Disjoining Pressure Gradient Research Articles

Contribution of solid–liquid–vapor interface to droplet evaporation

A solid–liquid–vapor (slv) interface is formed by a capillary and disjoining pressure gradient near the macroscopic contact line of a droplet. At a micro/nanostructured surface, the scale of the slv interface increases, such that it can significantly contribute toward droplet evaporation. However, the wettability of the micro/nanostructured surface and droplet volume may affect this contribution and warrant further clarification. In this study, the evaporation rate of a droplet at a macroscopic liquid–vapor (lv) interface was derived theoretically and that at fabricated surfaces was measured experimentally. Considering droplet mass conservation, the evaporation rate at the slv interface was estimated based on the difference between the experimental results and theoretical evaporation rate at the lv interface. The corresponding scale of the slv interface was estimated and further validated with experiments. It was found that the theoretical evaporation rate agreed with the experimental results, when the scale of the slv interface was negligible. For microstructured surfaces, the scale of the slv interface was estimated to be 253–940 µm for a 4 μL water droplet, significantly contributing toward the evaporation rate, in addition to the evaporation at the lv interface. The evaporation rate at the slv interface accounted for 16–48 % of the total droplet evaporation rate. The scale of the slv interface and evaporation rate increased with a decrease in the initial contact angle or an increase in the droplet volume. The results of this study are expected to contribute toward improvements in practical engineering and medical applications using micro/nanostructured surfaces for droplet evaporation.

Greenly Synthesized Magnetite@SiO2@Xanthan Nanocomposites and Its Application in Enhanced Oil Recovery: IFT Reduction and Wettability Alteration

Nanomaterials were used in enhanced oil recovery methods to improve the surface activity at the solid/oil/liquid contact line and remove oil through disjoining pressure gradient mechanism. The main objective of study is to prepare a green nanocomposites (NCs) and identify its effect on the interfacial tension (IFT) reduction and wettability alteration. A simple, economical and green technique was applied to synthesize Fe3O4@SiO2@xanthan NCs from the Alocasia macrorrhiza plant extract. The prepared NCs were identified employing X-ray diffraction, Fourier-transform infrared spectroscopy, and scanning electron microscopy. The fabricated NCs at different concentrations (250 to 1500 ppm) were dispersed in distilled water to prepare the nanofluid solutions which were characterized by analyzing their viscosity, pH and conductivity properties. Additionally, the effect of the prepared nanofluids on the IFT reduction and wettability alteration of the crude-oil/aqueous-phase/rock system was examined. Obtained experimental results demonstrated a significant decrease in the values of IFT and contact angle under the effect of the synthesized NCs, which were reduced from 28.3 mN/m and 134° to 4.35 mN/m and 28°, respectively. Thus, the wettability of the used carbonate rock was greatly altered from a strong oil-wet to a strong water-wet system.

Precise Wettability Characterization of Carbonate Rocks To Evaluate Oil Recovery Using Surfactant-Based Nanofluids

Free energy of nanoparticles can increase the surface activity at the solid/oil/liquid contact line and remove oil through the disjoining pressure gradient mechanism. Surfactants can also remove oi...

Structural disjoining pressure induced solid particle removal from solid substrates using nanofluids

Nanofluids comprising nanoparticle suspensions in liquids have significant industrial applications. Prior work performed in our laboratory on the spreading of a nanofluid on a solid substrate has revealed that the structural disjoining pressure gradient caused by the layering of the nanoparticles normal to the confining plane of the film with the wedge profile is a new mechanism for oily soil detachment from the solid substrate. In the present work, we explore the application of this new mechanism for the solid particle detachment using latex particles on glass and a copper-coated wafer substrate using nanofluids. In the experiment, we employed the nanofluids to observe the detachment of the latex particles adhered to the glass substrate.We found that the structural disjoining pressure exerted by the nanoparticles can detach the solid particles on the glass substrate. A video depicting this mechanism is provided. Our results showed that the detachment of the particulate solid particles on the solid substrate was clearly enhanced by the nanofluids, compared to using only pure liquids (such as water). The detachment efficiency was increased with the increase in the nanoparticle volume fraction.Our nanofluids also showed a greater detachment efficiency for the particulate soil removal from the copper-coated wafer substrates compared to that of pure liquids.We found that the detachment efficiency is well correlated with the calculated structural disjoining energy.Our findings in this paper provide new insights for the novel application of the structural disjoining energy mechanism for cleaning hard surfaces.

