Motion Of Water Droplets Research Articles

Water droplet impact on perfluoropolyether-based liquid-like surfaces

Water droplet impact on water-repellent surfaces is of relevance to many important engineering applications. Although the droplet impact behavior on water-repellent surfaces has been extensively investigated, in particular, textured superhydrophobic surfaces, that on water-repellent liquid-like surfaces has been barely reported. In this study, we experimentally investigated the oblique impact of water droplets on a surface with a liquid-like perfluoropolyether coating using a high-speed camera. The direct observation showed that all the impinging water droplets released from various heights exhibited similar outcomes, including an inelastic collision without rebound as well as a continuous sliding over the liquid-like surface. The liquid-like perfluoropolyether molecular chain on the surface was suggested to behave like a high viscosity oil layer that could cause more intense energy dissipation during the impact process. The crucial impact parameters of the maximum tangential droplet extension, the spreading time and the sliding length were further analyzed quantitatively by the methodology widely adopted in the exploration of the impact behavior in previous studies. It was indicated that the process of the impinging droplet spreading on the liquid-like surface was more likely dominated by the competition between the inertial and capillary forces although the increased viscous dissipation arising from the liquid-like layer significantly suppressed the rebound of the impinging droplet. Compared to superhydrophobic surfaces, the liquid-like surface could generate a stronger friction force for the slide motion of the impinging water droplet. The triboelectric effect arising from the water-solid contact was proven to inhibit the retraction of the impinging droplet with relatively higher impact velocities. This study is expected to improve the fundamental understanding of the droplet impact process and facilitate us to make more rational designs of liquid-like surfaces for relevant applications.

Facile fabrication of extreme-wettability contrast surfaces for efficient water harvesting using hydrophilic and hydrophobic silica nanoparticles

Freshwater shortages threaten ecosystems and human communities around the world. Inspired by the ability of cacti and desert beetles to collect water from the atmosphere, we successfully fabricated a wettability contrast surface with efficient water harvesting using a simple and low-cost process. A solution of isopropyl alcohol and a mixture of hydrophobic and hydrophilic silica nanoparticles was sprayed on a laminating film of ethylene vinyl acetate and polyethylene terephthalate, after which sandpaper was laid on the dry-coated film for hot-press lamination. After peeling the film from the sandpaper, the fabricated surface exhibited random wettability contrast patterns with nano-microstructures formed by nanoparticles and the sandpaper surface. The highest water-harvesting efficiency of the prepared surface was approximately 446.7 mg/cm2/h when the mass ratio of the hydrophobic to hydrophilic silica nanoparticles in the spraying solution was 75/25 due to the optimal performance between the generation of water nucleation, coalescence, and the movement of water droplets. An analysis of the effect of ultraviolet irradiation and adhesive damage on the surface revealed that the efficiency of water collection was nearly unchanged in the face of the ultraviolet irradiation and decreased slightly after the tape test. The proposed fabrication method can make flexible and wettable contrast surfaces at large scales and low costs. In addition, superhydrophobic, or nearly superhydrophilic surfaces can be fabricated with the same process by using solely hydrophobic or hydrophilic silica nanoparticles. Therefore, this process is versatile and can be applied for creating surfaces with a range of wettability properties.

The theoretical approach of how to predict the critical rotational speed of the rotating nozzle in flashing purification to increase the evaporation rate

We developed a new flashing method in this study to improve the evaporation rate in flashing purification. In this case, a stationary nozzle is replaced by a rotating nozzle during the flashing process. The use of a rotating nozzle may distribute water droplets in previously unreachable spaces; the more droplets, the greater the evaporation rate. The use of a rotating nozzle during flashing causes problems if the rotational speed causes the water droplets to collide. To avoid a collision, a rotating nozzle must rotate at an appropriate rate. The kinematics of drifting water droplets after being sprayed by a rotating nozzle can be used to calculate this speed. It could be calculated by periodically observing droplet movement to determine the critical rotational speed of a rotating nozzle. By studying the trajectories of water droplet movement after being sprayed by a rotating nozzle, we can obtain no droplet collision circumstances. Water droplet evaporation during drifting reduces droplet size, resulting in a lower nozzle rotational speed to avoid a collision. For a water particle diameter of 153.5 µm, the smallest nozzle rotational speed predicted is 85.4 rpm. A droplet with a diameter of 80 µm produces 82 rpm of nozzle rotational speed, while a droplet with a diameter of 40 µm produces 80 rpm of nozzle rotational speed. Evaporation may reduce the size of the water droplets during the flashing process. To avoid droplet collision and increase evaporation rate, finer water droplets require a lower nozzle rotational speed.

