Spreading Of Liquid Film Research Articles

Enhanced propagation of free films with fast spread-out phenomena under the influence of megahertz surface acoustic waves

In this study, a novel approach for enhancing the rapid spreading of free liquid film on lithium niobate (LiNbO3) substrate under megahertz (MHz) surface acoustic wave (SAW) excitation is presented by treating it with surfactants. Through the design of a specific interdigital transducer structure, it was discovered that exciting the SAW at a frequency of 32.3 MHz can achieve optimal spreading performance for water droplets on the surface of surfactant-treated LiNbO3 substrate. The maximum average velocity reaches 1.76 mm/s at position P2 = 1250 μm in the water film front, and the stable film spreading speed shows a 204.9% increase compared to the existing research. Simultaneously, through the investigation of the spreading experiment phenomenon of silicone oil and de-ionized water droplets at varying frequencies, we have discovered the dynamic mechanism of “reverse phase” propagation in liquid film for the first time. This entails that the advancing edge of the wetting film demonstrates a spreading motion law that is opposite to the traditional spreading phenomena, with the spreading velocity in the central exceeding that on both sides. Our research demonstrates that this microfluidic device developed by SAWs enhances the spreading efficiency of the free films, enabling rapid expansion of the target liquid to form a high-surface area film layer. This advancement holds promise for overcoming the limitations of low sensitivity and short response time in the field of rapid pathological diagnosis in contemporary medicine.

Aerodynamically Levitated Droplets as Small-Scale Chemical Reactors and Liquid Microprinters.

A thin liquid film spread over the inner surface of a rapidly rotating vial creates an aerodynamic cushion on which one or multiple droplets of various liquids can levitate stably for days or even weeks. These levitating droplets can serve as wall-less ("airware") chemical reactors that can be merged without touching-by remote impulses-to initiate reactions or sequences of reactions at scales down to hundreds of nanomoles. Moreover, under external electric fields, the droplets can act as the world's smallest chemical printers, shedding regular trains of pL or even fL microdrops. In one modality, the levitating droplets operate as completely wireless aliquoting/titrating systems delivering pg quantities of reagents into the liquid in the rotating vial; in another modality, they print microdroplet arrays onto target surfaces. The "airware", levitated reactors are inexpensive to set up, remarkably stable to external disturbances and, for printing applications, require operating voltages much lower than in electrospray, electrowetting, or ink jet systems.

Aerodynamically Levitated Droplets as Small‐Scale Chemical Reactors and Liquid Microprinters

AbstractA thin liquid film spread over the inner surface of a rapidly rotating vial creates an aerodynamic cushion on which one or multiple droplets of various liquids can levitate stably for days or even weeks. These levitating droplets can serve as wall‐less (“airware”) chemical reactors that can be merged without touching—by remote impulses—to initiate reactions or sequences of reactions at scales down to hundreds of nanomoles. Moreover, under external electric fields, the droplets can act as the world's smallest chemical printers, shedding regular trains of pL or even fL microdrops. In one modality, the levitating droplets operate as completely wireless aliquoting/titrating systems delivering pg quantities of reagents into the liquid in the rotating vial; in another modality, they print microdroplet arrays onto target surfaces. The “airware”, levitated reactors are inexpensive to set up, remarkably stable to external disturbances and, for printing applications, require operating voltages much lower than in electrospray, electrowetting, or ink jet systems.

Molecular Dynamics Simulation of Dual Nanodroplet Impacts on a Cylindrical Surface.

Droplet impact behavior is ubiquitous in various fields. However, the dynamics and spreading mechanisms of micro- and nanoscale droplet impact on curved surfaces, particularly in the case of multiple droplets, have yet to be fully elucidated. In this study, molecular dynamics (MD) methods are employed to investigate the dynamic evolution of double nanodroplet impact on a nano cylindrical wall. The effects of droplet spacing, initial impact velocity, and wall wettability on droplet impact characteristics are analyzed. The results demonstrate that there are five impact modes of nanoscale double-droplet impacts with nanocylinders: spreading-partial wrapping-splitting-complete detachment (SPSC), spreading-complete wrapping-complete attachment (SCC), spreading-partial wrapping-complete attachment (SPC), spreading-partial wrapping-partial attachment (SPP), and spreading-partial wrapping-fragmentation-partial attachment (SPFP). The droplet spacing has an insignificant effect on the impact modes but affects the droplets' spreading shape in both the axial and radial directions. The initial velocity and wall wettability have significant impacts on the droplet impact modes and liquid film spreading characteristics. As the initial velocity increases, the liquid film's radial and axial spreading distances gradually increase. Under hydrophobic conditions, the spreading of the droplet is dominant in the radial direction, while under hydrophilic conditions, the spreading is dominant in the axial direction. Properly reducing the droplet spacing, increasing the impact velocity, and enhancing the wall hydrophobicity can promote detaching the droplet from the cylindrical wall.

