Rupture Of Liquid Film Research Articles

Liquid film rupture beyond the thin-film equation: A multi-component lattice Boltzmann study

Under the condition of partial surface wettability, thin liquid films can be destabilized by small perturbations and rupture into droplets. As successfully predicted by the thin film equation (TFE), the rupture dynamics are dictated by the liquid–solid interaction. The theory describes the latter using the disjoining pressure or, equivalently, the contact angle. The introduction of a secondary fluid can lead to a richer phenomenology, thanks to the presence of different fluid/surface interaction energies but has so far not been investigated. In this work, we study the rupture of liquid films with different heights immersed in a secondary fluid using a multi-component lattice Boltzmann (LB) approach. We investigate a wide range of surface interaction energies, equilibrium contact angles, and film thicknesses. We found that the rupture time can differ by about one order of magnitude for identical equilibrium contact angles but different surface free energies. Interestingly, the TFE describes the observed breakup dynamics qualitatively well, up to equilibrium contact angles as large as 130°. A small film thickness is a much stricter requirement for the validity of the TFE, and agreement with LB results is found only for ratios ε=h/L of the film height h and lateral system size L, such as ε≲×10−3.

Study on the stability of foamed gel and its structural evolution characteristics

ABSTRACT Foamed gel is an efficient material for coal spontaneous combustion prevention and control, which effectively combines the fire prevention and extinguishing properties of foam and gel. In this paper, the formation principle, structural evolution, stability, and fluidity of foamed gel are elaborated. Foamed gel is mainly formed by foaming agent, cross-linking agent and water through physical foaming and gelling reaction, and its formation process is divided into two stages: the first stage is physical foaming and forming foam; the second stage is adding cross-linking agent to foam and generating foamed gel through gelling reaction. The first stage of foamed gel mainly reflects the fluidity and stacking of foam, and in the second stage, with the increase of gel amount, foamed gel reflects the temperature resistance of gel. The basic structure of the foamed gel is the same as that of the water-based foam, and the gel particles constitute the basic skeleton of the foamed gel, which slows down the gravitational drainage and foam disproportionation rate. The evolution process of foamed gel structure includes liquid film rupture, gel formation and skeleton collapse, which eventually forms a gel layer to fill the residual coal fissure. The flow of foamed gel in the mining area is subject to the combined effect of grouting pressure, gravity and resistance, and the lateral flow velocity is larger than the longitudinal flow velocity, and the contour line of its diffusion front peak is ellipsoidal.

Local deformation and coalescence between two equal-sized droplets in a cross-focused microchannel

• Three flow patterns of head-on coalescence of droplets are observed. • Effects of viscosity ratio and surfactant are explored. • Local nipple of droplet is conductive to coalescence. • Influence factors of the nipple are investigated systematically. • A novel model and a predictive equation of the nipple are proposed. The local deformation and the head-on coalescence dynamics of droplets in a cross-focused microchannel are experimentally investigated. The dynamic process of droplets after collision is divided into squeezing stage and decompressing stage, and the flow patterns are divided into squeezing coalescence, decompressing coalescence and non-coalescence based on the moment when the liquid film ruptures. The local nipples caused by the lubrication force in the thin film arise in the decompressing stage and facilitate coalescence. The effects of the dispersed to continuous-phase viscosity ratio and the concentration of surfactant to the dynamic process, operating range of each flow pattern, and the extent of local deformation are investigated, and the predictive equation of the nipple angle is proposed. This research combines the local deformation with coalescence and provides a reference for further understanding the mechanism of droplet coalescence.

固体平板と気泡間に形成される液膜の排水過程における中心膜厚に及ぼす水中の電解質の影響

The drainage and rupture of the thin liquid film formed between a bubble and a flat mica plate have been investigated. It is known that the bubble coalescence can be prevented in electrolyte aqueous solution, and its effects strongly depends on the type of electrolyte. In this study, by using two types of electrolyte aqueous solutions (0.05 M MgSO4 aqueous solution and 0.05 M CH3COONa aqueous solution) and surfactant aqueous solution (10 ppm Triton X-100 aqueous solution), evaluate the effects of impurities on the behavior of the liquid thin film. The effect of the approach velocity also is also evaluated. By using an optical interferometer and a high-speed video camera, interference fringe images are taken, and the shape of the liquid thin film formed between a bubble and a flat mica plate can be obtained. The effect of solute in transition on the thin film process in the center of the liquid film is investigated. Under conditions of 2 mm bubble diameter and the approach velocity of 50 μm/s or less, the thickness of the central film increases, and the spreading of the liquid film is suppressed when electrolytes are added into water.