Effect of the wetting behavior of water droplets on the output of a water-droplet-based electricity generator

Water-droplet-based electricity generators (DEGs) have attracted considerable attention owing to their simple metal substrate/dielectric/metal electrode structure and ability to generate high power instantaneously. However, their characteristics are significantly affected by the behavior of water droplets. In this study, to verify the relationship between water droplet movement and DEG output characteristics, we fabricated DEGs with two different structures. The electrodes were arranged vertically and horizontally with respect to the sliding direction of the water droplet, and DEG operation with different water droplet positions was verified. The results indicated that DEG output voltage varied according to water droplet behavior. Moreover, when the water droplet came in contact with an electrode, the wetting effect modified the shape of the droplet, thereby changing DEG output characteristics.

Relationship between water droplet motion and output characteristics in droplet-based electricity generator

Recently water droplet-based electricity generators (DEGs), which utilize triboelectricity between a droplet and a dielectric material, has a great attention. DEG generates an instantaneous high-power density despite its simple metal/dielectric/metal stack structure. In the DEG, the contact area between a droplet and a dielectric surface affects the output characteristics. We fabricated a DEG using a PTFE film as a dielectric material, measured the output voltage and evaluated the contact area in various dropping heights and positions. We demonstrated that the output peak voltage increased as the contact area increased related to spreading and rebounding of a droplet.

Directional Self-Transportation of Droplets on Superwetting Wedge-Shaped Surface in Air and Underliquid Environments.

The directional self-transportation of droplets has aroused great attention in microfluidic systems. However, most reported surfaces are mainly designed for driving water droplets to move in air, displaying low adaptability in complex environments. This work presents a wedge-shaped surface with multiple superwettability, i.e., superhydrophilicity/superoleophilicity and underwater superoleophobicity/underoil superhydrophobicity, fabricated by electrodeposition of a metal-organic framework on a copper sheet. This surface exhibited excellent performance for driving droplet self-transportation, regardless of the droplet type (water or oil) and environmental media (air or underliquids). In air, the wedge-shaped surface with wedge angle of 9.2° could move droplets of water and dodecane up to 24.5 mm and 17.9 mm, respectively. The movement of water droplet under dodecane, however, dropped from 24.5 mm to 22.1 mm, while the dodecane droplet underwater increased from 17.9 mm to 20.3 mm in moving displacement, indicating the underliquid environment is in favor of manipulation of oil droplets. Furthermore, the droplet convergence, transportation, and separation were achieved on the well-designed multiple wedge tracks in air with a total movement distance up to 60.0 mm. The test of micro-oil droplets collecting under water demonstrated that a sponge with two wedges has 2.1 times the oil droplet collection capacity over that of the sponge only, providing a new strategy for efficient treatment of the micro-oil droplets contaminated water.

Wetting-State-Induced Turning of Water Droplet Moving Direction on the Surface

The spontaneous directional movement of water droplets on a wedge-shaped groove has gained extensive attention due to the advantage of not requiring energy input and its potential wide applications. However, manipulating the direction of movement of water droplets on a wedge-shaped groove has been not fully achieved, and the fundamental understanding of its underlying mechanism remains unclear. Here, molecular dynamics simulations and theoretical analyses are combined to reveal the mechanism of movement in opposite directions of a water droplet at the same position on the wedge-shaped groove interface. It is shown that the moving direction of the water droplet is related to its wetting state on the surface, i.e., the Wenzel and the Cassie states. A water droplet initially in the Wenzel and Cassie states will move toward the diverging and the converging ends, respectively. This phenomenon is attributed to the opposite roles played by the groove substrate and the upper layers in the two wetting states. Moreover, it is found that the water droplet is likely to move faster on a surface with a higher groove, larger opening angle and stronger hydrophobicity. These findings are expected to be of benefit for fully understanding droplet movement and shedding light on the regulation of the direction of movement of the droplets on the groove surface.

DOES INTERNAL CONVECTION PROMOTE MICRO-EXPLOSION OF W/O EMULSION DROPLETS?

