Motion Of Water Droplets Research Articles

Switchable Isotropic/Anisotropic Wettability and Programmable Droplet Transportation on a Shape-Memory Honeycomb.

Programmable droplet transportation is required urgently but is still challenging. In this work, breath figure was employed to fabricate shape-memory poly(lactic acid) (PLLA) honeycombs in which tiny crystals and an amorphous network act as the shape-fixed phase and recovery phase, respectively. Upon uniaxial tension, circle pores from the breath figure were deformed to elliptical pores, producing contact angle differences and anisotropic wetting behaviors in two directions. Both pore geometry and anisotropic wettability can be tailored via the draw ratio. On the PLLA honeycomb surface with a lower draw ratio, the contact angle difference is too small to induce droplet transportation along the desired direction. In the case of a higher draw ratio, however, the movement of water droplets has been controlled absolutely along the tension direction. The transition between them can be achieved reversibly during uniaxial tension and recovery processes based on the shape-memory effect. The enhanced flow control, which can be attributed to the synergism between optimal hydrophobicity and enlarged anisotropic wetting behaviors, endows water droplets with the ability to turn a corner spontaneously on a V-shaped surface including two regions exhibiting different oriented directions.

Laser-induced Leidenfrost surfaces

Liquids in the Leidenfrost regime levitate on a cushion of vapour and have the ability to self-propel if they are placed on hot ratchets. However, potential applications of the Leidenfrost phenomenon are limited by the high temperatures required to trigger the drop movement. Here, we report on the development of hierarchically structured, functionalized surfaces, which aim to significantly lower the Leidenfrost temperature. For this purpose, saw tooth profiles (p≈1.5 mm, h≈200 µm) were milled from an aluminium alloy and their tips were irradiated with a femtosecond laser to generate laser-induced periodic surface structures (LIPSS) with a period of 840 nm. Subsequently, the surfaces were hydrophobized by silanization using 1H,1H,2H,2H-perfluorooctyltriethoxysilane. It is shown, that the temperature threshold required to trigger a directional movement of water droplets on hydrophobic structured surfaces decreases with increasing wetting angle and reaches a minimum of T = 120 °C at a contact angle of θ = 149°. Furthermore, we show for the first time that an acceleration of drops on superhydrophobic saw teeth is also possible during heating even if the drops have already been applied at room temperature.

Why Can Water Droplets Move Smoothly Even on Statically Hydrophilic Surfaces?

Studies on hydrophilic surfaces showing excellent water sliding properties are very rare, despite the numerous practical advantages they offer. One of the authors has recently developed a smooth, transparent and sufficiently-thick hydrophilic film with low contact angle hysteresis (5°) and small water tilt angles (6 ± 2°) through a simple sol-gel reaction of 2-[methoxy (ethyleneoxy)10propyl]trimethoxysilane (PEG-M) and tetraethoxysilane (TEOS). However, the origins of these unusual water sliding properties have not been clearly identified. As such, the impact of the addition of TEOS acting as a "nanospacer" or the identity of PEG terminal functional groups on the final static/dynamic wetting properties needs to be explored. In this study, we investigated the orientational/conformational states of PEG chains in PEG-M/TEOS and [hydroxy(ethyleneoxy)8-12propyl]triethoxysilane (PEG-OH)/TEOS hybrid films under wet and dry conditions using sum-frequency generation (SFG) spectroscopy. We found that PEG-M/TEOS hybrid film surfaces had no marked differences in the conformational states of PEG chains under wet or dry conditions, resulting in excellent water sliding properties as there was no energy barrier for water droplet motion. In contrast, PEG chains were completely disordered after contact with water in the PEG-OH/TEOS hybrid films due to hydration effects. This large conformational change between the liquid/solid and gas/solid interface at the three-phase contact line resulted in an unfavorable energetic barrier for water droplet motion, leading to the poor water sliding properties. TEOS did not physically work as a "nanospacer," but chemically worked as a binder to endow practically useful properties, such as good adhesion and versatility in substrates used, to our hybrid films.

