9704301 Gollub This is a renewal project which will extend our understanding of nonlinear physical phenomena, especially those involving granular materials and fluids, using small scale laboratory experiments and involving undergraduate students. An extensive study will be made of the phenomena of shear friction in granular layers. Delicate shear force measurements will be combined with simultaneous microscopic imaging. These improvements will allow the onset of stick- slip motion, precursors, and the transition to continuous slipping to be investigated in detail. One goal is to compare and contrast granular friction with solid-solid friction. A second focus will be the study of fluid mixing phenomena, in which a passive impurity is mixed by a flow. These experiments will include investigations of chaotic advection in random cellular flows, and transport in granular media. Finally, continuing experiments on pattern formation and dynamics of nonlinear waves will explore phenomena such as hexagonal superlattice patterns with multiple spatial periodicities. %%% This a renewal project which will extend our understanding of nonlinear physical phenomena that can give rise to complex or unpredictable behavior. The project will focus especially on granular or particulate materials and fluids, using small scale laboratory experiments. The training of undergraduate students in research will be an important aspect of this work. An extensive study will be made of the motion of layers of granular material when subjected to shear forces. Shear friction in granular layers is potentially important in connection with earthquakes, though these studies are aimed at achieving a basic understanding. Delicate force measurements will be combined with optical imaging to understand the onset of sudden slipping events, and the localized precursors that occur in advance of major events. A second area of study will involve fluid mixing phenomena, in which a passive impurity is transported or mixed by a fluid flow, which may contain a porous medium such as sand. Studies of this type may be relevant to the transport of impurities in soils. ***
