because at room temperature, xenon exists just above its critical point, so the density can be varied widely just by changing the pressure moderately. To the right you see frames from a high speed video showing the pinch-off of a near vacuum bubble (top), and very dense bubble (70% that of water, bottom). The images in rows 2 and 3 are at intermediate densities. The xenon gas is emerging from a submerged nozzle in a specially designed chamber capable of high pressures (~100 atmospheres). Not only does the morphology of the pinch-off process change as the density is increased, the asymptotic similarity solution and corresponding power-law exponent associated with the singularity undergoes a bifurcation when the density of the interior fluid is about 1/4 that of the exterior fluid. You can see more in the gallery section of the webpage for videos of this process, as well as checking out the upcoming article in the January 2009 version of Physics Today.
Also check out the blurb in the online version. 
Experiments and Theory of 2D Inviscid Pinch-off
Recently I have also been interested in the effects
of dimensionality on pinch-off singularities. In 3D there are two principal curvatures of a drop surface that affect the pressure distribution along the surface. In 2D, there is only one relevant curvature. However, realizing a 2D experimental fluid system with no solid boundaries is difficult. Our attempt at an experimental system that approached 2D was to use very thin liquid lenses on the surface of water. The liquid lenses are composed of a low-viscosity hydrocarbon liquid that forms a lens-like shape when placed on the surface, like drops of fat in chicken soup. In order to image the pinch-off of lenses, we use a flowing trough setup under a microscope coupled to a high-speed camera. The small flow rate of the water is enough to deform the lens from tip of a capillary containing the hydrocarbon. The figure on the left shows the pinching region of a decane liquid lens (see gallery) on the surface of water. Strikingly, a multiple, fractal-like breakup pattern is observed that is no present in the breakup of a decane drop submerged in water. In an attempt to determine if this behavior is characteristic of all 2D pinch-off, I performed numerical simulations of non-viscous drops in both 2D and 3D (below). It turns out that the characteristic power-law exponent describing pinch-off in 3D is 2/3, while in 2D the exponent is an anomalous, irrational exponent. This behavior is typically known as "self-similarity of the second kind." We are currently modifying our system to determine if this exponent can be experimentaly measured.
Helium-4 Superfluid Droplet Pinch-off (T=1.34 Kelvin)
Above is a sequence of pictures from a high-speed video of the pinch-off of a superfluid helium droplet at 1.34 Kelvin (1.34 degrees above absolute zero). The high-speed video was taken in an optical cryostat. The quantum mechanical nature of the superfluid state manifests itself hydrodynamically as a fluid with exactly zero viscosity. Unfortunately, the interesting effects of a zero-viscosity fluid manifests in the pinch-of process at lengh scales below the optical resolution of our camera, although this limit can be explored in conducting fluids using electrical techniques (Burton, Rutledge, Taborek, PRL 2004). Pinch-off in low-viscosity fluids (such as water) are characterized by the geometry of a cone structure piercing a sphere (third to last frame). The radius of the connecting neck between the drop and rest of the fluid shrinks in time as R~(t'-t)^(2/3), where t' is the time at which the singularity occurs. Although the advent of high-speed imaging and analysis has provided a very useful tool to examine these singular events, some of which occur in just a few milliseconds, my work has shown that optical imaging is not sufficient for direct measurements of pinch-off on small scales.