Effect Of Disjoining Pressure Research Articles

Collision of liquid drops: bounce or merge?

Whether colliding drops will merge with or bounce off each other is critical to numerous processes, and the physics involved is notoriously complex. In particular, experiments show that both sufficiently slow and fast head-on drop collisions lead to merging, but that there is often an intermediate regime in which bouncing is observed; these transitions in behaviour were recently discovered to be surprisingly sensitive to the radius of the drops and the ambient gas pressure. We show here that these transitions between bouncing and merging are governed by nanoscale phenomena; namely, gas-kinetic and disjoining pressure effects. To capture these crucial effects, a novel, open-source computational model is developed for the simulation of colliding drops. The model uses a hybrid approach, based on solving the Navier–Stokes equations in the drop with a lubrication approach for the unconventional physics of the gas film. Our simulations show remarkably good agreement with experiments of head-on collisions and also provide new experimentally verifiable predictions.

Capillary Condensation of Water in Graphene Nanocapillaries.

Recent experiments have revealed that the macroscopic Kelvin equation remains surprisingly accurate even for nanoscale capillaries. This phenomenon was so far explained by the oscillatory behavior of the solid-liquid interfacial free energy. We here demonstrate thermodynamic and capillarity inconsistencies with this explanation. After revising the Kelvin equation, we ascribe its validity at nanoscale confinement to the effect of disjoining pressure. To substantiate our hypothesis, we employed molecular dynamics simulations to evaluate interfacial heat transfer and wetting properties. Our assessments unveil a breakdown in a previously established proportionality between the work of adhesion and the Kapitza conductance at capillary heights below 1.3 nm, where the dominance of the work of adhesion shifts primarily from energy to entropy. Alternatively, the peak density of the initial water layer can effectively probe the work of adhesion. Unlike under bulk conditions, high confinement renders the work of adhesion entropically unfavorable.

Thermocapillary migration of a droplet on a substrate with wettability difference: A comparison between slip and precursor film models in three dimensions

Moving contact line dynamics calculations include two models: precursor film models and slip models. The lubrication approximation method is used to establish a three-dimensional mathematical model to analyze the droplet thermocapillary migration behavior on a non-uniformly heated solid substrate with a wettability track. The contact line dynamics in the slip model and the disjoining pressure effect in the precursor model are proposed to regulate the substrate wettability. Both models are numerically implemented to investigate droplet spreading for three cases: free spreading on an isothermal substrate, thermocapillary migration on a uniform wettability substrate, and thermocapillary migration on a wettability-confined track. For the case of free spreading on an isothermal substrate, the three-dimensional results of the slip and precursor contact line models are essentially consistent with two-dimensional slip model results. For the case of thermocapillary migration on a uniform wettability substrate, the results of the two models essentially agree with the experimental results. Decreasing the thermal gradient reduces the discrepancies between the two models that result from the coordinate transformation method used in the slip model, which reduces the contact angles measured in the y-direction and enlarges the advancing contact angle in the migration direction. For the case of thermocapillary migration on a wettability-confined track, the slip model gradually shows a “dynamic-pinning” behavior with increasing equilibrium contact angle in the hydrophobic region. By contrast, the precursor film model maintains a stationary pinning behavior but separates a residual liquid outside the track. The precursor film model is preferred over the slip model in lubrication approximations for three-dimensional fluids when calculating complex moving contact dynamics caused by wettability differences. However, the precursor film model must be further optimized to prevent numerical instability.

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

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

Impact of Brine on Disjoining Pressure and Rock Wettability of Oil, Brine, and Mineral Systems

The alteration of rock wettability is essential for improving the efficiency of low-salinity water flooding. However, the fundamental understanding of this process remains unclear. In this paper, an extended Derjguin–Landau–Verwey–Overbeek (DLVO) theoretical model was applied to study the effects of cation types, brine concentrations, and mineral types on rock wettability in an oil–brine system. The zeta potential was measured to analyze the electric double layer. The oil–rock contact angles were tested to evaluate the wettability. The disjoining pressure was also estimated to analyze the rock wettability changes. The divalent cation showed a greater ability to compress the electric double layer. The lower concentrations of cations led to a thicker water film, promoting the detachment of oil from the rock surface. Clay minerals showed a greater impact on the wettability of rocks than quartz. This study improves the fundamental understandings of alteration of wettability in brine, oil, and rock systems and elucidates the mechanisms of disjoining pressure effects on low-salinity water flooding while shedding some light on improving the performance of low-salinity water flooding.

