Properties Of Thin Liquid Films Research Articles

Phase transition properties of thin liquid films with various thickness on different wettability surfaces

Thin liquid film phase transition is often used to solve high heat flux heat transfer problem due to the high heat transfer coefficient. Both the surface wettability and the liquid film thickness are key factors affecting the phase transition behaviors of thin liquid films, however, the microscopic mechanism of the coupling effect is still unclear. Molecular dynamics method was employed to study the phase transition properties of liquid films with three thicknesses on six types of surfaces with different wettabilities, including hydrophilic, weak-hydrophilic, hydrophobic, and three hydrophilic-hydrophobic hybrid wettabilities. Results showed that explosive boiling occurred only when the temperature of the explosive boiling region reached the triggering temperature. The thermal resistance R1 at the solid-liquid interface, the thermal resistance R2 of the thin liquid film above the explosive boiling region, and their magnitude relationship determined the temperature rise rate of the explosive boiling region, then the phase transition mode and the onset time of explosive boiling (TOB) were further affected. Moreover, for hybrid surfaces, bubbles were easier to be generated in the hydrophobic region. The phase transition mechanism of thin liquid film on surface with different wettabilities was given in detail

Phase Change Properties of Thin Liquid Films with Various Thickness on Different Wettability Surfaces

Phase Change Properties of Thin Liquid Films with Various Thickness on Different Wettability Surfaces

Stability of Liquid Films Formed by a Single Bubble and Droplet at Liquid/Gas and Liquid/Liquid Interfaces in Bovine Serum Albumin Solutions.

The properties of thin liquid films are usually investigated under static conditions, isolated from external disturbances. Such studies provide vital information about the drainage mechanism of the thin liquid film, but the conditions of these measurements are vastly different from those that occur when a real dispersed system is created. In this paper, we present elaborated methodologies that allow qualitative and quantitative measurements of the stability of both the emulsion and foam films formed by a single bubble and droplet at liquid/gas and liquid/liquid interfaces, where the hydrodynamic factors are of crucial importance. The experiments were performed in a bovine serum albumin (BSA) solution at different pH values. The adsorption behavior of BSA under different pH conditions at the liquid/gas and liquid/liquid interface is described, and its implication for the single bubble/droplet motion and liquid film drainage is analyzed. The mechanism of thin-liquid-film stabilization by the BSA molecules is shown to be significantly different for the foam and emulsion films and depends significantly on the bubble history as well as the pH of the BSA solution. Additionally, the results obtained for BSA were compared to those acquired for a typical surface-active substance, sodium lauryl sulfate. The similarities and differences in the rising bubble/droplet dynamics (caused by different dynamic adsorption layer architectures) and foam and emulsion film stabilization by these two types of stabilizers under dynamic conditions are shown and discussed.

Effect of Velocity, Solid Wettability, and Temperature on Drainage Dynamics of C5PeC11-in-Toluene Liquid Films between Silica and Water Droplet

Properties of thin liquid films between two approaching droplets or between a droplet and a surface are critical for emulsion stability, oil and gas extraction, and mineral flotation. The dynamic force apparatus (DFA)—simultaneous measurement of interfacial film thickness, drainage time, and interaction forces at precisely controlled approach velocities—was used to study the interactions of a water drop with silica surfaces in toluene containing the model asphaltene compound N-(1-undecyldodecyl)-N′-(5-carboxypentyl)-perylene-3,4,9,10-tetracarboxylic bisimide (C5PeC11). A water droplet in toluene containing 0.1 g/L C5PeC11 was driven toward a silica surface of varying wettability (contact angles of 0 and 107°) at two different droplet approach velocities (0.1 and 1 mm/s) and temperatures (22 and 40 °C). Rupture of thin liquid films between a water droplet and silica surfaces was observed, with a moving three phase contact line and strong attachment for hydrophilic silica and minor, local attachment and an easily detachable water drop for hydrophobic silica. Increasing the approach velocity of water droplets toward solid surfaces resulted in a larger dimple and longer film lifetime. Interestingly, higher temperature led to a faster film rupture for hydrophilic silica, in line with industrially observed improvements in removal of water-in-oil emulsions at higher temperatures. The experimentally determined dynamics of the thin liquid film drainage were modeled successfully using the Stokes–Reynolds–Young–Laplace (SRYL) theoretical model.