W/O emulsions are considered as a beneficial alternative to the use of fossil hydrocarbon fuels and not treated vegetable oils. The presence of a dispersed phase composed of water allows to reduce pollutant emissions and also increases combustion efficiency because of the so-called micro-explosion phenomenon. This micro-explosion has a strong stochastic behavior, and its realization depends on several parameters such as water droplet size distribution and motion during heating. This paper aims to provide a better understanding of the impact of the water droplet behavior on the atomization rate. PLIF technique is used to visualize and track the motion of the water droplets dispersed inside heated emulsion drops. Natural convection of the water droplets is observed, and its impact on coalescence and atomization rate is investigated. Emulsion with small size distributions and high droplet velocity tends to show a low or nonexistent micro-explosion rate, while coarse emulsions show an important coalescence rate, resulting in a high micro-explosion occurrence. Finally, a dimensionless indicator measuring dispersed-phase motion intensity is proposed.

Oscillatory Motion of Water Droplets Both in Oil and on Superhydrophobic Surface under Corona Discharge.

Charged droplets driven by Coulomb force are an important part of a droplet-based micro reactor. In this study, we realized the rapid oscillatory motion of droplets both in oil and on superhydrophobic surface by injecting charges through corona discharge. Distinct from the oscillatory motion of water droplets under a DC electric field, charge injection can make the motion of water droplets more flexible. A droplet in the oil layer can move up and down regularly under the action of corona discharge, and the discharge voltage can control the movement period and height of the droplet. In addition, the left-right translation of droplets on a superhydrophobic surface can be achieved by injecting charges into the hydrophobic film surface through corona discharge. Two kinds of droplet motion behaviors are systematically analyzed, and the mechanism of droplet motion is explained. The present results could help establish new approaches to designing efficient machines in microfluidics and micromechanical equipment.

Bioinspired slippery asymmetric bumps of candle soot coating for condensation and directional transport of water

Natural species use their surface structures for water harvesting. It is the dire need of the hour to learn from nature, mimic its functions, and solve the water scarcity problem through dew condensation. However, the present microscale and nanoscale approaches still suffer from complicated fabrication methods and durability failure in harsh conditions. Here, we introduce a bioinspired slippery surface of candle soot particles which uses its bumps to enhance the growth of condensed water droplets. The Namib desert beetles-inspired convex topography, cacti-inspired asymmetric slope, and Nepenthes pitcher plant-inspired slippery surface of soot particles synergistically combines the large droplet nucleation density, fast droplet coalescence, and easy removal of droplet from the surface. The hydrophilic SLIPS of soot particles harvests water at a rate of 326.4 ± 9 mg/cm2 h-1 which is 70 % high as compared to hydrophobic SLIPS of soot particles. The fluorinated SLIPS reveals 32 % high water collection rate as compared to the methyl-terminated SLIPS of candle soot particles and the results are corroborated by calculating the intermolecular hydrogen bonding energies through DFT. The programmed motion of water droplet on a slippery surface of soot particles is also observed by applying laser light, heat, and magnetic field even against gravity. The slippery surface shows long-term stability due to nano-porosity, permeation of lubricant into soot particles and lubricant’s crosslinking ability with the binder. The developed surface is biocompatible, self-cleaning, and suitable for water harvesting particularly in cold and arid regions.

Collision Characteristics of Water Droplets in Icing process of Insulators

Icing on the insulators of transmission lines threats the power system security. The flashover properties of ice-covered insulators have been widely studied while there is a lack of research on the icing properties of insulators. In this paper, the collision process of supercooled water droplets on insulators is studied. Based on the boundary element method (BEM), a mathematical model of airflow and water droplet motion outside insulators is established. Taking advantage of vector calculation and array tracking, the 3D distribution of collision droplets on insulators is obtained. The results show that the droplets mainly collide with windward shed edge and steel cap of insulators. With the increase of wind velocity and median volume diameter of droplets, the number of captured droplets increases and the collision area expands from the shed edge and steel cap centre to the inner surface and two sides, leading to the increase of icing rate and icing area. Moreover, the related icing tests of insulator LXY-120 were conducted on Xuefeng Mountain, the recorded icing mass and shape are compared with the simulation results, and it is found that the model in this paper has a relative error of 13.5%.

Surface wettability of various phases of titania thin films: Atomic-scale simulation studies

In the paper, the wettability of different phases of TiO2 thin films (anatase, brookite, and rutile) have been studied using molecular-dynamics simulation. The principle of micro-wetting is discussed. The simulation results show that the contact angle decreases upon increasing the interaction energy between the water and the titanium dioxide interface during the wetting process. The values of contact angles from large to small are: rutile, brookite and anatase. The calculated equilibrium contact angles are 73.9°, 59.2°, and 43.7°, respectively. The reason is that the structural connection and the arrangement of the surface microtopography directly affect the movement of water droplets on the surface of the material, thus affecting the wettability. In addition, the amount of the interaction energy and the radial distribution function between these three interfaces and the droplets are calculated, and the density change of the droplet is analyzed further which indicate the difference in wetting between the three crystal structures. At the same time, by simulating and comparing the wettability of the trench surface and the original surface of anatase, it is inferred that the rough interface increases the contact angle with the droplet and reduces the wettability.

