Experimental Investigation of Settling-Initiated Instabilities in a Two-Layer Stably-Stratified Hele-Shaw Cell

Fingering instabilities can be initiated by settling of sediment in an initially stable step-stratification. Rayleigh-Taylor (RT) and Double-diffusive (DD) instabilities were investigated with initially stably stratified particle-laden fresh water over top of either salt water or dextrose water in a Hele-Shaw cell. Particle sizes were chosen such that settling rates were slower than, comparable to, or faster than diffusion rates of salt in order to separate out regimes likely to be dominated by either RT or DD or containing both simultaneously. Silicon dioxide nanoparticles with sizes 500 nm, 700 nm, 1000 nm or soda-lime glass microparticles of nominal sizes 3- 6 microns, 8 – 12 microns, or 35 – 45 microns were used. Considerable difficulties with aggregation of particles were encountered, especially with nanoparticles and with salt water.  These issues were largely overcome by a glycerol ligand exchange technique with the nanoparticles and substitution of dextrose solution for the saline, thus eliminating the interaction with the dissolved sodium and chlorine ions. Results are presented in light of linear stability analysis in the literature as well as in light of Darcy calculations.