Thin Liquid Film Research Articles

Impact of a nanodroplet on liquid surfaces

The present study investigates impact of nanodroplets on liquid films with varied film thicknesses over hydrophilic to hydrophobic surfaces via molecular dynamics simulations. A new crown pattern, hollow crown, is observed on medium liquid films for nanodroplets, whereas it is not reported for millimeter-sized droplets. Holes (or hollows) are attributed to capillary vibration of a nanometer-thick liquid film. The occurrence of holes is inhibited effectively for deep liquid films because the cavity thickness is much larger than the amplitude of capillary vibration. Surface wettability has a significant influence on thin and medium liquid films for nanodroplets, whereas it only affects thin liquid films for macrodroplets because the range of solid–liquid intermolecular forces is comparable to the size of droplets when the size of droplets reduces to the nanoscale. Subsequently, the accuracies of the existing macroscale models for predicting the time-dependent crown radius of nanodroplets are tested by simulated results. The results show that the macroscale models cannot capture the time-dependent crown radius of nanodroplets. On the basis of energetics analysis, we prove that the failure of macroscale models is attributed to the distinct difference in viscous dissipation mechanisms between macroscale and nanoscale droplets.

On interaction between a bubble with evaporation and heated pillar block in microchannel

As demand for managing high heat flux in specialized applications grows, flow boiling in microchannels has received escalating attention for its high efficiency and cost-effectiveness. The complex interaction between an evaporating bubble and a heated pillar in a microchannel is governed by a confluence of transport mechanisms, including bubble morphology, fluid convection, heat transfer, and phase change phenomena. This study develops a three-dimensional mathematical model, employing the saturated-interface-volume approach to simulate the complex interaction process effectively. The results indicate that the liquid film thickness between the bubble and the heated surface is the primary factor affecting heat transfer. A reduction in the Reynolds number as well as an increase in the initial bubble diameter lead to a decrease in the liquid film thickness and an increase in the temperature gradient within the thin liquid film, which enhance both the evaporation rate and heat transfer efficiency. The temperature of the surrounding fluid is also decreased. The bubble passage disrupts the flow structure, particularly impacting the boundary layer and vortex structure. These perturbations in temperature and flow structure constitute a secondary factor influencing heat transfer. The efficiency of heat transfer varies significantly across different surfaces; surfaces with a larger thin liquid film region exhibit the most significant improvement, followed by the downstream surface where the flow and temperature fields are most affected. This study advances the fundamental comprehension of the complex interaction between an evaporating bubble and a heated pillar in a microchannel, integrating a detailed analysis of the relevant transport mechanisms.

Equilibrium contact angles and dewetting in capillaries

In this work, we extend the model of contact angles that we have previously developed for sessile drops on a wetted surface to the case of a meniscus in a capillary. The underlying physics of our model describe the intermolecular forces between the fluid and the surface of the capillary that result in the formation of a thin, non-removable fluid layer that coats the capillary wall. We describe the shape of the meniscus using a Young–Laplace equation and an incompressible, two-phase, computational fluid dynamics (CFD) calculation, both modified to take into account intermolecular forces using the disjoining pressure model. We find that our numerical solutions of the Young–Laplace equation and equilibrium meniscus shapes obtained by CFD agree well with each other. Furthermore, for capillaries that are sufficiently larger than the thickness of the non-removable film, our numerical solutions agree well with the effective contact angle model that we previously developed for sessile drops. Finally, we observe that it is possible to tune the disjoining pressure model parameters so that the intermolecular forces between the liquid and solid molecules become so strong compared to the surface tension that our formula for effective contact angle gives an imaginary solution. We analyze this situation using CFD and find that it corresponds to dewetting, where the bulk liquid detaches from the walls of the capillary leaving behind the non-removable thin liquid film.

Capillary Bridge in Contact with Ice Particles Can Be Related to the Thin Liquid Film on Ice

Capillary Bridge in Contact with Ice Particles Can Be Related to the Thin Liquid Film on Ice

Two layer film flow on an unsteady stretching cylinder

Most of the earlier research works related to the thin film flow over a stretching cylinder are primarily limited to the boundary layer flow of a single liquid. Whereas, multilayer film coating is commonly used to coat microelectronic chips, circuits, etc. This scenario motivated us to investigate the thin double-layer liquid film development over a stretching cylinder. In the present work, the above limitations of earlier models have been removed to investigate the two-layer film flow over a stretching cylinder by considering a full set of momentum equations. The governing set of equations is transformed into a system of nonlinear partial differential equations (PDEs) by applying appropriate similarity transformation. The analytical solutions are obtained by using the asymptotic analysis method for smaller values of Reynolds number (i.e. Re1<<1). Whereas numerical solutions are obtained by implicit finite difference method for smaller as well as moderate values of Re1. It is found that the film thinning rate enhanced for both the liquid layers with raising values of the cylinder’s radius (A) and density ratio (m). It is further seen that both the liquid layers thin faster with a decreasing initial thickness ratio of both liquid layers (δ). It is also observed that the variation of film height for the first layer is very small in comparison to the second layer for increasing values of viscosity ratio parameter (n). The film height for both the liquid layers decreases very fast for a smaller Reynolds number (Re1) at the initial time but a reverse trend is noticed at large time.

