Session ZC03: Impact, Bouncing, Wetting and Spreading

Engulfment of a drop on solids coated by thin and thick fluid films

When an aqueous drop contacts an immiscible oil film, it displays complex interfacial dynamics. When the spreading factor is positive, upon contact, the oil spreads onto the drop’s liquid–air interface, first forming a liquid bridge whose curvature drives an apparent drop spreading motion and later engulfs the drop. We study this flow using both three-phase lattice Boltzmann simulations based on the conservative phase field model, and experiments. Inertially and viscously limited dynamics are explored using the Ohnesorge number Oh and the ratio between the film height H and the initial drop radius R. Both regimes show that the radial growth of the liquid bridge r is fairly insensitive to the film height H,and scales with time T as r∼T^1/2 for Oh≪1,and as r∼T^2/5 for Oh ≫ 1. For Oh ≫ 1, we show experimentally that this immiscible liquid bridge growth is analogous with the miscible drop–film coalescence case. Contrary to the growth of the liquid bridge, however, we find that the late-time engulfment dynamics and final interface profiles are significantly affected by the ratio H/R.   

Droplet impact of the ultrathin oil layer on smooth hydrophobic surfaces

This experimental study investigates the impact of water droplets on thin layers of oil covering different smooth hydrophobic surfaces. The selected oil and surface chemistry combinations exhibited low and very high affinities towards solids in the presence of water. Surfaces with strong oil affinity {van-der-Waal film(vdw)} displayed distinct behavior compared to surfaces with low affinity when subjected to water droplet impacts on the oil-coated surfaces. Smooth hydrophobic surfaces were prepared by coating silicone wafers with a thin layer of low surface energy materials such as octadecyl trichlorosilane (OTS) and trichloro-perfluorooctyl silane (FS). Subsequently, a thin layer of low viscous oil was coated uniformly on the hydrophobic surfaces using the dip coating method. Selected oil produced a very thin van-der-Waal (vdw) oil layer on the OTS functionalized surface and a thin non-vdw oil layer on the FS functionalized surface.  High-speed imaging cameras and image analysis methods were used to analyze the impact dynamics. The findings reveal a complex series of events during droplet impact, with initial deformation of the oil film causing droplets to rebound due to repulsion between water and the oil-coated surface. Measurements of interfacial energies and contact angles indicated the cloaking of water drops by the oil film. Notably, droplet impact behavior changed significantly under repeated drop impacts on the same spot, attributed to variations in oil film thickness due to drainage on vdw oil-coated surfaces and a combination of oil drainage and oil barrier rupture on non vdw oil-coated surfaces. Additionally, viscosity and oil film thickness played crucial roles in determining impact outcomes. The study provides important insights, including droplet rebound on smooth OTS surfaces, droplet adhesion on vdw oil-coated OTS surfaces, the effect of viscous oils on impact dynamics, and the impact of oil film thickness on drop motion resistance and film rupture on non vdw oil-coated smooth surfaces. These findings have implications in various industrial and biomedical applications.

  

High-velocity droplet impact dynamics on lubricant-infused surfaces

Since the pioneering work of Worthington in 1876, engineers and physicists including the public have been fascinated by the mesmerizing images of the crown that forms when a droplet impacts a rigid surface. Setting the beauty aside, droplet impact and breakup has broad technological implications in agriculture, inkjet printing, spray coating, combustion, and forensic science. The dynamics of a droplet impacting a rigid surface is determined by the complex interplay between surface tension, viscosity, inertia, and gravity. Oil-impregnated surfaces bring additional complexity since both the droplet and the underlying oil film deform during impact. In this study, we experimentally characterize and model droplet impact, spreading, retraction, ejection, and the instability-induced breakup and splashing mechanism of the radially expanding liquid lamella when a millimeter size droplet impinges on a thin lubricant film. Our preliminary results of high-velocity droplet impact show that the density and viscosity ratios between the droplet and the lubricant film play a pivotal role in the outcome of the droplet impact. The insights gained from this work advance our understanding of the fundamental physics that govern droplet-surface interaction during impact.   

Reverse Waterbells Formed During Drop Impact onto Immiscible Oil Layers on Water

The formation of a reverse waterbell (also known as a drop-impact canopy) is a fascinating splashing phenomenon, which is easily observed during heavy rain on puddles, but is yet to be completely understood. This research primarily deals with the impact of an oil droplet with a two-layered liquid consisting of a thin oil layer on top of a deep pool of water. This configuration is important in understanding the process of oil emulsification by rain after an oil-spill occurs on a lake or ocean. High-speed impacts give rise to the generation of a large crater along with an axisymmetric thin liquid sheet which rises out of the pool, initially expands, then retracts and finally collapses onto the axis of symmetry entrapping a hemisphere of air. The reverse waterbell or canopy is observed upon the completion of this whole process, by a bubble sitting on the pool surface. In our investigation, we varied the thickness and viscosity of the oil layer on the pool as well as the impact velocity of the oil droplet to explore where canopies are formed. Image analysis, obtained using two high-speed cameras, provides insights into how the controlling factors of fluid properties and film thickness affect these dynamics. We characterize the canopy size in Reynolds and Weber numbers space.   