Numerical Analysis of Transient Temperature Response of Soap Film

Measurements of thermophysical properties of thin liquid films are important to understand interfacial phenomena due to film structures composed of amphiphilic molecules in soap film, phospholipid bilayer of biological cell and emulsion. A transient hot-wire technique for liquid films less than 1 $$\upmu $$ m thick such as soap film has been proposed to measure the thermal conductivity and diffusivity simultaneously. Two-dimensional heat conduction equations for a solid cylinder with a liquid film have been solved numerically. The temperature of a thin wire with liquid film increases steeply with its own heat generation. The feasibility of this technique is verified through numerical experiments for various thermal conductivities, diffusivities, and film thicknesses. Calculated results indicate that the increase in the volumetric average temperature of the thin wire sufficiently varies with the change of thermal conductivity and diffusivity of the soap film. Therefore, the temperature characteristics could be utilized to evaluate both the thermal conductivity and diffusivity using the Gauss–Newton method.

Structures and Nanotribological Properties of Thin Liquid Films Confined between Smooth Surfaces

Structures and Nanotribological Properties of Thin Liquid Films Confined between Smooth Surfaces

Viscosity of Polyelectrolytes Solutions in Nanofilms

We use a thin film pressure balance to probe the rheological properties of thin liquid films. These films are made from mixed aqueous solutions of surfactants and polyelectrolytes. They drain under applied pressure in a noncontinuous way due to a stratification process of the polyelectrolytes network. The stratification kinetics was studied for films stabilized by different surfactants. Using a theoretical model, it is possible to examine the effect of both the surfactant and the film thickness on the local dissipation. On one hand, it was observed that dissipation depends on the polyelectrolyte concentration only, regardless whether the surfactant is neutral or bears electric charges opposite to those of the polyelectrolyte. On the other hand, it was found that dissipation is stronger in thinner films.

Correlation between adsorption isotherms, thin liquid films and foam properties of protein/surfactant mixtures: Lysozyme/C 10DMPO and lysozyme/SDS

The properties of thin liquid films and foams of lysozyme/C 10DMPO and lysozyme/SDS have been studied on the basis of their adsorption isotherms. The adsorption of mixed protein/surfactant solution at the air/water interface has a competitive nature and a gradual displacement of the protein from the interface is observed with increasing surfactant concentration. For this study three different surfactant concentrations of each type (non-ionic C 10DMPO and ionic SDS) have been chosen corresponding to a protein-like, mixed and surfactant-like adsorption layer. The addition of surfactant to the protein solution has a stabilizing impact on the foam properties.

Effects of surfactant adsorption and surface forces on thinning and rupture of foam liquid films

The properties of thin liquid films (TLF) are of paramount significance for colloidal disperse systems, and a number of industrial processes, including froth flotation. In flotation, the bubble–particle attachment is controlled by the thinning and rupture of the intervening liquid film between an air bubble and a mineral particle. The froth evolution and its transient stability are also a function of the drainage and rupture of liquid films between air bubbles. Surface-active substances (surfactants) are used as flotation reagents to control the behavior of the liquid films. This paper presents a review of our research in the area of surfactant adsorption, surface forces and liquid films. It mainly focuses on the validation, application and extension of the Stefan–Reynolds theory on the liquid drainage. The extension of the Stefan–Reynolds theory comprises surface forces (disjoining pressure), surface tension variation, caused by the adsorption and diffusion of surfactants. Both the experimental and theoretical results are mostly related to the free (foam) films formed between two bubbles but can be principally extended to emulsion films between two oil drops and wetting films between an air bubble and a solid surface.

Thermodynamic studies on thin liquid films. I.�General formulation

From the viewpoint that thermodynamic study is essential to elucidate the structure and properties of thin liquid films, thermodynamic equations based on a new convention and employing pressure as a thermodynamic variable are developed for adsorption at film interfaces of a plane-parallel film. The equations together with quasi-thermodynamic ones correlate the dependence of film tension on concentration, temperature, pressure, and disjoining pressure to film density, entropy and volume changes associated with adsorption, and thermodynamic film thickness, respectively. Based on the formulation adopting pressure as a variable, equations are also derived for the differences in thermodynamic quantity between the film and the bulk interfaces coexisting at equilibrium.

