Spreading Of Water Droplets Research Articles

Optimizing raindrop energy harvesting: Exploring water droplet spreading effects on IDE-based TENG for sustainable power generation

Harvesting clean energy from water has emerged as a promising approach to tackle sustainability challenges. Triboelectric nanogenerators (TENG) offer exceptional advantages, such as low-cost, low-frequency driving, and high output, making them a promising avenue for capturing energy from water droplets. However, there remains a dearth of research on the impact of water droplet spreading area on TENG electrical output and the underlying energy conversion mechanisms. Through a series of comprehensive experiments, we identified the optimal parameters associated with water droplet spreading area and validated the reliability of the experimental results through multi-physics field coupling. The findings underscore a direct correlation between the spreading area of water droplets and the electrical output of interdigital electrode-based triboelectric nanogenerators (IDE-based TENG). Moreover, the introduction of parameters characterizing droplet motion state and their influence on electrical output unveils the intricate energy conversion mechanisms at playing when water droplets interact with the surface of IDE-based TENG. The practical application of this research materialized in a raindrop energy harvesting device, successfully illuminating approximately 80 LED bulbs and powering a clock. This work provides a theoretical reference for analyzing the energy conversion mechanism between water droplet spreading area and TENG electrical output and can guide the design of micro-power generation devices. This work holds significant promise in advancing clean energy technologies and addressing the challenges of sustainable development.

Effects of heat treatment of silica-based glasses on their static and dynamic wettability

Heat treatment of silica-based glasses affects their wettability. In this study, the static and dynamic wettabilities of glass before and after heat treatment were investigated using different probe solutions to evaluate the effect of heat treatment on wettability. Commercially available silica-based glasses were subjected to heat treatment at various temperatures for 1 h. Heat treatment affected the surface roughness of the glass, which depended on the treatment temperature. The treated glass exhibited a higher surface free energy compared to the untreated glass, independent of the treatment temperature. Compared to the untreated glass, the static contact angles for water, ethylene glycol, and diiodomethane of the treated glasses decreased, and those of hexadecane increased. These results suggest the different interface formation, such as Wenzel or Cassie states, by the probe solutions. The dynamic behavior of water and ethylene glycol in the treated sample resulted in the spreading of droplets on the surface. The nanoscale surface roughness of the glass plays an important role in the spreading of water droplets on silica-based glasses. The change in the surface roughness of the silica-based glass and the selection of the probe solution affected the static and dynamic wettabilities after heat treatment.

Coupled Dynamics of Droplet Impact on a Flexible, Hydrophilic Cantilever Beam.

We experimentally study the droplet impact on a flexible, hydrophilic cantilever beam. Droplets of water, water-glycerol (1:1 v/v), and glycerol were considered on a copper beam. Side visualization of the droplet impact on the cantilever was carried out by using a high-speed camera. We systematically vary cantilever stiffness to obtain a characteristic time scale of the beam dynamics, that is, shorter on the same order and longer than the characteristic time scale of droplet dynamics. Water droplet spreading reduces with an increase in the beam's flexibility, due to the "springboard effect". Results reveal that a threshold cantilever length exists beyond which the droplet vibration mode "locks-in" to the cantilever vibration mode. During lock-in, the bending energy of the beam increases with an increasing length or decreasing stiffness. The time-varying cantilever deflection exhibits an oscillatory, exponentially decaying response. However, in the case where both time scales are almost the same, we found a two-stage damping in the measurements: a fast, initial damping followed by a slower, damped response. We explain this damped response by the interplay of droplet and cantilever early dynamics. As expected, increasing the droplet viscosity dampens the magnitude of droplet spreading and displacement of the cantilever's tip due to a larger dissipation of kinetic energy in the bulk of the droplet. The decay is exponential in all cases, and the time taken to reduce the spreading and displacement is shorter with a larger viscosity. The damping coefficient is found to inversely scale with the cantilever length or mass. We corroborated the measurements with available analytical models, confirming the hypotheses used to explain the results. Overall, the present study provides fundamental insights into controlling the coupled dynamics of droplet and flexible substrates, with potential applications such as the design of efficient agricultural sprays and wings of microaerial vehicles.

