Disjoining Pressure Gradient Research Articles

Microscopic spreading of nonreactive perfluoropolyalkylether film on amorphous carbon surfaces

A spreading mechanism of nonfunctional perfluoropolyalkylehter (PFPE) on carbon surfaces is proposed. For the thin thin-film regime, adsorption-desorption is a main driving force for spreading, and the surface diffusion coefficients increase as the film thickness increases. A two-dimensional virial equation is employed to explain the dependency of surface diffusion coefficient on the film thickness. For the thick thin-film regime, the spreading characteristic is determined by the disjoining pressure gradient. We adopt a slip boundary condition to analyze the thick thin-film regime. This modification of the boundary condition reasonably explains the dependence of surface diffusion coefficients on film thickness.

Complex terraced spreading of perfluoropolyalkylether films on carbon surfaces

Complex molecular layering of OH-terminated perfluoropolyalkylether films over amorphous carbon surfaces has been observed using spatially resolved microellipsometry. The first layer is diffusive in nature, and the subsequent layers exhibit sharp steps of about twice the thickness of the first layer. This behavior, characteristic of a coexisting two-dimensional gas, with cohesive, liquidlike multilayers, is in clear contrast with that of the ${\mathrm{CF}}_{3}$-terminated analog, which shows a smooth diffusive profile. In this particular case, spreading, for thickness greater than one monolayer, can be exactly described by a Poiseuille flow in a disjoining pressure gradient originating solely from van der Waals interactions. For both types of polymers, diffusion rate reaches a maximum at full monolayer coverage, and decreases in the submonolayer regime. Spreading for both types of polymers unambiguously follows a ${t}^{1/2}$ time dependence.

TRANSPORT PROCESSES AND INTERFACIAL PHENOMENA IN AN EVAPORATING MENISCUS

Abstract The chemical potential of an extended meniscus on an inclined flat plate is a function of its temperature, curvature, film thickness and height above a reference level. The meniscus thickness profile, which is related to the stress field in the liquid, was used to measure the sensitivity of the meniscus to the non-equilibrium effects associated with evaporation/condensation. The thickness profiles of a completely wetting film of a 1,1,2-Trichlorotrifluoroethane were measured using microcomputer based image processing of interferometric images. The automated data acquisition procedures were used to achieve enhanced resolution and thereby a better understanding of the transport processes occurring in the contact line region. The interfacial properties of the system were initially evaluated in situ and then used to describe the transport processes associated with a heated meniscus. Consistent with theoretical models, the curvature increased very rapidly from a value of zero in the adsorbed film at the leading edge of the intrinsic meniscus to a high value and then decreased rapidly until a thickness of about 1 μm. Flow results from a disjoining pressure gradient in the thinner region below the maximum curvature and from a curvature gradient in the thicker portion. At a higher power input, the meniscus was found to oscillate and the differences between the “advanced” and “receded” meniscus states were measured and analyzed. The curvature profiles in these two states are significantly different. The study showed that change of phase heat transfer and fluid flow in thin films are strongly coupled because of their common dependence on the intermolecular force field and gravity.

NUCLEATE POOL BOILING IN A CONFINED SPACE

Nucleate pool boiling of pure water and dilute aqueous sodium lauryl sulfate (SLS) solutions within a confined space between heated and unheated horizontal-parallel surfaces has been investigated under atmospheric pressure. The electrically heated lower surface is made of a sheet of stainless steel 304 with the dimension of 145 × 22 × 0.05 mm and the unheated upper plate is a stainless steel cylinder with a diameter of 20 mm. Periphery of the confined space is open and the size of the gap can be varied from 0 to 10 mm. Experimental results show that with the size of the gap between 0.3 and 3 mm, the heat transfer coefficient may increase to as much as 200 percent of those measured for conventional pool boiling. The addition of surfactant has no significant influence on the heat transfer coefficient in this case. Based on the experimental results, the microlayer evaporation with fluid flow driven by the gradient of disjoining pressure is discussed and considered as an important heat transfer mechanism for ...

Capillary dispersion in porous media at low wetting phase saturations

Spreading of wetting liquid over a surface by disjoining pressure gradient driven film flow and by capillary pressure gradient driven channel flow is well known. Bacri ( J. Phys. Lett. ( Paris) 46, L467–L473, 1985) reported “anomalously” rapid spreading of wetting liquid during imbibition into a prewet porous medium. We predict quantitatively this phenomenon, called “hyperdispersion,” from viscous flow along pore walls in thin films of thickness h governed by disjoining force and capillarity. Pore radius distribution, pore network topology, and disjoining pressure function Π( h) all determine the magnitude of capillary dispersion. At high capillary pressures, we find that capillary pressure P c , wetting phase relative permeability k rw , and capillary dispersion coefficient D c can obey power laws in saturation S, which is the sum of the thin-film and pendular-structure inventories: S = S film + S ps . If Π ∝ h − m so that S film ∝ P −1/ m c , if S ps ∝ P − b c , and if thin films control flow, two sets of power law behavior exist. In the case 1/ m < b, P c ∝ S − m , k rw ∝ S 3, and D c ∝ S 2− m . In the case 1/ m > b, P c ∝ S −1/ b , k rw ∝ S 3/ mb, and D c ∝ S(3− m− mb)/ mb. The exponents m and b dictate the behavior of D c as the saturation of the wetting phase approaches zero. More complex function Π( h) can lead to nonmonotonic D c ( S). Bacri measured D c values two or three orders of magnitude higher than we predict, implying a distribution of wetting liquid different from any we consider.

HOW LIQUIDS SPREAD ON SOLIDS

A theory of the wetting of solids by liquids is put forward. The theory accounts for capillary pressure gradient, gravitational potential gradient, surface tension gradient, disjoining pressure gradient driving forces of flow in thick thin-films and of surface diffusion in thin thin-films. Disjoining pressure stems from the way intermolecular forces aggregate in submicroscopically thin films. For thick thin-films of slowly varying thickness the lubrication approximation to velocity distributions is appropriate. With this approximation the spontaneous, unsteady, two-dimensional spreading of liquid is shown to be governed by a nonlinear convective-diffusion equation for the evolution of the film thickness profile. The predictions of the theory agree with Marmur and Lelah's (1980, 1981) observations of water drops spreading on glass and with Bascom, Cottington and Singleterry's (1964) and Ludviksson and Lightfoot's (1971) observations of oils spreading on high energy surfaces. The theory is used to analyze D...