Viscous droplet impingement on soft substrates (original) (raw)
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This article illustrates the spreading and receding characteristics of non-Newtonian droplets impinging on solid surfaces at different Weber numbers. A xanthan gum solution was used to generate non-Newtonian droplets. From digital images captured using a high speed camera, spreading diameters and dynamic contact angles (DCA) were measured during the impact process. Depending on impact velocity, distinct differences in spreading and receding motions were found between Newtonian and non-Newtonian droplets, which were highly associated with viscous energy dissipation. The maximum spreading diameters for Newtonian and non-Newtonian droplets were nearly the same, but a much slower receding motion was observed for non-Newtonian droplets because of the shear-thinning effect. Moreover, a rapid decrease of DCA in the spreading regime was observed for both non-Newtonian and Newtonian droplets, indicating that the inertial force became dominant. By contrast, measured DCAs for non-Newtonian fluid droplets in the receding regime were larger than those for Newtonian fluid droplets, demonstrating that cohesive surface forces were more dominant than inertial forces in this regime.
Bouncing-to-wetting transition for droplet impact on soft solids
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Soft surfaces impacted by liquid drops trap more air than their rigid counterpart. Due to the extended lifetime of this air film, the dynamics and eventual rupture of trapped air film depend on its interaction with the air cavity developed in the liquid during impact. In this work, we investigate the interaction between air cavity collapse and air film rupture for drops impacting on soft, hydrophobic surfaces using high-speed laser interferometry. We first reveal three different rupture dynamics of the trapped air film; the rupture may initiate either at the center of the dimple, at the edge of the dimple or at a random point in the dimple’s outer rim. We found that the transition between these rupture dynamics depends on both the impact velocity and the surface elasticity. Further, in the most special case of air-film rupture, the so-called dimple inversion, the rupture is directly caused by the collapsing air cavity in the droplet bulk. We further observe that in such cases, high-...
The Deformation of Single Droplets Impacting onto a Flat Surface
SAE Technical Paper Series, 2002
This paper presents an experimental study of the deformation of spherical liquid droplets impinging onto dry and flat surfaces making use of a CCD-camera with a high spatial resolution. The experiments consider different liquids (water and Diesel oil) and the effects of droplet velocity and diameter at the impact in a range of Weber numbers up to 1100 and Reynolds numbers up to 77400. Emphasis is put on the nature of the surface target. To consider this effect, two surface materiais were used (perspex and aluminium) with surface roughness varying from less than ôurn (considered as a smooth surface) up to Ra=66,6f.lm. For the range of droplet diameters considered in the experiments, the corresponding dimensionless values of Ra/Rdroplet vary from 1.5x1 0-5 and 2.5x1 0. 2. In a first step, the experiments focus on the spread of the liquid film and analysis of the results suggest that, provided that the Reynolds number of the droplet at the impact is large (Re>2000), the energy dissipated at the wall is not affected by the nature of the surface. The effect of surface roughness appears to be important for low Reynolds numbers, typically Re<1000. Depending upon the physical parameters at the impact, the droplet may splash at the first contact with the surface (prompt splash) or the liquid film at the wall may break-up on secondary droplets during spread. In a second step, the experiments emphasize the effect of surface roughness on the onset of splash for different liquids. The experimental results are analysed in terms of the critical Weber number for which splash occurs, and compared with correlations reported in the literature accounting for the effects of surface roughness and droplet liquid and suggest the likely influence of, not only the nature of the surface (e.g. surface profile and material), but also of the liquid.
On the impact of viscous drops onto dry smooth surfaces
Experiments in Fluids, 2012
An experimental study of the impact of glycerol/water drops onto a dry glass surface at Reynolds and Weber numbers around the splashing/deposition threshold is presented. Some new observed phenomena that may shed further light on the mechanisms underlying air bubble entrainment and splashing for high-viscosity liquids are presented and discussed. The experiments were recorded with a high-speed camera using two complementary lighting setups that enhance the visualization of different features of the air entrainment phenomena: backlighting with a light diffuser and oblique lighting without diffuser. Besides the ring of micro-bubbles surrounding the central entrapped bubble and the cloud of bubbles entrained as a result of the interaction between a levitated thin film and the solid surface, which have been studied by other authors in previous works, a second ring of micro-bubbles that delimits the outer cloud of bubbles has been detected in our investigation. Attention is mainly focused on analyzing the dependency of the behavior of the two rings of microbubbles on the drop impact velocity, the ranges of the relevant dimensionless numbers in which the rings are formed and the existence, in certain impact conditions, of an abrupt increase in the size of the second ring, which substantially modifies the impact outcome.