Experimental and model investigation of spreading characteristics of the liquid sheet formed by jet impinging on a circular plane

The focus of this paper is on the spreading characteristics of the liquid sheet formed by jet impinging on a circular plane. The liquid sheet is studied in two parts: the liquid film that spreads radially on the circular plane, and the free liquid sheet after leaving the circular plane, the former determines the initial thickness and velocity of the latter. Experimental results show that, for the liquid film on the circular plane part, as the jet velocity increases, the liquid film thickness increases due to the film velocity loss caused by the inelastic collision. The liquid film thickness is positively correlated with the jet radius and negatively correlated with the circular plane radius. Liquid viscosity slows the spreading of liquid film, forms the boundary layer, and thickens the liquid film. The surface tension inhibits the velocity loss due to the inelastic collision. For the free liquid sheet part, surface tension and gravity promote the bending of the liquid sheet. The larger the initial momentum of the liquid sheet leaving the circular plane, the larger the maximum spreading range of free liquid sheet reaching the equator. As for the theoretical model, a film velocity correction coefficient reflecting collision loss is introduced to improve the radial spreading theory of the liquid film on a solid surface, and a streamline function is assumed to solve the momentum integral equation of free liquid sheet. The predicted streamline agrees well with the experimental data.

Operating mechanism of pulsating heat pipe with different wettability from the perspective of thermo-hydrodynamic characteristics of vapor–liquid interface region

Pulsating heat pipe (PHP) is considered to be one of the most promising efficient heat sinks in electronic thermal management for good cooling capacity and compact structure. Understanding the complex operating mechanism of PHP from various angles is crucial for advancing its large-scale application. Among various factors, the thermo-hydrodynamic characteristics of the vapor–liquid interface region have a critical impact on the operation of PHP. Based on this, the transient numerical simulations were conducted in this work on PHPs with 20° and 150° contact angles (hydrophilic and hydrophobic, respectively), and the influence of different thermo-hydrodynamic characteristics of the vapor–liquid interface region on the operating mechanism of PHPs was analyzed. The results found that the presence of a widely spreading liquid film at the interface of hydrophilic PHP significantly changed its thermo-hydrodynamic characteristics compared to hydrophobic PHP. As a consequence, the hydrophilic PHP and hydrophobic PHP presented better heat transfer performance in the evaporation section and condensation section, respectively. Under low heat inputs, with regular vapor–liquid distribution in the interface region, the hydrophobic PHP presented better startup and heat transfer performance due to better hydrodynamic behavior caused by more easily formed pressure gradients. Under high heat inputs, the hydrophilic PHP showed significantly better heat transfer performance due to the intense evaporation and continuous spreading of new liquid film in the interface region, which also played a key role in stronger anti-dry-out performance.

Wetting and spreading of bulk liquid and precursor film of molten AgCuTi on ultrafast laser structured surface of Ti

In order to reveal the influence of special surface structures on the wetting and spreading of Ag/Ti system and the precursor film, micro-bumps, micro-pits, micro-grooves and micro-thread surface structures were machined on a grade 2 commercially-pure Ti (TA2) surface by using ultrafast laser. In situ wetting experiments of AgCuTi filler metal on TA2 surfaces with different special structures were conducted. The results showed that the wetting morphology of AgCuTi over original TA2 surface is special double ring (small circle inside and ring outside). The special surface structures can effectively change the wetting characteristics of Ag/Ti system, including the spreading rate and the final spreading area. It also affects the structure and composition of the wetting interface. In Ag/Ti system, different components are affected to different degrees by the surface structures, resulting in uneven distribution of the bulk liquid region and the precursor film region of the wetting interface. The micro-bumps and micro-pits structures can effectively promote the wetting and spreading of AgCuTi on TA2, while micro-thread structures can inhibit its wetting spread. For the micro-grooves, the AgCuTi spreads rapidly and almost completely along the direction of the groove, while in the vertical direction, it is difficult for the AgCuTi to spread through the groove. By studying the wetting spreading process and interface behavior of molten filler metal on complex surfaces, it is expected to provide important guiding significance for using special surface microstructure to control the wetting spreading process and interface reaction of brazing system.