Three-fluid simulation: Study of a Jet after impact of a drop on a pool of immiscible liquid

Multiphase problems usually consist of more than two fluids, for example fluid/gas mixtures in combustion engines as well as in industry in general. We also observe more than three fluids during phase changes. Furthermore, the ejection of small droplets or aerosols following an impact is also a crucial issue, in particular for the safety of personnel in the presence of hazardous fluids due to their toxicity (chemistry) or their properties (heat in the steel industry). We propose here to couple these two questions by simulating a jet due to a drop impact. The problem studied consists of a droplet surrounded by a gas, impacting an oil film above a deep pool of liquid. To perform these simulations, we use Basilisk. We consider here at least three different fluids, the gas, an oil and the same fluid in the drop and in the pool. Going from two to three fluids in the problem increases the number of physical parameters. To begin, we first decided to focus the study on the influence of the viscosity of the substrate on the maximum speed of the jet following the impact.   

Oil droplet impacting on the bulk water: energy transport depending on the oil viscosity

We experimentally investigate the interfacial phenomena (crater and jet) driven by the oil droplet impacting on the bulk water, while varying its viscosity. The silicon oil droplet (diameter of 2.7 mm), whose viscosity is varied as 0.65-100 cP, was dropped at different heights from the water surface to achieve the impact velocity of 1.5 - 3.5 m/s. The interaction of droplet and water surface was captured with a high-speed camera. Upon impacting, a crater is formed below the surface and as it retracts due to surface tension, a vertical jet appears above the surface. Various jet phenomena appear depending on the viscosity and impact velocity, which are categorized into four regimes based on the pinch-off, jet thickness, and secondary droplet composition. To understand the transition between each regime, we analyzed the temporal energy transport between gravitational potential energy, surface energy, and kinetic energy. The results show that there is less energy conversion (i.e., higher loss to viscous dissipation) to kinetic energy at higher viscosities than at lower viscosities, which leads to differences in the geometry of the jet (the maximum height of jet, the presence of pinch-off, and so on).   

Formation of downward Marangoni jet after ethanol droplet impact on water surface

When an ethanol droplet is released onto a quiescent water surface, immediately after the collision, the droplet penetrates the water and extends downwards before contracting. During the contraction phase, fresh water rises, forming a ring of ethanol on the water surface. Within this ring, the Marangoni convection occurs, resulting in the generation of downward ethanol jet. We experimentally investigated the conditions required for the generation of the jets. A glass container is filled with 500mL of ion-exchanged water. To visualize the jets, fluorescein dye (uranin) is dissolved in anhydrous ethanol. Ethanol droplets are generated and dropped into the container using a syringe pump connected to an injector with stainless steel pipes. Three types of stainless steel pipes with diameters of d=1.5mm, 2.5mm, and 3.5mm are used. The flow behavior of the ethanol droplets beneath the water surface is captured using a high-speed camera. In this work, we will show the dropping height from the liquid surface can significantly affect the generation of the jets.   

The impact and spreading dynamics of internal gel-phase emulsion drops on solid surfaces

The dynamics of drop impact, spreading, and evaporation on solid surfaces is fundamental in the design of many processes such as 3D printing, combustion, spray cooling, and coating. Such dynamics for an emulsion drop, i.e. a continuous phase containing many droplets of a second and immiscible phase is less known. Here, we formulate drops of heptane and sorbitan monooleate (Span80) as the continuous phase containing a diluted microemulsion phase made of either DI-water or silica-gel. In the latter, the microemulsion phase undergoes a gelation process, so the heptane drops host gelled droplets. We examine the heptane drop impacts and spreading on a glass surface.  We employ an automated dosing needle and a side-view camera to conduct and visualize the experiments in room conditions. The results manifest a two-stage process: the drop rebounds after the impact until spreading becomes dominant. It is found that the impact-rebound stage is identical for both DI-water and gel microemulsion phases. However, the presence of gel microemulsion slows the spreading dynamics and significantly reduces the final wetted area. Microscopic images of dried drops, after heptane evaporation, indicate the uniform deposition of gel droplets.   

Contact dynamics of an emulsion droplet impacting a solid surface

Liquid suspensions and emulsion droplets are common in agriculture and chemical processing. When impacting drops get close to the surface, the air deforms the drop resulting in a ring of contact with a thin disk of air trapped. Here, we used high-speed interferometry to study the initial dynamics of an emulsion drop impacting an ultra-smooth solid surface with a surface roughness of 0.5 nm. We mixed water with oil and used a probe sonicator to produce an oil-water emulsion. The air layer is unaffected for lighter oil-water emulsions (ρ_oil<ρ_w).  However, when heavier oil is used (ρ_oil>ρ_w), oil nano-lenses form on the water-air interface affecting contact dynamics. The present work gives insight into how emulsion droplets interact with the surface and make their first contact, which would apply to biological and chemical systems.   

Sliding, vibrating and swinging droplets on an oscillating fiber

We present an experimental study of the dynamics of water droplets on oscillating rigid fibers. As we vary the frequency and amplitude of the oscillations the droplet transitions between different modes: harmonic pumping, subharmonic pumping, a combination of rocking and pumping modes, and a combination of pumping and swinging modes. We characterize these responses and report how they affect the sliding speed of the droplet along the fiber. The swinging mode is rationalized through a minimal model making an analogy with a forced elastic pendulum.