Showing papers on "Laplace pressure published in 2016"

Fast Responsive and Controllable Liquid Transport on a Magnetic Fluid/Nanoarray Composite Interface

Abstract: Controllable liquid transport on surface is expected to occur by manipulating the gradient of surface tension/Laplace pressure and external stimuli, which has been intensively studied on solid or liquid interface. However, it still faces challenges of slow response rate, and uncontrollable transport speed and direction. Here, we demonstrate fast responsive and controllable liquid transport on a smart magnetic fluid/nanoarray interface, i.e., a composite interface, via modulation of an external magnetic field. The wettability of the composite interface to water instantaneously responds to gradient magnetic field due to the magnetically driven composite interface gradient roughness transition that takes place within a millisecond, which is at least 1 order of magnitude faster than that of other responsive surfaces. A water droplet can follow the motion of the gradient composite interface structure as it responds to the gradient magnetic field motion. Moreover, the water droplet transport direction can be co...

Thermally stable coexistence of liquid and solid phases in gallium nanoparticles

Abstract: A real-time investigation shows that Ga nanoparticles in the solid γ-phase coexist with liquid Ga at a broad range of temperatures, as a result of nanoscale confinement, Laplace pressure and epitaxial matching with the substrate.

Pressure-controlled formation of crystalline, Janus, and core-shell supraparticles.

Abstract: Binary mixtures of nanoparticles self-assemble in the confinement of evaporating oil droplets and form regular supraparticles. We demonstrate that moderate pressure differences on the order of 100 kPa change the particles’ self-assembly behavior. Crystalline superlattices, Janus particles, and core–shell particle arrangements form in the same dispersions when changing the working pressure or the surfactant that sets the Laplace pressure inside the droplets. Molecular dynamics simulations confirm that pressure-dependent interparticle potentials affect the self-assembly route of the confined particles. Optical spectrometry, small-angle X-ray scattering and electron microscopy are used to compare experiments and simulations and confirm that the onset of self-assembly depends on particle size and pressure. The overall formation mechanism reminds of the demixing of binary alloys with different phase diagrams.

Laplace Pressure of Individual H2 Nanobubbles from Pressure-Addition Electrochemistry.

Abstract: The Young–Laplace equation is central to the thermodynamic description of liquids with highly curved interfaces, e.g., nanoscale droplets and their inverse, nanoscale bubbles. The equation relates the pressure difference across an interface to its surface tension and radius of curvature, but the validity in using the macroscopic surface tension for describing curved interfaces with radii smaller than tens of nanometers has been questioned. Here we present electrochemical measurement of Laplace pressures within single H2 bubbles between 7 and 200 nm radius (corresponding, respectively, to between 200 and 7 atm). Our results demonstrate a linear relationship between a bubble’s Laplace pressure and its reciprocal radius, verifying the classical thermodynamic description of H2 nanobubbles as small as ∼10 nm.

Superhydrophobic Cones for Continuous Collection and Directional Transportation of CO2 Microbubbles in CO2 Supersaturated Solutions

Abstract: Microbubbles are tiny bubbles with diameters below 50 μm. Because of their minute buoyant force, the microbubbles stagnate in aqueous media for a long time, and they sometimes cause serious damage. Most traditional methods chosen for elimination of gas bubbles utilize buoyancy forces including chemical methods and physical methods, and they only have a minor effect on microbubbles. Several approaches have been developed to collect and transport microbubbles in aqueous media. However, the realization of innovative strategies to directly collect and transport microbubbles in aqueous media remains a big challenge. In nature, both spider silk and cactus spines take advantage of their conical-shaped surface to yield the gradient of Laplace pressure and surface free energy for collecting fog droplets from the environment. Inspired by this, we introduce here the gradient of Laplace pressure and surface free energy to the interface of superhydrophobic copper cones (SCCs), which can continuously collect and direct...

