Experimental investigations are crucial to the development of our understanding of complex systems, and the establishment of the CATS Center has made it possible to launch an experimental effort commensurate with the level of already existing theoretical activity. Currently we have ongoing experiments in chemical reactions (quenching analysis, invariant manifolds, chemical waves), fluid dynamics (turbulence in pipe flows, capillary waves, hydraulic jumps, thin film flows), acoustics (spectroscopy of high-Q resonators of various geometries), physics of biological systems (measurement of protein folding transitions), and granular flows.
One of the advances made possible by CATS SNF funding was the first mechanical measurements of protein folding transitions on nanometer scales. Using a mechanical technique developed here it was observed that the dynamics of protein folding is characterized by discrete steps in time. These measurements suggest a hierarchical organization within the protein molecule, motivating exploration of hierarchically structured statistical mechanics models pursued by CATS theorists.
Granular matter has properties that do not resemble those of other states of matter. In particular, granular flows do not behave like fluids, their discrete nature having manifest consequences. CATS experimentalists study the dynamics of rapid granular flows in simple geometries, focusing on the basic underlying mechanisms, such as steady flows, intermittent flows, and shock waves.
High resolution acoustic spectroscopy is used to study aspects of quantum chaos. The acoustic wave-equation is clearly very different from the Schrödinger equation (notably the displacement field is a vector field), but the basic features are similar enough to allow very useful analogies. For example Random Matrix Theory which is now used extensively to characterize chaos in quantum systems is tested to levels not previously inaccessible experimentally. In addition we study symmetries and symmetry-breaking which are specific to the elastic systems. We study parametric level dynamics where the parameter varied is either the shape of the object or the temperature. We measure wavefunctions of stationary states as well as the transient development of a wavefront following a sharp blow.
Particle Motion in Capillary Waves:
We have followed the motion of particles floating on a water surface in a cylindrical dish oscillated vertically so as to create capillary waves. For the single-particle motion, a cross-over in the diffusion motion is observed, from a strongly anomalous diffusion at length scales below the wave length to a motion closer to Brownian at larger length scales. For the relative particle motion, the non-Brownian character of the flow is very pronounced. We have determined the relative diffusivity and studied its dependence on distance.
Pipe flow turbulence:
Turbulence is produced in a pipe flow behind a grid, and the velocity fluctuations are measured using laser Doppler anemometry. The distributions and their moments of these fluctuations are determined at various temporal scales and Reynolds number. At intermediate Reynolds numbers the moments do not follow a power law in scale, but they still follow a power law when one moment is plotted versus another. We investigate how the exponents obtained in this way depend on the Reynolds number.
The circular hydraulic jump is used as a convenient laboratory for studying in detail the transition from laminar to turbulent flow. A sequence of states is observed involving different numbers of vortexes with varying degrees of symmetry breaking.