Energetics of drop deformation with application to turbulence.

Drop deformation in fluid flows is investigated here as an exchange between the kinetic energy of the fluid and the surface energy of the drop. We show analytically that this energetic exchange is governed only by the action of the rate-of-strain tensor on the surface of the drop, more specifically, by a mechanism analogous to vortex stretching. Our formulation allows to isolate the energetic exchange due to drop-induced surface effects from the external action of turbulence fluctuations. We perform direct numerical simulations of single drops in isotropic homogeneous turbulence and show that an important contribution to the energetic exchange arises from the stretching of the fluid-fluid interface by velocity fluctuations far from the drop surface. The identification of this mechanism, which is approximately independent of surface dynamics in a wide range of Weber numbers, sheds new light on drop deformation in turbulent flows, and opens the venue for the improvement and simplification of breakup models.