Surprising Lack of Influence on Water Droplet Motion by Hydrophilic Microdomains on Checkerboard-like Surfaces with Matched Contact Angle Hysteresis.

Chemically and spatially micropatterned surfaces have been successfully prepared for a number of diverse applications, including water/fog harvesting, screen printing, microfluidics, and cell/protein assays. While there have been quite some reports on micropatterned surfaces, less is known about the factors that influence dynamic surface wettability. To that end, smooth checkerboard-like micropatterned hydrophobic/(super)hydrophilic surfaces (2, 5, 10, 20 μm pattern sizes) with regions of matching/mismatching contact angle hysteresis (CAH) were prepared on the basis of a simple chemisorption/photopatterning of monolayers. The effects of regional wettability/CAH and pattern size on the overall dynamic wettability were then examined by measuring the dynamic contact angles (CAs) and substrate tilt angles (θT) of water. It was found that the dynamic wettability on samples with matching regional CAH remained unchanged, even when using hydrophilic regions or changing the pattern size. In contrast, surfaces containing mismatching CAH regions pinned water droplets, leading to overall dynamic wetting properties markedly dependent on pattern size. In addition, the experimental data did not match values predicted by the Cassie equation because dynamic wetting behavior is dominated not by interfacial area but by the interactions of the liquid and solid at the three-phase contact line.

Experimental Investigation on Droplet Deformation and Breakup under Uniform DC Electric Field

In order to understand the dynamic behaviors of the charged water droplet in the presence of electric field, the experimental investigation is conducted in this paper. A uniform DC electric field is produced by applying high voltage between two parallel copper plates which are placed horizontally in a silicon oil tank. The variation range of the electric field intensity is 0–6.67 kV/cm. Under the influence of the electric field, deformation, motion, breakup and other dynamic behaviors of the charged water droplet with the initial diameter of 5.36 mm are observed and analyzed. Results show that the increase in the electric field intensity results in the elongation of the charged droplet. When the electric field intensity increases to 2.5 kV/cm and 3.0 kV/cm, the droplet would deform into an unstable Taylor cone shape, and then the top of droplet breaks up into some daughter droplets. These daughter droplets move up and down between two copper plates. Further increasing the electric field intensity to 3.67 kV/cm, some of daughter droplets forms a liquid bridge linked the two plates which likes a pearl chain, and then discharge phenomenon occurs. For the enormous electric field intensity of 6.67 kV/cm, numerous tiny droplets, which distribute uniformly in the silicon oil, are produced by the breakup of larger daughter droplets and the large droplet located on the lower plate. This research has certain application value in the fields of oil treatment, electric demulsification and so on.

Simulation of Spray Cooling on Hot Steel Slabs in Continuous Casting

Abstract Continuous casting is an important process to solidify molten steel. Efficient and uniform heat removal by water spray without cracking or deforming steel slabs is a major challenge during continuous casting. With the aid of computational fluid dynamics (CFD), the current study presents a numerical model of spray cooling for process understanding and improvement. The Eulerian–Lagrangian approach is utilized to track the movement of water droplets that are injected from a hydraulic nozzle. The initial droplet size is predicted by the linear instability sheet atomization (LISA) model and the droplet distribution satisfies the Rosin-Rammler distribution. Droplet-slab impingement is modeled by the wall jet model and the associated heat transfer accounts for both heat conduction and radiation. Heat transfer coefficient (HTC) on slab surface is validated against a hot-plate benchmark experiment and shows good agreement. The significance of spray standoff distance and spray direction on heat transfer is also investigated. The results indicate the existence of the critical standoff distance beyond which the cooling effect becomes negligible. Spray direction plays an important role in determining the heat transfer rate as well, and insufficient cooling is observed when droplets are sprayed against gravity. Additional 10% to 15% of water will compensate for such low heat transfer. The current study should benefit the steel industry by providing fundamental insights into the process. The current model can also be applied to other types of spray nozzles and can further improve the efficiency of future simulations.