Wettability and spontaneous penetration of a water drop into hydrophobic pores

Abstract: The penetration of a water drop into hydrophobic pores reflects its instability on a porous surface. To understand the mechanism of penetration and to predict the behavior of such a drop, an investigation was conducted through experimental study combined theoretical analysis. Water drops with volumes from 0.5 to 15μL were examined on Polydimethylsiloxane (PDMS) substrates containing pores of 800μm and less in diameter. Results showed a critical condition at which a drop starts to penetrate into a certain sized pore. The critical condition presents a parabolic relationship between the volume of a water drop and the size of a hydrophobic pore. This behavior was due to a net force resulting from Laplace pressure, and capillary pressure. This force was found to be affected by the porosity, wetting angle, and there after the critical condition. The finding of this research will be beneficial for future design of structured surfaces.

Generating Soap Bubbles by Blowing on Soap Films.

Abstract: Making soap bubbles by blowing air on a soap film is an enjoyable activity, yet a poorly understood phenomenon. Working either with circular bubble wands or long-lived vertical soap films having an adjustable steady state thickness, we investigate the formation of such bubbles when a gas is blown through a nozzle onto a film. We vary film size, nozzle radius, space between the film and nozzle, and gas density, and we measure the gas velocity threshold above which bubbles are formed. The response is sensitive to containment, i.e., the ratio between film and jet sizes, and dissipation in the turbulent gas jet, which is a function of the distance from the film to the nozzle. We rationalize the observed four different regimes by comparing the dynamic pressure exerted by the jet on the film and the Laplace pressure needed to create the curved surface of a bubble. This simple model allows us to account for the interplay between hydrodynamic, physicochemical, and geometrical factors.

Cavitation inception of a van der Waals fluid at a sack-wall obstacle

Abstract: Cavitation in a liquid moving past a constraint is numerically investigated by means of a free-energy lattice Boltzmann simulation based on the van der Waals equation of state. The fluid is streamed past an obstacle and, depending on the pressure drop between inlet and outlet, vapor formation underneath the corner of the sack-wall is observed. The circumstances of cavitation formation are investigated and it is found that the local bulk pressure and mean stress are insufficient to explain the phenomenon. Results obtained in this study strongly suggest that the viscous stress, interfacial contributions to the local pressure, and the Laplace pressure are relevant to the opening of a vapor cavity. This can be described by a generalization of Joseph's criterion that includes these contributions. A macroscopic investigation measuring mass flow rate behavior and discharge coefficient was also performed. As theoretically predicted, mass flow rate increases linearly with the square root of the pressure drop. However, when cavitation occurs, the mass flow growth rate is reduced and eventually it collapses into a choked flow state. In the cavitating regime, as theoretically predicted and experimentally verified, the discharge coefficient grows with the Nurick cavitation number.

Liquid morphologies and capillary forces between three spherical beads.

Abstract: Equilibrium shapes of coalesced pendular bridges in a static assembly of spherical beads are computed by numerical minimization of the interfacial energy. Our present study focuses on generic bead configurations involving three beads, one of which is in contact to the two others while there is a gap of variable size between the latter. In agreement with previous experimental studies, we find interfacial ``trimer'' morphologies consisting of three coalesced pendular bridges, and ``dimers'' of two coalesced bridges. In a certain range of the gap opening we observe a bistability between the dimer and trimer morphology during changes of the liquid volume. The magnitude of the corresponding capillary forces in presence of a trimer or dimer depends, besides the gap opening, only on the volume or Laplace pressure of the liquid. For a given Laplace pressure, and for the same gap opening, the capillary forces induced by a trimer are only slightly larger than the corresponding forces in the presence of three pendular bridges. This observation is consistent with a plateau of capillary cohesion in terms of the saturation of a wetting liquid in the funicular regime, as reported in the experimental work [Scheel et al., Nat. Mater. 7, 189 (2008)].

New Drop Fluidics Enabled by Magnetic-Field-Mediated Elastocapillary Transduction.

