Spreading Of Liquid Droplets Research Articles

Dynamic Anti-Icing Performance of Flexible Hybrid Superhydropohobic Surfaces.

Normal superhydrophobic surfaces with a rough topography provide pocketed air at the solid-liquid interface, which guides the droplet to easily detach from the surface at room temperature. However, at low temperatures, this function attenuates obviously. In this research, a flexible hybrid topography with submillimeter (sub-mm) and microcone arrays is designed to adjust the impacting behavior of the droplet. The sub-mm cone could provide rigid support to limit deformation, leading to reduced energy consumption during impact processes. However, the microcone could maintain surface superhydrophobicity under different conditions, preventing droplet breakage and the change of the droplet contact state during impact processes by providing multiple contact points. Under the synergistic effect, such a hybrid structure could provide much more pocket air at the solid-liquid interface to limit the spreading of liquid droplets and reduce the energy loss during the impact process. At a low temperature (-5 °C), even if the impact height is reduced to 1 cm, the droplets still could be bound off, and the hybrid superhydrophobic surface presents excellent dynamic anti-icing ability. The special flexible hybrid superhydropohobic surface has potential application in fast self-cleaning and anti-icing fields.

Numerical study of the process of liquid droplets impacting the curved wall surface

The impact of droplets on the solid wall is a typical free surface flow phenomenon that widely exists in nature, industry, and daily life. According to the change of the droplet’s center thickness, the droplet, the spreading process of the droplet is usually divided into three unique stages: the initial droplet deformation stage, the inertia-dominated stage, and the viscosity-dominated stage with the increase of the impact time. The problem of the liquid droplet spreading under different impact conditions is simulated by the smoothed particle hydrodynamics (SPH) method. Firstly, the dynamic process of droplet spreading is simulated and compared with relevant experimental results to verify the effectiveness of the proposed method. Then, the effects of the Weber number and the diameter ratio of the droplet to the curved wall surface on the droplet’s maximum spreading diameter and central thickness were studied and analyzed. On this basis, the key coefficient in the existing relationship between the Weber number and maximum spreading diameter is fitted. Finally, the fitting equation of the center thickness and the impact time in the third stage of droplet spreading is assessed.

Glycerol Droplet Spreading on Growing Bacillus Subtilis Biofilms.

Bacterial biofilm is a three-dimensional matrix composed of a large number of living bacterial individuals. The strong bio-interaction between the bacteria and its self-secreted matrix environment strengthens the mechanical integrity of the biofilm and the sustainable resistance of bacteria to antibiotics. As a soft surface, the biofilm is expected to present different dynamical wetting behavior in response to shear stress, which is, however, less known. Here, the spreading of liquid droplet on Bacillus subtilis biofilm at its different growing phases was experimentally investigated. Due to the viscoelastic response of the biofilm to fast spreading of the droplet, three stages were identified as inertial, viscous stages, and a longer transition in between. The physical heterogeneity of growing biofilm correlates with the spreading scaling within the inertial stage, followed by the possible chemical variation after a critical growing time. By using the duration of inertial spreading, the characteristic time scale was successfully linked to the shear modulus of the elastic dissipation of the biofilm. This measurement suggests a facile, non-destructive and in vivo method to understand the mechanical instability of this living matter.

Wettability and droplet energetics on micro-scale wavy and V-grooved surfaces

The wetting physics and wetting transition on micro-wavy and micro V-grooved surfaces of similar topographic scales are investigated and compared with an emphasis on the droplet stability and anisotropic wetting that promotes liquid drainage. To develop 3D models of liquid droplets and to assess the morphology and spreading of liquid droplets across a wide range of parametric space, Surface Evolver (SE) has been used. The effect of roughness parameters of micro V-grooved surfaces such as groove height, groove width, pillar width, and groove angle on the wetting behavior is analyzed and compared to the same for the corresponding dimensional variation of pitch, amplitude, and depth of the asperities of micro-wavy surfaces. For all the comparable configurations, surfaces with micro-wavy textures exhibit higher wettability and smaller stable droplets than micro V-grooved surfaces. In addition, after specific threshold values of the aforementioned topographic parameters, the Cassie-Wenzel wetting transition is observed. Although wettability on micro-scale wavy and V-grooved surfaces have been widely examined because of their obvious importance, comparison of wetting physics and droplet energetics on them for similar roughness scale has not been conducted before. Therefore, the developed model may be useful in designing micro-structured V-grooved and wavy surfaces with tunable directional wetting properties.