Dynamics of liquid film rupture under local heating

Rupture dynamics of a horizontal liquid film non-uniformly heated from below was experimentally studied in a wide range of liquid viscosity. To visualize the liquid surface deformations and disruption of the film, an optical schlieren system was used. The instantaneous local film thickness was measured using a chromatic confocal sensor. In order to measure the thickness of thin liquid films, the substrate was covered with a black coating eliminating reflection of the incident light. Angular and frequency characteristics of the confocal sensor were studied in detail. The process of film rupture can be divided into three stages: 1) film thinning down to a residual film on the heater; 2) existence of a stable residual film for some time; 3) rupture and dry out of the residual film. The thickness of the residual film was found to strongly depend on the liquid viscosity: for water it is about 10 µm, whereas for silicone oil PMS-200 it is about 275 µm. The data obtained for various working liquids were generalized by a dimensionless correlation.

Thickness distribution and fluctuation characteristics of liquid falling film under turbulent conditions

The thickness distribution and fluctuation characteristics of turbulent falling film on vertical flat and corrugated plates were investigated by ultrasonic Doppler velocimetry and a high-speed camera under Rel (liquid phase Reynolds number) of 8.64 × 103–2.59 × 104, and a liquid film rupture model was established at the corrugated plate corner. The experimental results show that the crests and troughs in the corrugated plate produce “jet” and “reflux” effects compared with the flat plate, increasing the liquid film thickness and enhancing the fluctuation in the liquid film. The intensification of the liquid film fluctuation makes the probability density function of the instantaneous liquid film thickness tend to be a normal distribution. Plateau-Rayleigh instability hinders the growth of the liquid film thickness. The calculated values of the critical rupture velocity of the liquid film agree with the experimental values, and the relative error is within 15%.

A Novel Package-Integrated Cyclone Cooler for the Thermal Management of Power Electronics

Abstract In order for electronics packaging power density to increase, innovations and improvements in heat transfer are required. Electrification of transportation has the potential for significant fuel and energy savings. Changing to an electrified drive train requires reliable and efficient power electronics to provide power conversion between alternating current motors and direct current energy storage. For high power transportation systems like aircrafts or heavy vehicles, the power density of power electronics needs to be improved. Power density is also an enabler for high power military devices that must be used and transported via air, ground, and sea. This paper summarizes the outcome of a collaborative and multidisciplinary research effort aimed at co-designing a novel electronics cooling device that utilizes two-phase fluid flow. Two-phase flow cooling has been known for decades as well as the risks associated with it: critical heat flux (CHF), dry-out, and thermal runaway. Our research de-risks the two-phase cooling technology by swirling the flow to remove the bubbles from the wall and confining them at the core of the cooler. The combined effects of gas phase removal, enhanced nucleation, and dramatic liquid film agitation and rupture have been quantified by our experiments: double the heat transfer coefficient with only 13% increase in pressure drop. Besides advanced fluid-dynamics, our Package-Integrated Cyclone Cooler (PICCO) utilizes cutting edge packaging and additive manufacturing technology such as direct deposition of a metal substrate and circuits (dies) on a complex helical cooler that can only be manufactured via three-dimensional printing. By co-designing and testing the cooler, we have quantified the impact of the swirled flow on the junction temperature with respect to a conventional (non-swirl) two-phase-flow-cooled power electronics package. At steady-state, our post-test thermal simulations predict a junction temperature reduction from 185 °C to 75 °C at the same power dissipation. When the heat load is unsteady (United States Environmental Protection Agency Urban Drive Cycle), the junction temperature reduction is 140 °C to 60 °C.

