Levitation of nanofluid drops

In this experimental work, we investigate the influence of nanoparticle concentration and reduced surface tension on the terminal velocity and oscillation of free-falling drops. We employ a vertical wind tunnel to levitate drops of diameters 1 mm - 7 mm with silicon dioxide nanoparticle concentrations of 0.1, 0.5, and 1%m/m. The addition of 20-nm particles decreases the shape oscillation frequency observed in distilled water drops of equivalent diameter without affecting oscillation amplitude. Adding surfactant to distilled water and nanofluids stabilizes the drop by a reduction in the axis ratio oscillation, the absolute canting angle, and the absolute canting angular velocity. Surfactant produces flatter drops and a reduction in mean terminal velocity by 15\% versus distilled water drops of the same volume. Levitating drops exhibit coexisting modes of drop oscillation. The (2,0) axisymmetric mode occurs for all drops of varying sizes affirming previous studies. Smaller drop sizes introduce a (2,2) horizontal mode and a (2,1) transverse oscillation mode. In larger drops, the addition of nanoparticles introduces a (2,2) mode in addition to the (2,0) mode.