Abstract: This research introduces a new drop fluidics that uses a deformable and stretchable elastomeric film as the platform instead of the commonly used rigid supports. Such a soft film impregnated with magnetic particles can be modulated with an external electromagnetic field that produces a vast array of topographical landscapes with varying surface curvature, which, in conjunction with capillarity, can direct and control the motion of water droplets efficiently and accurately. When a thin layer of oil is present on this film that is deformed locally, a centrosymmetric wedge is formed. A water droplet placed on this oil-laden film becomes asymmetrically deformed, thus producing a gradient of Laplace pressure within the droplet and setting it in motion. A simple theory is presented that accounts for the droplet speed in terms of such geometric variables as the volume of the droplet and the thickness of the oil film covering the soft elastomeric film as well as material variables such as the viscosity of the oil...

Modulus--Pressure Equation for Confined Fluids

Abstract: Ultrasonic experiments allow one to measure the elastic modulus of bulk solid or fluid samples. Recently such experiments have been carried out on fluid-saturated nanoporous glass to probe the modulus of a confined fluid. In our previous work [G. Y. Gor et al., J. Chem. Phys., 143, 194506 (2015)], using Monte Carlo simulations we showed that the elastic modulus K of a fluid confined in a mesopore is a function of the pore size. Here we focus on the modulus-pressure dependence K(P), which is linear for bulk materials, a relation known as the Tait-Murnaghan equation. Using transition-matrix Monte Carlo simulations we calculated the elastic modulus of bulk argon as a function of pressure and argon confined in silica mesopores as a function of Laplace pressure. Our calculations show that while the elastic modulus is strongly affected by confinement and temperature, the slope of the modulus versus pressure is not. Moreover, the calculated slope is in a good agreement with the reference data for bulk argon and experimental data for confined argon derived from ultrasonic experiments. We propose to use the value of the slope of K(P) to estimate the elastic moduli of an unknown porous medium.

Modulus-Pressure Equation for Confined Fluids

Abstract: Ultrasonic experiments allow one to measure the elastic modulus of bulk solid or fluid samples. Recently such experiments have been carried out on fluid-saturated nanoporous glass to probe the modulus of a confined fluid. In our previous work [J. Chem. Phys., (2015) 143, 194506], using Monte Carlo simulations we showed that the elastic modulus $K$ of a fluid confined in a mesopore is a function of the pore size. Here we focus on modulus-pressure dependence $K(P)$, which is linear for bulk materials, a relation known as the Tait-Murnaghan equation. Using transition-matrix Monte Carlo simulations we calculated the elastic modulus of bulk argon as a function of pressure and argon confined in silica mesopores as a function of Laplace pressure. Our calculations show that while the elastic modulus is strongly affected by confinement and temperature, the slope of the modulus versus pressure is not. Moreover, the calculated slope is in a good agreement with the reference data for bulk argon and experimental data for confined argon derived from ultrasonic experiments. We propose to use the value of the slope of $K(P)$ to estimate the elastic moduli of an unknown porous medium.

Effect of Disjoining Pressure on Surface Nanobubbles.

Abstract: In gas-oversaturated solutions, stable surface nanobubbles can exist thanks to a balance between the Laplace pressure and the gas overpressure, provided the contact line of the bubble is pinned. In this article, we analyze how the disjoining pressure originating from the van der Waals interactions of the liquid and the gas with the surface affects the properties of the surface nanobubbles. From a functional minimization of the Gibbs free energy in the sharp-interface approximation, we find the bubble shape that takes into account the attracting van der Waals potential and gas compressibility effects. Although the bubble shape slightly deviates from the classical one (defined by the Young contact angle), it preserves a nearly spherical-cap shape. We also find that the disjoining pressure restricts the aspect ratio (size/height) of the bubble and derive the maximal possible aspect ratio, which is expressed via the Young angle.

Predicting the size of droplets produced through Laplace pressure induced snap-off

Abstract: Laplace pressure driven snap-off is a technique that is used to produce droplets for emulsions and microfluidics purposes. Previous predictions of droplet size have assumed a quasi-equilibrium low flow limit. We present a simple model to predict droplet sizes over a wide range of flow rates, demonstrating a rich landscape of droplet stability depending on droplet size and growth rate. The model accounts for the easily adjusted experimental parameters of geometry, interfacial tension, and the viscosities of both phases.

