Numerical model simulates dynamics of levitating droplet evaporation

With applications ranging from gas turbines to pharmaceuticals, understanding how liquid droplets evaporate has become increasingly important. Acoustic levitators leverage the power and finesse of ultrasonic fields to suspend droplets in midair while allowing them to rotate, free of heat conduction from filaments or substrates. In Physics of Fluids, mathematicians have announced a model that simulates the evaporation dynamics of these levitating droplets.

The model simulates all the features of droplet evaporation studied in acoustic levitators by employing a finite-element method, an analysis that reduces calculations into a system of discrete equations for nodal values of a triangular grid. The simulations work for theoretical and real levitators.

Unlike most numerical studies of such evaporating droplets that focused on single aspects of acoustic levitation, the team’s work takes several physical mechanisms into account. These changes, including the droplet’s position and shape, gas flow induced by acoustic streaming, and convective-diffusive heat and vapor transport, can be tailored for research scenarios.

Acoustic streaming not only significantly accelerated the evaporation, but the toroidal forces it induced did not entrap vapor, as had been presumed in the field. The team found that raising the sound pressure level of the levitator also accelerates the evaporation, allowing high ambient temperatures to be replaced by high sound pressure levels to achieve similiar evaporation rates. The results of the simulations were in close agreement with experiments utilizing the d2-law — where the rate change is constant — for the evaporation of spherical droplets and glass filaments.

Building on their work, the researchers next hope to extend their work to optimize the evaporation of droplets embedded with proteins for pharmaceutical uses.

Source: “Numerical study of droplet evaporation in an acoustic levitator,” by Eberhard Bänsch and Michael Götz, Physics of Fluids (2018). The article can be accessed at https://doi.org/10.1063/1.5017936 .