Observation of water droplet motion in a shear flow

Although numerous studies have investigated droplets in shear flows, the features of these droplets are incompletely understood. This paper reports our experimental visualization results of sessile droplets in fully developed turbulent shear flows and the onset of their movement. The visualization was performed from the side and bottom views. Furthermore, the droplet height is much smaller than the boundary-layer thickness, and the droplet motions are strongly affected by turbulent velocity fluctuation. The droplets were deformed toward the front and spread in a spanwise direction. The front and rear-side contact angles recover the balance position, that changes symmetrically over time and are not clearly related to sessile droplet oscillation. The droplet oscillation frequencies were well predicted by the model based on the resonant vibrations of the standing waves except for the lowest mode. Based on the critical air velocity and droplet height, the motion onset was adequately approximated by a simple empirical relation between the Weber and Reynolds numbers.

Droplet Navigation by Photothermal Pumping in an Optofluidic System.

Droplets with guided motion have potential applications as microreactors and delivery vehicles. Directing long-range migration powered solely by light is particularly advantageous since light can be applied remotely, patterned with a photomask, and readily translated to irradiate specified locations. Herein, we describe a universal platform that allows fast directional navigation and collective merging of droplets controlled by either ultraviolet or visible light. The guided motion of water and oil droplets follows density-driven convective flows arising from photothermal conversion at a light-absorbing amphiphobic substrate. Because of the relatively high photothermal efficiency, a low-intensity light beam can be employed. Further, we demonstrate that the moving droplets can function as carriers and on-demand reaction chambers.

Synergetic effect of absorber and condenser nano-coating on evaporation and thermal performance of solar distillation unit for clean water production

Two design modifications to increase the evaporation/condensation and freshwater yield of conventional solar stills (CSS) are presented – absorber coating with black paint (BP) in which 10 wt % reduced graphene oxide (rGO) was dispersed, and use of a nano-silicon-coated glass cover alongside the rGO coated absorber. Water contact angle analysis revealed that the hydrophilic glass/condenser hindered water droplet movement, whereas the hydrophobic silicon-coated glass enabled significantly faster water droplet movement. The rGO-coated absorber SS, with 99.2% solar absorptivity, improved the average water temperature by 11% leading to an augmented full-day freshwater yield of 3031 mL (versus 2160 mL for the CSS). The synergistic effect of the nano-silicon-coated glass and rGO-coated absorber further enhanced the full-day freshwater yield to 3410 mL. Energy/exergy analysis demonstrated enhanced efficiencies of the combined condenser and absorber coated SS of 37% (energy) and 112% (exergy), compared to the CSS. Non-linear characteristic equations described the instantaneous gain in energy efficiency of the combined absorber and condenser coated SS. Absorber/basin exhibited the highest rate of irreversibility in all three SS. The cost of water was 0.01 $/L for the dual-modified still. Water physicochemical quality analysis before and after desalination revealed that the purified water was suitable for drinking. • A solar still was fabricated with absorber and condenser coating for improved yield. • RGO coating improved the absorber average temperature by 10.9% compared to CSS. • Glass coated with nano-silicon altered condensation behavior to drop-wise from film-wise. • Modified SS showed augmentation in water yield compared to conventional SS. • Brackish water was converted into distilled water this is suitable for drinking.

An energy-conservative many-body dissipative particle dynamics model for thermocapillary drop motion

The thermocapillary motion of a drop on a solid substrate is a common phenomenon in daily life and many industrial fields. The motion can be significantly affected by the temperature gradient of the substrate and the properties of the liquid, such as surface tension, viscosity, thermal coefficient, density, and diffusivity. In this study, a numerical model based on modified many-body dissipative particle dynamics was developed to capture correctly the temperature dependence of a fluid. The momentum, thermal diffusivity, viscosity, and surface tension of liquid water at various temperatures ranging from 273 to 373 K were used as examples to verify the proposed model. The results calculated with this model for heat conduction in a liquid–solid system are in good agreement with those calculated with Fourier's law. The approach successfully modeled the thermocapillary motion of a liquid water droplet on a hydrophobic substrate with a temperature gradient. The migration of the droplet on a flat solid substrate was induced by the difference in surface tension due to the temperature gradient. The migration velocity increased with the temperature difference, which is in agreement with the present theoretical analysis and dynamic van der Waals theory. The modified numerical model proposed in this work could be used to study heat and mass transfer across a free interface, such as Marangoni convection in multiphase fluid flow.