Coalescence Behavior of Pure and Natural Fat Droplets Characterized via Micromanipulation

Abstract: For two approaching oil droplets, a region of arrested coalescence lies between full coalescence and total stability. Here the fusion of two droplets begins, but they are stopped from fully relaxing into one spherical droplet. The internal rigidity of the solid fat network within each droplet can provide the resistance necessary to arrest the shape change driven by Laplace pressure. These intermediate doublet structures lead to the partially-coalesced fat networks important for the desired physical properties of ice cream and whipped topping. The use of micromanipulation techniques allows coalescence events between two oil droplets to be microscopically observed. In this study, oil droplets composed of different fats were manipulated at varying elastic moduli, interfacial tension, and radii. It was seen that increasing the elastic moduli of the droplets or increasing droplet radii resulted in coalescence being arrested earlier. Under these experimental conditions, different interfacial tensions did not change the coalescence behavior between two oil droplets.

Power-free water pump based on a superhydrophobic surface: generation of a mushroom-like jet and anti-gravity long-distance transport

Abstract: Spontaneous anti-gravitational transportation of liquids across long distances has been widely discovered in nature, such as water transportation from the root to the crown of a tree. However, artificial liquid delivery remains a challenge. In this work, a new power-free pump composed of a superhydrophobic plate with a pore mounted on a leak-proof cylindrical container filled with water is presented for sustained anti-gravity and long distance transport. Water droplets can be spontaneously captured through the pore by the lower water column, forming a mushroom-like jet due to the energy transition from surface energy to kinetic energy. The spontaneously increased inside pressure in the container will push the water out, through another thin tube, realizing the energy transition from surface energy to gravitational potential energy. The dynamic driving and moving model of the pivotal mushroom-like jet were analyzed. The maximum transport height and transport abilities of the water pump were also discussed. The results show that Laplace pressure is the main driving pressure of the mushroom-like jet and that the developed power-free pump can effectively transport water to over 100 mm in height with an average transport speed of 4500 μL h−1, showing potential for application in microfluidic systems and medical devices where micropumps are needed.

Effects of Capillary Forces on a Hydrogel Sphere Pressed against a Surface.

Abstract: A theoretical treatment is provided for effects of capillary forces on a hemispherically shaped hydrogel sample pressed against a solid hydrophilic surface. It is pointed out that the adhesion of a hydrogel to a surface resulting from capillary forces is different from that of a nonporous solid because of the porous nature of the hydrogel. Because of this, the Laplace pressure subtracts from the osmotic pressure inside the gel. For neutral gels, it can exceed the osmotic pressure, causing the gel to deswell. For charged gels, since the counterions inside the gel generally provide much higher osmotic pressure than that due to monomers alone (which is the only source of osmotic pressure in neutral hydrogels), the Laplace pressure is less likely to make the gel deswell. The Laplace pressure can, however, be large enough to deswell asperities (due to surface roughness) on the gel surface, increasing the contact area. This could result in an increase in the friction and ionic electrical conductivity between the gel and the surface (if the surface is an electrical conductor).

Predicting the size of droplets produced through Laplace pressure induced snap-off

Abstract: Laplace pressure driven snap-off is a technique that is used to produce droplets for emulsions and microfluidics purposes. Previous predictions of droplet size have assumed a quasi-equilibrium low flow limit. We present a simple model to predict droplet sizes over a wide range of flow rates, demonstrating a rich landscape of droplet stability depending on droplet size and growth rate. The model accounts for the easily adjusted experimental parameters of geometry, interfacial tension, and the viscosities of both phases.

Dip-coating with prestructured substrates: transfer of simple liquids and Langmuir-Blodgett monolayers