Maximum spreading of liquid droplets impact on concentric ring-textured surfaces: Theoretical analysis and numerical simulation

It is fundamentally significant to predict the maximum spreading of liquid droplets impact on textured surfaces. However, relevant work is very limited. Considering the effects of contact line pinning and liquid penetration into textures, a theoretical model to predict the maximum spreading factor on concentric ring-textured surfaces is developed. Validated with numerical simulations conducted by an effective, sharp-interface, continuum level modeling method, this model has shown a good predicting performance. It is demonstrated that in the viscous regime, the viscous dissipation arising from liquid penetration into textures has a considerable effect on the maximum spreading factor, whereas the effects of the contact line pinning and surface energy are negligible. Meanwhile, as viscous dissipation becomes more dominant, the effect of texture gap on maximum spreading is less visible. The phase diagram of the wetting state at maximum spreading is also provided. We indicate that the intermediate wetting state is favorable under small Reynolds number and narrow texture gap, while the partial wetting state is supported under the opposite conditions.

Research on the influence of Micro-morphology on the hydrophobicity of material surface

In this study, numerical simulation and physical experiments are carried out to investigate the influence of material surface microstructure on its hydrophobicity. By observing the contact angle and liquid phase state of the droplet on various morphological surfaces, the relationship between the size and morphology of microstructures and the hydrophobicity of material surface is explored. The influence of microstructure on the hydrophobicity is reflected by:1. changes of the contact angle along with the changes of microstructure size, 2. anisotropy of liquid droplet spreading along with the changes of wire groove and cylindrical morphology. Driven by tension, droplets can move spontaneously on the surface of specific microstructures, and the moving speed and direction vary with the distribution of the microstructures.

Three dimensional modeling of liquid droplet spreading on solid surface: An enriched finite element/level-set approach

A physically consistent approach is introduced to simulate dynamics of droplets in contact with solid substrates. The numerical method is developed by introducing the molecular–kinetic model within the framework of the level-set/enriched finite element method and including the theoretically resolved sub-elemental hydrodynamics. The level-set method is customized to comply fully with the model acquired for the moving contact-line. The consistency of the proposed method is verified by comparing the simulation results with the theoretical predictions. In order to further validate the method, the spreading of a droplet is numerically modeled and compared rigorously with the experimental data reported in the literature. The proposed method is also employed to capture the evolution of a droplet trapped in a conical pore. All test-cases are simulated on three-dimensional computational domains.

Lateral Spreading of Gas Bubbles on Submerged Wettability-Confined Tracks.

Spreading of liquid droplets on wettability-confined paths has attracted considerable attention in the past decade. On the other hand, the inverse scenario of a gas bubble spreading on a submerged, wettability-confined track has rarely been studied. In the present work, an experimental investigation of the spreading of millimetric gas bubbles on horizontally submerged, textured, wettability-confined tracks is carried out. The width of the track is kept fixed along its entire length, and the spreading behavior of a gas bubble, dispensed at one end of the track, is studied. The effects of varying track width, bubble diameter, and ambient liquid are investigated. Post-contact, the gas bubble spreads along the track at a linear rate with time, while remaining pinned at its back end; the recorded spreading speed is O(0.5 m/s). An inertio-capillary force balance describes the experimentally observed spreading dynamics with excellent agreement.