Local flow dynamics in the motion of slug bubbles in a flowing mini square channel

For a slug flow flowing in a square capillary, the surrounding liquid flows through a thin film between the slug bubble and the wall and the corners of the channel and generates an asymmetric two-phase flow field with significant spatial variations. The current literature mainly discusses the bulk features of such a flow field, while local phenomena in these flows are not well known yet. In this study, the local dynamics of the liquid film and corner flows in a flowing square channel with a width of 3 mm were investigated by experiments and numerical simulations. Instantaneous velocity fields in the liquid flow were measured at the channel's central plane using particle tracking velocimetry (PTV). The obtained data were utilized to calculate the axial variations of the film and corner velocities. Three-dimensional numerical simulations with adaptive mesh refinement of the interface were performed for a series of flow conditions. Comparing the film velocity obtained from PTV and simulations demonstrates an overall good agreement between the results, with discrepancies close to the solid walls, where the PTV data is sporadic. Film and corner flows show rapid velocity changes adjacent to the minimum film area. Both experiments and simulations elucidate the backflow phenomena in the analyzed flow fields, where the liquid locally flows in the opposite streamwise direction. The backflow region shrinks as the liquid flow rate and speed of the bubble increase. Film flow shows significant pressure drops as it passes through the minimum film area. On the contrary, the corner flow does not demonstrate any rapid pressure fluctuations. The vortex structures’ identification revealed dominant vortex loops inside the gas and small-scale vortex filaments along the channel walls, generated from the liquid film rupture.

Study on the classification and floc-flotation process of Huangling coal slime

ABSTRACT This study primarily investigates the effect of particle classification on flotation of Huangling (HL) coal slimes with aiming to recover more combustible and clean fuels. The ash composition of fresh slime was analyzed using XRD and ICP-AES. The results showed that HL coal slime had high ash content and difficult washability. Conventional flotation and floc-flotation tests of coarser fraction (0.500 − 0.125 mm) and finer fraction (−0.125 mm) were performed to investigate comprehensively the flotation parameters. Results indicated that particles classification could effectively improved the flotation capacity of coarser fraction. Conventional flotation was suitable for coarser fraction, whereas floc-flotation was suitable for finer fraction. However, finer fraction has a large specific surface area, poor attachment with the air bubbles and entrained a large amount of impurity mineral particles. These features caused conventional flotation ineffective in finer fraction, whereas the addition of Polyacrylamide (PAM) anionic flocculant significantly enhanced flotation recovery. The excessive flocculant could not be continuously improved flotation effect, possibly because the excessive PAM molecules were adsorbed on the surface of coal particles, which accelerated the rupture of liquid film between coal particles and bubbles. Furthermore, the floc-flotation could effectively increased the particle size and reduced specific surface area of finer fraction.

Understanding the mechanism of froth flotation of molybdenite using oily collectors from a perspective of thinning and rupture of thin liquid film

Froth flotation of molybdenite is commonly achieved using hydrocarbon oils as the collector. The insoluble hydrocarbon oils adsorb on molybdenite surfaces, resulting in an increase in its surface hydrophobicity and thereby its flotation recovery. However, the underlying mechanism on a microscopic scale was poorly understood. In this study, the effect of insoluble oily collectors on thinning and rupture of thin liquid films (TLFs) between air bubbles and molybdenite face surfaces has been investigated by monitoring the spatial and temporal evolution of the TLFs using a synchronized tri-wavelength reflection interferometry microscope (STRIM) as well as by drop size and contact angle measurements. Two hydrocarbon oils, namely kerosene and dodecane, have been studied. The result from the STRIM measurements shows that the TLF is stable in a 1 mM NaCl solution between an air bubble and a molybdenite face surface despite the molybdenite face being hydrophobic. The TLF becomes unstable when the surface is treated in both dodecane and kerosene emulsion solutions and ruptures at a critical rupture thickness of 60–340 nm. The film rupture is predominantly attributed to the instability of the TLFs between air bubbles and oil droplets adsorbed on the molybdenite surfaces. It is found that both the thinning kinetics and the rupture thicknesses of the TLFs correlate with both the coverage and sizes of the oil droplets on molybdenite surfaces. Kerosene exhibits a better dispersibility in DI water than dodecane, and therefore performs better as the collector than dodecane from a perspective of film stability. The present work provides new insight into the underlying mechanism of oily collectors in the froth flotation of molybdenite and other naturally hydrophobic minerals. The result obtained from this study enables the development of new oil collectors in the future.

