Session E12: Drops: Impact, Bouncing, Wetting and Spreading I

Contact line solidification of a spreading drop

Contact line dynamics of water advancing on a solid substrate below the freezing temperature is a today's challenging question. Despite its relevance in various applications, such as airplane icing or ice accretion on wires or roadways due to freezing rain, the fundamental mechanism of contact line arrest is still not fully understood. In this work, we investigate the spreading of a deposited water drop on a cooled substrate, for temperature ranging down to -45°C. By visualizing the growth of the dendritic patch formed in contact with the solid, we show how it pins the advancing contact line and arrests the drop spreading. Then, both dynamics of the contact line and the dendritic front are compared on the whole temperature range. These allow us to bring strong arguments in order to enlighten the debated mechanism on the contact line arrest due to solidification.   

Splashing of drops impacting rough substrates

Here, we present experimental results of drops of low viscosity fluids of radii R, density ρ and interfacial tension coefficient σ, impacting with a velocity V over different types of sandpapers characterized by their grit size, ε. It is shown that the transition from spreading to splashing at normal atmospheric conditions can be classified depending on the value of the parameter ε/H_t = We_ε = We(ε/R), with We = ρV²R/σ the Weber number and H_t the initial thickness of the thin film which is ejected along the substrate once the drop touches the solid. When We_ε ≤ 1 and the liquid wets the substrate, the critical value of the Weber number above which the drop splashes, We_c, can be predicted using the results in Gordillo & Riboux (J. Fluid Mech., vol. 871, 2019, R3). Moreover, if the liquid does not wet the substrate, it is also shown that the splash velocity can be predicted using the theory for superhydrophobic substrates in Quintero, Riboux & Gordillo (J. Fluid Mech., vol. 870, 2019, 175−188). For those cases in which We_ε ≥ 1 and the liquid wets the substrate, we demonstrate that the critical Weber number for splashing decreases with ε as We_c ∝ (R cosθ₀ / ε)^3/5, with θ₀ the value of the Young contact angle.   

Microdroplet impact onto a commensurate topographical Features

As droplet sizes decrease in the search of increasing resolution in inkjet printing and other applications, substrate features of commensurate size start to have a significant effect on the printed footprint. In particular, they can affect whether consecutive droplets coalesce or not. Here, the deposition of multiple droplets onto a topographical feature of commensurate size is simulated using a 3-D GPU-accelerated multiphase lattice Boltzmann model validated against published experimental work and theoretical predictions. A regime map of coalescence vs. splitting depending on the width and height of the feature is constructed. The effects of substrate properties (advancing and receding contact angles), flow parameters (captured by the Reynolds number), and fluid properties (captured by the Ohnesorge number) on the regime map are also explored.   

Wettability Behaviour of Droplets and Larvae on Plants Leaf

In nature, we observe beautiful patterns in plants leaves and animal furs, feathers, and butterfly wings. The pattern on the leaf affects the wettability of plant leaves. These patterns are large-scale and uniform on the leaf. Therefore, this pattern helps to inspire the design of smart superhydrophobic to superhydrophobic materials for energy to the environment field. The wettability behaviour can be theoretically predicted by modified Young's equation. Some time larvaes of whitefly on the leaves are also observed which also form different contact angles. This larva is also composed of a complex liquid with different components but forms a spherical shape to variable contact angle on the leaf surface. This study will explain about the droplet and larva wettability behaviour on the leaves surface which will explore new potential applications.    

Wetting phenomena when freezing a rivulet

We experimentaly explore the freezing of a water capillary river flowing down a cooled inclined plane. In the first moments, we observe two different wetting phenomena taking place transverse to the streamwise direction. First, an ice footprint is left behind by the main droplet running down the inclined plane. Second, on top of the ice footprint, the remaining water film surprisingly dewets the ice on which it flows. The unsteady dewetting dynamics ultimately relaxes towards a steady state. We characterize the dependency of these two events against the substrate temperature and the water flow rate. Finally, we bring new understandings to two fundamental debated questions: the arrest of a contact line in the presence of solidification and the contact angle of water on ice.   