Impingement dynamics of [EMIm]Ac ionic liquid drops on heated porous surfaces

The impingement dynamics of a promising ionic liquid absorbent, namely, 1-ethyl-3-methylimidazolium acetate ([EMIm]Ac) in droplet falling film flow modes on heated porous surfaces in absorption refrigeration were investigated experimentally. The influences of impact velocity and surface temperature on the ionic liquid droplet spreading diameter, dynamic contact angle, spreading height, and residual volume against the heated porous surfaces were explored. The results indicated that the residual volume generally decreased in linear with increasing contact time at 323.15 K, while the dependence exhibited a polynomial behavior at higher heating temperatures. Compared to the impingement behaviors of deionized water droplets, such as oscillations and capillary waves were not observed in [EMIm]Ac droplets. The residual volume of the deionized water droplets varied exponentially with the contact time owing to the combined effects of the evaporation, penetration, and variation of the thermophysical properties of the droplet. Moreover, increases in the impact velocity and surface temperature enlarge the spreading diameter and accelerate the droplet penetration for the ionic liquid.

Early stage of liquid drop spreading on tunable nanostructured surfaces

In this study, the early stage of liquid droplet spreading on tunable nanostructures was investigated through high resolved and fast imaging visualization. It is well-known that the early stage of water droplet spreading on a solid surface features an inertia dominance on resistive droplet motion in a short period (~0.01 sec). The spreading dynamic law of this stage differs greatly from those of the classical spreading stage, after the inertia regime, being determined by the balance between a spreading force and viscous friction. The effect of triple line physics associated with the surface properties is generally underestimated in the early inertial regime by a relatively short period and strong inertia momentum, and such physics result in the nature of the spreading (Dt1/2). Here, we designed tunable nanostructures that can modify the nature law of the early droplet spreading. The results suggest that the increase of initial spreading momentum attribute to the capillary wicking effect controlled by the nanostructure morphology.

Contact Line and Adhesion Force of Droplets on Concentric Ring-Textured Hydrophobic Surfaces.

Advances made in fabrication of patterned surfaces with well-defined dimensions of topographic features and their lateral dissemination drive the progress in interpretation of liquid spreading, adhesion, and retreat on engineered solid surfaces. Despite extensive studies on liquid droplet spreading and adhesion on textured surfaces in recent years, conformation of the three-phase contact line and its effect on macroscopic contact angle and droplet adhesion remain the focus of intensive debate. Here, we investigate the effect of surface topography on the adhesion force of Cassie-Baxter-state droplets on concentric ring-textured hydrophobic surfaces having rings with lateral dimensions of 5, 10, and 45 μm and separated by 5, 6, and 7 μm trenches, respectively, with fixed depth of 15 μm. Unlike mostly tested surfaces textured with straight ridges, pores, and pillars, where the droplet base contact line is anisotropic and its conformation varies along the apparent boundary, concentric rings are symmetrical and reinforce the microscopic contact line to align to a circular one that reflects the shape of the pattern. In this study, adhesion forces were calculated based on surface tension and Laplace pressure forces and were compared with the experimental forces for both water and ethylene glycol droplets having a varying contact diameter on the concentric ring-pattern at the point of maximum adhesion force. Results show that the microscopic contact line of the liquid retains its circular shape controlled by circular rings of the pattern, irrespectively of the droplet base diameter larger than 0.8 mm, and there is a good agreement between the experimental and calculated adhesion forces.

Effect of interfacial mass transport on inertial spreading of liquid droplets

In this work, the early time dynamics of low-viscosity liquid drops spreading in their saturated vapor on partially wetting surfaces are investigated by lattice Boltzmann numerical simulations. Attention is paid to the effect of vapor transport through condensation on the spreading process. We observe that the condensation current resulting from the slight supersaturation of the liquid vapor near the dynamic wetting meniscus contributes to the motion and affects the spreading dynamics. Our results indicate that, in order to properly capture the initial dynamics of inertial spreading of a relatively volatile liquid drop, it is important to account for the vapor transport through condensation in the immediate vicinity of the contact line. A direct qualitative and quantitative comparison with experimental data of spontaneously wetting liquid drops is presented.

