Wetting Of Droplets Research Articles

Many-body dissipative particle dynamics simulation of the anisotropic effect of droplet wetting on stripe-patterned heterogeneous surfaces

The droplet anisotropic wetting on chemically heterogeneous stripe-patterned substrates after impact is studied by using many-body dissipative particle dynamics in this work. After a low-velocity impact on substrates with different length ratio (ratio of stripe width to the initial droplet size) and Cassie area fraction, the droplet can show various shapes and contact angles in parallel and orthogonal directions to the stripes. The elongation of the droplet can be increasingly evident when increasing the length ratio. Also, the contact angles at both directions follow the Cassie-Baxter predicted values well at low length ratio while deviate from them at high length ratio. Both the capillary and kinetic effects have a significant influence on the anisotropic wetting. When impact with higher velocity, the droplet leaves some residues on the hydrophilic stripes of the substrates. Surprisingly, the residues for substrates with certain length ratio (≤0.316) are distributed in circular regions with almost identical radii, indicating the spreading stages of them are independent on the surface properties and dominated by the kinetic effect. When the droplet reaches a stable state, the main body shows anisotropic wetting behavior, indicating the capillary effect dominates the wetting instead of the kinetic effect at the retraction stage.

Sources of the Measurement Error of the Impact Pressure in Sloshing Experiments

Sloshing experiments have increasingly received academic attention. Understanding the measurement errors in the sloshing impact pressures is an important parts of the sloshing experiments since these errors, which arise from experimental conditions, affect the subsequent results. As part of the research on the sources of the measurement errors, focused on the effects of surface conditions of pressure sensors on the measurement of impact pressures. Thirty-six integrated circuit piezoelectric pressure sensors were placed on the upper surfaces of a two-dimensional tank to measure the sloshing impact pressures under surge or pitch motions. For each motion, the experimental conditions were divided in two based on whether the surfaces of the sensors were dry or wet. The peak pressures of each test were measured as twenty repeated experiments to ensure reliability. The flow in the tank was visualized using a high-speed camera to observe and analyze macroscopic and microscopic phenomena along the sensor surface. Thermal shock effects were confirmed by varying the experimental temperature and that of the sensor surface. The effects of the wet surface and droplets formed on the sensor surface on pressure measurements are discussed.

Experimental study of the onset of downstream motion of adhering droplets in turbulent shear flows

In this study, the dynamics of adhering liquid droplets on various solid surfaces in shear flow, which is driven by a controlled air flow, are investigated experimentally. A series of experiments are carried out to understand the effects of fluid properties, wetting characteristics, droplet sizes and flow velocities. A rectangular Plexiglas-channel is used for the experiments. Droplets are placed on the bottom wall of that channel. The droplet shape and contour position are measured by the transmitted light technique and are further analyzed by using image processing. The shear flow leads to a deformation and oscillation of the droplet and eventually to a movement downstream. Three regimes are identified: regime I corresponds to a deformation and oscillation, whereas in regime II the deformed and still oscillating droplets moves downstream. Regime III characterizes a continuous downstream movement in a gliding manner. The start of regime II, i.e. the onset of downstream motion is defined by a critical air velocity. Results show that the contact angle hysteresis and surface energy have a strong influence on the critical air velocity for the onset of droplet motion, as well as the wetting characteristics and droplet volume. To find a global empirical law for the critical velocity, a dimensionless approach is derived based on the droplet Reynolds number and a modified Laplace number. The empirical law is in good agreement with experimental data and may serve as an estimator of the critical velocity for the onset of droplet motion.

Droplet Spreading and Wettability of Abrasive Processed Aluminum Alloy Surfaces

One of the main cause of a droplet metastable state is found to be surface roughness. This state is characterized by a large contact angle hysteresis and condition when the static contact angle is larger than the advancing dynamic contact angle. Besides the texture, other factors can influence the deviation from the equilibrium state, in particular, the fluid flow rate (the growth rate of a droplet) affecting the contact line speed. An experimental study was done to determine the effect of roughness and fluid flow rate on wetting of aluminum-magnesium alloy surfaces with random roughness processed by abrasive polishing. Three-dimensional roughness parameters were used to evaluate their texture. The correlations between these parameters, static, advancing and receding dynamic contact angles, hysteresis, and contact line speed were obtained. The molecular-kinetic theory of wetting was used to interpret the dynamic contact angle data.