Hot Tearing Susceptibility and Solidification Behavior of Mg-xGd-(4-x)Y-2Ni-0.5Zr Alloys

The hot tearing susceptibility (HTS) of Mg-xGd-(4-x)Y-2Ni-0.5Zr (x = 0, 1, 2, 3, 4, wt%) alloys was evaluated using a T-shape mold method. When the Gd element increased and the Y element decreased, the HTS of the alloys decreases gradually. The Mg-4Y-2Ni-0.5Zr has the highest while Mg-4Gd-2Ni-0.5Zr has the lowest HTS value. Microstructure observation reveals that the grains of the magnesium alloy were refined to a certain extent when the Gd element increased and the Y element decreased. The ‘pinning’ effect of the LPSO phase precipitated along the grain boundary effectively increases the solid skeleton strength of the alloy and reduces the HTS of the magnesium alloy to a certain extent. The thin and uniform liquid film at the fracture can disperse the solidification shrinkage stress. In addition, the binary phase precipitated in the alloy forms a fast channel of residual liquid phase feeding on the liquid film, which is beneficial to feeding hot cracks. Solidification behavior of the alloys by thermal analysis showed that with the increase of the Gd element and the decrease of the Y element, the dendrite coherency temperature of the dendrites of the alloy decreases, and the residual liquid phase feeding is easier. The solid fraction of dendrite coherency increases, the skeleton strength of the alloy increases, and the HTS of the alloy decrease. In Mg-xGd-(4-x)Y-2Ni-0.5Zr (x = 0, 1, 2, 3, 4, wt%) alloys, when x is 1, 2, 3, 4, the HTS of the alloy is reduced by 25%, 49%, 63%, 81%, respectively compared with that when x is 0.

Fifth-order evolution equation for a liquid film with odd viscosity

A fifth-order nonlinear wave equation (generalized Korteweg–deVries equation) that arises in the theory of thin liquid films is introduced with the odd viscosity by the long-wave approximation regime. The presence of the odd viscosity is considered to be a non-dissipative factor in the evolution of the film flow. Sinusoidal and travelling waves arise in systems with non dissipation and instability. The introduced wave equation includes quadratic and cubic nonlinearities. The linear instability of sinusoidal waves is analyzed. The stability and bifurcation of travelling waves are discussed.

Carbon-Induced Changes in the Morphology and Wetting Behavior of Ionic Liquids on the Mesoscale.

Thin films of ionic liquids (ILs) have gained significant attention due to their unique properties and broad applications. Extensive research has focused on studying the influence of ILs' chemical composition and substrate characteristics on the structure and morphology of IL films at the nano- and mesoscopic scales. This study explores the impact of carbon-coated surfaces on the morphology and wetting behavior of a series of alkylimidazolium-based ILs. Specifically, this work investigates the effect of carbon coating on the morphology and wetting behavior of short-chain ([C2C1im][NTf2] and [C2C1im][OTf]) and long-chain ([C8C1im][NTf2] and [C8C1im][OTf]) ILs deposited on indium tin oxide (ITO), silver (Ag), and gold (Au) substrates. A reproducible vapor deposition methodology was utilized for the deposition process. High-resolution scanning electron microscopy, atomic force microscopy, and X-ray photoelectron spectroscopy were used to analyze the morphological and structural characteristics of the substrates and obtained IL films. The experimental data revealed that the IL films deposited on carbon-coated Au substrates showed minor changes in their morphology compared to that of the films deposited on clean Au surfaces. However, the presence of carbon coatings on the ITO and Ag surfaces led to significant morphological alterations in the IL films. Specifically, for short-chain ILs, the carbon film surface induced 2D growth of the IL film, followed by subsequent island growth. In contrast, for long-chain ILs deposited on carbon surfaces, layer-by-layer growth occurred without island formation, resulting in highly uniform and coalesced IL films. The extent of morphological changes observed in the IL films was found to be influenced by two crucial factors: the thickness of the carbon film on the substrate surface and the amount of IL deposition.