Spreading Kinetics of Lubricant Droplets on Magnetic Recording Disks

The spreading of droplets of perfluoropolyether lubricants is studied on unlubricated, carbon-overcoated disk surfaces, the type used in magnetic recording disk drives. As the number of hydroxyl end groups on the Fomblin-Z backbone is increased from zero to two to four (Z → Zdol → Ztetraol), the spreading kinetics slows down dramatically for each addition of two hydroxyl end groups, and with eight hydroxyl groups, the lubricant ZTMD no longer spreads. Faster spreading kinetics is observed initially when a central droplet is still available to feed lubricants into the molecularly thin lubricant film. Much slower spreading kinetics is observed once this central droplet has dissipated to form a pancake-shaped film less than a monolayer thickness. During the initial spreading stage with the central droplet, the radius R of the area covered by the spreading film grows as R ~ t ν , with ν ~ 1/3, while during the final spreading stage, ν ~ 0.1. The faster spreading mechanism during the initial spreading stage is attributed to the en masse flow of lubricant molecules in the lubricant film driven by the backing pressure of the lubricant remaining in the droplet. The slower spreading kinetics once the central droplet has dissipated is well described by a thickness-dependent diffusion coefficient D(h) obtained by modeling lubricant migration as Poiseuille flow driven by the disjoining pressure gradient.

Wetting and Spreading of Nanofluids on Solid Surfaces Driven by the Structural Disjoining Pressure: Statics Analysis and Experiments

The wetting and spreading of nanofluids composed of liquid suspensions of nanoparticles have significant technological applications. Recent studies have revealed that, compared to the spreading of base liquids without nanoparticles, the spreading of wetting nanofluids on solid surfaces is enhanced by the structural disjoining pressure. Here, we present our experimental observations and the results of the statics analysis based on the augmented Laplace equation (which takes into account the contribution of the structural disjoining pressure) on the effects of the nanoparticle concentration, nanoparticle size, contact angle, and drop size (i.e., the capillary and hydrostatic pressure); we examined the effects on the displacement of the drop-meniscus profile and spontaneous spreading of a nanofluid as a film on a solid surface. Our analyses indicate that a suitable combination of the nanoparticle concentration, nanoparticle size, contact angle, and capillary pressure can result not only in the displacement of the three-phase contact line but also in the spontaneous spreading of the nanofluid as a film on a solid surface. We show here, for the first time, that the complete wetting and spontaneous spreading of the nanofluid as a film driven by the structural disjoining pressure gradient (arising due to the nanoparticle ordering in the confined wedge film) is possible by decreasing the nanoparticle size and the interfacial tension, even at a nonzero equilibrium contact angle. Experiments were conducted on the spreading of a nanofluid composed of 5, 10, 12.5, and 20 vol % silica suspensions of 20 nm (geometric diameter) particles. A drop of canola oil was placed underneath the glass surface surrounded by the nanofluid, and the spreading of the nanofluid was monitored using an advanced optical technique. The effect of an electrolyte, such as sodium chloride, on the nanofluid spreading phenomena was also explored. On the basis of the experimental results, we can conclude that a nanofluid with an effective particle size (including the electrical double layer) of about 40 nm, a low equilibrium contact angle (<3°), and a high effective volume concentration (>30 vol %) is desirable for the dynamic spreading of a nanofluid system with an interfacial tension of 0.5 mN/m. Our experimental observations also validate the major predications of our theoretical analysis.

Liquid–solid interaction at nanoscale and its application in vegetal biology

The water ascent in tall trees is subject to controversy: the vegetal biologists debate on the validity of the cohesion–tension theory which considers strong negative pressures in microtubes of xylem carrying the crude sap. This article aims to point out that liquids are submitted at the walls to intermolecular forces inferring density gradients making heterogeneous liquid layers and therefore disqualifying the Navier–Stokes equations for nanofilms. The crude sap motion takes the disjoining pressure gradient into account and the sap flow dramatically increases such that the watering of nanolayers may be analogous to a microscopic flow. Application to microtubes of xylem avoids the problem of cavitation and enables us to understand why the ascent of sap is possible for very high trees.