Retraction dynamics of aqueous drops upon impact on non-wetting surfaces
Journal of Fluid Mechanics, 2005
We study the impact and subsequent retraction dynamics of liquid droplets upon highspeed impact on hydrophobic surfaces. Performing extensive experiments, we show that the drop retraction rate is a material constant and does not depend on the impact velocity. We show that when increasing the Ohnesorge number, Oh = η/ √ ρR I γ, the retraction, i.e. dewetting, dynamics crosses over from a capillaro-inertial regime to a capillaro-viscous regime. We rationalize the experimental observations by a simple but robust semi-quantitative model for the solid-liquid contact line dynamics inspired by the standard theories for thin film dewetting.
Pre-impact dynamics of a droplet impinging on a deformable surface
Physics of Fluids
The nonlinear interaction between air and a water droplet just prior to a high-speed impingement on a surface is a phenomenon that has been researched extensively and occurs in a number of industrial settings. The role that the surface deformation plays in an air cushioned impact of a liquid droplet is considered here. In a two-dimensional framework, assuming small density and viscosity ratios between the air and the liquid, a reduced system of integrodifferential equations is derived governing the liquid droplet free-surface shape, the pressure in the thin air film, and the deformation of the surface, assuming the effects of surface tension, compressibility, and gravity to be negligible. The deformation of the surface is first described in a rather general form, based on previous membrane-type models. The coupled system is then investigated in two cases: a soft viscoelastic case where the surface stiffness and (viscous) damping are considered and a more general flexible surface where all relevant parameters are retained. Numerical solutions are presented, highlighting a number of key consequences of surface deformability on the pre-impact phase of droplet impact, such as reduction in pressure buildup, increased air entrapment, and considerable delay to touchdown. Connections (including subtle dependence of the size of entrapped air on the droplet velocity, reduced pressure peaks, and droplet gliding) with recent experiments and a large deformation analysis are also presented.
Influence of wetting behavior on the morphology of droplet impacts onto dry smooth surfaces
Physics of Fluids, 2021
The influence of wettability on the morphology of droplet impacts onto dry surfaces is often neglected in the literature, despite its significant effect on the resulting morphology. In this work, the role of wettability is investigated systematically by considering droplet impact processes on smooth dry surfaces of two different materials. The wetting behavior is varied not only by employing two different fluids, but most importantly by varying the surface properties by plasma activation and polymerization. Overall, this leads to four different wetting behaviors for each surface. The changes in impact morphology are visualized by means of a three-perspective experimental facility. In particular, the bottom view employs a total internal reflection-configuration for visualizing the exact droplet contact area and contact time. This enables us to characterize the main features of the different wetting behaviors. Overall, we found that surface wettability mainly influences the receding p...
Sessile droplets on deformable substrates
Wetting of deformable substrates has gained significant interest over the past decade due to its extensive applications and uses. This interest has developed due to technological advances which are able to capture interfacial behavior taking place when a liquid droplet is placed on a deformable substrate. Researchers have developed different theories to explain processes taking place in the process of wetting of deformable/soft substrates. For the scope of this review, we will consider the fluid to be Newtonian, partially wetting, and surface forces are incorporated with the help of disjoining/conjoining pressure acting in the vicinity of the apparent, three-phase contact line. The following subjects are briefly reviewed: (i) Equilibrium of droplets on soft substrates. It is shown that properties of the disjoining/conjoining pressure isotherm and properties of the deformable substrate determine both the shape of the liquid droplet and deformation of the substrate; (ii) Equilibrium c...
International Journal of Heat and Fluid Flow, 2008
The objective of this work is to develop and validate a numerical model to study wetting during the impact of millimeter-size drops on a flat, smooth, solid substrate under isothermal or nonisothermal conditions. A finite-element modeling is used to simulate the transient fluid dynamics and heat transfer, considering Laplace forces on the liquid-gas boundary. The Lagrangian scheme allows a very precise tracking of the free surface deformation. In this work, the numerical model is extended to account for a temperature-dependent viscosity and for dynamic wetting at the contact line. Numerical results are presented to study the influence of the kinetic wetting parameter on the wetting incipience and behavior. Our results show the influence of wetting on the spreading and the transient drop shape. Also, numerical results are compared with high-speed visualization, for cases of isothermal and non-isothermal impact. Matching between simulations and high-speed visualization allows the determination of the value of the kinetic 2 wetting parameter. Our main finding is that warm drops spread more than cold drops because of a reduction of viscous forces, and not because of an increase of wetting.