Rayleigh-Taylor and Double-Diffusive Instabilities from Sediment Settling in a Two-Layer Stably-Stratified Hele-Shaw Cell

Double Diffusive (DD) and Rayleigh Taylor (RT) Instabilities play critical roles in numerous natural and technological phenomena where they can significantly increase transport rates over Stokes settling. Research over the past decade has demonstrated how sediment settling can initiate these and other fluid instabilities in otherwise initially stable stratifications. Herein, DD and RT instabilities were investigated in a Hele-Shaw cell containing an initially stable stratification of particle-laden fresh water over aqueous dextrose. Particle sizes were chosen such that settling rates were slower than, comparable to, or faster than diffusion rates of dextrose based on the premise that these regimes might prove to be dominated by DD, RT, or contain both simultaneously. Particles used included silicon dioxide nanoparticles with sizes 500 nm, 700 nm, and 1000 nm, soda-lime glass microparticles of nominal sizes 3- 6 microns, and 8 – 12 microns, and nominally 2-micron Cerium Oxide microparticles. Some ability to predict fingering is demonstrated based on a product of a Rayleigh number and a ratio of a particle settling time to a dextrose diffusing time.