Liquid Droplet Impact Research Articles

Liquid Droplet Impact Over Hydrophobic Mesh Surfaces and Assessment of Weber Number Dependent Characteristics

Abstract Impacting droplets and droplet ejection from hydrophobic mesh surfaces have interest in biomedicine, heat transfer engineering, and self-cleaning of surfaces. The rate and the size of newborn droplets can vary depending on the droplet fluid properties, Weber number, mesh geometry, and surface wetting states. In this study, impacting water droplets onto hydrophobic mesh surface is investigated and impact properties including, spreading, rebounding, and droplet fluid penetration and ejection rates are examined. Droplet behavior is assessed using high recording facilities and predicted in line with the experiments. The findings reveal that the critical Weber number for droplet fluid penetrating/ejecting from mesh screen mainly depends on the droplet fluid capillary length, and hydrophobic mesh size. The contact time of impacting droplet over mesh surface reduces with increasing droplet Weber number, which opposes the case observed for impacting droplets over flat hydrophobic surfaces. The restitution coefficient attains lower values for impacting droplets over mesh surfaces than that of flat surfaces. The rate and diameter of the ejected droplet from the mesh increases as droplet Weber increases. At the onset of impact, streamline curvature is formed inside droplet fluid, which creates a stagnation zone with radially varying pressure at the droplet fluid mesh interface. This reduces the ejected droplet diameter from mesh cells as mesh cells are located away from the impacting vertical axis.

Nanowall Textured Hydrophobic Surfaces and Liquid Droplet Impact.

Water droplet impact on nanowires/nanowalls’ textured hydrophobic silicon surfaces was examined by assessing the influence of texture on the droplet impact dynamics. Silicon wafer surfaces were treated, resulting in closely packed nanowire/nanowall textures with an average spacing and height of 130 nm and 10.45 μm, respectively. The top surfaces of the nanowires/nanowalls were hydrophobized through the deposition of functionalized silica nanoparticles, resulting in a droplet contact angle of 158° ± 2° with a hysteresis of 4° ± 1°. A high-speed camera was utilized to monitor the impacting droplets on hydrophobized nanowires/nanowalls’ textured surfaces. The nanowires/nanowalls texturing of the surface enhances the pinning of the droplet on the impacted surface and lowers the droplet spreading. The maximum spreading diameter of the impacting droplet on the hydrophobized nanowires/nanowalls surfaces becomes smaller than that of the hydrophobized as-received silicon, hydrophobized graphite, micro-grooved, and nano-springs surfaces. Penetration of the impacted droplet fluid into the nanowall-cell structures increases trapped air pressure in the cells, acting as an air cushion at the interface of the droplet fluid and nanowalls’ top surface. This lowers the droplet pinning and reduces the work of droplet volume deformation while enhancing the droplet rebound height.

Impact of Solid Particles and Liquid Droplets on Foams – Cold Model and High Temperature Experiments

Impact of Solid Particles and Liquid Droplets on Foams – Cold Model and High Temperature Experiments

Investigation of Dynamic Characteristics of Liquid Nitrogen Droplet Impact on Solid Surface

Liquid nitrogen spray cooling technology exhibits excellent heat transfer efficiency and environmental protection performance. The promotion of this technology plays an important role in improving the sustainable development of the refrigeration industry. In order to clarify its complex microscale behavior, the coupled Level Set-VOF method was adopted to study the dynamic characteristics of liquid nitrogen droplet impact on solid surface in this paper. The spreading behaviors under various factors (initial velocity, initial diameter, wall temperature, and We number) were systematically analyzed. The results show that the spreading behaviors of liquid nitrogen droplet share the same process with the normal medium, which are rebound, retraction, and splashing. For the droplet with smaller velocity and diameter, Rebound is the common phenomenon due to the smaller kinetic energy. With the increase of droplet diameter (0.2 mm to 0.5 mm) and velocity (0.1 m/s to 5 m/s), the spreading factor increases rapidly and the spreading behaviors evolve into retraction and splashing. The increase of wall temperature accelerates the droplets spreading, and the spreading factor increases accordingly. For the liquid nitrogen droplets hit the wall, the dynamic behaviors of rebound (We < 0.2), retraction (0.2 < We < 4.9), and splashing (We > 4.9) will occur with the droplet weber number increased, which are consistent with the common medium. However, due to liquid nitrogen having lower viscosity and surface tension, the conditions of morphological transformations are different from the common media. The maximum spreading diameter has a power correlation with We, the power index of We is 0.306 for liquid nitrogen, lager than common medium (0.25). The reasons are: (1) the better wettability of liquid nitrogen, and (2) the vapor generated by the violent phase change ejects along the axial direction. The article will provide a certain theoretical basis for liquid nitrogen spray cooling technology, and can also enrich the flow dynamics of cryogenic fluids.