An experimental and simulation study on the velocity distribution of the falling film on a horizontal tube in jet mode

The velocity characteristics of falling film on horizontal tube affect the heat transfer efficiency. To date, experimental studies on velocity distribution of liquid film on tube surface are very scarce. In this paper, the mean velocity of liquid film is measured using particle tracking velocimetry (PTV). A simulation is also conducted to investigate the flow characteristics of falling film. Results show that the spreading of liquid film undergoes two stages: circular spreading and circumferential flow. In circumferential direction, the liquid film tangential velocity shows different law in planes of L* = 0–0.3 and L* = 0.4/0.5. The tangential velocity decreases and then increases as axial distance increases. Simulation results agree well with the experimental results within experimental angle range. The liquid film axial velocity is always zero at L* = 0. When L* = 0.1–0.5, the axial velocity decreases as circumferential angle increases and gradually tends toward zero.

Spreading and splashing of liquid film on vertical hot surface by inclined jet impingement

In this paper, the liquid film formed by inclined jet impingement on the vertical hot wall is investigated by a high-speed camera. The effects of the jet velocity (5.5 m/s ≤ u0 ≤ 14.6 m/s, 3804 ≤ Re ≤ 10098) and initial wall temperature (25 ℃≤T0 ≤ 250 ℃) on the spreading and splashing of the liquid film are explored. Although the jet velocity and initial wall temperature have little effect on the spreading velocity of the liquid film, the wetting front position and liquid film area increase. For T0 ≥ 150 ℃, a unique liquid film deflection phenomenon is observed due to the boiling of the liquid film, which results in a substantial amount of splashing. Based on the characteristics of the deflection splashing, three modes are categorized, i.e., annular splashing, banded splashing, and non-splashing. The splashing of the liquid film on the hot surface is owing to the rupture of boiling bubbles, the breakup of surface waves, and the liquid splashing in the boiling zone. The splashing rate of the liquid film is measured, and it is found that an increase in T0 improves the splashing rate significantly, and the splashing rate even exceeds 70 % at T0 = 250 °C.

Numerical Simulation of Liquid Film Characteristics during Atomization of Aluminum Alloy Powder

The process of atomizing aluminum alloy powder using a rotating disk was studied by numerical simulation and experimental verification. The motion characteristics of the molten metal thin liquid film and the evolution law of atomization into droplets were systematically studied with different disk shapes and speeds. The results showed that the slippage of the liquid film on the surface of the spherical disk was smaller, the liquid film spread more evenly, and the velocity distribution was more uniform. Under the same working condition, the boundary diameter of the continuous liquid film on the spherical disk was 21–29% larger, and the maximum liquid film velocity increased by approximately 19%. In other words, the liquid film obtained more energy at the same rotational speed, the energy utilization rate was higher, and the liquid filaments produced by the splitting region of the disk surface were finer and greater in number. The data showed that the average thickness of the liquid film on the surfaces of different disk shapes was more affected by the speed of the flat disk, and the thickness on the spherical disk was relatively stable and uniform, but the difference in thickness between the two disk shapes decreased from 4.2 μm to 0.3 μm when the speed increased from 10,000 rpm to 60,000 rpm. In particular, the influence of the disk shape on the liquid film thickness became smaller when the speed increased to a certain range. At the same time, the characteristics of the liquid film during the spreading movement of molten metal on the disk and the mechanisms of the primary and secondary breakage of the liquid film were obtained through this simulation study.

