Session HS: Drops IX: Impact

Beneath a splash: interference imaging of the air below a spreading drop

Viscous splashing produces a distinct morphology: an impacting drop spreads as a thick lamella, which at late times ejects a thin liquid sheet that subsequently breaks into drops. We describe an interference technique to measure the air layer beneath the spreading drop using high-speed, high-resolution video imaging. We use this technique to visualize the dynamics of the entrapped central air bubble. In addition, we measure the air gap between the thin sheet and the substrate, and find that, within experimental resolution, there is no air gap beneath the spreading lamella. At the lamella/thin sheet interface we find a surprising feature: small regions of the expanding thin sheet, approximately 1 $\mu$m above the substrate, frequently touch down and wet the surface. Pockets of air become trapped among these contacts, and form air bubbles. Air entrainment occurs only when the thin sheet is present; when it is suppressed the lamella spreads as a smooth wetting front.   

Experimental and theoretical study of pattern identification in physical systems on circular domains

This study is motivated by a recent finding of systems in the context of fluid dynamics (e.g. drop splash phenomena) where instabilities of different wavenumbers may co-exist and thus lead to several single-wavenumber patterns superimposed with random phase-shifts between them. A new experimental setup of stereo high-speed photography enables accurate data acquisition of these patterns. However, identification of the wavenumber structure of such patterns from experimental data is complicated by the lack of a theoretical basis as well as the presence of experimental uncertainties and possible missing points in the data. We present both a new theoretical framework and an example of application -- the crown structure analysis in the drop splash problem.   

The Generation of Secondary Droplets due to Drop Impact on a Water Surface in the Presence of Wind and Surfactants

The impact of single water drops on a water surface was studied experimentally in a wind tunnel. Water drops were generated from a needle oriented vertically from the top surface of the wind tunnel test section. The wind speed ranged from 0 to 10.0 m/s. After leaving the needle, the drops move downward due to gravity and downstream due to the effect of the wind, and eventually hit a pool of water on the bottom of the test section. The drop impacts were recorded simultaneously from the side and from above with two high-speed movie cameras set at 1,000 frames per second. It is shown that the water drop obliquely impacts the water surface and the impingement angle relative to vertical increases with increasing wind speed. After the drop hits the water surface, a chain of secondary drops are formed and move in the leeward direction. This is followed by a stalk formation at the location of water drop impact. The effects of wind speed and initial drop size on the number, diameter and total mass of secondary drops and the shape of the stalk were investigated. The effects of surfactants on these parameters were also studied.   

Qualitative Observations of Droplet Impact on Superhydrophobic Surfaces with Micro-ribs for Three Fluids

Droplet impingement experiments on superhydrophobic surfaces with micro-ribs and a hydrophobic coating were performed using three fluid types: water, a 50/50 water/glycerine mixture, and ethanol. Also tested, for comparison, were patterned uncoated, smooth coated, and smooth uncoated surfaces. For surfaces with rib and cavity features, the droplet spread and retraction were observed to be asymmetric and at high Weber numbers the spread along the rib direction is greater than the spread transverse to the ribs. The onset of peripheral splashing was observed to be contact angle dependant and preferential along the ribs. The occurrence of two-pronged and oscillating jets during droplet retraction for water/glycerine tests was observed when the surface was micropatterned. The oscillating and splitting jets were stronger on the superhydrophobic surfaces than on the surfaces with patterning but no hydrophobic coating. Further, an interesting spread pattern with four liquid droplets clustered at about 30\r{ } from the perpendicular direction was observed for all fluid types on both coated and uncoated patterned surfaces.   

Analogies between a drop impacting a solid surface, an oscillating sessile drop, and two coalescing drops

Numerous industrial processes involve the formation of drops which then collide with a solid substrate or another drop. When a drop impacts a solid surface without splashing, it may spread on the surface and then recoil, much like an oscillating sessile drop whose contact line is free to move. Were the impacting drop to make a fixed contact angle of 90 degrees with a substrate that exerts negligible viscous drag on the drop, the situation approximates well the aftermath of what happens once two drops have just started coalescing. Given the aforementioned analogies between these apparently quite distinct physical problems, the dynamics of each process is analyzed by solving numerically the 3D axisymmetric or 2D Navier-Stokes system using a well-benchmarked ALE algorithm based on the Galerkin/Finite Element Method (G/FEM) for spatial discretization and adaptive finite differences for time integration.   

Capillary waves on a periodically supported liquid cylinder in low gravity

The impact of a droplet on a capillary channel consisting of a helical wire filled with water generates capillary wave packets that propagate away from the impact zone helping to dissipate the droplet kinetic energy. A simplified model is presented for the channel consisting of a periodic array of wire rings that penetrate the surface of the liquid. The multiple scattering problem for monochromatic capillary waves on a liquid cylinder impinging on an array of concentric rings is solved by the finite element method (FEM) with radiation boundary conditions at each end. Contact lines are taken to be pinned on the wires. In the limit of wires of infinitesimal thickness the FEM results agree with a semi-analytical theory. The results also allow the determination of an effective wave speed. Finally, results of experimental measurements of wave-packet speed on small horizontal capillary channels will be discussed.   

Thermal Boundary Layer Dynamics in Multiple Droplet Impingement

The impingement of a stream of HFE-7100 droplets striking a pre-wetted and heated surface is studied for droplet Weber and Reynolds numbers ranging from 285 to 427 and 1250 to 4850, respectively, and for a film depth to droplet diameter ratio varying from 0.4 to 1.5. After a short period, a quasi-steady state is achieved; in which the liquid crown formed during continuous droplet impact remains nearly stationary. Temporal averages of the velocity, temperature, and liquid fraction fields suggest that the boundary layer can be categorized as consisting of a stagnation point flow region, a linear growth section, and a jump region, similar to a hydraulic jump, near the liquid crown. Results of the average radial temperatures are compared to experiments for various heat fluxes yielding good agreement. Additionally, it is shown that a sub-layer is present in all cases considered, which is categorized by low values of the local Peclet and Reynolds numbers. The heat transfer mode in this sub-layer domain is governed to a great degree by conduction, and experiences a delayed cooling effect.   

Viscous drop collisions on surfaces of varying wettability

We present an experimental study of increasingly viscous acetone rich and Newtonian equivalent liquid drops colliding on surfaces of varying wettability. This class of liquids applies directly to spray coating processes in pharmaceutical industries. The results from this study will elucidate the physics in a regime where resisting viscous forces and the restoring forces of capillarity are balanced, $Oh\sim$ 1. Early spreading dynamics $\tau=Ut/D\ll$ 1 indicate negligible dependence on contact angles while longer times demonstrate deviations from Tanner's law, $D\sim t^{1/10}$. We will compare our results with recent theory to demonstrate the feasibility of modelling complex rheology spreading characteristics over short and long time scales. Preliminary results indicate an intermediate spreading regime following the inertial phase where the diameter, $D\sim t^{n}$ with 1/7 $< n <$ 1/5.   

Drop fragmentation due to hole nucleation during Leidenfrost impact

Drop impact on a smooth plate heated above the Leidenfrost temperature is investigated in the range of large Weber number. Liquid fragmentation due to the rupture of the expanding lamella during the impact is studied experimentally. This rupture can be triggered by the presence of a small defect on the surface, which acts as a nucleation site for the hole formation, whereas the liquid does not contact the substrate. The rupture is shown to take place above a critical impact velocity, the lower when the defect size is the larger. This mechanism of rupture is compared to classical splash. It is shown to be relevant if the drop size $R_0$ and the size of the defect $d$ are below a critical ratio $R_0/ d \le 40$.