Liquid droplet impact on a sonically excited thin membrane.

The characteristics of droplet impact on hydrophobic surfaces can be altered by introducing surface oscillations. The contact duration, spreading, retraction, and rebounding behaviors of the impacting water droplet are examined at various sonic excitation frequencies of the hydrophobic membrane. Membrane oscillation and droplet behavior are analyzed by utilizing a high-speed camera. The restitution coefficient and membrane dynamics are formulated and the findings are compared with those of the experiments. It is found that the mode of membrane oscillation changes as the sonic excitation frequency is changed. The droplet spreading and retraction rates reduce while the rebound height and restitution coefficient increase at a sonic excitation frequency of 75 Hz. However, further increase of the excitation frequency results in reduced rebound height because of the increased energy dissipation on the impacted surface. The droplet contact (transition time) duration reduces as the excitation frequency increases. Increasing droplet Weber number enhances the droplet contact period on the membrane, which becomes more apparent at low frequencies of sonic excitation.

Dynamic study of [Emim]Ac ionic liquid droplet impact on mildly heated solid surfaces

The impingement dynamics of the 1-ethyl-3-methylimidazolium acetate ([Emim]Ac) ionic liquid (IL) droplets on mildly heated solid wall were investigated. Comparison of the impingement dynamics of water, 60 wt% IL aqueous solution, and pure IL droplets were made, and the spreading characteristics of the 60 wt% IL aqueous solution droplets under different impact velocities and surface temperatures were discussed. A constant contact radius with a sharp decrease subsequently and a linearly decreasing residual volume were observed for water droplet. By contrast, a gradually increased spreading factor was observed for the pure IL and 60 wt% IL aqueous solution droplets, and the residual volume stays the same and goes through a polynomially decreasing for the pure IL and 60 wt% IL aqueous solution droplets, respectively.

Electrohydrodynamics of dielectric droplet collision on different wettability surfaces

In this article, we report the experimental and semi-analytical findings to elucidate the electrohydrodynamics (EHD) of a dielectric liquid droplet impact on superhydrophobic (SH) and hydrophilic surfaces. A wide range of Weber numbers (We) and electro-capillary numbers (Cae) are covered to explore the various regimes of droplet impact EHD. We show that for a fixed We ∼ 60, droplet rebound on a SH surface is suppressed with increase in electric field intensity (increase in Cae). At high Cae, instead of the usual uniform radial contraction, the droplets retract faster in an orthogonal direction to the electric field and spread along the direction of the electric field, inducing large electrical stresses at the liquid rim facing the electrodes. This prevents the accumulation of sufficient kinetic energy to achieve the droplet rebound phenomena. For certain values of We and Ohnesorge number (Oh), droplets exhibit somersault-like motion during rebound. Subsequently, we propose a semi-analytical model to explain the field induced rebound phenomenon on SH surfaces. Above a critical Cae ∼ 4.5, EHD instability causes a fingering pattern via evolution of a spire at the rim. Further, the spreading EHD on both hydrophilic and SH surfaces is discussed. On both wettability surfaces and for a fixed We, the spreading factor shows an increasing trend with increase in Cae. We have formulated an analytical model based on energy conservation to predict the maximum spreading diameter. The model predictions hold reasonably good agreement with the experimental observations. Finally, a phase map was developed to explain the post impact droplet dynamics on SH surfaces for a wide range of We and Cae.

Effects of liquid droplet volume and impact frequency on the integrity of Al alloy AW2014 exposed to subsonic speeds of pulsating water jets

This study focuses on the analysis of the surface material integrity of the polished, rigid, solid surface of an aluminium alloy after periodical impingement of liquid droplets with variable volume and impact frequency distributions normal to the solid surface. The volume of water droplets was determined for the pressures of 20 and 40 MPa. By increasing the traverse speed of the ultrasonic pulsating water jet head with respect to the stationary tested specimen, the number of water droplet impacts on one area was controlled in order to reach the early erosion stages. Also, for the comparative study, aluminum alloy was exposed to continuous water jet for both supply pressure 20 and 40 MPa with traverse speed of 1 mm/s over the material surface. Systematic testing focused on material integrity in the early erosion stages in the interval between the elastically deformed surface and material disintegration was conducted with the aid of microhardness measurements, X-ray analysis of stress state, and microstructural analysis by SEM. The motivation for carrying out this experiment was to verify the effects of periodic drops on the integrity of the material and to identify the parameters leading to surface strengthening without erosion as compared to continuous jet. Such surface treatments can improve fatigue life, similarly to shot peening.