Determination of the Dielectrophoretic Force Induced by the Photovoltaic Effect on Lithium Niobate.

The actuation of droplets on a surface is extremely relevant for microfluidic applications. In recent years, various methodologies have been used. A promising solution relies on iron-doped lithium niobate crystals that, when illuminated, generate an evanescent electric field in the surrounding space due to the photovoltaic effect. This field can be successfully exploited to control the motion of water droplets. Here, we present an experimental method to determine the attractive force exerted by the evanescent field. It consists of the analysis of the elongation of a pendant droplet and its detachment from the suspending syringe needle, caused by the illumination of an iron-doped lithium niobate crystal. We show that this interaction resembles that obtained by applying a voltage between the needle and a metallic substrate, and a quantitative investigation of these two types of actuation yields similar results. Pendant droplet tensiometry is then demonstrated to offer a simple solution for quickly mapping out the force at different distances from the crystal, generated by the photovoltaic effect and its temporal evolution, providing important quantitative data for the design and characterization of optofluidic devices based on lithium niobate crystals.

Investigation of water motion in a fuel cell channel with a reservoir

This study presents a novel gas distribution channel configured with a reservoir and an extra outlet to prevent channels from being clogged by long slugs in the vicinity of the outlet port in fuel cells. The motion of water droplets in this channel was investigated using the volume of fluid method. The behavior of the water droplets in the channel is examined under varying outlet pressures, droplet size, the initial position of the droplets, and the direction of the gravitational acceleration. The results showed that the proposed design yields the water droplets to break up while preventing channel clogging and facilitating water removal. It has also been observed that water discharge accelerates as the reservoir outlet relative pressure decreases, and discharge occurs to a large extent when the gauge pressure is lower than −1000 Pa. The proposed intermediary water evacuation approach and design have been determined to have the capability of active water discharge from multiple points via an appropriate pressure differential. It is seen that liquid water propagation to larger areas, especially in long flow lengths, can be prevented via the proposed new design and approach.

Self-cleaning of superhydrophobic nanostructured surfaces at low humidity enhanced by vertical electric field.

Self-cleaning is the key factor that makes superhydrophobic nanostructured materials have wide applications. The self-cleaning effect, however, strongly depends on formations and movement of water droplets on superhydrophobic nanostructured surfaces, which is greatly restricted at low humidity (< 7.6 g·kg−1). Therefore, we propose a self-cleaning method at low humidity in which the pollution is electro-aggregated and driven in the electric field to achieve the aggregation and cleaning large areas. The cleaning efficiency of this method is much higher than that of water droplet roll-off, and will not produce “pollution bands”. A simplified numerical model describing pollution movements is presented. Simulation results are consistent with experimental results. The proposed method realizes the self-cleaning of superhydrophobic nanostructured surfaces above dew point curve for the first time, which extends applications of superhydrophobic nanostructured materials in low humidity, and is expected to solve self-cleaning problems of outdoor objects in low humidity areas (< 5.0 g·kg−1).Electronic Supplementary MaterialSupplementary material (experimental procedures, computational details, modeling process, supplementary figures, tables, and videos) is available in the online version of this article at 10.1007/s12274-022-4093-0.

Development of a Coating-Less Aluminum Superhydrophobic Gradient for Spontaneous Water Droplet Motion Using One-Step Laser-Ablation.

Nature shows various approaches to create superhydrophobicity, such as the lotus leaf, where the superhydrophobic (SHPB) surface arising from its hierarchical surface consists of random microscale bumps with superimposed nanoscale hairs. Some natural systems, such as the hydrophilic silk of some spider's webs, even allow the passive transport of water droplets from one part of a surface to another by creating gradients in surface tension and Laplace pressure. We look to combine both ideas and replicate the superb water repellence of the lotus leaf and the surface tension gradient-driven motion of the spider silk to form an all-metal, coating-less surface that promotes spontaneous droplet motion. We present the design, fabrication, and investigation of such superhydrophobic gradient surfaces on aluminum, which are aimed at spontaneous water droplet movement for improved surface water management. One surface demonstrates a droplet travel distance of almost 2 mm for a 11 μL droplet volume. We also present surfaces that map the theoretical ranges of the surface tension gradient surfaces tested here.