Axial and azimuthal development of disturbance waves in annular flow in a horizontal pipe

Transformation of gas-liquid flow in a horizontal pipe is investigated during the transition from stratified to annular flow pattern. Using Brightness-Based Laser-Induced Fluorescence technique, spatiotemporal evolution of liquid film thickness is analyzed over the downstream distance range of about 900 mm (45 pipe diameters), starting from the inlet. The measurements are carried out for three values of azimuthal angle θ: 0 (pipe bottom), 90°, and 180°, to track the circumferential spreading of liquid film and disturbance waves. At large gas velocities, thin liquid film is dragged upwards before the formation of large waves. The disturbance waves are created at the pipe bottom and spread circumferentially as they propagate downstream, over the already-wetted pipe walls. At large enough liquid flow rates, the spreading disturbance waves reach the pipe ceiling and form full rings around the circumference. The frequency and velocity of the disturbance waves eventually become the same around the pipe circumference; the disturbance wave amplitude and the base film thickness decrease with θ. The base film is more uniform around the circumference compared to the wave amplitude. At lower liquid flow rates, the disturbance waves cover only a part of pipe circumference. Their edges demonstrate oscillatory circumferential spreading, which ends by deceleration and decay of the edges. The upper part of the pipe may be wetted by a thin base film layer, covered only with ripples, or remain dry. At low gas speeds and large liquid flow rates, occasional splashing of large waves over the pipe ceiling without pre-wetting by the thin film is possible; however, the remaining film drains downward and no stable annular film is maintained. Liquid droplets, entrained from the pipe bottom and depositing in the upper parts of the pipe, are gathered in trains of creeping pendant droplets at the very top part of the pipe; no continuous wetting is achieved due to droplet deposition.

High-Efficient Microdroplet Harvesting and Detaching Inspired from Sarracenia Lid Trichome.

Fog harvesting plays a pivotal role in harnessing atmospheric water resources and holds significant promise for alleviating global water scarcity. Nonetheless, enhancing harvesting efficiency remains a persistent challenge, especially concerning the rapid detachment of droplets from surfaces. In this study, we discovered that the trichomes of Sarracenia not only efficiently harvest and transport liquid but also quickly drain harvested liquid. We have elucidated the augmentation mechanism behind effective fog harvesting and drainage within the lid of Sarracenia. The trichomes facing the counterflow can enhance fog harvesting efficiency by 80% through air-flow-assisted spreading of liquid film. The wedge corner generated by the interface between hydrophilic and hydrophobic surfaces, coupled with the reduction of cross-sectional angles, diminishes the adhesive force of liquid droplets, fosters droplet spheroidization, and substantially facilitates droplet detachment. In addition, the quantitative detachment of droplets can be achieved by adjusting the cross-sectional angle and wetting gradient. This integrated structure combining efficient condensation and detachment has diverse applications in cooling towers and seawater desalination.

Mesoscopic Model for Disjoining Pressure Effects in Nanoscale Thin Liquid Films and Evaporating Extended Meniscuses.

Disjoining pressure effect is the key to describe contact line dynamics, micro/nanoscale liquid-vapor phase change heat transfer, and liquid transport in nanopores. In this paper, by combining a mesoscopic approach for nanoscale liquid-vapor interfacial transport and a mean-field approximation of the long-range solid-fluid molecular interaction, a mesoscopic model for the disjoining pressure effect in nanoscale thin liquid films is proposed. The capability of this model to delineate the disjoining pressure effect is validated. We demonstrate that the Hamaker constant determined from our model agrees very well with molecular dynamics (MD) simulation and that the transient evaporation/condensation mass flux predicted by this mesoscopic model is also consistent with the kinetic theory. Using this model, we investigate the characteristics of the evaporating extended meniscus in a nanochannel. The nonevaporating film region, the evaporating thin-film region, and the intrinsic meniscus region are successfully captured by our model. Our results suggest that the apparent contact angle and thickness of the nonevaporating liquid film are self-tuned according to the evaporation rate, and a higher evaporation rate results a in larger apparent contact angle and thinner nonevaporating liquid film. We also show that disjoining pressure plays a dominant role in the nonevaporating film region and suppresses the evaporation in this region, while capillary pressure dominates the intrinsic meniscus region. Strong evaporation takes place in the thin-film region, and both the disjoining pressure and capillary pressure contribute to the total pressure difference that delivers the liquid from the intrinsic meniscus region to the evaporating thin-film region, compensating for the liquid mass loss due to strong evaporation. Our work provides a new avenue for investigating thin liquid film spreading, liquid transport in nanopores, and microscopic liquid-vapor phase change heat/mass transfer mechanisms near the three-phase contact line region.