Abstract: When a plate is withdrawn from a liquid bath, either a static meniscus forms in the transition region between the bath and the substrate or a liquid film of finite thickness (a Landau-Levich film) is transferred onto the moving substrate. If the substrate is inhomogeneous, e.g., has a prestructure consisting of stripes of different wettabilities, the meniscus can be deformed or show a complex dynamic behavior. Here we study the free surface shape and dynamics of a dragged meniscus occurring for striped prestructures with two orientations, parallel and perpendicular to the transfer direction. A thin film model is employed that accounts for capillarity through a Laplace pressure and for the spatially varying wettability through a Derjaguin (or disjoining) pressure. Numerical continuation is used to obtain steady free surface profiles and corresponding bifurcation diagrams in the case of substrates with different homogeneous wettabilities. Direct numerical simulations are employed in the case of the various striped prestructures. The final part illustrates the importance of our findings for particular applications that involve complex liquids by modeling a Langmuir-Blodgett transfer experiment. There, one transfers a monolayer of an insoluble surfactant that covers the surface of the bath onto the moving substrate. The resulting pattern formation phenomena can be crucially influenced by the hydrodynamics of the liquid meniscus that itself depends on the prestructure on the substrate. In particular, we show how prestructure stripes parallel to the transfer direction lead to the formation of bent stripes in the surfactant coverage after transfer and present similar experimental results.

Flow-induced phase inversion of emulsions in tapered microchannels

Abstract: Phase inversion emulsification (PIE) is a process of generating emulsions by inverting the continuous and dispersed phases of pre-existing emulsions. Although PIE is conventionally performed in batch processes, flowing emulsions through precisely engineered channels (i.e. flow-induced phase inversion emulsification (FIPIE)) can induce PIE and potentially enable continuous processing. In this study, we demonstrate flow-induced phase inversion of oil-in-water (O/W) emulsions using microfluidic channels with gradual constriction. We investigate the importance of wetting properties and geometric characteristics of microfluidic channels on FIPIE. We show that two dimensionless groups, Ca and the ratio of droplet-size to channel dimensions determine the outcome of the process. In situ observation of individual droplets undergoing FIPIE reveals that the rupture of films of the continuous (water) phase between oil droplets and a wetting oil layer on the surface of microchannels is the most crucial step for phase inversion. Finally, we compare our experimental observations with a scaling relationship that is based on the force balance between disjoining pressure and Laplace pressure, which provides insights into the underlying physical phenomena responsible for the rupture of the aqueous film and the occurrence of FIPIE. We believe our work provides critical insights and parameters for designing channels and pores that can be used for continuous PIE.

Rheology and Kinetics of Pressure Sintering

Abstract: The rheological viscous flow model of deformable, irreversibly compressible, porous body based on mechanics of continua, and creep theory of crystalline materials, is used to describe quantitatively the sintering of powder materials with pressure in isothermal and nonisothermal conditions. Densification of the porous body occurs under action of Laplace pressure, generated by surface tension, and applied pressure. The densification kinetics of porous metals and crystalline compounds in initial and intermediate stages of sintering with static external pressure represent nonlinear steady-state creep controlled by a climb dislocation mechanism in solid matrix forming porous material. Activation energies of this mechanism are consistent with the bulk diffusion. A diffusional creep controls the pressure sintering kinetics in a later stage. The rheological models of deformable viscoelastic bodies and the associated dynamic strain theory for viscoelastic irreversibly compressible bodies, based on the energy conservation law, enable a quantitative description of their densification under dynamic loading. At the same time it is taking into account the internal energy of deformable body. The solutions of dynamic systems involve the mechanical interaction of compacting machine with this body. The simulation of impact sintering of porous metals shows that the viscosity of the matrix, that forms the porous body, and the activation energy of viscous deformation dramatically decrease with increasing initial impact velocity. This promotes the compaction of the material to practically nonporous state and enhances its mechanical properties.

Transmission of pressure head through the zone of tension saturation in the Lisse effect phenomenon

Abstract: The problem of transmission of pressure head through the zone of tension saturation in the Lisse effect (LE), i.e., the rapid response of groundwater level to pressurized pore air in the unsaturated zone, is investigated theoretically and experimentally. From the law of conservation of energy and the continuity equation, a one-dimensional diffusion equation is derived for transmission of pressure head through the zone of tension saturation. The solution to the equation is the pressure head at any point below the upper boundary of the zone of tension saturation and at any time after the compressed pore air pressure is imposed on the boundary. The key parameter, which determines the behaviour of transmission of pressure head, is the newly proposed pressure head diffusivity coefficient. The theoretical results agree with the experimental results, obtained from laboratory column experiments in three physically different soils. Editor Demetris Koutsoyiannis Associate editor Xi Chen