Contact line motion on heated patterned surfaces

We develop a lubrication-type model of contact line motion on a surface patterned by an array of square patches of decreased wettability. The model accounts for evaporation of the liquid due to surface heating, as well as effects of surface tension and disjoining pressure. Stick-slip motion and strong interface deformations are found from the simulations. The height of the capillary ridge becomes spatially nonuniform when the stable contact line travels over the region of decreased wettability. Coupling between the surface pattern and intrinsic contact line instability is investigated. For the weak evaporation limit, the model predictions are compared to previous studies of contact line dynamics under isothermal conditions.

Distribution of a water film confined in inorganic nanopores in real shale gas reservoirs

Distribution of the water film and initial water saturation within inorganic nanopores of shale are key problems for the evaluation and prediction of recoverable gas resources. Numerous research on the distribution and quantitative characterization of water film confined in nanopores have been conducted to the present. However, quantitation of water confined in inorganic nanopores of shale gas reservoir still remains challenging due to the complexity of porous media and reservoir conditions. In the present study, a novel model with disjoining pressure, interfacial tension and real gas effect taken into consideration, was proposed to disclose the distribution of water film confined inside circular or elliptic pores. In addition, the phenomena of capillary condensation and partial condensation have been revealed in elliptic pores. The proposed model was verified by comparing the calculation results with data from previous paper. Results show that the capillary condensation behavior of the water film is remarkably affected by the real gas effect. Water condenses easily into water films under the actual gas reservoir conditions, especially for small pores. However, the interfacial tension and high temperature can inhibit the formation of the water film in the confined space at a nanometer scale. Finally, the partial condensation is more likely to occur at both ends of elliptic pores.

Bowtie-Shaped Deformation Isotherm of Superhydrophobic Cylindrical Mesopores.

Deformation of superhydrophobic cylindrical mesopores is studied during a cycle of forced water filling and spontaneous drying by in situ small-angle neutron scattering. A high-pressure setup is put forward to characterize the deformation of ordered mesoporous silanized silica up to 80 MPa. Strain isotherms of individual pores are deduced from the shift of the Bragg spectrum associated with the deformation of the hexagonal pore lattice. Due to their superhydrophobic nature, pore walls are not covered with a prewetting film. This peculiarity gives the ability to use a simple mechanical model to describe both filled and empty pore states without the pitfall of disjoining pressure effects. By fitting our experimental data with this model, we measure both the Young's modulus and the Poisson ratio of the nanometric silica wall. The measurement of this latter parameter constitutes a specificity offered by superhydrophobic nanopores with respect to hydrophilic ones.

Probing Hydrophobic Interactions between Polymer Surfaces and Air Bubbles or Oil Droplets: Effects of Molecular Weight and Surfactants.

Hydrophobic interaction plays an important role in numerous interfacial phenomena and biophysical and industrial processes. In this work, polystyrene (PS) was used as a model hydrophobic polymer for investigating its hydrophobic interaction with highly deformable objects (i.e., air bubbles and oil droplets) in aqueous solutions. The effects of polymer molecular weight, solvent (i.e., addition of ethanol to water), the presence of surface-active species, and hydrodynamic conditions were investigated, via direct surface force measurements using the bubble/drop probe atomic force microscopy (AFM) technique and theoretical calculations based on the Reynolds lubrication theory and augmented Young-Laplace equation by including the effect of disjoining pressure. It was found that the PS of low molecular weight (i.e., PS590 and PS810) showed slightly weaker hydrophobic interactions with air bubbles or oil droplets, as compared to glassy PS of higher molecular weight (i.e., PS1110, PS2330, PS46300, and PS1M). The hydrophobic interaction between PS and air bubbles in a 1 M NaCl aqueous solution with 10 vol % ethanol was weaker than that in the bare aqueous solution. Such effects on the hydrophobic interactions are possibly achieved by influencing the structuring/ordering of water molecules close to the hydrophobic polymer surfaces by tuning the surface chain mobility and surface roughness of polymers. It was found that the addition of three surface-active species, i.e., cetyltrimethylammonium chloride (CTAC), Pluronic F-127, and sodium dodecyl sulfate (SDS), to the aqueous media could suppress the attachment of the hydrophobic polymer and air bubbles or oil droplets, most likely caused by the additional steric repulsion due to the adsorbed surface-active species at the bubble/polymer/oil interfaces. Our results have improved the fundamental understanding of the interaction mechanisms between hydrophobic polymers and gas bubbles or oil droplets, with useful implications on developing effective methods for modulating the related interfacial interactions in many engineering applications.