The effect of sharp solid edges on the droplet wettability

Understanding the role of pinning force in droplet dynamic wetting is of critical importance for surface science studies. Generally, the pinning force is only related to the surface tension and the change of contact angle. However, there is an obvious correlation between the pinning force and the surface geometry. In this paper, the relation between the surface geometry and contact line pinning was studied with systematical experiments and theoretical analysis. We compared the samples with different edge angles and carried out plenty experiments with different liquids. Meanwhile, the theoretical analysis and molecular simulation were carried out. The results show that the sharp edge has a strong pinning effect on the contact line and can significantly change the contact angle and wetting state of droplets. The maximum contact angle of droplet has a linear relation with the edge angle of substrate. The formula of pinning force was revised to consider the impact of surface topography. According to the relationship between surface defect and contact line, we proposed a model to classify the cases of contact line pinning for the first time. Our research will deepen the understanding of contact line pinning and provide help for potentially industrial production designs.

Microstructure and wear performance of Ni-10 wt.%Al coatings plasma sprayed on Ni-based superalloys with a sound field

We investigated the microstructure and wear performance of Ni-10wt.%Al coatings deposited by plasma spraying on a Ni-based superalloy loaded with a 100-Hz sound field. Applying the sound field greatly increased both the compactness and bonding strength of the coating. The coatings sprayed with and without a sound field both showed a lamellar structure. Electron backscattering diffraction analysis showed that the sound field helped to align the long axis of the γ-Ni grain perpendicular to the substrate rather than along the fine equiaxed crystal, as present in the coating sprayed without the sound field. Additionally, spraying with the sound field produced a γ-Ni phase with strong 〈001〉 orientation. The wear performance of the Ni-10 wt.%Al coating sprayed with the sound field was much better than that sprayed without the sound field. We attributed the microstructure transformation, preferred orientation, and performance enhancement of the coating to the application of the sound field during plasma spraying. Sound pressure and acoustic streaming enhanced the wettability of molten droplets on the substrate as well as the filling ability of molten droplets in the gaps between solidified particles. Applying the sound field enhanced the heat loss of molten droplets close to the substrate, causing a temperature gradient during solidification.

Capillary interactions between droplets and ideal roughness: Attractive protrusion and repulsive trench

The interactions between liquid drops and ideal defects (protrusion or trench) on a contact angle hysteresis (CAH) free surface were studied by experiments and Surface Evolver (SE) simulations. The CAH-free characteristic was achieved by the slippery liquid-infused porous surface (SLIPS) and the ideal defects were constructed on such a CAH-free surface. Both hydrophobic (water) and lyophilic (hexadecane) systems were considered. The force (attraction and repulsion) between a drop and a defect was determined by the inclination experiments. Regardless of the surface wettability, the drop tends to adhere to the protrusion on a horizontal plane. A critical force corresponding to a critical tilted angle is required to detach the drop from the protrusion to overcome the attraction between them. In contrast, during the encounter of a drop with a trench, the pass of the drop is resisted by the trench, irrespective of the surface wettability. When the external force exceeds the critical value, the drop crosses the trench directly in the hydrophobic system but impregnates the trench in the lyophilic system. For all cases studied, the experimental results agree well with the simulation outcomes. This work provides insights into the influence of surface roughness on droplet wetting.