A Study on the Drift of Spray Droplets Dipped in Airflows with Different Directions

The European Directive concerning pesticide sustainable use establishes regulations to reduce the environmental drift throughout treatments to agricultural crops, particularly in nearby sensitive areas, such as water bodies, natural reserves and urban areas. The drift represents the fraction of mixture delivered by the sprayer that is not caught by the crop, and is the clearest cause of environmental pollution. Anti-drift nozzles are usually employed, and buffer zones are also maintained along the edges of the sprayed field to reduce drift production. The aim of this work was the theoretical study of the motion of the spray droplets delivered by a nozzle, dipped in downwards and/or lateral forced air flows. A mathematical model has been developed, consisting of a system of 2nd order differential equations, to simulate the motion of water droplets of different diameters within simultaneous different directions of air flow. The graphs, obtained by means of the numerical solution of the model, allow us to analyze the level of the droplets’ drift, according to their diameter and to the speed of the lateral and the downward air flows, respectively. A lateral airflow at a speed of 5 m · s − 1 produced a drift in its direction until 1.70 m for droplets from 100 to 500 μm in diameter. For larger drops, the impact of the downward airflow is not very significant. The results obtained by the numerical solution of the mathematical model have been compared with the results of experimental tests carried out to evaluate the drift of spray produced by different nozzles.

Impact of Carbon Paper Anisotropy on Water Droplet Movement through the Electrodes of Proton-Exchange Membrane Fuel Cells

Green vehicles which employ proton-exchange membrane (PEM) fuel cells as the power source of their propulsion systems have been developed to a high stage, in the recent years. Nevertheless, additio...

A water droplet motion energy harvester with wafer-level fabrication method

Water droplets are ubiquitous in nature and harvesting droplet kinetic energy has attracted a great deal of attention to meet the increasing worldwide energy demands. In this work, we report a water droplet motion energy harvester with the wafer-level fabrication method, which is based on the water–solid surface contact electrical double layer effect. A theoretical model is established to illuminate the water droplet motion energy harvesting mechanism. Guided by the model, an energy harvester is developed based on Micro Electro-Mechanical Systems compatible process technology, which achieve a wafer-level fabrication. The output performance of the energy harvester has been optimized by decreasing the electrode width and its interval. Experimental results also show that the output voltage can be improved by increasing the droplet volume and decreasing the ionic concentration. The optimized energy harvester exhibits an open circuit output peak voltage of 57.5 mV. This research provides an in-depth theoretical study and a practical guidance on water droplet kinetic energy harvesting.

Numerical investigation of drop dynamics in presence of wettability gradient inside a serpentine channel of proton exchange membrane fuel cell

Water management significantly affects the performance of a proton exchange membrane fuel cell (PEMFC). Therefore, interest is felt to numerically investigate water droplet movement and slug formation inside the microchannel (gas) of PEMFC. Two important parameters—water coverage ratio and pressure drop have been studied in detail. A U-shaped geometry with a round corner is used for this purpose. 3D unsteady-state models are used to study the drop dynamics using commercial CFD software ANSYS FLUENT 18. For tracking of water drop dynamics, the volume of fluid model is used. Two different situations are simulated. In the first case, the investigation of hydrodynamics of the 0.4 mm drop adhered to the surface of the gas diffusion layer (GDL) has been done. In the second case, simulation of air-water slug flow has been done. GDL surfaces at upstream and downstream of bend are modified using user-defined functions, such that the GDL surface has a dynamic contact angle with respect to the direction of flow. This makes it a continuously hydrophilic surface at upstream and continuously hydrophobic surface at downstream with respect to the direction of flow. The impact of GDL wettability on water retention and removal has been discussed. It is noted that the presence of a gradient facilitates the removal of water drop adhered to the GDL surface. For the case of a suspended drop with an increase of 1°/mm in the magnitude of the gradient, a decrease of 30% is observed in water coverage ratio and pressure drop observed in the channel. Such modified surfaces aids in the conversion of slugs to film at the downstream of bend that reduces maldistribution. The pressure fluctuations and average pressure drop are reduced by 66% when subjected to the aforementioned hybrid gradient.

