Session M12: Drops: Impact, Bouncing, Wetting and Spreading IV

Wetting or freezing? Investigating the spreading dynamics of water on ice

When capillary flows (spreading or impacting drops, films, rivulets) solidify, the inert substrate is replaced by a growing ice layer. In such cases, the adhesion of the liquid on the solid is modified, which can have a major impact on the flow geometry and constraints. In our experiments, we investigate the spreading of a liquid drop, hotter than the water melting temperature, on ice. This allows us to observe directly the combined and competing effects of capillarity and solidification. We find that the spreading and arrest features are different than in isothermal cases, but also different from similar experiments made on non-ice cold substrates. Our results point towards the idea that liquid water does not totally wet ice, and hence the existence of an ice-water contact angle, which has already been suggested in the literature.   

Water film solidification

Solidification of water is present in many situations, from the formation of frost and ice in natural flows, to the icing of structures such as airplane wings, of crucial importance for flight safety. Here, we study experimentally ice formation by flowing of a water film on a cold substrate. The experience consists in a closed loop where water flows down a plate cooled by a cryostat. We analyse the influence of inlet water and plate temperatures, water flow rate, and plate inclination on the mechanism of freezing. According to previous works, we expect to observe a first step of ice growing on the substrate and then a stationary stage where ice will isolate the flowing water from the cold plate. Finally, this experience will also help to characterize the wettability of water on ice.   

Drop bouncing dynamics on ultra-thin films

Drops impacting liquid films tend to irreversibly coalesce and spread on a film made of the same substance. However, in this work, new dynamics of drops initiating contact yet carrying enough momentum to completely lift off of the substrate are called contact bouncing drops. An ultra-thin film is generated by making the ratio of the liquid films to the drop diameters to be less than 0.1 by either creating a smooth draining or flowing film via a syringe pump. High-speed interferometry is also used to visualize the interfacial gas layer over various film thicknesses and Weber numbers. While the increased mobility of the thin liquid films increases the gas entrainment early on, it concomitantly accelerates the rate of the gas purging underneath the drops, enhancing the probability of contact before the gas film retraction and the subsequent drop lift-off sequence. Drops which contact the liquid film during the retraction stage are able to bounce with negligibly small mass loss where the residual drop deposition may be of use to various controlled nanoliter range drop deposition applications as well as understanding the physics of dropwise contamination routes on various surfaces.   

Droplet bouncing and merging on a wet spherical surface

The impact of droplets on surfaces of varying characteristics has seen a plethora of investigations owing to its practical relevance in many natural and industrial processes. Of particular interest is liquid droplets that either coalesce or completely rebound when impacting a thin layer of the same liquid which typically occurs at low impact Weber numbers. Several studies have established regimes of impact outcomes for which the transition between bouncing and merging may occur upon changing film thickness normalized by drop radius, h^*. These studies however focus on impact on a thin film on a planar surface. Herein we present an experimental investigation on liquid droplets impinging a thin film on curved surface with h^* ranging from 0.01-0.05 and discuss the effects of viscosity on the location of the inertial limit transition boundary.   

Modelling high-speed droplet impact onto an elastic membrane

The impact of a high-speed droplet onto an elastic membrane is a highly nonlinear process and poses a formidable modelling challenge due to both the multi-scale nature of the flow and the fluid-structure interaction between the droplet and the membrane. We present two modelling approaches for droplet impact onto elastic membranes: matched asymptotics and direct numerical simulations (DNS). Inviscid Wagner theory is used in the former to derive analytical expressions which approximate the behaviour of the droplet during the early stages of impact, while the DNS builds on the open-source volume-of-fluid code Basilisk. We demonstrate the strong influence that the substrate motion has on the dynamics of the droplet, in particular in terms of altering its internal pressure distribution. We also quantitatively show that the speed the droplet spreads across the substrate is notably decreased when the membrane is more compliant, which is consistent with experimental findings that splashing can be inhibited by impacting onto a soft substrate. We conclude by showing how these methods are complementary, as a combination of both can lead to a thorough understanding of the droplet impact across timescales.   