Superwetting metal-organic frameworks optimized interface for high-performance hydrogen-water isotopic exchange for tritiated water treatment

A metal-organic frameworks (MOFs) modified interface with superhydrophilic and superaerophobic properties was successfully prepared via the solution process for tritiated nuclear wastewater (HTO) treatment. The superwetting performance of the packing surface which facilitated the liquid phase catalytic exchange was discussed in detail for the hydrogen-water isotope exchange reaction. Superhydrophilic property originated from the microporous hydrophilic nature of MOFs resulting in a rapid spreading and phase-transfer process of the tritiated water droplet, and superaerophobic property accompanied by superhydrophilic water thin film favored the effective exchange between gaseous reactants. Computational fluid dynamic (CFD) simulations revealed the influence of contact angle and liquid flow rate on the gas-liquid two-phase interface and catalytic performance. High performance with the lowest height equivalent to a theoretical plate (HETP) and long-term stability is achieved for this superwetting packing, which holds promise for the detritiation of nuclear wastewater with higher performance and stability.

Computational and Analytical Investigation of Droplet Impingement and Spreading Dynamics around the Right Circular Cone.

Numerical computations are performed to elucidate the water droplet impingement and spreading dynamics around a small right-angled circular cone suspended in the air. An axisymmetric model employing the volume of fluid approach describes the engrossing impact, spreading, and detachment behavior of droplets around the solid substrate. Influence of various dimensionless pertinent factors, like Weber number (We), contact angle (θ), Ohnesorge number (Oh), Bond number (Bo), and cone base-to-droplet diameter ratio on maximum deformation factor (βf) is demonstrated thoroughly to understand droplets' hydrodynamic and morphological behavior. An increase in We shortens the droplet's interaction duration with the substrate for a particular value of θ, Oh, and Dc/Do. Moreover, the interaction time reduces drastically with the increase of Oh when θ, We, and Dc/Do remain constant. Moreover, correlations are developed for both free (We = 0) and forced (We ≠ 0) falling of the droplet to determine the deformation factor as a function of various relevant dimensionless parameters, which operates satisfactorily within 0.8% of the computational data. Lastly, the maximum deformation factor for the droplet is calculated analytically, and it demonstrates an extremely good matching with simulated data.

A new preparation scheme for the wettability-patterned copper surfaces and its applicability

Droplet control technology has broad application prospects and the wettability-patterned surface is a popular way to realize it. However, the existing preparation methods are either not suitable for copper or not suitable for its batch fabrication. In this paper, three specific preparation schemes with different effects are proposed for the wettability-patterned copper surfaces. Then the influences of preparation conditions on the contact angle are investigated. Using the selected preparation conditions, the effects among different preparation schemes are compared and the wettability-patterned copper surface prepared following preparation scheme 3 shows the best effect. Subsequently, according to different design purposes, the motions and spreading of water droplets on wettability-patterned copper surfaces with four new patterns are observed. It indicates that the hydrophilicity/hydrophobicity and directional liquid transport functions basically reach the pre-designed effects, which verifies the applicability of preparation scheme 3 on the wettability-patterned copper surfaces with different patterns for droplet control. Finally, compared with the existing preparation schemes, only the new preparation scheme proposed in this paper is conducted under room temperature and it does not require any special equipment. What’s more, its preparation time is the shortest (18 min). Hence, it is suitable for batch fabrication, which can extend the applications of wettability-patterned copper surfaces.

Experimental study on freezing characteristics of water droplets on cold surfaces

The freezing of supercooled water droplets (SCWD) on the aircraft surface is the cause of aircraft icing, which seriously affects the safe flight of the aircraft. In order to explore safe, efficient and energy-saving anti-icing method, this paper conducts experimental study on the impact freezing characteristics (heat and mass transfer) of water droplets on cold surfaces with different materials (hydrophilic/superhydrophobic surface). The effects of water droplet impact velocity and different surface temperature on water droplet morphology evolution, water droplet spreading factor (WDSF), water droplet height factor (WDHF) and freezing time are analyzed. It can be found that the freezing characteristics of water droplets impacting different materials are different. When water droplets impact the hydrophilic surface at the same temperature, the MSF of water droplets increases significantly with the increase of We number, and the oscillation time of water droplets decreases, and the amplitude decreases gradually. And the surface temperature is less than -10 °C, the water droplet spreading time is less than 10 ms. Then, when the superhydrophobic surface temperature is 10 and 20 °C, the water droplets bounce many times; the surface temperature is 0 and -5 °C, the water droplets bounce only once and then adhere to the surface for continuous oscillation and the surface temperature is less than -10 °C, water droplets cannot effectively bounce off the surface. Meanwhile, singular jet phenomena are observed in the experiment. Additionally, with the decrease of surface temperature, the freezing rate of water droplet decreases. Compared with the hydrophilic surface, the decrease of water droplet freezing rate is more pronounced with the decrease of superhydrophobic surface temperature. Finally, the superhydrophobic surface temperature is low (such as Ts = -20 or -25 °C), its hydrophobic ability decreases.