Time-dependent measurements of length and area of the contact line in contact-boiling regime

During the impact of a liquid droplet on a sufficiently heated surface, bubble nucleation reduces the contact area between the liquid and the solid surface. Using high-speed imaging combined with total internal reflection, we measure and report how the contact area decreases with time for a wide range of surface temperatures and impact velocities. We also reveal how formation of the observed fingering patterns contributes to a substantial increase in the total length of the contact line surrounding the contact area.

Analytical Consideration for the Maximum Spreading Factor of Liquid Droplet Impact on a Smooth Solid Surface

Based on the energy conservation approach, this study develops a universal model to predict the maximum spreading factor of liquid droplet impact on a smooth solid surface. Validated with the present simulations and experiments in the literature, this model effectively overcomes the limitation of previous models in the viscous regime and greatly reduces the computing errors from over 30% to below 6%. It is demonstrated that the underestimated maximum spreading factor by previous models results from the overestimation of viscous dissipation. By replacing the conventional model of spreading time, tm = 8D0/3U0, with a more precise one, tm = 1.47τiWe-0.44, the formulation to compute the viscous dissipation of entire spreading is improved. Finally, we examine the applicability of present model in the capillary regime and good performance is also shown.

Post-Impact Behavior of a Droplet Impacting on a Permeable Metal Mesh with a Sharp Wettability Step.

The impact of liquid droplets on permeable substrates is important for a number of applications, such as fog collection, liquid atomization, and interaction of liquids with filters and textiles. When a water droplet impacts a wettable mesh, it penetrates the mesh easily with a part of the liquid remaining pinned. On the other hand, when striking a superhydrophobic mesh, part of the water droplet may penetrate and detach from the parent droplet, depending upon the impact velocity and the relative length scales of the droplet and the mesh. In most cases, the remaining droplet would rebound from the top of the superhydrophobic mesh. In this work, we study the impact of a water droplet on a wettability-patterned mesh, with the droplet centrally impacting the wettability-contrast line between the superhydrophobic and superhydrophilic semi-infinite domains. Half of the droplet seeing the superhydrophobic domain responds to it in a fashion that differs from the half hitting the superhydrophilic mesh side. This creates a wide range of post-impact scenarios, depending on the impact conditions and the relative characteristics of thedroplet and the mesh. The difference in mesh wettability leads to a net unbalanced surface-tension force that makes the droplet rebound with a horizontal momentum component directed from the non-wettable to the wettable side. Some part of the droplet may even detach during such directional rebounding (i.e., vectoring). Along with the experimental results, a simplified analytical model is presented, which differentiates the cases of detachment or no detachment during vectoring.

Pressure generated at the instant of impact between a liquid droplet and solid surface.

The prime objective of this study is to answer the question: How large is the pressure developed at the instant of a spherical liquid droplet impact on a solid surface? Engel first proposed that the maximum pressure rise generated by a spherical liquid droplet impact on a solid surface is different from the one-dimensional water-hammer pressure by a spherical shape factor (Engel 1955 J. Res. Natl Bur. Stand. 55(5), 281–298). Many researchers have since proposed various factors to accurately predict the maximum pressure rise. We numerically found that the maximum pressure rise can be predicted by the combination of water-hammer theory and the shock relation; then, we analytically extended Engel’s elastic impact model, by realizing that the progression speed of the contact between the gas–liquid interface and the solid surface is much faster than the compression wavefront propagation speed at the instant of the impact. We successfully correct Engel’s theory so that it can accurately provide the maximum pressure rise at the instant of impact between a spherical liquid droplet and solid surface, that is, no shape factor appears in the theory.