Sessile droplets shape response to complex body forces

The majority of studies on forced wetting of sessile droplets refers to application of a steadily increasing normal or tangential force or to a specific combination of these forces arising from tilting the substrate. The above constitute well-defined test conditions but are not representative of what is encountered in most industrial applications. To approach realistic industrial conditions and so also expand existing wetting theories, an evaluation of droplet shape deformation under the influence of a complicated evolution of body forces, is performed herein. To this aim, two sets of experiments are carried out in Kerberos device creating force fields by combination of gravitational and centrifugal forces: i) by oscillations of the tilting angle at constant rotation speed and ii) by alternating small step-increases of the rotation speed and the tilting angle, one after the other, so as to trail the symmetric side profile (SSP) curve of a droplet. The latter is done in three or six steps following two different paths: increasing first either the rotation speed or the tilting angle. The aim of the experiments is, on one hand, to explore droplet deformation under cycles of increasing/decreasing tangential forces and, on the other hand, to analyze the effect of residual tangential forces on droplet shape. Several features related to the droplet shape evolution are investigated such as contact angles, side and top contour profiles and droplet 3-dimensional shape. The resulting droplet profiles are analyzed using numerical solutions of the Young-Laplace equation. It is found that the 3-dimensional Young Laplace equation can describe very accurately the experimental profiles.

Vortex-induced dispersal of a plant pathogen by raindrop impact.

Raindrop impact on infected plants can disperse micron-sized propagules of plant pathogens (e.g., spores of fungi). Little is known about the mechanism of how plant pathogens are liberated and transported due to raindrop impact. We used high-speed photography to observe thousands of dry-dispersed spores of the rust fungus Puccinia triticina being liberated from infected wheat plants following the impact of a single raindrop. We revealed that an air vortex ring was formed during the raindrop impact and carried the dry-dispersed spores away from the surface of the host plant. The maximum height and travel distance of the airborne spores increased with the aid of the air vortex. This unique mechanism of vortex-induced dispersal dynamics was characterized to predict trajectories of spores. Finally, we found that the spores transported by the air vortex can reach beyond the laminar boundary layer of leaves, which would enable the long-distance transport of plant pathogens through the atmosphere.

Study on Aerosol Characteristics of Electrostatic Minimum Quantity Lubrication and Its Turning Performance

The technology of electrostatic minimum quantity lubrication (EMQL) can not only enhance the depositing rate of lubricant droplets on the cutting area, but also increase their wetting and penetration to the contact interface, which thus improve droplet lubrication and cooling, as well as reduce concentration of the oil mist floating in the cutting environment. As a EMQL cutting system is developed, the effect of charging voltages on the diameter and distribution of lubricant droplets are investigated, and droplet wettability and deposition on the cutting interface are analyzed. The heat transfer capacity of EMQL in steady stage and its machining performance and oil-mist concentration after turning are comparatively studied. By use of tool wear analysis, the lubrication mechanism of EMQL in the cutting process is revealed. Results indicate that as lubricant charged, droplet diameter decrease and its distribution improve with an enhancement in the wettability and deposition. The cutting temperature, oil mist concentration, tool wear and workpiece surface roughness of EMQL are 14%, 8%, 37% and 28% lower than those of the traditional MQL, respectively. When EMQL is used, the improved wettability and reduced droplet diameter allow lubricant droplets to enter and cover the tool-workpiece interface more easily, which would help lubricate and cool the cutting interface, and thus achieve a better cutting performance.

Wetting and spreading of droplets on rough aluminum surfaces

The wetting and spreading of distilled water droplets on abrasion-treated aluminum alloy AlMg 6 surfaces were studied. Using the shadow method, the dependences of the dynamic contact angle and hysteresis on the arithmetic mean of the profile deviation were obtained. Based on the analysis of the height and hybrid 3D roughness parameters, a relationship between the texture and spreading of the droplet is established.

Droplet leaping governs microstructured surface wetting.

Microstructured surfaces that control the direction of liquid transport are not only ubiquitous in nature, but they are also central to technological processes such as fog/water harvesting, oil-water separation, and surface lubrication. However, a fundamental understanding of the initial wetting dynamics of liquids spreading on such surfaces is lacking. Here, we show that three regimes govern microstructured surface wetting on short time scales: spread, stick, and contact line leaping. The latter involves establishing a new contact line downstream of the wetting front as the liquid leaps over specific sections of the solid surface. Experimental and numerical investigations reveal how different regimes emerge in different flow directions during wetting of periodic asymmetrically microstructured surfaces. These insights improve our understanding of rapid wetting in droplet impact, splashing, and wetting of vibrating surfaces and may contribute to advances in designing structured surfaces for the mentioned applications.