Fabrication of superhydrophilic self-cleaning SiO2–TiO2 coating and its photocatalytic performance

Generally, superhydrophilic self-cleaning coatings are prepared from semiconductors with photocatalytic properties. Organic pollutants attached to the coating surface can be degraded by its photocatalytic performance realizing a self-cleaning goal. Herein, SiO2–TiO2 composite particles were fabricated by the hydrolysis and precipitation of TiOSO4, and SiO2 microspheres were chosen as carriers, which are inexpensive and environmentally friendly. Then, superhydrophilic self-cleaning SiO2–TiO2 coatings were fabricated by spraying the composites on the surfaces of substrates. The morphology, structure and self-cleaning performance of the SiO2–TiO2 coating were characterized and tested. The results revealed that nano-TiO2 was loaded on the surfaces of SiO2 microspheres uniformly forming a hierarchical micro/nanostructure. The SiO2–TiO2 composite particles exhibited excellent photocatalytic degradation performance, and the degradation rate of methyl orange (10 ppm) was more than 98% under UV irradiation for 40 min. Furthermore, the coating prepared with the SiO2–TiO2 composite particles exhibited superhydrophilicity. A water droplet spreads completely on the coating surface in 0.35 s, and the contact angle reaches 0°. In addition, rhodamine B (RhB) and methylene blue (MB) on the coating surface can be degraded efficiently under sunlight irradiation. The SiO2–TiO2 composite particles can be sprayed directly on the surfaces of concrete, brick, wood, and glass slides. Therefore, the particles showed good adaptability to different substrates. The superhydrophilic property was due to the hydrophilicity of SiO2 and TiO2, the hierarchical micro/nanostructure of the SiO2–TiO2 composites, and the photoinduced superhydrophilicity of TiO2. The above experimental results show that the as-prepared superhydrophilic self-cleaning SiO2–TiO2 coating has a large application potential.

AI predicts water droplet spreading dynamics and icing patterns on cold surfaces

The techniques could lead to improvements in aircraft anti-icing, spray-freeze drying, and influenza vaccine storage.

Influence of conditions of liquid dosing on water droplet spreading

The work presents the experimental studies of the influence of surface roughness on the dynamic characteristics of distilled water droplet spreading. As solid surfaces, copper samples treated with abrasive materials (average grain size of 10–100 μm) and aluminum samples polished to a mirror finish were used. The advancing dynamic contact angles obtained using DSA technique were found. The effect of the droplet formation rate and surface roughness on the characteristics of water droplet spreading, in particular, dynamic contact angle, was analyzed.

Impingement dynamics of droplets on mildly heated walls at initial and later stages

The impingement dynamics of water droplets on a heated wall at initial and later stages are experimentally investigated. First, the effects of the wall temperature and the Weber number on the water droplet spreading characteristics are considered. A constant contact radius evaporation mode is observed during most of the evaporation. The wall temperature has little influence on the spreading characteristics at the initial stages. The Weber number greatly influences the spreading characteristics, such as the spreading dynamic behavior, maximum spreading time, spreading height, diameter, and contact angle, at the initial stages. At the later stages of spreading, the heating temperature has a relatively greater impact on the rate of linear change of the residual volume, whereas the impact of the Weber number on that is relatively small. Subsequently, the effect of evaporation on the impingement dynamics is investigated. The spreading characteristics of the water droplet are compared with those of an ionic liquid droplet (which does not experience evaporation), whereby the spreading factor increases and the volume remains unchanged with the increasing contact time in the absence of the evaporation effect.

