Stability Of Thin Liquid Films Research Articles

Nanoscale Investigation into Dynamics of Thin Liquid Films during Bouncing and Attachment of Rising Air Bubbles on Hydrophilic and Hydrophobic Surfaces.

Investigations on bouncing and attachment of free-rising air bubbles on hydrophobic surfaces have been limited to side-view, high-speed photography of the bubble-plate attachment process. In this work, an investigation of the dynamics as well as stability of thin liquid films (TLFs) between free-rising air bubbles and quartz surfaces was performed using a newly developed multiple-wavelength synchronized reflection interferometry microscopy (SRIM) technique. The effect of surface hydrophobicity on both the stability and critical rupture thickness of TLFs was investigated. Results showed that the TLF ruptured at a critical rupture thickness of 100-1000 nm or beyond during bubble's impact on hydrophobic quartz surfaces. The critical rupture thicknesses varied depending on the surface hydrophobicity as well as surface asperity. A higher surface hydrophobicity, in general, contributed to a higher critical rupture thickness. In addition, the effect of n-octanol on the stability of the TLFs was investigated. Results showed that film stability increased with increasing the concentration of n-octanol, which was accompanied by a significant decrease in the critical rupture thickness. The present result illustrates, for the first time, the dynamics of TLFs on hydrophobic surfaces under a dynamic condition compared with previous studies under a quasi-equilibrium condition. The information revealed from the present work has a significant implication to many industrial applications, including froth flotation and other biological and semiconductor applications.

Insight into the influence of surface wettability on flotation properties of solid particles – Critical contact angle in flotation

Bubble-particle attachment is a fundamental element of the flotation process, and its occurrence depends on the stability of the thin liquid film separating these objects. In this study, the effect of surface wettability on the stability of liquid film and subsequent attachment was investigated by flotation of glass microspheres (70–110 μm) with controlled contact angles (9–89°) in a Hallimond tube. Additionally, pH was varied to determine the electrostatic interactions between the bubble and particles. The results showed that for the flotation process to take place, the minimum (critical) value of the contact angle must be exceeded. This value was found to be ca. 25° when the electrostatic interactions were either attractive or weakly repulsive and as high as 62° for strong repulsive electrostatic interactions. These findings highlight the role of attractive hydrophobic interactions, whose importance increases with increasing surface contact angle in the context of stability of thin liquid films.

Oppositely charged surfactants and nanoparticles at the air-water interface: Influence of surfactant to nanoparticle ratio

HypothesisThe interactions between oppositely charged nanoparticles and surfactants can significantly influence the interfacial properties of the system. Traditionally, in the study of such systems, the nanoparticle concentration is varied while the surfactant concentration is kept constant, or vice versa. However, we believe that a defined variation of both components' concentration is necessary to accurately assess their effects on the interfacial properties of the system. We argue that the effect of nanoparticle-surfactant complexes can only be properly evaluated by keeping the surfactant to nanoparticle ratio constant. ExperimentsZeta potential, dynamic light scattering, high amplitude surface pressure and surface tension measurements are employed synergistically to characterize the interfacial properties of the nanoparticle-surfactant system. Interferometric experiments are performed to highlight the effect of surface concentration on the stability of thin liquid films. FindingsThe interfacial properties of surfactant/nanoparticle mixtures are primarily determined by the surfactant/nanoparticle ratio. Below a certain ratio, free surfactant molecules are removed from the solution by the formation of surfactant-nanoparticle complexes. Surprisingly, even though the concentration and hydrophobicity of these complexes do not seem to have a noticeable impact on the surface tension, they do significantly affect the rheological properties of the interface. Above this ratio, free surfactant monomers and nanoparticle-surfactant complexes coexist and can co-adsorb at the interface, changing both the interfacial tension and the interfacial rheology, and thus, for example, the foamability and foam stability of the system.

Frequency-Dependent Dewetting of Thin Liquid Films Using External ac Electric Field.

The stability of thin liquid films on a surface can be controlled by using external stimuli, such as an electric field, temperature, or light, by manipulating the total excess free energy of the system. It has been previously shown that thin lubricating films on slippery surfaces can be destabilized via the spinodal mechanism using an external electric field, which returns to the original stable configuration upon the electric field. However, the role of the frequency of the applied ac electric field is not clear, which is the main topic of study in this report. When an ac electric field of fixed voltage and varying frequency is applied across thin lubricating films of slippery surfaces, a different dewetting behavior is observed. Characteristic length and time scales of dewetting depend strongly on the frequency of the applied voltage, which is primarily due to the change in the dielectric behavior of the lubricating fluid. In addition, the interplay of various time scales involved in the dewetting process also depends on the frequency.