Oxidation effect on pool boiling critical heat flux enhancement of Cr-coated surface for accident-tolerant fuel cladding application

The oxidation of an accident-tolerant fuel (ATF) cladding alters the surface roughness, which, in turn, affects the capillary wicking characteristics and critical heat flux (CHF) during the boiling process. To evaluate the effect of oxidation on the boiling performance of ATF coating, this study fabricated Cr-coated surfaces by varying the substrate temperature, resulting in various sizes of deposited particulate nanostructures by using a direct-current magnetron sputtering technique. To consider the oxidation effect, sputtered test specimens were oxidized at 400 °C dry air for 20 days, forming a Cr2O3 oxide layer with a thickness of approximately 0.1 μm. Pool boiling CHF experiments were conducted in deionized water under atmospheric pressure. The resultant CHF on the fresh and oxidized Cr-coated surfaces was enhanced by up to 36% and 48%, respectively. A higher CHF enhancement was achieved with a lower substrate temperature during sputtering, which resulted in a higher nanoscale surface roughness. The spreading velocities on each specimen were measured through the characterization experiments of liquid droplet spreading, which can be interpreted as the capillary wicking momentum. The pool boiling CHF was predicted with spreading velocity and roughness parameters. The predicted CHF showed a good agreement with the measured CHF. This study suggests that the oxidation of Cr-coated ATF specimens has a positive effect on the enhanced CHF, which can be confirmed through the altered surface roughness and capillary wicking momentum.

Predicting the maximum spreading of a liquid drop impacting on a solid surface: Effect of surface tension and entrapped air layer

The spreading of liquid droplets impacting a surface at high speed is well understood by now. However, when a droplet impacts a surface at relatively low impact velocities (<1 m/s), the wetting properties of the fluid become important, and the entrapped air layer between the impacting drop and the solid surface prevents the immediate wetting of the surface. To determine the influence of both wetting and the entrapped air, we perform experiments by systematically varying the surface tension of the liquid and the air pressure. Drop impact measurements at reduced air pressures show that the spreading is independent of the pressure; dynamic contact-angle measurements indicate that this happens because the air film breaks rapidly. By varying the surface tension and surface wettability, we show that droplet spreading at low velocities can be predicted from the wetting properties of the surface and the known energy balance for the impact.

Influence of negative ambient temperature on the morphology of blood droplet traces

The aim of this study was to identify the peculiarities of blood droplets on the street in frosty weather on the surfaces of objects with a negative temperature. The effect of negative ambient temperature (-19 °C) on the morphology of blood droplet traces was experimentally established. On cold trace surfaces, the droplet footprint was smaller, relatively flat and with wave-shaped edges; there was no splashing. Such a morphological presentation in frosty weather arises from the rapid cooling of the falling drop of blood which crystallizes in contact with the cold surface. There was no complete spreading of liquid droplets. The data obtained should be taken into account when assessing the traces of blood droplets detected at the scene of the incident on the street in winter, in order to avoid expert errors in determining their mechanism and the height of the blood source.

Heat transport for evaporating droplets on superhydrophilic, thin, nanoporous layers

Previous studies of droplet spreading on superhydrophilic nanostructured surfaces have demonstrated that enhanced wetting and wickability of such surfaces can produce more rapid and extensive spreading of liquid droplets. There is also ample evidence from prior studies that these types of superhydrophilic nanostructured surfaces can exhibit enhanced vaporization heat transfer performance in for droplet evaporation and pool boiling processes. This study specifically explored the mechanisms of droplet vaporization on superhydrophilic nanoporous thin layers, at a fundamental level, for conditions in which liquid viscous forces and pore capillarity forces dominate, and liquid inertia forces are low. The investigation, summarized here, experimentally explored the droplet evaporation heat transfer performance on superhydrophilic, nanoporous, thin layers on a heated metal substrate. A thermal growth process was used to fabricate a layer of ZnO nanopillars a few microns thick on a copper substrate. The resulting nanoporous layer exhibited ultra-low contact angles and high wickability. The experiments indicate that, on these surfaces, a deposited droplet undergoes an initial rapid spreading phase followed by a slower vaporization phase, which shrinks the droplet until evaporation is complete. Although droplet evaporation without bubble nucleation is the primary focus of this work, the experiments also examined the added effects of bubble nucleation on droplet evaporation at high surface superheats. Based on experimental observations and data, a model of heat transport during the evaporation process was developed. The model predictions are shown to agree well with droplet evaporation time measurements for droplets of different initial sizes and a range of surface temperatures. The experiments and model indicate that the surfaces tested enhance droplet evaporation heat transfer performance by as much as a factor of three compared to an ordinary copper surface at the same conditions. The model is also used to explore the potential for more extreme augmentation of droplet evaporation heat transfer by further enhancement of surface wetting and wickability. The results of this study provide a clearer picture of the interconnection of droplet spreading mechanisms and evaporation heat transfer for these circumstances.