Numerical study on dynamic wettability of micro-structure surface

Wettability plays a vital role in many fields. In this paper, the shortcomings of current experimental measurement of dynamic wetting characteristics are described. The dynamic wettability of droplets on the simplified triangular wave micro-structure surface is studied by volume of fluid method. The results are compared with the dynamic wettability of droplets on the smooth surface and the rectangular wave micro-structure surface. The results show the similarities and differences of the motion characteristics of droplets on smooth surface and different micro-structure surface. Under the action of volume force, the droplets are first deformed and then moved. The droplets move as a whole on the smooth surface. But the droplets partially slip on the micro-structure surface, and the three-phase contact point on the left side does not move.

Droplets Capped with an Elastic Film Can Be Round, Elliptical, or Nearly Square.

We present experiments that show that the partial wetting of droplets capped by taut elastic films is highly tunable. Adjusting the tension allows the contact angle and droplet morphology to be controlled. By exploiting these elastic boundaries, droplets can be made elliptical, with an adjustable aspect ratio, and can even be transformed into a nearly square shape. This system can be used to create tunable liquid lenses and, moreover, presents a unique approach to liquid patterning.

Patterning Bubbles by the Stick–Slip Motion of the Advancing Triple Phase Line on Nanostructures

The stick-slip motion of the triple phase contact line (TCL) has wide applications in inkjet printing, surface coatings, functional material assembly, and device fabrication. Here, for the first time, we report that on an alumina substrate with nanostructures, the stick-slip motion of the advancing TCL during spreading of an emulsion droplet can serve as an effective nanopatterning process. Air enclosed in the substrate nanostructures can be exchanged with liquid during the "stick" phase, resulting in the formation of bubbles arranged in a ring pattern. The process takes place in two stages: rings of air form first and then, as the volume of air increases, they separate into air bubbles as a result of the Plateau Rayleigh instability. During the first stage, the rings form due to the stick-slip of the advancing TCL and are ascribed to hydrogen-bonding interactions. Ultimate bubble size is dependent on the substrate pore dimensions. The process was simulated using finite-element analysis to elucidate the mechanism associated with subsequent bubble formation. The simulations corroborate well with the experimental results. This stick-slip motion of the advancing TCL provides new insights into the phenomena associated with droplet spreading and wetting, and the ability to control the formation of patterned bubbles will be promising in applications ranging from microfluidics to printing of functional materials and devices based on bubble templates and applications requiring submerged hydrophobic surface.

Anisotropic Wetting of Droplets on Stripe-Patterned Chemically Heterogeneous Surfaces: Effect of Length Ratio and Deposition Position.

The equilibrium state of a droplet deposited on chemically heterogeneous surfaces is studied by using many-body dissipative particle dynamics. The length ratio covers 2 orders from 0.01 to 1 and allows a systematical inspection of the changes of the droplet shape, contact angle, and aspect ratio with this parameter. Moreover, a new parameter, global aspect ratio, is introduced to better characterize the distortion of the droplet. It is found that the droplet shape at the equilibrium stage strongly lies on the deposition position when the length ratio is beyond 0.1. Additionally, the lateral displacement is observed when depositing the droplet on the border of two stripes at large length ratios (over 0.1). On the other hand, the Cassie area fraction also has a significant effect on the wetting behaviors. When the droplet is driven by a body force with a 45° inclined angle to the stripes, the moving direction could be strictly in line with the force direction, deviating from the force direction, or totally in line with the stripes, depending on the length ratio.

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.

Branched InAs nanowire growth by droplet confinement

Anisotropy in crystal growth of III-V semiconductor nanowires can be enhanced by the assistance of a liquid particle. During the past decades, selected scientific works have reported a controlled change in the nanowire growth direction by manipulation of the assisting droplet. Although these results are interesting from an engineering point of view, a detailed understanding of the process is necessary in order to rationally design complex nanostructures. In this letter, we utilize our understanding of the growth-assisting droplet to control the morphology and direction of gold-assisted wurtzite-phase InAs nanowires, using controlled droplet displacement followed by resumed growth. By confining the droplet to the nanowire sidewall using zincblende inclusions as barriers, epitaxial growth of horizontal branches from existing nanowires is demonstrated. This is done by tailoring droplet wetting of the nanowire and using identical conditions for the nanowire “stem” and branch growth. This work demonstrates the importance of the droplet dynamics and wetting stability, along with the benefits of crystallographic control, for understanding the growth along different directions. Controlled branched growth is one way to achieve designed nanowire networks.