Rupture of bar-coated liquid films on wettability-patterned surfaces

Rupture of bar-coated liquid films on wettability-patterned surfaces

Experimental investigation on sliding bubble coalescence of subcooled flow boiling in rectangular narrow channel

The behavior of bubble coalescence in subcooled boiling has a significant influence on the void fraction and heat transfer capacity. In this paper, a visual flow boiling experiment is carried out to investigate the phenomenon of bubble coalescence and the effect of sliding bubbles on coalescence. The phenomenon of bubble coalescence, the velocity of bubble and interface evolution characteristics after coalescence are analyzed in detail. The annular waves caused by the rupture of the thick liquid film and the severe deformation caused by bubble coalescence are observed during the experiment. The propagation of annular waves on bubble surfaces makes bubbles unstable and then easily breaks down to produce secondary bubbles. Unlike the secondary bubbles produced by air bubble coalescence, the critical diameter of initial average bubbles that produce secondary bubbles after bubble coalescence is about 0.87 mm. This difference is because the viscosity of the fluid at high temperature in boiling two-phase flow is different from that in adiabatic water-air two-phase flow. Besides, it is found that there is an oscillation adjustment zone after bubble coalescence. This stage is divided into the violent oscillation region and moderation region, and their duration is basically the same. The relative velocity of sliding bubbles during collision is obtained according to the velocity difference between bubbles and the growth rate of the sliding bubble. Also, the effect of sliding bubbles on bubble coalescence is analyzed in this paper.

Chemical and physical modification of low rank coal floatability by a compound collector

Effective collectors are essential for improving coal floatability. In this investigation, contact angle, X-ray photoelectron spectroscopy (XPS), optical micrograph, field emission scanning electron microscope (FE-SEM), and flotation tests were used to characterize chemical and physical modification of low rank coal floatability by a compound collector consisting of coal tar and diesel. XPS showed that the C-C/C-H content on coal surface after compound collector adsorption was higher than that on coal surface after diesel adsorption. Optical microscope image results showed that compound collector produced more flocs than diesel, which increased flotation efficiency. Contact angle results showed compound collector enhanced coal surface hydrophobicity more significantly than diesel. FE-SEM images illustrated that coal surface after compound collector adsorption was rougher than that after diesel adsorption, which accelerated the rupture of liquid film between coal particle and bubble. Therefore, coal floatability after compound collector adsorption was much better than that after diesel adsorption.

The role of inertia in the rupture of ultrathin liquid films

Theory and numerical simulations of the Navier–Stokes equations are used to unravel the influence of inertia on the dewetting dynamics of an ultrathin film of Newtonian liquid deposited on a solid substrate. A classification of the self-similar film thinning regimes at finite Ohnesorge numbers is provided, unifying previous findings. We reveal that, for Ohnesorge numbers smaller than one, the structure of the rupture singularity close to the molecular scales is controlled by a balance between liquid inertia and van der Waals forces, leading to a self-similar asymptotic regime with hmin ∝ τ2/5 as τ → 0, where hmin is the minimum film thickness and τ is the time remaining before rupture. The flow exhibits a three-region structure comprising an irrotational core delimited by a pair of boundary layers at the wall and at the free surface. A potential-flow description of the irrotational core is provided, which is matched with the vortical layers, allowing us to present a complete parameter-free asymptotic description of inertia-dominated film rupture.

Generation of the bubble-free layer during the metal foaming process

This study focuses on the generation of the bubble-free layer during the fabrication of the metal foam using the direct melt foaming method. The theoretical models for the three stages during the foaming process have been established, and the boundary conditions have been discussed. The results from the theoretical study indicated that the bubble number of the liquid foam played a critical role in the generation of the bubble-free layer, thus, improving the bubble number retards the velocity of the bubble-free layer generation. The liquid film rupture led to the expansion of the local cross-sectional area of the Plateau border channel and increased the drainage velocity, thus, resulting in an enhancement in the bubble-free layer. Additionally, the verification experiments were conducted by employing the graphene nanoflakes (GNFs) as the heterogeneous nucleation agent. The findings indicated that the generation of the bubble-free layer was hindered, and the bubble-free layer was altogether absent in case the content of the heterogeneous nucleation agent was enhanced.

Breakup of Thin Liquid Films: From Stochastic to Deterministic.

The thinning and rupture of thin liquid films is a ubiquitous process, controlling the lifetime of bubbles, antibubbles, and droplets. A better understanding of rupture is important for controlling and modeling the stability of multiphase products. Yet literature reports that film breakup can be either stochastic or deterministic. Here, we employ a modified thin film balance to vary the ratio of hydrodynamic to capillary stresses and its role on the dynamics of thin liquid films of polymer solutions with adequate viscosities. Varying the pressure drop across planar films allows us to control the ratio of the two competing timescales, i.e., a controlled hydrodynamic drainage time and a timescale related to fluctuations. The thickness fluctuations are visualized and quantified, and their characteristics are for the first time directly measured experimentally for varying strengths of the flow inside the film. We show how the criteria for rupture depend on the hydrodynamic conditions, changing from stochastic to deterministic as the hydrodynamic forces inside the film damp the fluctuations.