A disjoining pressure study of n-dodecyl-β-D-maltoside foam films

The disjoining pressure Π as function of film thickness h of aqueous n-dodecyl-β-D-maltoside (β-C12G2) solutions has been measured using a newly assembled thin film pressure balance. The theoretical analysis of the experimental Π(h) curves in terms of the DLVO theory confirms the existence of electrical charges and provides numerical values for their density at the water/air interface of this nonionic sugar surfactant system. The origin of the charges is discussed on the basis of a recently proposed adsorption model for OH− ions. The surface charge density is found to decrease with increasing surfactant concentration, to increase with increasing electrolyte concentration and stay constant in the range 4 < pH < 8. Furthermore, it is shown that the molecular structure of sugar surfactants has no influence on the magnitude of the long-range repulsive forces stabilizing the common black film. However, the structure influences the stability of the film as well as its ability to form a Newton black film. The good agreement between the results presented and those for nonionic alkyl polyglycol ethers (CiEj) suggests a common origin of the properties of thin liquid films stabilized by nonionic surfactants.

Restoration of protein foam stability through electrostatic propylene glycol alginate-mediated protein–protein interactions

The action of propylene glycol alginate in the enhancement of foam stability of a destabilised Tween 20/bovine serum albumin mixed system was evaluated. A significant increase in the foam stability was observed in the presence of low concentrations of propylene glycol alginate. A pseudo-plateau level of foam stability was obtained in the presence of approximately 0.8 μg/ml propylene glycol alginate in the solution used to form the foam. Foam stability enhancement due to bulk viscosity changes and surface effects were elucidated. The increase in foam stability was investigated by reference to the properties of thin liquid films and the macroscopic interface of test solutions. Propylene glycol alginate was found to slow the rate of thin film drainage, increase the equilibrium thickness of the films, slow the lateral diffusion of a fluorescent probe molecule located in the adsorbed layer and increase the elasticity of the interface. Data are consistent with propylene glycol alginate-induced crosslinking of protein in the adsorbed layer. This polysaccharide presents a means for controlling protein foam stability.

Enhancement of Protein Foam Stability by Formation of Wheat Arabinoxylan‐Protein Crosslinks

ABSTRACTThe foam stability properties of a defined mixed solution of Tween 20 and bovine serum albumin was evaluated as a function of arabinoxylan concentration. A marked increase in the foam stability was observed with low concentrations of arabinoxylan. Maximum improvement in the foam stability was obtained with 0.2–0.3 mg/mL of arabinoxylan. Enhancement of foam stability due to a combination of bulk viscosity changes and surface effects was identified. The relative contribution of arabinoxylan to bulk viscosity and adsorbed layer structure was studied by examination of the properties of thin liquid films and the macroscopic air‐water interface. Arabinoxylan reduced the rate of thin film drainage, increased the equilibrium thickness of the films, slowed the lateral diffusion of a fluorescent probe molecule located in the adsorbed layer, and increased the surface elasticity. These data are congruent with arabinoxylan‐mediated crosslinking of adsorbed protein. These observations may be of significance in gas retention during breadmaking. In addition, this naturally occurring polysaccharide offers potential for use in the control of protein foam stability.

Molecular Diffusion and Drainage of Thin Liquid Films Stabilized by Bovine Serum Albumin−Tween 20 Mixtures in Aqueous Solutions of Ethanol and Sucrose

The drainage characteristics and surface lateral diffusion properties of thin liquid films (foam lamellae) formed by BSA (bovine serum albumin) and Tween 20 were studied using optical microscopy including epi-illumination and fluorescence recovery after photobleaching. The concentrations of the surfactant and protein at the aqueous phase interface, the composition of the interface (BSA, Tween 20, and BSA-Tween 20 mixtures), and the aqueous phase composition (1 M ethanol and 0.5 M sucrose) were the variables studied. This study has revealed many complicated effects arising from film-film and film-subphase interactions as a function of the variables studied. The interactions between Tween 20 and BSA at the interface were found to dependent not only on the interfacial composition but also on the subphase composition.

Rheological properties of thin liquid films by molecular dynamics simulations

In this paper we present the molecular dynamics simulations of thin fluids films sheared in Couette flow geometry between two structured plane walls. An NVT ensemble of atoms was chosen and simulation conducted in isothermal conditions. To keep the temperature at the required level a Gaussian thermostat was employed. This method was shown to be superior to the simple velocity rescaling method, especially at high shear rates. The Gaussian thermostat method gave results for viscosity in good agreement with the results of other researchers who used the reservoir method. The results for density and velocity profiles were obtained for a wide range of simulation parameters. The effects of shear rate and wall-fluid interaction strength were investigated in detail over a wide range of parameters. The material functions and normal stress differences were also obtained and the effects of shear rate and wall strength parameter on these properties were studied. The effect of film thickness on the viscosity was investigated and was compared with what we found for bulk fluid using the SLLOD algorithm. the existence of a non-Newtonian region with shear-thinning effect is found and examined for various films. The results suggest an increase in viscosity for thinner films in the Newtonian regime, though this is valid only for a limited range of wall-fluid interaction strength. A decrease in viscosity was also observed when the attraction force of the wall was increased.