Breakup of thin liquid films with viscous interfaces

Thin liquid films are ubiquitous in nature and have many practical applications. From biological films to the curtain coating process, thin films are present in both large and small scales. Despite their importance, understanding the stability of these films remains a significant challenge due to the fluid–fluid interface that is free to deform, affected by interfacial tension and complex rheological behavior. Instabilities in thin films are often caused by van der Waals attractions, which can lead to the rupture of the layer. To investigate the rupture dynamics, numerical methods are commonly used, such as asymptotic derivations of the lubrication theory or interface tracking methods. In this paper, we present a computational study of the breakup dynamics of a stationary thin liquid sheet bounded by a passive gas with a viscous interface, using the arbitrary Lagrangian–Eulerian method and the Boussinesq–Scriven constitutive law to model the rheological behavior. Our results demonstrate that the stability of thin liquid films is influenced by both surface rheology and disjoining effects and that the viscous character of the interface can delay sheet breakup, leading to more stable films.

Electric Field Driven Reversible Spinodal Dewetting of Thin Liquid Films on Slippery Surfaces

AbstractThe stability of thin liquid films on a surface depends on the excess free energy of the system involving various short‐range and long‐range interactions. When forced to wet an unfavorable surface, thin liquid films dewet into multiple small‐sized droplets via spinodal, homogeneous, or heterogeneous nucleation process. However, if the total excess free energy of the system can be manipulated using external stimuli, for example, electric field, temperature, or light, one can control the stability of thin liquid films on demand. Here the electric‐field induced reversible dewetting and rewetting of thin liquid films underneath aqueous drops on slippery surfaces is studied. Upon applying voltage, hundreds of nanometer thick stable liquid films dewet in a manner identical to the spinodal dewetting of few nanometer‐thick liquid films following the linear stability analysis. Upon removing the applied voltage, the dewetted droplets spread, coalesce with neighboring ones, and form a uniform film again, albeit taking significantly longer times. The characteristic features defined via spatial and temporal evolution of the dewetting and rewetting processes are present over multiple cycles suggesting the complete reversibility of the process.

Linear Stability of Thin Liquid Films Flows Down on an Inclined Plane Using Long-Wave Theory

The Long-Wave Theory is applied to investigate the dynamic stability of free thin fluid films flowing down an inclined plane. We assume that thin supported films have a thickness of 𝐻̅ and less than or equal to one hundred nm. Equations of Navier and Stokes, continuity-equation, and related boundary conditions are used to represent a two-dimensional stream demonstrated as a continuum. Under long-wave approximation, the governing equations for the film interface have been rescaled and simplified to obtain a highly non-linear condition of development for the film interface. A procedure for evaluating the magnitude of the effects of the high-order effects is also used to formulate simplified governing equations. In the future, we can study this problem by adding heat transfer over the stretching plate. In addition, we can also study the stability analysis to two-dimension flow of a viscous liquid within a horizontal thin liquid film with neglecting the inertia terms of Navier-Stokes equations.

Effect of electrolyte and flotation collector on foam films in the presence of frothers

The growing complexity of mineralogy and environmental considerations requires mineral concentrators to maximise the synergistic effect of reagent mixtures and salts on flotation performance. There is thus an increasing need to study those interactions. This paper examines the stability of thin liquid films formed from frother solutions in the presence of a collector, i.e., potassium amyl xanthate (PAX) and an electrolyte (NaCl). Two different frothers were used: methyl isobutyl carbinol (MIBC) and di polypropylene glycol (DPG). The film thinning process was recorded to determine the film lifetime. The rupture thickness was calculated, and the surface potential was obtained from the films that reached an equilibrium state. The presence of 1 ppm PAX significantly improved the stability of films in certain solutions of DPG (10–2 × 103 ppm) and MIBC (1 × 102–2 × 103 ppm), where the films could be stable at a very small thickness. The changes in the surface potential of films generated in DPG and DPG/PAX solutions suggested that the addition of PAX influenced the frother adsorption on the film surface and supported a molecular interaction between xanthate and frother molecules at the air–water interface. Overall, adding salt decreased the stability of films with frother/PAX. Higher salt concentrations reduced the film stability, and the films generally ruptured at a greater thickness. Specifically, in the presence of 2 M NaCl, the stability of the films formed from the frother/PAX solutions significantly declined at higher frother concentrations, probably due to the generation of microbubbles or oil droplets. The cooperative effect between the frother, PAX and NaCl was evidently seen from the film behaviour of the ternary system. The stabilities of those films were more affected by the presence of salt than PAX. Closer attention needs to be paid to the presence and concentration of salt in monitoring the flotation pulp in a plant.