Dynamic effects in thin liquid films containing ionic surfactants

This paper is dedicated to studying dynamic effects in thin liquid films (TLF) containing ionic surfactants. The standard theory of TLF drainage has been developed without considering the electrical double layer (EDL) in the hydrodynamic equations, although EDL always exists. In addition, it has been found that this theory very well describes the drainage of TLF containing non-ionic surfactants in the presence of electrolytes. The inclusion of EDL into the hydrodynam ics of TLF complicates the theory , producing additional dynamic effects during film drainage. For example, a gradient of electrostatic disjoining pressure across the film arises, thus causing non-uniform electrostatic repulsion between the film surfaces. This paper analyzes the hydrodynamics of TLF with EDL. A new equation of drainage was derived. This equation accounts for the non-uniform distribution of surface charges during the films drainage, which is coupled with non-uniform electrostatic repulsion between the film surfaces and results in faster film drainage. The theory was tested with drainage experiments on TLF with ionic surfactants. Foam films containing sodium dodecyl sulfate (SDS) in the presence and in the absence of added electrolyte were studied and the experimental data compared to the theoretical predictions. The experimental results, however, disagree with the theory. For example, the kinetic equation predicted faster film drainage for foam films at low ionic strength; at high ionic strength the theory tends to “Reynolds drainage”. Inversely, the experiment exhibited slower drainage than predicted by the Reynolds equation in both cases of low and high ionic strengths. Numerical simulations yielded V / V Re < 1 . In addition, cases of “positive” and “negative” velocity of film surfaces were shown. Despite the sign of the velocity the dependence V / V Re < 1 remained. The analysis showed similarity between the experimental data and the prediction of the Manev–Tsekov–Radoev (MTR) drainage model at R < R c r . The analysis showed rather complex dynamics of the film drainage. The paper calls upon further development on this topic.

Rupture of Draining Foam Films Due to Random Pressure Fluctuations

A generalized formalism for the rupture of a draining foam film due to imposed random pressure fluctuations, modeled as a Gaussian white noise, is presented in which the flow inside the film is decomposed into a flow due to film drainage and a flow due to imposed perturbation. The evolution of the amplitude of perturbation is described by a stochastic differential equation. The rupture time distribution is calculated from the sample paths of perturbation amplitude as the time for this amplitude to equal one-half the film thickness and is calculated for different amplitudes of imposed perturbations, film thicknesses, electrostatic interactions, viscosities, and interfacial mobilities. The probability of film rupture is high for thicker films, especially at smaller times, as a result of faster growth of perturbations in a thick film due to a smaller disjoining pressure gradient. Larger viscosity, larger surface viscosity, higher Marangoni number, and smaller imposed pressure fluctuation result in slower growth of perturbation of a draining film, thus leading to larger rupture time. It is shown that a composite rupture time distribution combining short time simulation results with equilibrium distribution is a good approximation.

Second law analysis for microscale flow and heat transfer

In phase change transport devices, capillary forces drive overall circulation of working fluid from an evaporator section to a condenser section, whereas the thin film flows at the evaporating meniscus are driven by capillary and disjoining pressure gradient. An analysis has been provided for the microscale fluid flow and heat transfer in an evaporating extended meniscus. Using the second law of thermodynamics, the entropy generated has been investigated. The geometric configuration that corresponds to the minimization of entropy generated and minimization of fluid flow resistance is identified.

Functional perfluoropolyether spreading on a solid substrate

Molecular dynamics simulations based on a coarse-grained, bead-spring model are adopted to investigate the spreading of functional perfluoropolyether on a solid substrate. The spreading phenomenon has been described by the thickness profiles and molecule movements which evolve with time. The spreading mechanism has been analyzed in terms of both the driving term from the gradient of disjoining pressure and the resistance term from the bead-substrate friction and the film viscosity by dividing the spreading film into three regions.