Thin liquid film drainage mechanism for bubble − particle attachment interactions in the low-rank coal flotation in the presence of salt and surfactants

Bubble-particle interaction is a fundamental process in froth flotation, where the attachment of bubbles and particles is the crucial process in effective flotation. However, modeling bubble-particle attachment interaction is challenging. This is because advanced methods like AFM (Atomic Force Microscope) characterizations are difficult to quantify hydrophobic interactions reproducibly for air bubbles and non-spherical particles. This study presents a methodology to obtain the hydrophobic force constants for bubble-particle attachment interaction in the presence of salt (KCl) and surfactants (DTAC and SDS). The hydrophobic interaction constants of bubble-particle interactions were acquired from first principles with the Stefan − Reynolds theory and Glembotsky experimental device. The Stefan − Reynolds theory was successfully applied in the determination of the real hydrophobic constants for bubbles and particles interaction in DI water (deionized water). However, in the coexistence of salt and surfactants, the real hydrophobic constants for bubbles and particles interaction cannot be obtained while the fictive hydrophobic constants were acquired with a linear extrapolation method. The results showed that the addition of inorganic salt and surfactants compressed the electronic double layer’s thickness and presented a higher hydrophobic force constant, enhancing the interaction between a bubble and a low-rank coal particle. In addition, the evaluations of hydrophobic constants reveal that the hydrogen bond energy and salt ions destruction energy should be considered to the comprehension of attachment interaction mechanism and the hydrophobic force source for bubble − particle attachment interactions.

Droplet impacting dynamics on a thin liquid film on the rice-leaf microstructured surface

Although spraying pesticides onto stems and leaves is the most effective method to prevent plant pests and diseases, the splashing of the droplets upon impact with the leaf surface is one of the main ways that the liquid agent is lost during the application process. Moreover, the collision and impact of flowing droplets with the liquid film are complex owing to the presence of microstructures on the leaf surface. Therefore, understanding the droplet impact dynamics is crucial for improving the pesticide application process. In this paper, a simulated rice leaf with a microstructured surface was established, and the impacting dynamics of a liquid droplet on this surface with thin liquid film were investigated using numerical methods. Specifically, the influences of different droplet diameters, droplet velocities, liquid film thicknesses, and leaf surface microstructure dimensions on the impacting dynamics were analyzed. The results showed that the behavior of spreading, jetting, rebound, and splashing occurred upon when droplets impact the simulated surface of rice leaves with thin liquid film. Moreover, a larger droplet diameter and a higher velocity lead to an increase in both the maximum height and diameter of the formed crown. Additionally, the thicker liquid films result in crowns with a smaller base radius. When the jetting angle increases, the crown height increases while its base radius decreases. With the increase in the rib spacing of microstructure, the maximum crown height initially increases and then decreases, while the crown base radius decreases. However, the rib depth has no significant influence on the base radius of the crown. Equations for the quantitative expression of the transition relationships between the deposition, jetting, and splashing phenomena were established on the basis of the Reynolds number, Ohnesorge number, and Weber number as well as the dimensionless liquid film thickness.

Invariant analysis, exact solutions, and conservation laws of time fractional thin liquid film equations

This present paper investigates Lie symmetry analysis, one-dimensional optimal system, exact solutions and conservation laws of the (2 + 1)-dimensional time fractional thin liquid film equations (TFTLFE) with Riemann–Liouville fractional derivative. Explicitly, we obtain six vector fields and the one-dimensional optimal system admitted by TFTLFE. Then, we perform the symmetry reductions with the help of Erdélyi–Kober fractional differential operator and (2 + 1)-dimensional TFTLFE is reduced into (1 + 1)-dimensional fractional partial differential equations (FPDEs). Additionally, by means of compound variable transformation and the power series expansion method, the solution of reduced FPDEs is obtained and its convergence is verified. Moreover, we derive other solutions for the reduced equations taking advantage of the invariant subspace method. Furthermore, the conservation laws are also established utilizing generalized Noether's theorem. Finally, we construct the exact solution using the method of conservation laws.