Impact of Amphiphilic Nanostructures on Formation and Rheology of Interfacial Layers and on Foam Film Drainage

The aim of the present studies is to clarify how the surfactant adsorption layer properties are related to the course of the drainage parameters of microscopic foam films in the special case of aqueous solutions containing premicellar amphiphilic nanostructures. The scope of the research covers the adsorption dynamics, construction of equilibrium adsorption isotherms, surface rheology of interfacial layers, and foam film drainage kinetics. It is established that, in the premicellar domain, there are concentration intervals, where the considerable irregularities of adsorption layerproperties are observed: several plateau regions in the surface tension isotherms, unusual changes of the surface rheological characteristics, etc. The systematic investigation of the drainage of foam films obtained from these solutions show that the dependences of basic kinetic parameters of the films on the amphiphile concentration run in synchrony with changes in the adsorption layer properties. Thus, the presence of smaller loose aggregates (premicelles) plays a significant role for the kinetic stability of films. The importance of this research is related toproviding a better insight into the initial stages of self-assembling phenomena and into the factors determining thedrainage and the stability of thin liquid films. The results have implications for the understanding and the correct prediction of properties of foam systems.

Stability of gravity-driven liquid films overflowing microstructures with sharp corners

We are concerned with the stability of thin liquid films overflowing single microstructures with sharp corners. The microstructures were of rectangular and triangular shape. Their heights and widths were 0.25, 0.5 and 0.75 times the Nusselt film thickness. To observe smooth, wavy and very unstable films we performed simulations with Reynolds numbers ranging from 10 to 70. The dynamics of the liquid film and the overflowing gas phase were described by the coupling between the Cahn-Hilliard and Navier-Stokes equations. The resulting model forms a very tightly coupled and nonlinear system of equations. Therefore we carefully selected the solution strategy to enable efficient and accurate large-scale simulations. Our results showed that the formation of waves was shifted to higher Reynolds numbers compared to the film on a smooth surface. If waves were finally formed the microstructures led to irregular waves. Our results indicate a great influence of the microstructure's shape and dimension on the stability of the overflowing liquid film.

A Review on Hydrodynamical Stability of Thin Film Flowing Along an Inclined Plane

The dynamics and stability of thin liquid films have fascinated scientists over many decades. Thin film flows occur over a wide range of length scales and are central to numerous areas of engineering, geophysics and biophysics. These include nanofluidics and microfluidics, lava flows, coating flows, tear-film rupture, dynamics of continental ice sheets and surfactant replacement therapy. Study of falling film instability has its wide applications in the practical field of industry and engineering. Practical applications in industrial processing motivate the recent research to investigate the factors which may affect the formation of waves on the surface of the coating layers and/or to determine the ways to overcome or to minimize the unwanted factors within the desired limit of tolerance. The dynamics of a liquid film flowing down a plane under the action of gravity is a problem which appears in many technological and natural systems, namely large scale geophysical environments such as lava flows or spillways, daily life scenarios such as water flowing down a window pane or a slippery road on a rainy day, chemical engineering processes such as evaporators, heat exchanges and falling film reactors or surface coating. The aim of this paper is to throw light on the studies conducted on hydrodynamical stability.

Molecular simulation of thin liquid films: Thermal fluctuations and instability.

The instability of a thin liquid film on a solid surface is studied both by molecular dynamics simulations (MD) and a stochastic thin-film equation (STF), which models thermal fluctuations with white noise. A linear stability analysis of the STF allows us to derive a power spectrum for the surface fluctuations, which is quantitatively validated against the spectrum observed in MD. Thermal fluctuations are shown to be critical to the dynamics of nanoscale films. Compared to the classical instability mechanism, which is driven by disjoining pressure, fluctuations (a) can massively amplify the instability, (b) cause the fluctuation wavelength that is dominant to evolve in time (a single fastest-growing mode does not exist), and (c) decrease the critical wavelength so that classically stable films can be ruptured.

Quantitative imaging of the complexity in liquid bubbles\u2019 evolution reveals the dynamics of film retraction

The dynamics and stability of thin liquid films have fascinated scientists over many decades. Thin film flows are central to numerous areas of engineering, geophysics, and biophysics and occur over a wide range of lengths, velocities, and liquid property scales. In spite of many significant developments in this area, we still lack appropriate quantitative experimental tools with the spatial and temporal resolution necessary for a comprehensive study of film evolution. We propose tackling this problem with a holographic technique that combines quantitative phase imaging with a custom setup designed to form and manipulate bubbles. The results, gathered on a model aqueous polymeric solution, provide unparalleled insight into bubble dynamics through the combination of a full-field thickness estimation, three-dimensional imaging, and a fast acquisition time. The unprecedented level of detail offered by the proposed methodology will promote a deeper understanding of the underlying physics of thin film dynamics.