Not Participating

Drops sliding onto viscoelastic substrates dissipate energy in both liquid and solid. So far, studies focused on substrates much more viscous than liquids. In accordance with recent theoritical predictions, we assume dissipation ratio between solid and liquid is a key parameter of soft wetting systems. We conducted sliding experiments onto thick viscoelastic substrates and changed the dissipation ratio increasing liquid viscosity. A huge dissipation in the solid rises an apparent hysteretic behaviour, which confirms theoretical predictions; this so called “soft hysteresis” results in longer drops. Increasing dissipation in the solid also changes the relation between weight and running speed. Thus, we experimentally showed that dissipation ratio controls dynamical soft wetting systems. This key parameter could optimise liquid runoff processes such as dew collectors.   

Droplet settling on soft layers

We study the gravitational settling of a droplet in air onto a soft substrate using a three-phase lubrication model. We consider for this soft substrate either a thin viscous film, a thin elastic compressible material, or an elastic sheet supported by a thin viscous film. We combine scaling analysis, analytical methods, and numerical simulations to describe the resulting dynamics. In particular, when the response of the soft layer is linear, we show that the air layer takes longer to drain as compared to the case of a droplet settling onto a rigid substrate. Our results provide new insight into the coupled interactions between droplets and solids coated by a thin film of a soft material.   

Transient cavity evolution and morphology after the impact of a spherical gel drop on a deep gel pool

Impact craters appear in many natural and industrial processes. Their dynamics are widely studied using Newtonian fluids or granular media. However, in some processes the yield strength of the impactor and the target is important. In this work, motivated by planetary impacts, we present results from an experimental study where both the impactor and the target are Carbopol gel. We use a high-speed camera to record the falling of the impactor, which is dyed to be distinguishable from the pool material after impact, and a second high-speed camera to record the cavity evolution through a transparent tank. The experimental parameters of our lab impacts are in the lowest energy limit of planetary impacts and surface tension effects are negligible. We perform analysis of the transient morphology of the cavity and find dependence on the material yield strength. Also, whereas for high-Reynolds-number craters in Newtonian fluids there is near perfect conversion of the impactor kinetic energy into the potential energy of the maximum transient cavity, here we find that some of the energy is expended to yield the material. Before reaching a final depth in the pool, the impactor material oscillates vertically in a damped oscillation with period that depends on material elasticity.   

Bouncing dynamics of oil droplets in strongly stratified liquids

Remarkably rich physics is involved when an oil droplet is submerged in a stably stratified ethanol-water mixture. Whereas some droplets may levitate above their density matched position due to a stable Marangoni convection along its surface, others may show an oscillatory bouncing motion. This transition is due to a Marangoni instability that happens either when the stratification is strong enough (low viscous oil) or when the droplet is large enough (high viscous oil) [Li, et al., PRL 126, 124502 (2021)] [Li, et al., JFM, Under Review (2021)]. In the present work, we experimentally study the bouncing dynamics of silicone oil droplets of different viscosities and characterize how the jumping characteristics (jumping height and amplitude, rising and sinking time) depend on the control parameters (droplet radius, stratification strength, viscosity). In addition, we performed numerical simulations with the boundary element method, allowing for one-to-one comparisons between numerical and experimental results. In general, we find good overall agreement, apart from a small over-prediction of the Marangoni force, which we ascribe to surfactants in the experiments. The present study helps to better understand the rich phenomenology of physicochemical hydrodynamics out of equilibrium.   

Polymer filaments from bouncing drop

Polymer filaments and their production are of primary importance in biology, tissue engineering, medicine, and pharmacology. Herein we use high-speed video imaging to study the impact and bouncing of a polymer drop to generate such filaments. Different filament structures can be pulled from the drop during the bouncing, depending on the varying super-hydrophobic surfaces.  The liquid drops comprise distilled water and a small amount of high molecular weight (4 MDa) polymer - poly(ethylene oxide) (PEO) ¹.  We also present a novel method of depositing fine polymer and DNA filaments on super-hydrophobic pillared surfaces, differing from previous methods using an evaporating drop ^2–6.  Different impact velocities and substrate inclination angles are investigated to optimize the thickness and length of the filaments.

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