Magnetic field induced motion of water droplets and bubbles on the lubricant coated surface

Diamagnetic water droplets placed on the silicone-oil-lubricated solid surfaces were displaced by a permanent magnetic field B≅0.4T. The motion was sensitive to the hydrophobicity of the solid surface. Maximal velocities vmax≅0.1-0.2mms were registered on the silicone-oil-lubricated hydrophobic surfaces. The maximal velocities were insensitive to the volume of water droplets for the volumes in the range of 2μl<V<12μl. The maximal velocities of the droplets were also only slightly dependent on the thickness of the oil layer. A model of the motion of diamagnetic droplets driven by the permanent magnetic field is suggested. The qualitative model describes satisfactorily scaling of the experimental findings. The reported process involves purely diamagnetic materials. Motion of silicone oil bubbles driven by the magnetic field is also reported.

Self-propelled water droplet movement on a laser-etched radial gradient copper surface

This paper reports the behavior of water droplets on a topography-based, radial gradient on a metallic surface. The radial gradient was designed to concentrate water in the center region through the spontaneous motion and coalescence of individual droplets on the gradient pattern. In this work, parallel microchannels having a fixed width (~25 μm) and depth (100 μm) but variable spacing were used to impart a net surface tension force to the droplet for actuation. The diameter of the radial gradient (i.e. circular test region) was approximately 18.9 mm (or, ~0.75 in.). The surfaces were fabricated using a 355 nm YVO4 laser system and then characterized using droplet injection (5–38 μL) and spray testing techniques. Injected droplets were observed to move spontaneously away from the hydrophobic outer ring region of the circle towards the hydrophilic center as designed by the gradient. Droplet travel distances as large as 4 mm were measured depending on the droplet volume and injection site location. In application, such a design could be useful for promoting droplet coalescence on a surface and subsequently removal by gravity through the formation of sufficiently large droplets from two or more otherwise small droplets.

Highly ordered water droplet arrays on a volatile solvent surface

The condensation of water vapor on cold surfaces is termed as breath figures. It is fascinating that the highly ordered breath figure arrays can be formed by simple evaporation of a volatile polymer solution. It has been speculated that the ordered breath figure arrays originate from ordered water droplet arrays induced by the evaporation of volatile solvents. However, it is unknown what the real formation process of ordered water droplet arrays on volatile surfaces of pure solvents is. Herein, we report the real-time observation of the growth process of water droplets on the volatile surface of the pure solvent of carbon disulfide by using an optical microscope. We observed the highly ordered water droplet arrays on the volatile surface. We found six distinct stages during the evaporation process. The growth mechanism of water droplets obeys individual growth before coalescing. In addition, a few defects of large water droplets, vacancies and irregular boundaries were observed. The rapid movement of water droplets indicated the strong convective current in the solvent, promoting the rearrangement of water droplets to form more ordered arrays. The highly ordered water droplet arrays only need a moist environment, which is very convenient for ordered structures. Our findings not only provide direct evidence for ordered water droplet arrays but also quantitatively clarify their dynamic process, which helps to understand the mechanism of ordered breath figure arrays.

Quadri-directionally anisotropic droplets sliding surfaces fabricated by selective laser texturing of aluminum alloy plates

A periodic array patterned superhydrophobic surface consisted of asymmetric multi-directional stairway units was prepared by selective laser texturing of aluminum alloy plate, which was endowed for the first time with the quadri-directionally anisotropic droplets sliding characteristic. Through the layer-by-layer subtractive laser ablation procedure, the unique unequal tread depths of the micron-sized stairway-structure in four directions were ingeniously constructed. The achieved surface with an arbitrary desired sliding angle in any of the four directions constructed a transitional superhydrophobic state between Wenzel’s and Cassie’s states of the supported water droplets. The comprehensive mechanism of the supernatural quadri-directional anisotropy was verified to be mainly summarized into the synergy and balance between the droplet’s resistance effect of gravity force and the pinning-effect controlled adhesion force. The two obstructive effects were verified to be in exact accordance with each other on the movement of water droplets. The quadri-directionally anisotropic sliding droplets characteristic induced by the simple but powerful selective laser texturing method can provide a fresh cognition of anisotropic wettability and enormous potential applications in microfluidic systems for biochemical separation.

