  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.