Dynamics of the Remote and Contact Interaction of Liquid Droplets with Film-Like and Fibrous-Porous Polymeric Materials

The problem of inkjet printing quality was considered for the case of synthetic polymer-based film-like and fibrous-porous materials. Physical modeling of inkjet printing on polypropylene using water-based ink revealed remote interaction of liquid droplets with films and nonwoven webs, responsible for the droplet path deviation from the vertical fall and the droplet moving across the polymer film surface after the contact or holding at the fiber ends. Relationships were established and controlling techniques were proposed for the processes of absorption and spreading of liquid droplets in the materials to be printed with the view of increasing the accuracy of printing information symbols and reducing ink consumption.

Visualization study on capillary-spreading behavior of liquid droplet in vertically aligned carbon nanotube array

Vertically aligned carbon nanotube (CNT) arrays exhibit distinct wetting characteristics. Thus, they are a promising solution for many applications such as heat-pipe wicks. In this study, the dynamic behavior of liquid-droplet spreading via capillary action in a vertically aligned CNT array has been investigated experimentally and theoretically. Two groups of CNT array with heights of 150 μm and 250 μm were tested. Two organic solvents, namely ethanol and acetone, were selected as the wetting liquids because of their good wettability with the as-grown CNTs. A visualization system equipped with a high-speed camera was used to record the capillary-spreading process. The results of the CNT array with different array heights showed two distinct modes of spreading: a steady uniform mode and an unsteady random mode. In the unsteady mode, the liquid predominantly spreads above the array. In the steady mode, the spreading process can be divided into three phases based on the different dominating effects. After the first flooding phase, the spreading in the second phase was found to follow the Lucas–Washburn model, whereas the third phase can be described using a constant evaporation-rate model. The self-assembly behavior was observed using an SEM after the wetting experiments. Honeycomb-shaped microstructures were formed spontaneously in the precursor-film region, whereas striped lines were observed on the front end of the contact line.

Dynamics of spreading of impinged droplets on the curved-grooved surface

Impact of droplets on rough surfaces has many industrial applications. Various types of splashing and spreading of liquid droplets result due to impingement of the droplets on the different surfaces incorporating formation of satellite droplets. Dynamics of splashing and spreading is different with respect to the point of impact—groove or hill—on the grooved surface. Physics of spreading of impacted droplets on the curved-grooved surfaces is not known so far. Impact of droplets on curved-grooved surface has been studied for a narrow range of Weber number to know the abrupt change in projected area of splashed droplets. Such abrupt change in the geometry of splashed droplets due to exaggerated elongation in the direction of groove was observed for narrow range of Weber numbers from 42 to 45. The present findings explore new direction for further fundamental research work to comprehend the physics of dynamics of impacted droplets for development of new theory helpful for spray painting, printing, and designing of shielding structures.

Water Droplet Spreading and Wicking on Nanostructured Surfaces.

Phase-change heat transfer on nanostructured surfaces is an efficient cooling method for high heat flux devices due to its superior wettability. Liquid droplet spreading and wicking effect then dominate the heat transfer. Therefore, this study investigates the flow behavior after a droplet touches a nanostructured surface focusing on the ZnO nanowire surface with three different nanowire sizes and two array types (regular and irregular). The spreading diameter and the wicking diameter are measured against time. The results show that the average spreading and wicking velocities on a regular nanostructured surface are both smaller than those on an irregular nanostructured surface and that the nanowire size affects the liquid spreading and capillary wicking.