On the applicability of Young–Laplace equation for nanoscale liquid drops

Abstract: Debates continue on the applicability of the Young-Laplace equation for droplets, vapor bubbles and gas bubbles in nanoscale. It is more meaningful to find the error range of the Young-Laplace equation in nanoscale instead of making the judgement of its applicability. To do this, for seven liquid argon drops (containing 800, 1000, 1200, 1400, 1600, 1800, or 2000 particles, respectively) at T = 78 K we determined the radius of surface of tension R (s) and the corresponding surface tension gamma (s) by molecular dynamics simulation based on the expressions of R (s) and gamma (s) in terms of the pressure distribution for droplets. Compared with the two-phase pressure difference directly obtained by MD simulation, the results show that the absolute values of relative error of two-phase pressure difference given by the Young-Laplace equation are between 0.0008 and 0.027, and the surface tension of the argon droplet increases with increasing radius of surface of tension, which supports that the Tolman length of Lennard-Jones droplets is positive and that Lennard-Jones vapor bubbles is negative. Besides, the logic error in the deduction of the expressions of the radius and the surface tension of surface of tension, and in terms of the pressure distribution for liquid drops in a certain literature is corrected.

Dynamic equilibrium model for stable nano- and micro-bubbles partly covered with hydrophobic material

Abstract: In the dynamic equilibrium model, a bubble is only partly covered with hydrophobic material in contrast to the skin model that a bubble is completely covered with organic material. On the surface of the hydrophobic material in water, gas is highly concentrated due to the presence of depletion layer. Then, gas diffuses into a bubble near the peripheral edge of the hydrophobic material, which may balance with the gas diffusion out of a bubble from the other part of the uncovered bubble surface due to the higher internal gas pressure than the partial pressure of gas dissolved in the liquid water by the Laplace pressure. In the present study, not only the condition of the mass balance but also that of stability is numerically calculated. The results have indicated that a nanobubble could be stable when the fraction of the surface coverage by the hydrophobic material is more than 0.5 in water both supersaturated and under-saturated with gas (air). In slightly degassed water, a microbubble could be stabilized when the fraction of the surface coverage is on the order of 10-4 or less. These bubbles could play an important role in underwater acoustics as well as in acoustic cavitation.

Thermodynamics of superheated phase-change agents

Abstract: Superheated phase-change nanodrops are particles comprised of a fluorocarbon (PFC) core and lipid monolayer coating that stably exist in the liquid state at temperatures beyond their boiling point. Currently, these nanodrops are being researched as cavitation nuclei for ultrasound-mediated biomedical applications. The nanodrops are either fabricated at room temperature in the liquid state using a perfluoropentane (C 5 F 12 ) core, or they can be formed by the condensation of gas-filled microbubbles using either perfluoropropane (C 3 F 8 ) or perfluorobutane (C 4 F 10 ) cores. These droplets have been shown to be stable as a superheated liquid under physiological conditions. Originally, it was suggested that the large Laplace pressure experienced inside the sub-micron sized particles was responsible for this high resistance to vaporization. However, recent research proposes that the large energy barrier associated with homogeneous nucleation is responsible. New vaporization conditions were calculated based on homogeneous nucleation theory and compared to experimental results. The findings in this study suggest the energy barrier associated with homogenous nucleation is responsible for nanodrop superheat stability.

Microfluidic Transport Through Microsized Holes Treated by Nonequilibrium Atmospheric-Pressure Plasma

Abstract: In the field of microfluidics, it is possible to facilitate liquid transport through microsized holes with large slip lengths by lowering the friction at the interface between the flow and the inner surface of the holes. In this paper, we discuss the use of nonequilibrium atmospheric-pressure plasma to modify the surface wettability of microsized holes in glass substrates that are similar to those used as flow channels in glass microfiltration devices. In our experiments, liquid transport flows were driven by internal Laplace pressure differences based on the surface tensions of droplets placed on the front and back sides of the tested substrates.