Control of Surfactant-Induced Destabilization of Foams through Polyphenol-Mediated Protein-Protein Interactions

The effectiveness of (+)-catechin in the control of foam stability of a deliberately destabilized Tween 20//3-lactoglobulin mixed system was investigated. Significant improvement in the foamability and foam stability was obtained in the presence of low concentrations of (+)-catechin. The maximum improvement was observed at a molar ratio of (+)-catechin to protein of approximately 0.1. The mechanism of action of (+)-catechin was investigated by careful study of the properties of thin liquid films (isolated foam lamellae). This polyphenolic compound was found to increase the equilibrium thickness was located in the adsorbed layer. The data are consistent with (+)-catechin-induced cross-linking of proteins in the adsorbed layer. Such molecules offer considerable potential protein foam stability.

Competitive adsorption of l-α-lysophosphatidylcholine/β-lactoglobulin mixtures at the interfaces of foams and foam lamellae

The stability of foams formed with the protein β-lactoglobulin as a function of increasing concentration of the lipid analogue l-α-lysophosphatidylcholine were investigated using a microconductivity technique. The drainage, surface diffusion and thickness properties of thin liquid films (foam lamallae) were also studied using optical microscopy including epi-illumination, fluorescence recovery after photobleaching and film interferometry techniques. In addition, the surfactant binding properties of the protein were examined. The addition of small quantities of l-α-lysophosphatidylcholine to β-lactoglobulin (molar ratio, R < 7:1) increased the foam stability, whereas a slightly higher concentration of surfactant in the mixture ( R = 10) caused foam destabilisation. The explanation of these observations is based on changes in the composition and structure of the adsorbed interfacial layers of the thin films caused by competitive displacement of the protein by the surfactant.

Influence of surface forces on hydrodynamics of wetting

It is well known that properties of thin liquid films deviate from those of a bulk liquid. Disjoining pressure was introduced by Derjaguin to describe thin liquid film behavior. The motion of a long oil drop or bubble under the action of an imposed pressure difference in a thin capillary filled with another immiscible liquid is investigated theoretically, taking into account the action of disjoining pressure. The velocity of the drop motion as a function of the pressure difference and the thickness of the film intervening between the drop and the capillary walls are calculated. The solution obtained coincides with Bretherton's equation at high velocities and deviates substantially at low velocities, which is a manifestation of disjoining pressure action in the thin film. In the same way, a problem concerning the thickness of a thin liquid film covering a thin solid thread drawing out from a liquid container is considered. The thickness of the film is calculated as a function of the thread velocity. The theoretical results obtained are in good agreement with known experimental data.

Steady-state foaming and the properties of thin liquid films from aqueous alcohol solutions

The role of individual foam films in the stabilization of steady-state foams (froths) obtained under dynamic conditions was studied. n-Amyl alcohol (AmOH) was chosen as the frothing agent. It was assumed that the stability of such foams is determined by the stability of the bubbles in the top layer, because the liquid content in the foam volume is high and the gas bubbles do not contact one another. The role of the foam films was found through parallel investigations of steady-state foams, single foam films, and foam bubbles at the surface of a solution at appropriately chosen geometrical parameters and at the same composition of the solution. The results showed that the bubbles in the surface layer of the stationary froths rupture at relatively large thickness (larger than 300 nm) where the specific thermodynamic and kinetic properties of the foam films are not yet operative.

Dielectric Properties of Liquid Surface Zones, I. Experimental Method

AbstractA new method for the determination of the dielectric properties of thin liquid films between metal surfaces is reported. The distance dependence of the dielectric constant εs, near the solid surface is introduced to be equation image where εb is the bulk value of the dielectric constant: x is a distance coordinate and α and β are parameters which can be evaluated on the basis of a theory founded on the model of a spherical/flat condenser plate system.A detailed description of the experimental set‐up is given. First results show that the dielectric constant in thin liquid layers can be altered when these exhibit a molecular long‐range orientation, which was detected by other methods.