Spreading and freezing of supercooled water droplets impacting an ice surface

The present study conducts supercooled droplet impact experiments on flat ice surfaces with varied impact velocities, initial diameters, droplet supercoolings, and ice surface temperatures. Droplet spreading at room temperature can be described by equating the inertial force to capillary and viscous forces. A universal scaling on maximum spreading can be obtained by interpolating between the laws in the capillary and viscous regimes where the dominant forces are capillary and viscous, respectively. During supercooled droplet spreading on ice surfaces, dendritic solidification occurs, forming a moving ice–water interface and leading to new droplet spreading mechanisms. The maximum spreading factor is sensitive to droplet supercooling, whereas surface temperature has a minor influence on spreading. A new dimensionless number, the icing number, is proposed to scale icing effects on a flow field, refining the law in the viscous regime for supercooled droplets. Nevertheless, icing does not affect the scaling in the capillary regime. The total influence of icing is related to the magnitude of the icing number and the competition between capillary and viscous forces. The icing number can then form a simple criterion for determining whether icing is negligible. The newly derived scaling performs well when modeling the present experimental results.

Water droplet rapid spreading transport on femtosecond laser-treated photothermal and superhydrophilic surface

Rapid water droplet spreading transport on responsive surfaces has attracted much recent interest. Most current methods for transporting water droplets rely on external electric or magnetic fields, which limits their practical application. Here, a photothermal and superhydrophilic stainless steel surface is prepared by femtosecond laser direct writing technology. By virtue of the synergistic functions of the light absorption and superhydrophilicity, water droplet rapid spreading transportation can be achieved on the as-prepared surface by simply exposing to light. Moreover, the relationship between water droplet spreading transportation time and cycles, droplet volumes, temperatures were systemically investigated, respectively. And the spreading transportation mechanism is discussed. This finding offers a simple method for fabricating a light-responsive interface material for rapid water droplet spreading transportation, which would be useful in microfluidics and water treatment.

Effect of wettability on droplet impact: Spreading and splashing

This study experimentally investigates the dynamic behaviors of water droplets over wide ranges of diameters and velocities (200 μm < D0 < 2600 μm, 50 < We < 3000) impacting on surfaces with various wettability (ranging from hydrophilic to superhydrophobic), and focuses on the effect of surface wettability on water droplet spreading and splashing. The experimental results show that the surface wettability contributes to the lift of the lamella, related to the cosine of the advancing angle (cosθa), that significantly influence the spreading and splashing. The lifting lamella promotes the spreading speed by reducing the surface friction and the viscous dissipation in the boundary layer. The mean spreading speed is linear to the impact velocity on each surface type, and the contact angle slightly affects the mean spreading velocity on the hydrophilic or hydrophobic surface. While, the mean spreading velocity on the superhydrophobic surface is much larger than the other surfaces due to the fewer viscous dissipation. The splashing behavior is dominated by the force from the air surrounding the lamella. The experimental results demonstrate that the splashing threshold K = OhRe1.25 is independent of the advancing angle for the case of θa < 90°, but is proportional to cosθa for the case θa > 90°. Furthermore, we put forward a prediction model of droplet splashing threshold, considering the effect of surface wettability, as K/K0 = 1 + αcosθa, which could quantitatively express the effect of surface wettability on water droplet splashing.

Spreading of Water Droplets on Cellulose-Based Papers: the Effect of Back-Surface Coating.

Regulation of wetting and spreading of liquid on porous material plays an important role in a variety of applications, such as waterproofing, anti-icing, antioxidation, self-cleaning, etc. In this work, we reveal the role of back-surface coating with superhydrophobic nanoparticles in controlling the spreading of water droplets on cellulose-based papers. A layer of superhydrophobic polydivinylbenzene (PDVB) nanoparticles is spin-coated on the back surface of different types of papers. The spreading of a water droplet on the top, uncoated surface is dependent on the size of the PDVB nanoparticles in the coating. Using a relationship derived from Darcy's law, we observe that the energy barrier for the spreading of water droplets on three types of papers (heavy-weight, light-weight, and slight-weight papers) decreases with the decrease of the nanoparticle size in the back-surface coating. The spreading of the water droplet is dependent on the porous structure, permeability, and compressibility of the papers. The method presented in this work provides a feasible approach to use the back-surface coating to control the wettability of papers.