Cloaked Droplets on Lubricant-Infused Surfaces: Union of Constant Mean Curvature Interfaces Dictated by Thin-Film Tension.

It has been recently shown that small-volume droplets on lubricant-infused surfaces (LISs) can be analytically modeled using rotationally symmetric constant mean curvature (CMC) surfaces. While such an approach is available for noncloaked droplets, a similar approach is missing for cloaked droplets that are ubiquitous in a number of LIS-related applications. The presence of a thin cloaking film on the top spherical cap portion and its gradual transition to a bulk meniscus remain unaddressed for its implications on the interfacial profile of cloaked droplets. Here, we take into account the cloaking film tension and the disjoining pressure to present a mean curvature-based framework for modeling cloaked droplets on LISs. The transition of the bulk meniscus to a thin film is formulated as a transition regime, which is then modeled as a single imaginary point akin to the Neumann point of noncloaked droplets. We next show that the shape of a small droplet on a LIS essentially obeys a simple fundamental mean curvature relation that changes forms depending on the regimes of lubrication and whether the droplet is cloaked or noncloaked. We validate our framework with the droplet profiles recorded during the evaporation of cloaked droplets in our experiments, as well as those published in the literature. In addition, we also demonstrate the ability to model the shapes of floating droplets on LISs reported in the literature. In addition to quantifying the effect of disjoining pressure on interfacial profiles, we importantly unmask the behavior of the contact line, which is optically covered by the lubricant meniscus around the droplets on LISs.

Positive coupling effect in gas condensate flow: Role of capillary number, Scheludko number and Weber number

The coupled flow in gas condensate reservoirs is investigated using a purely theoretical approach for the description of velocity effect on relative permeability in the absence of inertial effects. A combination of linear stability analysis and dynamic wetting theory is used to describe this coupled flow under the effect of viscous resistance and disjoining pressure. The role of capillary number and Scheludko number (i.e. the dimensionless ratio of disjoining pressure to capillary pressure) is clearly expressed through closed relative permeability formula for the limits of negligible and strong disjoining effect. The model predicts no velocity dependence at very low capillary numbers and a positive velocity effect at high capillary numbers but the negative velocity effect at very high velocities due to strong inertial effects is not predicted due to the model's basic assumptions. Although the model was not intended to serve as a substitute for the experiment, reasonable quantitative agreement is observed between its predictions and available experimental data as long as the Weber number (the dimensionless ratio of inertial resistance to capillary pressure) is smaller than 5 × 10−5, with an average and a maximum relative deviation of 16% and 67% respectively, for a total number of 49 data points.

The effect of surface morphology on the rate of phase change of micron and sub-micron sized 2-D droplets

ABSTRACT Heat transfer via phase change is a major contributor to heat removal in numerous engineering applications. Thin films of liquid result in increased heat transfer due to a reduction of conduction resistance, in addition the pressure jump at the liquid-vapor interface also affects the rate and direction of the rate of phase change. Because of these effects the morphology of the substrate surface is expected to affect the film shape, hence heat transfer, especially in thin films. In this study, the influence of surface characteristics on the rate of phase change from micron- and submicron-sized 2D droplets – i.e. films extending to infinity – forming on a substrate are modeled. Surface film profiles are generated on both flat and nonflat surfaces, triangular or wavy in nature, and a kinetic model for quasi-equilibrium phase change is applied. In the case of wavy surfaces, the surface is assumed to be a harmonic wave with an amplitude equal to the surface roughness and a wavelength corresponding to values commonly encountered in applications. Due to the presence of intermolecular forces at the contact line, which renders the solution of the augmented Young-Laplace equation stiff, an implicit scheme is employed for the numerical integration. To verify the method, the predictions of a molecular dynamics (MD) simulation of a nano-sized droplet present on a V-grooved surface are compared to the continuum model. The augmented Young-Laplace equation is solved numerically along with a phase change model originating from kinetic theory to calculate the shape of the two-phase interface forming the droplet and study the effect of various parameters on the rate of phase change. Results are obtained for droplets with liquid pressures higher and lower than that of vapor, resulting in opposite contribution to phase change due to the pressure jump at the interface. The results show that the heat-transfer rate can be substantially altered due primarily to the combined effects of surface morphology and disjoining pressure. It is also concluded that wavy surfaces with short amplitudes are preferable to ones with longer amplitudes for enhancing the rate of evaporation or condensation.