Gunwale bobbing and the quantum canoe

Recently, various authors have shown experimentally that droplets, bouncing in a liquid bath, can be made to 'walk' at constant horizontal velocity, and in so doing exhibit a wealth of analogous quantum phenomena. A life-size application of this phenomena takes the form of a person, or many people, jumping up and down on a canoe to achieve forward motion through the surfing of their own wave field, a sport known to those who have found themselves up the lake without a paddle. After an initial transient, the canoe achieves a cruising velocity which satisfies a balance between the thrust generated from pushing downwards into the surface gradients of the wave-field and the resistance due to a combination of skin, form and wave drag. By superposing the linear wave theories of Havelock (1919) for steady cruising and Helmholtz for a bouncing source we demonstrate that such a balance can be sustained, and calculate the optimal parameter values to achieve maximum canoe velocity. A comparison is made to accelerometer data taken from an enthusiastic Gunwale bobber, and various perspectives are discussed, including possible analogies with quantum systems.   

The restitution coefficient of bouncing binary droplet collisions

Bouncing droplets abound in nature and industrial applications. Droplets can bounce in different impact scenarios, such as in binary droplet collisions and droplet impact on super-hydrophobic surfaces or liquid surfaces. In bouncing process, initially the impacting droplet spreads, reaching a maximum extent before retracting due to surface tension and leaving the surface accompanied by oscillation in shape. During this process, kinetic energy is transferred to surface energy and lost due to viscosity. Thus, the coefficient of restitution is less than unity and depends both on the magnitude of the initial viscous loss and that occurs during post-collision oscillation. Here, we study these physics using extensive experimental measurements of head-on, equal-size binary droplet collisions imaged using a high-speed camera. A semi-theoretical model as a function of Weber number (kinetic energy vs. surface energy) and Ohnesorge number (viscosity vs. surface tension) was developed, which agrees well with the measured coefficient of restitution. Data of impacts on super-hydrophobic surfaces reported in the literature is also compared.   

Viscous dissipation dictates Taylor-Culick type retractions

Surface tension minimizes the surface area of a ruptured liquid film resulting in a spontaneous retraction. For a long film surrounded by passive gaseous medium (like air), the retraction speed is known to approach a constant Taylor-Culick velocity (given by v_TC = (2γ/(ρh₀))^0.5 for a sheet of thickness, h₀, density ρ, and surface tension coefficient γ). This velocity is independent of the viscosity of both the film as well as the surrounding. In the present work, we study similar retraction dynamics of a liquid film (i) surrounded by a viscous medium (oil) and (ii) sandwiched between a passive (air) and a viscous media (oil). We show that the film in these configurations still retracts with a constant velocity that scales linearly with v_TC. However, the prefactor depends on the viscosity of the oil bath, η_s. The dependence is stronger in case of first configuration (i, v ∽ 1/η_s) as compared to the second (ii, v ∽ 1/η_s^0.5). This difference arises due to the localization of the viscous dissipation near the three-phase contact line in the second configuration. We look at the overall energy budgets to provide insights into the dynamics of this process.   

Revisiting Taylor-Culick retractions

A freely suspended liquid film, upon rupture, spontaneously retracts under the action of surface tension. If the film is surrounded by a passive medium, the retraction velocity of the film is known to approach the classical Taylor-Culick velocity, which is independent of the viscosity of the film. However, if the external medium is viscous, the dissipation within that medium can play a role in determining the magnitude of the retraction velocity. In the present work, we experimentally study the retraction of a liquid (water) film sandwiched between air and another viscous liquid (oil). The measured retraction velocity is found to have a weaker dependence on the viscosity of the oil phase as compared to the configuration where the water film is completely covered by oil. We reveal an unprecedented regime in the We-Oh (or Ca-Oh) phase space for retracting liquid films, governed by the localized viscous dissipation in the vicinity of the three phase contact line. Furthermore, the retraction velocity was found to depend only on the viscosity of the surrounding medium and not on that of the film itself.   

Effect of viscosity ratio on the drop impact dynamics

Drop impact on a liquid pool is ubiquitous in many natural and industrial processes. Theliquid viscosity plays an important role in changing both the global and local drop impactdynamics. The impacts with same type of liquids for the drop and the liquid pool have beenstudied extensively. Here, drop impact on a liquid pool with mismatched viscosity isnumerically simulated and various behaviors emerge during the drop impact process. Theenergy distribution during the impact process obtained from the simulation provides insighton the dynamics. In addition, the interstitial layer between the drop and liquid pool is criticalfor the bouncing to merging transition as well as the bouncing dynamics. The effect ofviscosity ratio on the interstitial layer dynamics is also studied. The scrutiny of the detaileddynamics gained from the simulation provides better understanding of the drop impactprocess and lays the foundation for further studies.