Adding Konjac Glucomannan for Enhancing the Whole Spraying Performance on Superhydrophobic and Hydrophilic Surfaces.

Spraying is a common way to coat solutions onto surfaces evenly. Improving spraying effectiveness can avoid wasting solutions and reduce pollution. In this study, a trace amount of natural polysaccharide, konjac glucomannan (KGM), was added into solutions to regulate the spraying performances including the breakup of liquid jets, size of produced droplets, and collision and spreading of droplets on both superhydrophobic and hydrophilic surfaces. The shear viscosity, extensive viscosity, and surface tension of the KGM solutions were tested. The results of spraying experiments showed that adding KGM inhibited the liquid jet from breaking into small droplets, avoided the breakage of droplets on superhydrophobic surfaces, and promoted the spreading of liquid films on hydrophilic surfaces. The numerical simulation showed the stretching of single macromolecules and quantified the energy stored in molecular chains in a shear-dominated flow field during the spreading of droplets on surfaces and an elongational-dominated flow field during the breakage of a liquid bridge. The storage and dissipation of energy during the stretching and relaxing of KMG macromolecules were important origins of the increase in the colloid viscosity and molecular mechanisms of the effect of the KGM additive on spraying performances. This study provided an understanding and a strategy for optimization and application of spraying additives.

Spreading Characteristics of Volatile Liquid Film on the Liquid-solid Substrates

The spreading characteristics of volatile liquid film on the liquid-solid substrate have been experimentally investigated. Unlike the liquid film spreading on the liquid substrate, there exists a dip at the top of the solid substrate, as a result, the spreading behaviors are inhibited. It is found that dip characteristics of R’/ R slightly decrease at the beginning then it keeps constant R’/ R=0.83 until the end. Moreover, the comparison of spreading behaviors on two different substrates is investigated. When the volatile film spreads on the liquid-solid substrate, the maximum radii of the outer and inner rings are smaller, but the process lasts longer. Although the evolutions of radii are different, the variation trend of nondimensional ring width is consistent. We further study the spreading rate; results show there exist three stages due to the relative importance of different forces. Our results provide important missing pieces to the rich mechanisms of the multiple-phase flow driven by the Marangoni effect.

3D Solar Evaporation Enhancement by Superhydrophilic Copper Foam Inverted Cone and Graphene Oxide Functionalization Synergistic Cooperation.

Solar evaporation has become a promising and sustainable technique for harvesting freshwater from seawater and wastewater. However, the applicability and efficacy of solar evaporation need further improvement to achieve high production closer to theoretical limits in compact systems. A 3D (three-dimensional) hierarchical inverted conical solar evaporation is developed, which consists of a 3D copper foam skeleton cone decorated with micro-/nano-structures functionalized with graphene oxide, fabricated via easy and scalable wet oxidation, impregnation, and drying at room temperature. The proposed configuration empowers high-efficiency solar absorption, continuous liquid film spreading and transport, enhanced interfacial local evaporation, and rapid vapor diffusion through the pores. More notably, the 3D conical evaporator realizes thermal localization at the skeleton interface and allows evaporation to occur along the complete structure with unimpeded liquid and vapor rapid diffusion. The solar-thermal evaporation efficiency under 1-Sun is as high as 93% with a maximum evaporation rate per unit area of 1.71 kg·m-2 ·h-1 . This work highlights the benefits of synergistic cooperation of an easily scalable 3D hierarchical functiomicro-/nano-structured copper foam skeletons and functionalized graphene oxide for high-efficient solar evaporation of interest to numerous applications.