Dynamics of single droplet impact on cylindrically-curved superheated surfaces

Liquid droplet impact on superheated symmetric surfaces such as flat and spherical geometries has been extensively investigated in the past. However, the effect of a superheated asymmetric curvature on the collision dynamics remains unknown. In this study, experiments are performed to elucidate the thermo-hydrodynamic behavior of milli-metric water droplets impacting on superheated cylindrical surfaces having convex and concave profiles. These geometries offer an asymmetric zone for droplets to expand and retract. Mono-dispersed water droplets are impinged on convex and concave surfaces maintained in the temperature range of 125–290 °C. The impact Weber number is varied between 5 and 65 and droplet evolution is visualized using the high-speed imaging technique. It is observed that, droplet morphology and spreading characteristics are entirely different for convex and concave surfaces compared to a flat substrate due to momentum anisotropy. The maximum spread factor and residence time of the water droplet are significantly influenced by the impact Weber number while the surface superheat has negligible effect. Importantly, the asymmetric distribution of momentum facilitated 16% and 24% reduction in contact time for convex and concave surfaces respectively. The preferential extension of fluid in the azimuthal direction assisted by the gravity causes greater spreading of droplets on a convex surface. In contrary, these conditions enforced a reduction in contact time for droplets on a concave topography.

Spread and recoil of liquid droplets impacting on solid surfaces with various wetting properties

Thermally sprayed coatings are manufactured by molten or semi-molten droplets impinging onto the target substrate. The properties of coatings created (porosity, conductivity, cracks, ...) depend largely on the dynamics of the droplets which is an outcome of joint aspects, such as properties of liquid droplet and surface wettability. The present work investigates the normal impact of liquid droplets (molten ceramic or metallic droplets) onto dry solid surfaces of various wetting properties. Use is made of the Volume-Of-Fluid (VOF) interface tracking method with a single set of mass and momentum conservation equations. The wettability of a solid surface, characterised by a contact angle, is taken as part of the boundary conditions in the Smooth VOF algorithm. Introducing the dimensionless time t* = tV0/D0 for a droplet with the initial speed V0 and initial diameter D0, its spread factor ξ (= D/D0) is found to be proportional to the square root of t* when 0.2 < (t*)1/2 < 0.35. There exists a transitional zone where the changing rate of the spread factor decreases with the ascending contact angle. In addition, the wetting effects on the spreading can be ignored when 0.2 < (t*)1/2 < 0.35 but have to be accounted for once (t*)1/2 < 0.2. Further, the droplet jetting time is found to be inversely proportional to the impact velocity but not affected by the solid surface wettability. Finally, in the recoil stage, the influences of Weber number, Ohnesorge number and contact angle are studied; in particular, the spread factor ξ turns out to be well described by a second-order polynomial equation for t*≥0.5We.

Crown formation and atomization in burning multi-component fuel droplets

We report the observation of a crown-like sheet expansion and its subsequent atomization in burning multi-component fuel droplets. The droplets consisting of constituents with significant volatility differential are chosen in this study. Through homogeneous nucleation, the breakup of high-pressure vapor bubble results in the formation of radially expanding crown-like hemispherical sheet. The expansion of liquid sheet and the development of radial ligaments on its destabilized rim results in the creation of several secondary droplets. This pattern of breakup bears a close resemblance to the universally recognized Worthington-Edgerton crown configuration, which forms when a liquid droplet impact on a deep pool/thin film of same liquid or another liquid. Two distinct modes of liquid sheet fragmentation are identified during the experiments: (i) unstable sheet fragmentation and (ii) stable sheet fragmentation. In particular, the breakup characteristics of stable sheet mode is comparable to the commonly observed crown breakup by drop impingements. The modes of sheet breakup are primarily influenced by the proportion of volatile component in the mixture, which in turn dictates the onset of fragmentation and the intensity of breakup.

3D lattice Boltzmann simulation for a saturated liquid droplet at low Ohnesorge numbers impact and breakup on a solid surface surrounded by a saturated vapor

In this paper, new simulation results of a saturated liquid droplet impact dynamics on hydrophilic (θ = 80°) and hydrophobic (θ = 170°) solid surfaces surrounded by a saturated vapor are obtained based on a 3D lattice Boltzmann method (LBM). A novel piecewise relaxation time is proposed to overcome numerical instability at high liquid/vapor density ratios and low liquid droplet viscosities. Simulations are carried out for low Ohnesorge (Oh) numbers in the range from 0.01 to 0.05, corresponding to a water droplet diameter in the range of 1 µm to 130 µm. The impinging droplet deformation process including breakup phenomena is illustrated on hydrophilic and hydrophobic surfaces at different Weber (We) numbers. Droplet spread factors obtained from simulations match well with existing experimental data and theoretical values, validating the accuracy of the present 3D LBM. It is found that a water droplet, after reaching its maximum spread factor, breaks up into a toroid shape with a vapor cavity or a liquid film left at its center at large We numbers on a hydrophilic and a hydrophobic surface, respectively. A map in terms of We number versus Oh number is obtained for predicting the breakup occurrence. The critical We number is monotonically increasing with the increase of the Oh number, meaning that the smaller diameter water droplet requires a larger We number for breakup. Droplet breakup is more likely to occur on a hydrophobic surface than on a hydrophilic surface at larger Oh numbers, while the behavior is opposite at lower Oh numbers.

