Contactless generation and trapping of hydrodynamic knots in sessile droplets by acoustic screw dislocations

Abstract

Helicity is an important quantity in fluid mechanics that indicates the presence of linked or knotted hydrodynamic vortex filaments. Such flow structures are not only promising elementary structures to study mass and momentum transfer in turbulent flows but also potent analogs for other topological problems arising in particle physics, liquid crystals, and plasma physics. However, experimental studies of knots and links are highly challenging due to the limited control over helicity generation and difficult observation of the resulting fast-paced multiscale flow evolution. In this paper, we propose using acoustic streaming to link hydrodynamic filaments in fluids. The method is contactless, almost instantaneous, and relatively insensitive to viscosity. Importantly, it allows starting from quite arbitrary three-dimensional flow structures without relying on external boundary conditions. We demonstrate our approach by using an acoustic screw dislocation to link two hydrodynamic vortex filaments in a sessile droplet. We observe an inversion of the flow chirality (measured by the hydrodynamic helicity) as the topological charge of the screw dislocation is increased. Combined with recent progress in acoustic field synthesis, this work opens a window to study more complex hydrodynamic knots and links topology at a broader range of space and time scales.