Experimental investigation of the effects of oil asphaltene content on CO2 foam stability in the presence of nanoparticles and sodium dodecyl sulfate

Foam stability tests were performed using sodium dodecyl sulfate (SDS) surfactant and SiO2 nanoparticles as foaming system at different asphaltene concentrations, and the half-life of CO2 foam was measured. The mechanism of foam stability reduction in the presence of asphaltene was analyzed by scanning electron microscope (SEM), UV adsorption spectrophotometric concentration measurement and Zeta potential measurement. When the mass ratio of synthetic oil to foam-formation suspension was 1:9 and the asphaltene mass fraction increased from 0 to 15%, the half-life of SDS-stabilized foams decreased from 751 s to 239 s, and the half-life of SDS/silica-stabilized foams decreased from 912 s to 298 s. When the mass ratio of synthetic oil to foam-formation suspension was 2:8 and the asphaltene mass fraction increased from 0 to 15%, the half-life of SDS-stabilized foams decreased from 526 s to 171 s, and the half-life of SDS/silica-stabilized foams decreased from 660 s to 205 s. In addition, due to asphaltene-SDS/silica interaction in the aqueous phase, the absolute value of Zeta potential decreases, and the surface charges of particles reduce, leading to the reduction of repulsive forces between two interfaces of thin liquid film, which in turn, damages the foam stability.

The Effect of the Type of Planting Media and The Addition of O2 Nanobubbles on the Growth of Dendrobium burana Green × Ong Ang Ai Boon Orchid Plantlets in Vitro

Dendrobium orchids come in a variety of colors, shapes, and beauty that are in great demand. The process of growing orchids requires special handling of an appropriate growing environment, such as proper planting media and nutrients that can stimulate orchid growth. This type of thin liquid film media is the use of liquid media with an immersion system while vermiculite and perlite media have almost the same properties. However, vermiculite has a higher water absorption capacity than perlite. Nanobubbles are bubbles with 1-100 nm that can live more stably in water. This study aims to influence the type of media and the addition of Nanobubbles O2 on the growth of Dendrobium burana Green × Ong Ang Ai Boon orchid plantlets. This research was carried out from November to December 2022 at the PT. Java Indo Arjuna Singosari, Malang. This study used a completely randomized design (CRD) with 10 treatments, namely solid MS, gaviota solid, thin liquid film, thin liquid film + NBs O2, vermiculite, vermiculite + NBs O2, perlite, perlite + NBs O2, vermiculite + Perlite, vermiculite + perlite + NBs O2 with 4 replications. The data were analyzed descriptively and the Manova test. The results showed that the vermiculite + NBs O2 treatment had the potential to increase the length of plantlets by 13.80 mm, the wet weight of plantlets by 0.30 g, the percentage of live plantlets by 100% whereas in the vermiculite + perlite treatment has the potential for the first root growth to appear, namely 5 HST. The vermiculite treatment had the potential to increase the highest number of leaves by 5 leaves, whereas in the leaf color treatment the solid MS treatment had a more intense green color.

Effect of dynamic contact angle behavior on droplet topological structure and film thickness evolutions in droplet mode

The droplet mode falling film evaporation was an area of intense research due to its thin liquid film and large interfacial area. A two-dimensional mathematical model based on the dynamic contact angle (DCA) model was developed to quantitatively analyze the effect of DCA behavior on droplet topological structure and film thickness evolutions of droplet mode in falling-film evaporation. Due to the movement of the three-phase contact line (vc) considered in the DCA model, the fact that the calculated DCA was significantly greater than the static contact angle (SCA) resulted in the phenomena of the liquid ring and droplet pulsation captured by the DCA model. The observed fluctuations of the DCA and vc might be attributed to the break-up of the liquid neck. For the impact stage, the peaks of the transient film thickness calculated by the DCA model were thicker than those by the SCA model and their differences increased with the increasing circumferential angle. For the metastable stage, the average film thickness predicted by the DCA model was thicker than that by the SCA model due to the captured droplet pulsation and calculated shorter metastable state by the DCA model.

Numerical investigation on heat transfer enhancement of spray cooling by surface structure optimization

A numerical model of spray cooling for enhanced surfaces in the non-boiling regime is created with deionized water as cooling medium. This model is validated by contrasting the simulated data with the corresponding experimental data, and the relative deviation is within 12 %. Compared with a smooth surface, the area of the enhanced surface is expanded, the cooling performance is significantly improved, but the surface temperature distribution is less uniform. Compared to the straight groove surface with the same area, the heat flux of the radial groove surface is higher because its surface structure is more conducive to the spreading and leaving of the liquid film. Studies of the effects of the structure parameters (groove width and groove number) of the radial groove surface on the heat transfer show that there is an optimal groove width (0.2 mm) and groove number (45) for the strongest cooling capacity, and both the velocity and thickness of the liquid film are significant factors in cooling performance. The faster the liquid film velocity, the thinner the liquid film and, thus, the better the heat transfer. Motivated by these findings, three novel enhanced surfaces are machined to increase liquid film fluidity and heat transfer area even further. The cooling performance of the three novel enhanced surfaces is further improved, and the slope groove surface acquires the strongest cooling capacity due to the acceleration of liquid film flow by gravity, although its heat transfer area is not the largest. Research on all enhanced surfaces indicates that thinner liquid film and faster liquid film flow should be the primary design goals of the enhanced surface, followed by an increase in heat transfer area.