Development of silicon quantum dots based nano-fluid for enhanced oil recovery in tight Bakken cores

A novel silicon quantum dots (SiQDs) based nano-fluid was first developed for effectively recovering Bakken Oil. The Bakken Formation is one of the largest contiguous deposits of unconventional oil in the USA and Canada. However, the oil recovery rate is only 5%−10% due to the existence of abundant tiny pores and ultra-low permeability. In this work, a novel nano-fluid was developed using ultra-small sized silicon quantum dots (SiQDs) coated with a zwitterionic surfactant. The nano-fluid exhibited good stability in synthetic high salinity brine containing divalent cations at high temperature and excellent capabilities in reducing interfacial tension and altering wettability, making it a good enhanced oil recovery (EOR) agent for tight Bakken Formation. Experimental results showed that the developed nano-fluid recovered 25.72 %OOIP (original oil-in-place) from Bakken core samples by spontaneous imbibition, which was 7.49 %OOIP higher than using surfactant alone. Similar results were obtained in the core flooding tests, where 41.98 %OOIP was finally recovered by the nano-fluid, compared to 30.28% in surfactant case. Herein, the mechanisms of disjoining pressure and synergistic effects between surfactant and nanoparticles have also been put forward. In short, the developed quantum dots based nano-fluid has played an instructive and a reference role in the application of oil field and would be a potential oil displacing agent for EOR.

Role of nanoscale roughness in the heat transfer characteristics of thin film evaporation

Thin film evaporation yields high local heat fluxes that contributes significantly to the total heat transfer rate during various two-phase transport processes including pool boiling, flow boiling, and droplet evaporation, among others. Recent studies have shown a strong correlation between the roughness of a surface and its two-phase heat transfer characteristics, but the underlying role of nanoscale surface roughness in thin film evaporation is not fully understood. In the present work, a thin film evaporation model is developed that accounts for the role of the roughness-affected disjoining pressure and flow permeability in determining the film thickness profile and heat transfer rate. Nanoscale surface roughness leads to a flatter evaporating meniscus profile when the effect of disjoining pressure is more pronounced of the two and promotes evaporation, consistent with previous experimental observations. However, our results reveal that surface roughness may also inhibit evaporation and lead to a steeper evaporating meniscus profile when flow permeability has the more pronounced influence on thin film evaporation. It is important to identify the specific surface roughness characteristics that determine whether disjoining pressure or flow permeaiblity has the stronger influence. To this end, a parametric study is performed that analyzes thin film evaporation on V-grooved surfaces of different depths and pitches. While the heat transfer rate increases monotonically with groove depth, there exists an optimal groove pitch that leads to a maximized evaporation rate. Also, when the groove pitch is smaller than a critical value, surface roughness inhibits thin film evaporation.

Predictive ion-electrostatic model of disjoining pressure of synthetic gel structures in coarse-dispersed soil substrates

The mechanism of disjoining pressure controlling the swelling and water absorption of gel structures for soil conditioning is discussed. Its quantitative description is based on the fundamental ion-electrostatic model of soil water sorption isotherms containing the phase variable of gel swelling degree and specific surface area index of the soil substrate. The model adequately reproduces the s-shape of soil water sorption isotherms in a wide range of water activity from 0.05 to 0.999 depending on the working doses of hydrogels in the range of 0.05-0.3 mass%. Reinforcement of gel structures with amphiphilic dispersed particles increases the effect of disjoining pressure and water absorption.

Modeling of Moving Liquid-Vapor Interfaces in the Constrained Vapor Bubble System

We use a lubrication-type approach to investigate the dynamics of moving contact lines observed during droplet evaporation in the constrained vapor bubble experimental set-up. The effects of capillarity, viscous flow, and disjoining pressure are considered, while gravity is neglected. The disjoining pressure is described by a two-component model. Direct comparisons to the experimental recordings of receding contact line dynamics for n-butanol in a quartz cuvette are made.