Dynamics of dissolutive wetting: Physical mechanics investigations

Dissolutive wetting, i.e. droplet wets and simultaneously dissolves its solid substrates, is of great significance in both academic research and practical applications. The diffusion interface at the moving contact line is one of the answers to the Huh-Scriven paradox. Besides, dissolutive wetting is the bottleneck problem in many practical fields, such as metal alloy processes, shale gas exploitation, drug release, etc. Furthermore, dissolutive wetting involves complex physical processes, i.e. wetting, diffusion and convection. The coupling transport of mass and momentum as well as the internal convection in the droplet increases the difficulty in studying dissolutive wetting. Even though previous works have been done on metal/metal, metal/ceramic systems, there has been no experimental observation on the flow details in droplets and changes of the solid-liquid interface due to the opacity of the materials in dissolutive wetting. Thus, the physical mechanism of dissolutive wetting is still far from being well understood. Based on the problems mentioned above, in this paper, dynamics of dissolutive wetting of droplets on solid surfaces is investigated by physical mechanics. From a new perspective, we carry out the molecular dynamic simulations of glucose and water dissolution pairs. we match the parameters that dominate the dissolutive wetting in our simulations with real experimental parameters. By this method, the simulation systems are simplified and the glucose molecules can be regarded as Lennard Jones particles. Experiments are also designed to show the in situ real time measurement of the dissolutive wetting. In order to obtain the flow details in the droplet, we select three pairs of high transparent materials. Beyond that, we built a dimensionless number space that divides the dissolutive wetting into two types. For small Pelect number, the convection effect is ignored. But when Pelect number is greater than one, the convection effect that leads to the uniform of the solute distribution must be considered. Morever, we obtain that the droplet follows different scaling laws in different dissolutive wetting stages. In the early stage, the surface tension dominates and the scaling law satisfies the Tanner law. Whereas scaling law in dissolutive wetting changes while diffusion and convection effect are more obvious with the dissolution of solutes. Compared with the non-convective process, the internal convection increases the energy dissipation within the dissolution pairs in our experiments and inhibits the spreading of the droplets to a certain extent. We also find that the coupling effect of dissolution and wetting process will have important influences on the molecular structure of the droplet. Overall, we establish multiscale dissolutive wetting model and reveal the scaling laws of droplet spreading in dissolutive wetting by physical mechanics. Our findings may help to further understand the mechanisms of dissolutive wetting and provide theoretical guidance of practical applications that involve dissolutive wetting.

Magnetically Responsive Superhydrophobic Surface: In Situ Reversible Switching of Water Droplet Wettability and Adhesion for Droplet Manipulation.

A smart, magnetically responsive superhydrophobic surface was facilely prepared by combining spray coating and magnetic-field-directed self-assembly. The surface comprised a dense array of magnetorheological elastomer micropillars (MREMPs). Benefitting from the magnetic field-stiffening effect of the MREMPs, the surface exhibited reversible switching of the wettability and adhesion that was responsive to an on/off magnetic field. The wettability and adhesion properties of the surfaces with MREMPs were investigated under different magnetic fields. The results revealed that the adhesion force and sliding behaviors of these surfaces were strongly dependent on the intensity of the applied magnetic field and the mixing ratio of poly(dimethylsiloxane) (PDMS), iron particles, and solvent (in solution) used for preparation of the magnetically responsive superhydrophobic surfaces. The adhesion transition was attributed to the tunable mechanical properties of the MREMPs, which was easily controlled by an external magnetic field. It was also demonstrated that the magnetically responsive superhydrophobic surface can be used as a "mechanical hand" for no-loss liquid droplet transportation. This magnetically responsive superhydrophobic surface not only provides a novel interface for microfluidic control and droplet transportation, but also opens up new avenues for achieving smart liquid-repellent skin, programmable fluid collection and transport, and smart microfluidic devices.