A two–dimensional model for the interaction between lubricant droplet on the slider surface and air flow within the head/disk interface of disk drives

We numerically study the interaction phenomenon between a lubricant droplet located on the slider’s air bearing surface and the air flow within the head/disk interface of disk drives. A very strong interaction between the lubricant droplet motion and air flow is discovered for ultra-low-flying slider design. The lubricant droplet is driven to migrate towards the slider’s trailing edge by a net effect of air shearing, air bearing pressure gradient, and disjoining pressure gradient. The droplet simultaneously undergoes significant shape deformation. The moving and deforming droplet in turn modifies the effective air bearing surface profile and induces a corresponding air flow adjustment. For a simplified two-dimensional (2D) slider with a fixed flying attitude, our results show that the moving and deforming lubricant droplet induces significant air bearing force and pitch moment change. The interaction becomes stronger when the size of the droplet and the disk’s linear velocity are increased, and when the flying height and pitch angle of the slider are reduced. Our results indicate that a lubricant droplet contamination entering the air bearing surface may impose a non-negligible disturbance on the slider’s flying dynamics.

Rupture of equilibrium foam films due to random thermal and mechanical perturbations

A generalized formalism for the rupture of an equilibrium foam film due to imposed random thermal and mechanical perturbations, modeled as a Gaussian white noise, is presented. The evolution of amplitude of perturbation is described by a stochastic differential equation. The average film rupture time is the average time for the amplitude of perturbation to equal to one half the film thickness and is calculated by employing a first passage time analysis for different amplitudes of imposed perturbations, wavenumbers, film thickness, van der Waals and electrostatic interactions and interfacial mobilities. A critical film thickness is identified based on the sign of the disjoining pressure gradient, below which the film is unstable in that the rupture time is very small. There exists an optimum wavenumber for minimum rupture time only for an unstable film. The calculated values of rupture time as well as the optimum wavenumber in the present analysis agree well with the results of linear stability analysis for unstable film with immobile film interfaces. For stable films, the rupture time is found to increase dramatically with film thickness near the critical film thickness. As expected, the average rupture time was found to be higher for smaller amplitudes of imposed perturbations, larger surface potentials and smaller Hamaker constants. The interfacial mobility is accounted for through surface viscosity as well as Marangoni effect. The mean rupture time for partially mobile gas–liquid interface, as characterized by two dimensionless groups, dimensionless surface viscosity and dimensionless Marangoni number, increased with the two dimensionless groups and approached the limiting value for immobile interfaces. The second moment of the rupture time distribution was also evaluated and a beta distribution was fitted to the first two moments. The fitted rupture time distribution was found to be broader for larger film thickness, smaller pressure amplitude, lower Hamaker constant and smaller surface potential.

Rupture of thin stagnant films on a solid surface due to random thermal and mechanical perturbations

A generalized formalism for the rupture of a nondraining thin film on a solid support due to imposed random thermal and mechanical perturbations, modeled as a Gaussian white noise, is presented. The evolution of amplitude of perturbation is described by a stochastic differential equation. The average film rupture time is the average time for the amplitude of perturbation to equal to the film thickness and is calculated by employing a first passage time analysis for different amplitudes of imposed perturbations, wavenumbers, film thickness, van der Waals and electrostatic interactions and surface tensions. The results indicate the existence of an optimum wavenumber at which the rupture time is minimum. A critical film thickness is identified based on the sign of the disjoining pressure gradient, below which the film is unstable in that the rupture time is very small. The calculated values of rupture time as well as the optimum wavenumber in the present analysis agree well with the results of linear stability analysis for immobile as well as completely mobile gas–liquid film interfaces. For stable films, the rupture time is found to increase dramatically with film thickness near the critical film thickness. As expected, the average rupture time was found to be higher for smaller amplitudes of imposed perturbations, larger surface potentials, larger surface tensions and smaller Hamaker constants.

Spreading of nanofluids driven by the structural disjoining pressure gradient.

This paper discusses the role of the structural disjoining pressure exerted by nanoparticles on the spreading of a liquid film containing these particles. The origin of the structural disjoining pressure in a confined geometry is due to the layering of the particles normal to the confining plane and has already been traced to the net increase in the entropy of the system in previous studies. In a recent paper, Wasan and Nikolov (Nature, 423 (2003) 156) pointed out that the structural component of the disjoining pressure is strong enough to move a liquid wedge; this casts a new light on many applications-most notably, detergency. While the concept of spreading driven by the disjoining pressure is not new, the importance of the structural disjoining pressure arises from its long-range nature (as compared to the van der Waals' force), making it an important component of the overall force balance near the contact line. In this paper, we report on a parametric study of the spreading phenomena by examining the effects of nanoparticle size, concentration and polydispersity on the displacement of an oil-aqueous interface with the aqueous bulk containing nanoparticles. The solution of the extended Laplace-Young equations for the profile of the meniscus yields the position of the nominal contact line under the action of the structural disjoining pressure. Simulations show that the displacement of the contact line is greater with a high nanoparticle volume fraction, small particles for the same volume fraction, monodispersed (in size) particles rather than polydispersed particles and when the resisting capillary pressure is small, i.e., when the interfacial tension is low and/or the radius of the dispersed phase drop/bubble is large.