Characteristics of liquid film rupture at wave plate corner: A short communication

WPD (Wave plate dryer) is a very vital SWS (steam-water separation) equipment for nuclear engineering. It is meaningful to study the rupture characteristics and mechanism of the liquid film on WP (wave plate) wall. However, there are few studies on rupture phenomenon at WP corner. In this short communication, the LFR (liquid film rupture) model at the corner of the wave plate is established. In addition, the calculation equation of the relative PLFR (position of the liquid film rupture) at the WP corner is deprived. Additionally, PLFR is measured by PLIF (Planar Laser Induced Fluorescence) method. The theoretical results agree well with the experimental results, which provides vital prediction method for LFR phenomenon in WP of SG (steam generator).

Study on the Breakdown Mechanism of Water Film on Corrugated Plate Wall Under the Horizontal Shear of Airflow: A Short Communication

The corrugated plate dryer is an important steam-water separation equipment in the steam generator in the secondary loop of nuclear power plants. Therefore, it is very important to understand the process and mechanism of liquid film rupture on the wall surface of the corrugated plate dryer. In this paper, for the first time, the process of rupture of the liquid film on the surface of the corrugated plate is studied. The mechanism of liquid film rupture on the wall surface of vertical corrugated plate was studied experimentally. The high-speed camera was used to collect the image of the broken liquid film, and the experimental research on the broken position, the broken process and the broken shape of the liquid film was carried out. The results show that the rupture of the liquid film on the wall surface of the corrugated plate under the horizontal shear of air flow mostly occurs at the upper middle position of the main flow of the liquid film. And the rupture position moves up with the increase of the Reynolds number of the liquid film. Liquid film rupture needs to go through the process of stable flow, single strip-shaped liquid film detachment, two strip-shaped liquid film formation, and finally chaotic disordered rupture. The angle between the two strip-shaped liquid films is positively related to the Reynolds number of the liquid film.

Analysis of the Flow Characteristics of the Liquid Film on the Wall Surface of the Corrugated Plate Dryer Based on PSD Method: A Short Communication

Corrugated plate dryer is an important steam-water separation equipment in steam generator of secondary loop of nuclear power plant. Its steam-water separation efficiency has a certain influence on economic index of nuclear power plants. It is meaningful to understand flow characteristics of liquid film on corrugated plate wall. Besides, the fluctuation of the liquid film may cause the occurrence of liquid film rupture, thus the research on the fluctuation of the liquid film on the wall of the corrugated plate is very important. This paper uses PLIF (Plane laser induced fluorescence) method to measure liquid film thickness. Curve of film thickness with time is obtained. Calculation formula of average value of liquid film thickness is given. Based on power spectral density (PSD) method, nonlinear characteristics of liquid film thickness are studied. Results show that the relationship between film thickness and Reynolds number conforms to Nusselt theory. Nevertheless, the results show deviation from Nusselt as well as Karapantsios estimation. When Reynolds number is in the range of 600 to 3000 and surface roughness is 0.03, the fluctuation of the power spectrum of the liquid film thickness caused by gravity can be ignored, which provides a reference for study on steam-water separation.

Lattice Boltzmann study of the interaction between a single solid particle and a thin liquid film

The isothermal single-component multi-phase lattice Boltzmann method (LBM) combined with the particle motion model is used to simulate the detailed process of liquid film rupture induced by a single spherical particle. The entire process of the liquid film rupture can be divided into two stages. In Stage 1, the particle contacts with the liquid film and moves into it due to the interfacial force and finally penetrates the liquid film. Then in Stage 2, the upper and lower liquid surfaces of the thin film are driven by the capillary force and approach to each other along the surface of the particle, resulting in a complete rupture. It is found that a hydrophobic particle with a contact angle of 106.7° shows the shortest rupture duration when the liquid film thickness is less than the particle radius. When the thickness of the liquid film is greater than the immersed depth of the particle at equilibrium, the time of liquid film rupture caused by a hydrophobic particle will be increased. On the other hand, a moderately hydrophilic particle can form a bridge in the middle of the liquid film to enhance the stability of the thin liquid film.