Effects of surface subcooling on the spreading dynamics of an impact water droplet

Spontaneous spreading of a liquid droplet upon a cold solid surface is ubiquitous in nature as well as critical to many industrial technologies, while the mechanism of which still remains elusive. The role of surface subcooling in a water droplet spreading behavior upon impacting on a smooth silicon surface has been experimentally investigated. Under the subcooling condition of the substrate, in the low Weber number region, the non-dimensional maximum spreading diameter decreases with the surface subcooling due to a larger viscosity dissipation and higher surface tension. However, in the case of a high Weber number, the maximum spreading factor first descends and then increases with the increasing surface subcooling. This non-monotonic tendency is attributed to the competition between the increased maximum fingering length and the reduced maximum interior spreading diameter with an increase of the surface subcooling. A sufficiently large Weber number is the prerequisite for forming fingering patterns, and a high subcooling reinforces them due to the enhanced deceleration caused by a larger surface tension and viscosity. The time at maximum spreading barely changes with the impact velocity and slightly decreases with the surface subcooling depending on the droplet size. An improved correlation of the time at maximum spreading as a function of the maximum spreading factor, droplet size, impact velocity, and surface subcooling is proposed.

Wetting transition of a nanodrop on switchable hydrophilic-hydrophobic surfaces

The dynamic modulation of contact angle on switchable hydrophilic-hydrophobic surfaces is one of the important factors in designing miniaturized optical devices. A spreading and retention of a nano water droplet placed on an insulator surface covering two planar electrode layers and perpendicularly in contact with a conductive tip is studied using atomistic molecular simulation. The change in the droplet shape is initiated by modifying interaction energy between the insulator atoms and the water molecules. The contact angle analysis shows a unique dependence on the nature of the underlying substrate. Time correlation function analysis based on the calculation of variation of the height of the drop’s center of mass with time estimates the rate of relaxations for wetting-dewetting behavior subject to the condition of surface hydrophilic and hydrophobic nature. For the hydrophilic case, the relaxation rate of the drop is found to be twice slower than the hydrophilic one. The difference in the rate of relaxations is analyzed by considering the effects of the frictional force on the contact line motion of the drop. Friction coefficients are calculated on the basis of continuum hydrodynamic (HD) and molecular kinetic theory (MKT) theories. The dependence of the friction coefficients on the surface properties depicts the strong dominance of the liquid-substrate friction on the drop’s perimeter motion based on molecular kinetic theory.

Super-wetting enabled by an array of SU-8 micro-pillars etched with ion-beam

The wettability transition of an ion-beam-etched micro-pillar array surface is reported in the present study. The micro-pillar array, made of negative photoresist SU-8, was fabricated by ultraviolet lithography. The results indicate that the combination of the micro-pillar array and ion-beam etching technique can easily regulate surface wettability, which can be changed from hydrophobicity to super-hydrophilicity. Moreover, the super-hydrophilic property of the textured surface can be significantly affected by the etching time and etching energy, because the ion-beam etching could alter the shape and height of the micro-structure as well as the chemical composition of the surface. When a 2 µl water droplet spreads on the etched micro-pillar array surface, the contact angle (CA) decreases to almost 0° within 144 ms, indicating ultrafast spreading of the water droplet on the textured surface. It was found that the CA of the fabricated surface remained the same after two weeks’ exposure in the air, showing undependable wettability stability of the textured surface. The results of the present study can open a gate for potential applications of proposed super-wetting surfaces for micro-fluidics and anti-counterfeiting technologies.

Spreading Dynamics of Water Droplets on a Completely Wetting Surface

The spreading of water droplets of varying sizes on a completely wetting surface where the kinetics of spreading are controlled by hydrogen bonding between substrate and water molecules is modeled ...

Directional liquid spreading on laser textured aluminum surface

The wettability property of a material surface is highly dependent on the topography of the surface micro/nanostructure. Femtosecond (fs) laser can be used to fabricate various kinds of surface micro/nanostructures. We present parallel microgroove arrays with unique cross-sectional profiles on the surface of an aluminum foil induced by varying the laser fluence and the scanning spacing. The laser textured aluminum surface shows directional spreading of water and inherently hydrophilic aluminum surface becomes superhydrophilic. A water droplet spreads highly anisotropically on the processed area and flows preferentially along the microgrooves. The maximum average spreading velocity of water droplet along the laser textured microgrooves is ~ 200 mm/s, while it is much slower in the direction perpendicular to the microgrooves, which is 41 mm/s. Moreover, the spreading distance of water and time t in both directions have a linear relation with t1/2. The fast-self-spreading motion of water along the microgroove is due to its capillary pressure and a local energy barrier at the boundary between two parallel microgrooves. In addition, laser-modified surface roughness and laser-altered chemical composition on the aluminum surface play a role in enhancing the superhydrophilicity and the directional water spreading behavior. Our research can be applied in the fields of microfluidics, lab-on-chip technology, chemical and biological sensors and heat transfer devices.