The review deals with drop impacts on thin liquid layers and dry surfaces. The impacts resulting in crown formation are referred to as splashing. Crowns and their propagation are discussed in detail, as well as some additional kindred, albeit nonsplashing, phenomena like drop spreading and deposition, receding (recoil), jetting, fingering, and rebound. The review begins with an explanation of various practical motivations feeding the interest in the fascinating phenomena of drop impact, and the above-mentioned topics are then considered in their experimental, theoretical, and computational aspects.

Drop Impact Dynamics: Splashing, Spreading, Receding, Bouncing ...

A drop hitting a solid surface can deposit, bounce, or splash. Splashing arises from the breakup of a fine liquid sheet that is ejected radially along the substrate. Bouncing and deposition depend crucially on the wetting properties of the substrate. In this review, we focus on recent experimental and theoretical studies, which aim at unraveling the underlying physics, characterized by the delicate interplay of not only liquid inertia, viscosity, and surface tension, but also the surrounding gas. The gas cushions the initial contact; it is entrapped in a central microbubble on the substrate; and it promotes the so-called corona splash, by lifting the lamella away from the solid. Particular attention is paid to the influence of surface roughness, natural or engineered to enhance repellency, relevant in many applications.

https://hal.archives-ouvertes.fr/hal-01398138/document

Drop Impact on a Solid Surface

The spread and rebound of droplets upon impact on flat surfaces at room temperature were studied over a wide range of impact velocities (0.5–6 m/s), viscosities (1–100 mPa.s), static contact angles (30–120°), droplet sizes (1.5–3.5 mm), and surface roughnesses using a fast-shutter-speed CCD camera. The maximum spread of a droplet upon impact depended strongly on the liquid viscosity and the impact velocity. The tendency of a droplet to deposit or to rebound is determined primarily by the liquid viscosity and the liquid/substrate static contact angle. A model more broadly applicable than existing models was developed to predict maximum spread as a function of the Reynolds number, the Weber number, and the static contact angle. Based on the conservation of energy, a rebound model is proposed that predicts the tendency to rebound as a function of maximum spread and static contact angle.

The maximum-spread model prediction agrees to within 10% with more than 90% of the experimental data from different sources. In the current study, the rebound model successfully predicts the tendency of a droplet to rebound.

Spread and rebound of liquid droplets upon impact on flat surfaces

This paper presents results of an experimental investigation of a single drop impact onto a dry, partially wettable substrate and its numerical simulation Particularly, the drop spreading diameter and the dynamic contact angle are measured at different time instants after impact Two surfaces, wax (low wettability) and glass (high wettability), are used to study the effect of surface wettability (static contact angle) on the impact dynamics It is shown that existing empirical models for the dynamic contact angle (eg, Hoffman–Voinov–Tanner law) do not predict well the change of the dynamic contact angle, especially at high capillary numbers In addition to the experimental investigations, the drop impact was studied numerically, focusing primarily on the contact angle treatment The singularity in the neighborhood of the moving contact line is removed from the computational domain and replaced by a local force with some dependence on the instantaneous advancing/receding contact-line velocity The predicted time dependence of the drop spreading diameter and of the dynamic contact angle agrees well with the experimental data for both the advancing and receding phases of the impact process

Dynamic contact angle of spreading droplets: Experiments and simulations

Advances and challenges in explaining fuel spray impingement: How much of single droplet impact research is useful?

Impact of water droplets on a flat, solid surface was studied using both experiments and numerical simulation. Liquid–solid contact angle was varied in experiments by adding traces of a surfactant to water. Impacting droplets were photographed and liquid–solid contact diameters and contact angles were measured from photographs. A numerical solution of the Navier–Stokes equation using a modified SOLA‐VOF method was used to modeldroplet deformation. Measured values of dynamic contact angles were used as a boundary condition for the numerical model. Impacting droplets spread on the surface until liquid surface tension and viscosity overcame inertial forces, after which they recoiled off the surface. Adding a surfactant did not affect droplet shape during the initial stages of impact, but did increase maximum spread diameter and reduce recoil height. Comparison of computer generated images of impacting droplets with photographs showed that the numerical model modeled droplet shape evolution correctly. Accurate predictions were obtained for droplet contact diameter during spreading and at equilibrium. The model overpredicted droplet contact diameters during recoil. Assuming that dynamic surface tension of surfactant solutions is constant, equaling that of pure water, gave predicted droplet shapes that best agreed with experimental observations. When the contact angle was assumed constant in the model, equal to the measured equilibrium value, predictions were less accurate. A simple analytical model was developed to predict maximum droplet diameter after impact. Model predictions agreed well with experimental measurements reported in the literature. Capillary effects were shown to be negligible during droplet impact when We≫Re1/2.

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Capillary effects during droplet impact on a solid surface

The spreading of a water droplet impacting on a flat solid surface was studied using both numerical models and experiments. Surface tension was varied in experiments by adding traces of a surfactant to water. Droplet impact was photographed, and liquid-solid contact diameter and contact angle measured from photographs . A numerical solution of the Navier Stokes equation using a modified SOLA-VOF method was used to model droplet deformation during impact. Measured values of contact angle were used as a boundary condition for the numerical model. Under the condition of low We numbers (We<40) impacting droplets spread on the surface until surface tension forces overcame liquid inertia, after which they recoiled off the surface. Decreasing surface tension did not affect droplet spreading, but did reduce the height of recoil. Comparison of computer generated images of impacting droplets with photographs showed that the numerical model correctly modelled droplet shape during deformation. Accurate predictions were obtained for droplet contact diameter during spreading, and at equilibrium. The model overpredicted droplet contact diameters during recoil . When the contact angle was assumed constant in the model, equal to the measured equilibrium value, predictions were less accurate. INTRODUCTION A study of the impact dynamics of liquid drops on a solid surface is required in modelling industrial. processes such as Spray cooling of hot surfaces, flre extinguishment by 53 sprinkler systems, plasma coating, spray forming, and pesticide spraying. Simple analytical models of droplet impact have been proposed (e.g., Madejski 1976; Chandra & Avedisian 1991; Karl, Anders & Frohn 1993), based on an energy balance that equates initial droplet kinetic energy to the change in surface energy due to droplet deformation, and the work done in overcoming liquid viscosity during impact. Bennett & Poulikakos (1993) have reviewed several such models and discussed their use in predicting the maximum diameter of droplet spread, after which further spreading is restrained by surface tension and viscous forces . However, calculations of heat transfer between the surface and droplet require detailed information about droplet shape during impact, which can be obtained only by a complete numerical solution of the continuity, momentum and energy equations. Harlow and Shannon (1967) were the flrst to obtain a numerical solution to the problem of droplet impact, using the so called "Marker-and-Cell" (MAC) finite difference method to solve the Navier-Stokes equations. They neglected the effect of surface tension and viscosity, so that their results were applicable only to the initial stages of droplet impact when these forces are negligible compared to inertial effects. Their solution could not predict the maximum extent of liquid spread, but proved useful in research on erosion of turbine blades by high speed impinging droplets, where it predicted peak liquid pressures immediately after impact (Huang, Hammitt & Yang 1973; Pidsley 1983). Modelling heat transfer within the droplet required modifications to the MAC code to include surface tension and viscous effects, which was done by Tsurutani

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Y. M. Qiao

Papers

Capillary effects during droplet impact on a solid surface

The effect of surface tension and contact angle on the spreading of a droplet impacting on a substrate

The Morphology and Doping Effects for Ice adhesion on Alumina Surface