Nature–Inspired self–cleaning surfaces: Mechanisms, modelling, and manufacturing

Nature-inspired self-cleaning surfaces have attracted considerable attention from both fundamental research and practical applications. This review adopts a chemical-engineering point of view and focuses on mechanisms, modelling, and manufacturing (M3) of nature-inspired self-cleaning surfaces. We will introduce six nature-inspired self-cleaning mechanisms: The Lotus-effect, superhydrophobic-induced droplet jumping, superhydrophobic-induced unidirectional movement of water droplet, underwater-superoleophobic-based self-cleaning, slippery-based self-cleaning, and dry self-cleaning. These mechanisms of nature self-cleaning examples are popular and well-known as well as have been widely applied or exhibited potential applications in our daily life and industrial productions. The mathematical and numerical modelling of the identified self-cleaning mechanisms will be carefully introduced, which will contribute to the rational design and reproducible construction of these functional self-cleaning surfaces. Finally, we will discuss how these materials can be produced, with a focus on scalable manufacturing. We hope this review will strengthen the understanding on nature-inspired self-cleaning surfaces and stimulate interdisciplinary collaboration of material science, biology and engineering.

Directed motion of an impinging water droplet—seesaw effect

This study discovered a “seesaw effect” in the underlying physics of the directional bouncing behavior of a droplet on superhydrophobic heterogeneous surfaces, leading to projectile motion.

Photo-switching of surface wettability on micropatterned photopolymers for fast transport of water droplets over a long-distance

Long distance movement (&gt;20 mm) of water droplets across thiol–acrylate photopolymers with inscribed wettability and Laplace pressure gradient is demonstrated.

Light-driven motion of water droplets with directional control on nanostructured surfaces.

Discrete droplet transport has drawn much interest in a broad range of applications. Controlling the motion direction in droplet transport, however, is a long-lasting challenge. In this work, a simple yet efficient approach is demonstrated to realize the motion of droplets with directional control on nanostructured surfaces with predefined channels. Light is used as the external stimulus to induce the uneven thermal expansion of the substrate, which leads to the tilting of nanostructured channels so that the droplet is driven to move along the channel. Due to the easy manipulation of light, including both the light position and power density, this study demonstrates the controllable entrance of static water droplets into targeted channels and the simultaneous control of the motion of multiple droplets in multi-channel systems, using just one light source. Besides static droplets, this approach can also be applied for the directional control of moving droplets in multi-channel systems. As a proof-of-concept, such an approach has been utilized for efficient multiplexed reactions for chemical sensing or microreactor applications. This work offers an alternative approach for the manipulation of droplet movement in applications that involve the control of droplet motion.

The light-driven macroscopic directional motion of a water droplet on an azobenzene-calix[4]arene modified surface.

A novel photo-responsive surface was constructed via modifying azobenzene-calix[4]arene (ABC4) on a microstructured silicon surface. Asymmetric UV light irradiation could drive the macroscopic directional motion of a water droplet on this photo-responsive surface.

Finger directed surface charges for local droplet motion.

Water droplets are expected to be employed as animated soft matter to mimic the behaviours of both nonliving objects and small living organisms. Local water droplet motion has attracted considerable interest and has expanded into various application areas because of its close relationship with processes associated with life. However, few approaches have been capable of independently manipulating local droplet motion without loss on a substrate due to the difficulty in shaping and focusing the motion route. Here, we demonstrate a non-contact electrostatic-powered local water motion strategy. The gradient of electrostatic charges in space guides the local drop motion without liquid loss in a controlled motion path. The local droplet motion on surfaces with varied wettabilities is discussed and compared. A unipolar electrostatic field is theoretically simulated. This work can introduce a finger-directed surface charge pattern and local droplet motion as a new variable in many droplet robot schemes and inspire next-generation liquid devices.