MICROSCOPIC SPREADING CHARACTERISTICS OF NONPOLAR PERFLUOROPOLYETHER FILMS

The surface diffusion characteristics of nonpolar perfluoropolyether (PFPE) Z on carbon surfaces are investigated in two regimes, submonolayer and multilayer, for nano-thin films. For the submonolayer regime, the two-dimensional, cubic van der Waals equation of state is applied to determine the dependence of the surface diffusion coefficient on the film thickness, as experimental surface diffusion coefficients increase with increasing film thickness. For the multilayer regime, a conventional fluid mechanics analysis with position dependent viscosity and a van der Waals disjoining pressure gradient is applied to investigate the surface diffusion characteristics. The present theoretical analysis qualitatively agrees with the experimental results.

Evaporation heat transfer of steady thin film in micro capillary tubes of micron scale

AbstractThis paper reports that the heat transfer mechanism of phase change in a capillary tube belongs to liquid film conduction and surface evaporation. The surface evaporation is influenced by vapor temperature, vapor‐liquid interfacial temperature, and vapor‐liquid pressure difference. In the vapor‐liquid flow mechanism, flow is effected by both the gradient of disjoining pressure, and the gradient of capillary pressure. The mechanism of vapor‐liquid interaction consists of the shear stress caused by momentum transfer owing to evaporation, and frictional shear stress due to the velocity difference between vapor and liquid. In the model presented for a capillary tube, the heat transfer, vapor‐liquid flow, and their interaction are more comprehensively considered. The thin film profile and heat transfer characteristics have close relations with a capillary radius and heat transfer power. The results of calculation indicate that the length of the evaporating interfacial region decreases to some extent with decreasing capillary radius and increasing heat transfer power. © 2002 Wiley Periodicals, Inc. Heat Trans Asian Res, 31(7): 513–523, 2002; Published online in Wiley InterScience (www.interscience.wiley.com).DOI 10.1002/htj.10050

Dewetting Induced by Density Fluctuations

We consider the stability with respect to dewetting of thin films deposited on a substrate they thermodynamically wet. We show that if density fluctuations with a sufficiently large amplitude appear in a film, the resulting gradient of disjoining pressure in the film destibilizes the free surface eventually leading to the breaking up of the film in droplets. This dewetting mechanism is expected to play a role in films undergoing significant restructuring after deposition or for materials close to a critical point.

Mobility of Z-Dol lubricant thin film on carbon overcoat surface

We simulate the slider/disk interaction induced Zdol lubricant depletion and flow-back dynamics by time-of-flight secondary ion mass spectrometry (TOF-SIMS). Experimental results show that the lubricant flow rate is inversely proportional to original lubricant thickness. The flow rate on partially removed lubricant layer is much higher than that on lubricant depleted carbon surface. These could be attributed to the increase in disjoining pressure gradient with aid of ion beam sputtering on lubricant surfaces.

The Molecular Spreading of Nonpolar Perfluoropolyether Films on Amorphous Carbon Surfaces

The spreading mechanism of nonpolar perfluoropolyether films on carbon surfaces is examined in the mesoscopic regime, including both submonolayer and multilayer films. For the submonolayer film, adsorption–desorption is a main mechanism for spreading, and the surface diffusion coefficients increase as the film thickness increases. The driving force for the spreading in the submonolayer regime is the gradient of the disjoining pressure, which is described by the two-dimensional virial equation. For the multilayer film regime, the spreading characteristics are determined by the molecular weight and the disjoining pressure gradient, which is assumed to be purely van der Waals in nature. We adopt a partial slip boundary condition to analyze the multilayer film, which qualitatively explains the dependence of the surface diffusion coefficient on film thickness.