Direct observation of hydrodynamic instabilities in a driven non-uniform colloidal dispersion

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.