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.

Flow pattern-based heat transfer analysis of microchannel flow boiling: An experimental investigation

The present study investigates the flow boiling heat transfer and flow pattern of R245fa in a smooth, horizontal microchannel with an inner diameter of 1 mm. The experiments cover a range of mass flux from 300 to 700 kg/m2s and heat fluxes varying between 10 and 94 kW/m2, with corresponding saturation temperature of 23–54 °C. Heat transfer experiment results indicate that the local heat transfer coefficient (HTC) presents two distinct variation trends under different heat and mass flux conditions. In addition, the heat flux is positively correlated with the local HTC at each measure point. Visualization experiment results show that annular flow forms at low vapor quality and significantly influences heat transfer as the primary flow pattern. The flow boiling heat transfer of R245fa in the 1 mm microchannel can be characterized by three regimes: bubble/slug regime, liquid film evaporation regime, and intermittent dryout regime. Thin liquid film evaporation and intermittent dryout during annular flow predominately contribute to heat transfer. Finally, the experimental HTC is compared with five different predictive methods, revealing that the three-zone model proposed by Thome et al. [46] holds immense potential for estimating the local HTC in the microchannel.

Transient simulation of oscillatory gas-liquid Taylor flow and its effects on heat transfer

Microchannel-based gas-liquid Taylor flow has been increasingly used in various cutting-edge applications due to its outstanding flow mixing and heat transfer performance. Previous simulations have been limited to the discussions of flow characteristics under stable flow regimes although several experimental studies have observed the occurrence of fluctuated Taylor bubble tails at relatively higher Reynolds numbers. This study has developed a numerical approach to simulate three-dimensional transient Taylor flow and heat transfer with large bubble tail oscillations. A two-stage modelling procedure combining both fixed and moving frame of reference systems has been adopted to ensure a fully-developed flow is obtained. Simulation results of a water-nitrogen system have been compared with experiments and correlation-predicted results, and good agreements have been achieved. A transient oscillation of the bubble tail has been found to occur as the Reynolds number is greater than 700, approximately, and the fluctuation has been more pronounced as the Reynolds number increases further. The above transient effect has been found to be closely related to the enhanced jet flow which originates from the thin liquid film and later merges into the downstream liquid slug, promoting both small and large-scales flow mixing, enhancing heat transfer up to four times of that for a pure liquid flow.

Falling liquid films on a uniformly heated compliant substrate with broken time-reversal symmetry

This study aims to analyze the dynamics of a thin Newtonian liquid film on a uniformly heated compliant substrate. We consider the violation of time-reversal symmetry in the liquid, resulting in an additional non-zero term in the liquid stress tensor. Using the long-wave expansion technique, we derive a set of coupled equations governing the film thickness and substrate deformation, accounting for inertia, surface tension, thermocapillarity, and odd viscosity. Through linear stability analysis and spatiotemporal simulations, we observe that the compliant substrate enhances instability, while wall heating exacerbates it. However, the introduction of odd viscosity effectively suppresses these instabilities, as confirmed by the agreement between simulation and theoretical predictions.

Soap-Film Membranes for CO2/Air Separation.

Thin liquid films are a potential game changer in the quest for efficient gas separation strategies. Such fluid membranes, which are complementary to their solid counterparts involving porous materials, can achieve complex separation by combining permeability and adsorption mechanisms in their liquid core and at their surface. In addition, unlike porous solid membranes that must be regenerated between separation steps to recover a gas-free porosity, thus preventing continuous operation, liquid membranes can be regenerated using continuous liquid flow through the fluid film. Here, building on the self-sustained mobile film technique, we propose a simple experimental setup allowing direct quantitative assessment of the gas permeability of soap films stabilized by different surfactant types. Using a simple prototypical example involving O2/N2 mixtures, the measurement principle is first presented to establish a proof of concept. As the gas solubilities and diffusivities are known, the results of such experiments can be compared with microscopic models to disentangle the liquid core and surface permeabilities from a direct macroscopic transport response of the film subjected to a gas concentration difference. The same dynamical experiments performed for air enriched in CO2 indicate that the permeability of the soap film varies with the molar fraction in the gas compartment, a feature not observed for O2/N2. These experimental findings pave the way for the design of novel separation technologies in fields and situations where porous solid membranes are of limited efficiency.