Stability of thin liquid film flowing down a rotating horizontal or inclined plane by momentum-integral method

Abstract In this study we investigate the linear as well as the weakly nonlinear stability of a thin liquid film flowing down a rotating horizontal or inclined plane. The analysis is carried out using the momentum-integral method, which is valid for small as well as large Reynolds number ( R e ). A single free surface equation is obtained considering self-similar velocity profiles for specific ranges of flow parameters. Considering sinusoidal perturbation of the free surface and taking into consideration the linear terms from the free surface equation we obtain the linear phase speed along x -direction c x and the critical wave number k c . It is found that the linear phase speed decreases with the increasing value of the parameter G which is directly proportional to the square root of the Coriolis force, the angle of propagation of the wave ϕ and the angle of inclination ( θ ) of the inclined plane but it increases with the increasing value of Taylor number ( T a ). The study also points out the variation of the region of instability with the variation of G , ϕ , θ , R e and T a . It is found that as G increases, the region of instability decreases, which reflects the stabilizing role of Coriolis force. It is observed that as the angle of propagation of the wave ϕ increases from 0 to 0.6725, the unstable region decreases but increases if ϕ varies from 0.6725 to 2.2, thereafter again decreases up to ϕ = π . Also it is shown that for ϕ = 0, the region of instability decreases as θ increases but the situation becomes reverse if ϕ = π 4 . The Taylor number T a plays a double role. For ϕ = 0, π 6 , the linear unstable zone increases as T a increases but for ϕ = π 4 , the situation reverses. Also it is found that the region of instability always decreases for the variation of small as well as moderate Reynolds number, up to a critical value of the propagation angle ϕ c . But for small values of R e when ϕ > ϕ c , the unstable region first decreases up to a critical value of R e , then again increases up to another critical value of R e and ultimately decreases for moderate values of R e . Weakly nonlinear stability analysis reveals the dependence of J 2 in the Landau’s equation and imaginary part of the complex frequency ω i on the variation of the Coriolis and centrifugal forces. Both the forces ensure the existence of supercritical and unconditional stable zones. The nonlinear study confirms the stabilizing role of the Coriolis force and the destabilizing role of the centrifugal force as obtained in the linear stability analysis. We also obtained that in the supercritical stable zone the threshold amplitude decreases (increases) as the Coriolis force (centrifugal force) increases. Finally we find that the nonlinear phase speed decreases with the increase of the Coriolis force but increases with the small increment of the centrifugal force.

Development of a 3D-printed modified Scheludko-cell: Potential application for adsorption and thin liquid film study

A systematic study on the formation of thin liquid films (TLFs) of n-dodecane, as an intervening liquid phase in aqueous surfactant solution, was performed using a modified 3D-printed Scheludko-cell (MSC). The formation and stability of TLFs are electrically detected using the current density-voltage (J-Ψ) characteristics of the system, and it is confirmed by in-situ visualization of TLFs. The conductivity of the TLFs is measured using the J-Ψ curve and resistors in a series model of the system. The induced conductivity of the film is measured for different concentrations of potassium chloride (KCl) and sodium dodecyl sulfate (SDS) aqueous solutions. We monitored an increase in TLF conductivity due to the accumulation of inorganic hydrophilic ions like (K+ or Cl−) and adsorption of amphiphilic anion of dodecyl sulfate to the interface. The higher rate of increase in the TLF conductivity in the case of SDS is attributed to the ability of amphiphilic ions to pass through a so-called unscreened potential in the vicinity of oil-aqueous solution interface leading to an unscreenable potential at the boundary of the TLF. The predicted adsorption time-scale of surfactants using the MSC cell in ionic liquid (IL) phase, at O/W interface, is found to be in good agreement with the dynamic interfacial tension test results.

