Direct observation of hydrodynamic instabilities in driven non-uniform   colloidal dispersions
  A Rayleigh-Taylor-like instability of a dense colloidal layer under gravity in a capillary of microfluidic dimensions is considered. We access all relevant lengthscales with particle-level microscopy and computer simulations which incorporate long-range hydrodynamic interactions between the particles. By tuning the gravitational driving force, we reveal a mechanism whose growth is connected to the fluctuations of specific wavelengths, non-linear pattern formation and subsequent diffusion-dominated relaxation. Our linear stability theory captures the initial regime and thus predicts mixing conditions, with important implications for fields ranging from biology to nanotechnology. 
Adam Wysocki, C. Patrick Royall, Roland G. Winkler, Gerhard Gompper,   Hajime Tanaka, Alfons van Blaaderen and Hartmut L\"owen