Footprint of droplets after impact onto paper surfaces with a hydrophobic barrier

This paper presents findings from a study of the impact of liquid droplets onto papers which have been treated to incorporate a hydrophobic barrier. Such papers are currently being explored as new paper-based microfluidic technology for chemical, biological and medical applications, where discrete volumes of liquid (i.e. droplets) are deposited on the paper. We experimentally capture the impingement stage with the aid of high-speed videography and analyze the spreading, retraction and final footprint of the droplets. Understanding the maximum spread and final footprint is important for paper-based devices because it can determine whether or not a droplet that impinges upon them will reach the hydrophilic wicking matrix.We conclude that the final contact area (footprint) could be tuned simply by varying the impact energy of the droplet and vapor deposition time. In contrast to untreated papers, droplets impinging on treated papers impregnate the porous structure but there is no subsequent wicking, i.e. the contact line always pins, which is explained by the interaction of the droplet with fibers of the paper.

Elliptical spreading characteristics of a liquid metal droplet impact on a glass surface under a horizontal magnetic field

The spreading characteristics of a liquid GaInSn alloy droplet on a glass surface with the action of a horizontal magnetic field have been experimentally investigated in the present paper. With changing the impact velocity from 0.1 m/s to 1.2 m/s and increasing the magnetic field from 0 T to 1.6 T, we focus on studying the influence of the horizontal magnetic field on the spreading characteristics of a liquid metal droplet using the shadow-graph method. The elliptical spreading pattern of a liquid metal droplet induced by the horizontal magnetic field was discovered by experiments. By introducing a numerical method in getting the distribution of current lines and the Lorentz force inside the droplet, we give a detailed explanation on the mechanism of elliptical spreading. Furthermore, some quantitative results on a maximum spreading factor and time at moment of maximum spreading varied with the Hartmann number and Weber number are shown to give us a comprehensive understanding of the elliptical spreading. With the increasing of the magnetic field, the maximum spreading factor in the front view is reduced while that in the side view is increased, which reveals a larger deformation happened during the spreading process. While with the increasing of impact velocity, the spreading factor increased. Finally, we present a non-dimensional parameter to get scaling laws for the averaged maximum spreading factor and the aspect ratio of the maximum spreading factor; results show that the predict data can agree with experimental data in a certain degree.

Immiscible impact dynamics of droplets onto millimetric films

The impact of liquid droplets onto a film of an immiscible liquid is studied experimentally across a broad range of parameters [ $$Re = O(10^{1}-10^{3})$$ , $$We = O(10^{2}-10^{3})$$ ] with the aid of high-speed photography and image analysis. Above a critical impact parameter, $$Re^{1/2}We^{1/4} \approx 100$$ , the droplet fragments into multiple satellite droplets, which typically occurs as the result of a fingering instability. Statistical analysis indicates that the satellite droplets are approximately log-normally distributed, in agreement with some previous studies and the theoretical predictions of Wu (Prob Eng Mech 18:241–249, 2003). However, in contrast to a recent study by Lhuissier et al. (Phys Rev Lett 110:264503, 2013), we find that it is the modal satellite diameter, not the mean diameter, that scales inversely with the impact speed (or Weber number) and that the dependence is $$d_\mathrm{{mod}} \sim We^{-1/4}$$ .

Thermodynamic wetness loss calculation in nozzle and turbine cascade: nucleating steam flow

Rapid expansion of steam in turbines and nozzles cause condensation. The formation of liquid droplets due to condensation results in wetness losses, which include aerodynamic losses (due to friction between liquid droplets and the vapour), thermodynamic losses (due to irreversible latent heat addition), and braking losses (due to the impact of liquid droplets on the turbine blade). In this study, a numerical investigation of the thermodynamic loss in a nucleating steam flow is performed. The thermodynamic loss is calculated using the change in entropy due to condensation. The effect of different operating conditions on the thermodynamic loss is estimated for a nozzle and turbine cascade in a nucleating flow. The non-equilibrium condensation in high-speed steam flows is modelled using Eulerian-Eulerian approach.