Paul Umbanhowar Assistant Professor PhD, University of Texas, 1996 Paul Umbanhowar's research interests lie in the area of nonlinear and complex physics with a focus on the dynamics of granular media. Complex behavior often arises when dissipative systems are driven from equilibrium -- weather is a familiar example. Diverse nonlinear systems exhibit strikingly similar, even universal, behavior. In systems with a large number of degrees of freedom, such as fluids or grains, a typically featureless base state spontaneously self-organizes into a spatial pattern. Much of Umbanhowar's research has focused on the dynamics of grains in vibrated granular materials. When a layer of sand is shaken hard enough it undergoes a subcritical transition to standing-wave patterns of squares at low frequencies, and to stripes at high frequencies. When the layer is sufficiently deep, localized wave structures, called "oscillons", appear at onset in the frequency range between squares and stripes. Oscillons are stable and come in two "flavors". Like oscillons repel while opposites attract and bind to form larger ensembles which resemble simple molecules. Because general equations governing granular dynamics do not yet exist, there is no solid theoretical understanding of why waves and oscillons form or what determines their interaction. Umbanhowar is currently working on experiments investigating relaxation and flow in dense granular ensembles. Selected Publications P. N. Segre, F. Liu, P. B. Umbanhowar, and D. A. Weitz Effective Gravitational Temperature for Sedimentation Nature 409, 594 (2001) P. B. Umbanhowar and H. L. Swinney Wavelength Scaling and Square/Stripe and Grain Mobility Transitions in Vertically Oscillated Granular Layers Physica A 288, 345 (2000)