Role of particle flux on effective mass diffusion in high-Schmidt number environments

Experiments were conducted to study the role of particle flux in altering the effective mass diffusivity across a stable interface between two liquid layers. To create the interface, a mixture of Rhodamine 6G, distilled water, and 3% Glycerine (by volume) was injected into the bottom of the test tank, and pure distilled water was layered on top. Barium titanate spherical particles (SG 4.5) were released to free-fall normal to the interface at various drop rates in a controlled manner in the range of 0.25-1.5 mg/sec, using a computer-controlled stepper motor and a screw-driven dropping mechanism. The particles and the dropping mechanism were carefully characterized for consistency, and the particles were found to have a mean diameter of d_p=228 µm, standard deviation of 19 µm, mean circularity of 0.955. Continuous concentration measurements were made using an sCMOS camera imaging a synchronized strobing of an LED backlight and measuring the absorbance. These absorbance measurements were converted into average concentration profiles normal to the interface to extract the change in mass diffusivity in time with and without the particle-flux. The results show upto an order of magnitude increase in the effective diffusivity between particle-laden and particle-free flows. The role of particle drop rate, Reynolds number, and density ratio will be explored to quantify their role in altering the effective diffusivity.