Capillary-driven horseshoe vortex forming around a micro-pillar

Hypothesis: Horseshoe vortices are known to emerge around large-scale obstacles, such as bridge pillars, due to an inertia-driven adverse pressure gradient forming on the upstream-side of the obstacle. We contend that a similar flow structure can arise in thin-film Stokes flow around micro-obstacles, such as used in textured surfaces to improve wettability. This could be exploited to enhance mixing in microfluidic devices, typically limited to creeping-flow regimes.Experiments: Numerical simulations based on the Navier–Stokes equations are carried out to elucidate the flow structure associated with the wetting dynamics of a liquid film spreading around a 50 μm diameter micro-pillar. The employed multiphase solver, which is based on the volume of fluid method, accurately reproduces the wetting dynamics observed in current and previous (Mu et al., Langmuir, 2019) experiments.Findings: The flow structure within the liquid meniscus forming at the foot of the micro-pillar evinces a horseshoe vortex wrapping around the obstacle, notwithstanding that the Reynolds number in our system is extremely low. Here, the adverse pressure gradient driving flow reversal near the bounding wall is caused by capillarity instead of inertia. The horseshoe vortex is entangled with other vortical structures, leading to an intricate flow system with high-potential mixing capabilities.

Spreading dynamics of the viscous droplet impacting on a spherical particle

The dynamic behavior of droplets in the impingement process with particles has attracted extensive interest due to its widespread industrial applications. In this study, collision experimentation was carried out to investigate the spatial and temporal variation of droplets on a target particle surface by utilizing high-speed photography. Energy conversion and force analysis were conducted through theoretical analysis. Moreover, we captured the microscopic and evolutionary features of droplets in detail by image processing. The dimensionless liquid film thickness in the maximum spreading state is primarily determined by its spreading area. The Weber number can be used to calculate the maximum spreading area of droplets for liquids with a specific viscosity and droplet-to-particle size ratio. The time-evolved liquid film morphology and spreading area of a highly viscous liquid show a different trend compared to that of the low-viscous liquid. When the liquid film is not broken, the maximum dimensionless spreading area is linearly related to the Weber number. At low Weber numbers and high Reynolds numbers, droplets exhibit more pronounced oscillation characteristics. The oscillation period of the collision is related to the droplet-to-particle size. The liquid film thickness decreases as the Weber and Reynolds numbers rise. As for the low-viscous liquid, a low Weber number leads to a periodic change in the dynamic contact angle. A decrease in the Reynolds number for the highly viscous droplets generates a greater dynamic contact angle. The recoiling of the liquid film results in a more significant reduction in the dynamic contact angle.

Numerical study of the cross-vapor stream effect on falling film heat transfer performance over a horizontal tube

Vapor shearing can cause a falling film (FF) evaporator to exhibit various unusual phenomena. Under a cross-vapor stream, numerical simulations were conducted to examine the FF heat transfer performance outside a horizontal tube. The liquid film spreading, temperature fields, as well as time-averaged peripheral and axial, peripheral- and axial-averaged, and average heat transfer coefficients (HTCs) were presented, respectively. According to the results, we found that: (1) the cross-vapor stream produces deflections in liquid film spreading; (2) several zones of the tube surface have distinct distributions of HTCs, velocity vector fields, and liquid film thicknesses (LFT); (3) the cross-vapor causes apparent asymmetry for the axis-averaged peripheral HTC, especially at the higher vapor velocities; (4) the profiles of time-averaged peripheral and axial HTC on the upwind side are quite different from the one on the downwind side due to the cross-vapor; (5) within the current range of liquid flow rates, three trends of average HTCs can be observed with increasing vapor velocity; and (6) larger liquid flow rates can tolerate higher cross-vapor velocities to guarantee a better FF heat transfer performance.

Numerical study of inlet eccentricity on liquid film spreading and splitting in centrifugal granulation assisted thermal energy recovery

Centrifugal granulation assisted thermal energy recovery(CGATER) is a method of recovering waste heat from molten slag by centrifugal granulation on a rotating disc. Disturbance and positioning errors can cause the incoming liquid column to deviate from the center of the disc. In this study, numerical simulation was conducted to investigate the effect of inlet eccentricity on liquid film flow spreading, liquid filament formation and fragmentation, and droplet size distribution during CGATER. The study shows that the eccentric granulation results in a new splitting pattern in which the liquid filament grows to a certain length with the rotation of the disc and then breaks from the bottom; the thickness of the liquid film on the eccentric granulation disc is consistent with the radial velocity distribution of the liquid film. Finally, the general dimensionless prediction correlation of particle median diameter is established for the eccentric situation. • A simulation of centrifugal granulation under eccentric entry was carried out. • Eccentric entry creates a new mode of liquid filament splitting. • Consistent liquid film thickness and radial velocity distribution on the disc. • A dimensionless particle size prediction equation with eccentricity was proposed.