Vortex-induced buckling of a viscous drop impacting a pool

Abstract

CitationLi EQ, Beilharz D, Thoroddsen ST (2017) We study the intricate buckling patterns which can form when a viscous drop impacts a much lower viscosity miscible pool. The drop enters the pool by its impact inertia, flattens, and sinks by its own weight while stretching into a hemispheric bowl. Upward motion along the outer bottom surface of this bowl produces a vortical boundary layer which separates along its top and rolls up into a vortex ring. The vorticity is therefore produced in a fundamentally different way than for a drop impacting a pool of the same liquid. The vortex ring subsequently advects into the bowl, thereby stretching the drop liquid into ever thinner sheets, reaching the micron level. The rotating motion around the vortex pulls in folds to form multiple windings of double-walled toroidal viscous sheets. The axisymmetric velocity field thereby stretches the drop liquid into progressively finer sheets, which are susceptible to both axial and azimuthal compression-induced buckling. The azimuthal buckling of the sheets tends to occur on the inner side of the vortex ring, while their folds can be stretched and straightened on the outside edge. We characterize the total stretching from high-speed video imaging and use particle image velocimetry to track the formation and evolution of the vortex ring. The total interfacial area between the drop and the pool liquid can grow over 40-fold during the first 50 ms after impact. Increasing pool viscosity shows entrapment of a large bubble on top of the drop, while lowering the drop viscosity produces intricate buckled shapes, appearing at the earliest stage and being promoted by the crater motions. We also present an image collage of the most intriguing and convoluted structures observed. Finally, a simple point-vortex model reproduces some features from the experiments and shows variable stretching along the wrapping sheets.

Dynamic wrinkling of freely floating smectic films

“…The driving forces are gravitation and lateral confinement by the bubble geometry [10][11][12]. The formation of wrinkles of fluid interfaces have also been described during the impact of droplets onto a viscous liquid [13] and for droplets rising in a liquid [14]. A related phenomenon is the buckling of viscous fluid filaments [15,16].…”

Two jets during the impact of viscous droplets onto a less-viscous liquid pool

“…Thus, the droplet can be assumed to be a soft and hydrophilic body that deforms gradually. Compared to the impact of a solid hydrophilic sphere, the upward flow of the liquid layer not only needs to overcome the gravity and the inherent viscous damping at the interface, but also converts a portion of its energy into a vortex flow inside the droplet [29]. Therefore, the viscous damping at the interface will increase and the speed threshold V * will be larger than that in the sphere impact model.…”

“…The curve tends to be flat around 0.9 ms after the impact, indicating that the rim diameter of the liquid layer remains almost unchanged after that moment. Because of the low viscosity of the droplet, the droplet deforms greatly and spread along the surface of the pool liquid to form a toroidal viscous sheet [29]. The droplet starts to mix with the pool liquid and there is no clear interface between them.…”

“…Their results show that the liquid corolla spreading out from the impact region consists of both an ejecta and a Peregrine sheet, which, as the pool viscosity increases, evolve on separated timescales. Li et al studied the intricate buckling patterns formed when a viscous drop impacts onto a miscible pool of much lower viscosity, and found that the viscous droplet will stretch into a hemispheric bowl, which would be pulled by the rotating motion around it and gradually form toroidal viscous sheets [29]. Wetting property has a profound effect on the impact dynamics of liquid droplets and solid spheres [30], and investigated the role of wettability of the impacting sphere on the degree of splashing.…”

Nonsphere Drop Impact Assembly of Graphene Oxide Liquid Crystals

“…To date, the exploration on 3D LC topological structures has focused on confinement in high symmetrical space, such as spheres, ellipsoids, and toroids. − However, LC topological structures confined in spaces with broken spherical symmetry still remain mysterious, due to the experimental difficulty to restrict fluid LCs within nonspheres, confinement that usually induces a highly distorted topological configuration. Drop impact, referring to the dynamic process when a liquid droplet strikes a liquid surface, − has been recognized as a simple means to achieve rich nonspherical geometries that are induced by complex liquid movements. − Isotropic molecules and colloids have been studied in the vortex rings generated by drop impact for a century, − but capturing the intriguing textures of the LC within the generated nonspheres still remains difficult.…”