Stability of thin liquid films subjected to ultrasonic vibration and characteristics of the resulting thin solid films

Thin solid films have ubiquitous presence in many existing and emerging technologies. Solution-processed thin solid films may be fabricated by casting a thin liquid film followed by a drying step. It has been shown that by imposing a low-amplitude ultrasonic vibration on the substrate, the physical and structural characteristics of the resulting thin solid films is improved, significantly. Therefore, in this study, to investigate and rationalize the aforementioned findings, the evolution and stability of thin and ultrathin films of Newtonian liquid solutions, subjected to vertical and horizontal ultrasonic vibration is studied, using the long-wave approximation and negligible inertia forces. An explicit criterion is obtained for the film instability. Consistent with established theories, it is found that the vertical vibration tends to destabilize the thin liquid film, although for ultrasonic vibrations with low-amplitude, the term contributing to the perturbation growth rate decays rapidly with time and the film may remain stable. However, the vertical ultrasonic vibration is found as a significant destabilizing force, if the film thickness is near a critical value in which case the destabilizing van der Waals and stabilizing gravity and surface tension forces balance one another. To validate the model, experiments on thin liquid films of dilute polymeric solutions are performed. It is found that while imposing ultrasonic vibration may potentially destabilize and breakup the thin film, imposing a low-power vibration can significantly improve the homogeneity, electrical properties, and uniformity of the film, whereas a large-amplitude vibration may have a detrimental effect, because of excessive mixing and agitation of the liquid film or cracking of the resulting thin solid film.

Thermo-Electrohydrodynamic Patterning in Nanofilms

To improve the electrically assisted patterning process and create smaller sized features with the higher active surface area, the combined thermocapillary-electrohydrodynamic (TC-EHD) instability of liquid nanofilms is considered. First, the 3-D thin film equation is rederived for nonisothermal films and then the influential factors on the dynamics and stability of thin liquid film are found using linear stability (LS) analysis. Nonlinear studies are also conducted to investigate the long-time evolution of the interface using an in-house developed Fortran code employing high order finite difference and adaptive time step solver for the spatial and time derivatives. The number density of pillars (columnar raised structure) formed in 1 μm(2) area is significantly increased compared to the EHD base-case and nanosized pillars are created due to the thermocapillary effects. Relative interface area increases of up to 18% due to this pattern miniaturization are realized. It is also found that increase in the thermal conductivity ratio of layers changes the mechanism of pattern formation resulting in nonuniform and randomly distributed micro pillars being generated.

Model of Meniscus Shape and Disjoining Pressure of Thin Liquid Films on Nanostructured Surfaces with Electrostatic Interactions

The effect of electrostatic interactions on the stability of thin liquid films on nanostructured surfaces is important in lubrication, wetting, and phase change but is poorly understood. In this study, a general, closed-form model is developed to account for both the effects of electrostatic and van der Waals interactions on meniscus shape and disjoining pressure for thin liquid films on nanostructured surfaces based on the minimization of free energy, the Derjaguin approximation, and the disjoining pressure theory for flat surfaces. The model is verified using the molecular dynamics (MD) simulations for a water–alumina system with both triangular and square nanostructures of varying depth and film thickness. Good agreement is obtained between MD results and model predictions, demonstrating the robustness of the analytical model. The results show that the electrostatic interactions enhance the disjoining pressure, thereby making the meniscus more conformal to the nanostructured surfaces. In addition, the electrostatic disjoining pressure is shown to increase with the nanostructure depth but decrease with the thin film thickness.

Effect of nanostructures on the meniscus shape and disjoining pressure of ultrathin liquid film.

The stability of thin liquid films on nanostructured surfaces is important but poorly understood. Here, we develop a general model of the meniscus shape and disjoining pressure for thin liquid films on nanostructured surfaces based on the minimization of the free energy and the Derjaguin approximation. This model is then compared with molecular dynamics simulations for a water-gold system with triangular and square nanostructures of varying depth and film thickness, demonstrating the robustness of the analytical model.

Stabilization of thin liquid films by repulsive van der Waals force.

Using high-speed video recording of bubble rise experiments, we study the stability of thin liquid films trapped between a rising bubble and a surfactant-free liquid-liquid meniscus interface. Using different combinations of nonpolar oils and water that are all immiscible, we investigate the extent to which film stability can be predicted by attractive and repulsive van der Waals (vdW) interactions that are indicated by the relative magnitude of the refractive indices of the liquid combinations, for example, water (refractive index, n = 1.33), perfluorohexane (n = 1.23), and tetradecane (n = 1.43). We show that, when the film-forming phase was oil (perfluorohexane or tetradecane), the stability of the film could always be predicted from the sign of the vdW interaction, with a repulsive vdW force resulting in a stable film and an attractive vdW force resulting in film rupture. However, if aqueous electrolyte is the film-forming bulk phase between the rising air bubble and the upper oil phase, the film always ruptured, even when a repulsive vdW interaction was predicted. We interpret these results as supporting the hypothesis that a short-ranged hydrophobic attraction determines the stability of the thin water film formed between an air phase and a nonpolar oil phase.