EXPERIMENTS IN ACOUSTIC LEVITATION - SURFACE-TENSION MEASUREMENTS OF DEFORMED DROPLETS

Acoustic levitation permits the observation of individual oscillating liquid droplets at low to moderate temperatures, Measurements of the oscillatory behavior of such droplets may be used to obtain thermophysical property measurements without the contaminating effects of a solid container. For example, for a droplet of spherical equilibrium shape, each mode of oscillation has been analytically shown to exhibit a single natural frequency that is a function of droplet size, mode number, and the surface tension and density of the liquid. In levitation experiments performed in Earth's gravity, however, this single frequency splits into multiple distinct frequencies, a phenomenon that may be attributed to the distorted equilibrium shape and the rotation of the droplet. In addition, the natural frequencies of droplets of highly volatile liquids change rapidly over time due to evaporation. This work presents experimental data and observations on the frequency splitting and surface tension of acoustically levitated samples of water and ethyl alcohol; the surface tension measured for both liquids came within 5% of the published value. In addition, a novel and relatively simple photodetection scheme for measuring oscillations as well as an improved method for modeling the ellipsoidal static shape deformation of the droplet are presented.