The nonequilibrium crystallization force

The forces exerted by growing crystals on the surrounding materials play a major role in many geological processes, from diagenetic replacement to rock weathering and uplifting of rocks and soils. Although crystallization is a nonequilibrium process, the available theoretical predictions for these forces are based on equilibrium thermodynamics. Here we show that nonequilibrium effects can lead to a drop of the crystallization force in large pores where the crystal surface dissociates from the surrounding walls during growth. The critical pore size above which such detachment can be observed depends only on the ratio of kinetic coefficients and cannot be predicted from thermodynamics. Our conclusions are based on a physical model which accounts for the nonequilibrium kinetics of mass transport, and disjoining pressure effects within the thin liquid film separating the crystal and the surrounding walls. Our results suggest that the maximum size of the pores that can sustain crystallization forces close to the equilibrium prediction ranges from micrometers for salts to a millimetre for low-solubility minerals such as calcite. These results are discussed in the light of recent experimental observations of the growth of confined salt crystals.

Inertial impedance of coalescence during collision of liquid drops

The fluid dynamics of the collision and coalescence of liquid drops has intrigued scientists and engineers for more than a century owing to its ubiquitousness in nature, e.g. raindrop coalescence, and industrial applications, e.g. breaking of emulsions in the oil and gas industry. The complexity of the underlying dynamics, which includes occurrence of hydrodynamic singularities, has required study of the problem at different scales – macroscopic, mesoscopic and molecular – using stochastic and deterministic methods. In this work, a multi-scale, deterministic method is adopted to simulate the approach, collision, and eventual coalescence of two drops where the drops as well as the ambient fluid are incompressible, Newtonian fluids. The free boundary problem governing the dynamics consists of the Navier–Stokes system and associated initial and boundary conditions that have been augmented to account for the effects of disjoining pressure as the separation between the drops becomes of the order of a few hundred nanometres. This free boundary problem is solved by a Galerkin finite element-based algorithm. The interplay of inertial, viscous, capillary and van der Waals forces on the coalescence dynamics is investigated. It is shown that, in certain situations, because of inertia two drops that are driven together can first bounce before ultimately coalescing. This bounce delays coalescence and can result in the computed value of the film drainage time departing significantly from that predicted from existing scaling theories.

Evaporation and interface dynamics in microregion on heated substrate of non-uniform wettability

Modeling of evaporation in the microregion separating an adsorbed film and a macroscopic meniscus is important for a number of applications such as pool boiling and micro heat pipes. We develop a model of a moving microregion incorporating the effects of evaporation, viscous flow, surface tension, and two-component disjoining pressure and apply it to study motion of microregions over heated surfaces with wettability defects or patterns. Substantial heat transfer enhancement is found when a receding microregion passes over the portion of the substrate with higher wettability than the surrounding areas. The effect is explained in physical terms by widening of the part of the microregion with low thermal resistance. To achieve sustained heat transfer enhancement, we then consider a configuration in which the substrate is patterned by an array of high-wettability stripes and investigate the evaporative flux as a function of the geometry of the pattern. The effects of Marangoni stress at the interface are also studied and found to result in slight reduction of the average evaporative flux, as the tangential stress at the interface reduces the liquid supply into the microregion.

Limitations of Matching Condensing Film Profile on a Micro Fin with the Groove: Critical Effect of Disjoining Pressure

ABSTRACTCondensation on a fin top terminating with a groove involves several simultaneous phenomena including vapor–liquid boundaries whose shapes are unknown a priori, fluid flow due to capillary and disjoining pressure gradients, and condensation over thin films. This problem occurs in grooved heat pipes, where the condensation is predominantly present on fin tops due to the thinner liquid film – having a lower thermal resistance compared to inside the groove where the fluid is substantially thicker. Majority of the studies in the literature assume an approximate profile for the liquid film surface and apply an integral balance for conservation laws, accounting for the effect of the capillary pressure only. In addition, this approximate profile is matched with the liquid profile inside the groove, which serves as a boundary condition. Although intuitive, validity of the matching is not straightforward, and its limitations have never been discussed in the literature, despite the presence of experimental findings to the contrary. In the current study, the effect of disjoining pressure and matching conditions with the groove is investigated using a comprehensive model. The results suggest that for small temperature differences and small slopes, the effect of disjoining pressure is non-negligible, and beyond limiting values of edge angles, the effect of disjoining pressure precludes solutions where the fin top film matches the groove in a smooth transition.