Disjoining pressure of thin nonfreezing interlayers

At subzero temperatures, it has been found that nonfreezing water interlayers form between the inner smooth surface of a thin quartz capillary and the ice in the capillary core. The dilatometry method has been used to measure the thicknesses of these nonfreezing interlayers over a range of temperatures between -1 and -0.14 degrees C and for applied pressures up to 8 MPa. The measured data are used to calculate the disjoining pressure isotherms of nonfreezing water interlayers. It is shown that structural forces caused by structural changes of nonfreezing water are the main contribution to the disjoining pressure. The structural changes are also responsible for the phenomenon of the nonfreezing interlayers.     

黑膜厚度与膜表面张力关系的研究

用微干涉测量技术直接测定楔压等温线,研究了电解质浓度对阳离子表面活性剂TTAB在浓度大于cmc时形成黑膜厚度的影响及膜表面张力与溶液表面张力之间的差别.结果显示,黑膜厚度取决于楔压和电解质浓度,随着楔压的增加,液膜厚度减少至一定程度后几乎保持不变,表明黑膜类型的转化是阶跃式的,而电解质屏蔽了液膜两个表面电荷层间的排斥作用,故电解质浓度增加,液膜厚度变小.由楔压等温线得出的膜表面张力的结果说明一般黑膜的表面张力与溶液的表面张力并无明显差别.     

Disjoining pressure in a symmetric circular slit

Local disjoining pressure has been calculated for dispersion forces inside a symmetric circular plane-parallel empty slit. In contrast to the asymmetric case, the local disjoining pressure in a symmetric slit is defined unambiguously. The edge effects in the symmetric slit have been shown to be substantially stronger than those in the asymmetric one. The data obtained have been compared with the published results of the approximate calculation of averaged disjoining pressure. The approximate approach has been revealed to underestimate the absolute value of the correction to the averaged disjoining pressure by nearly 10%.     

Untersuchungen des spaltdruckes dünner filme, deren entwicklung, ergebnisse und zu lösende aktuelle probleme

Considered is the development of the notions of the equilibrium disjoining pressure, beginning from its first experimental discovery for the films between solid surfaces, wetting or free. Considered is also the role of the disjoining pressure in the thermodynamics of liquid films, which is regarded as the main thermodynamic characteristic of the latter. The application of the notion of the disjoining pressure to the hydrodynamics of thin films has been mentioned. Considered are the components of the disjoining pressure, dependent upon the dispersion forces, ionic-electrostatic forces, and upon the structural peculiarities of boundary layers. In conclusion, one has considered the importance of the disjoining pressure for the stability of colloids.     

Correction to the paper “the condition of mechanical equilibrium on the surface of a nonuniform thin film” [1]

The correction is introduced to the definition of the disjoining pressure of wedge-shaped film formulated in the authors’ previous publication. In refined definition, the nondiagonal pattern of pressure tensor even in cylindrical coordinates is taken into account and the disjoining pressure is introduced by analogy with the case of the transition zone of wetting meniscus. The alternative procedure of the definition of the disjoining pressure through the diagonal pressure tensor in the middle of the wedge is also considered.     

Nonflat equilibrium liquid shapes on flat surfaces

The hydrostatic pressure in thin liquid layers differs from the pressure in the ambient air. This difference is caused by the actions of surface forces and capillary pressure. The manifestation of the surface force action is the disjoining pressure, which has a very special S-shaped form in the case of partial wetting (aqueous thin films and thin films of aqueous electrolyte and surfactant solutions, both free films and films on solid substrates). In thin flat liquid films the disjoining pressure acts alone and determines their thickness. However, if the film surface is curved then both the disjoining and the capillary pressures act simultaneously. In the case of partial wetting their simultaneous action results in the existence of nonflat equilibrium liquid shapes. It is shown that in the case of S-shaped disjoining pressure isotherm microdrops, microdepressions, and equilibrium periodic films exist on flat solid substrates. Criteria are found for both the existence and the stability of these nonflat equilibrium liquid shapes. It is shown that a transition from thick films to thinner films can go via intermediate nonflat states, microdepressions and periodic films, which both can be more stable than flat films within some range of hydrostatic pressure. Experimental investigations of shapes of the predicted nonflat layers can open new possibilities of determination of disjoining pressure in the range of thickness in which flat films are unstable.     

Equilibrium thin liquid films

The latest results are reviewed and a number of new concepts of the thermodynamics of thin films are formulated. Current definitions of disjoining pressure and their applications for introducing disjoining pressure into thermodynamics of phase equilibria, as well as the new thermodynamic definition of the thickness of thin film, are considered. New approaches to the rigorous definition of disjoining pressure in curved films and films with nonuniform thickness, including transition zones of wetting films, are analyzed. The modulus of Gibbs’ elasticity is derived for the case of a thin film. The role of the elasticity of this type in thin films and its correlation with traditional transverse (Derjaguin) elasticity related to the disjoining pressure are explained.     

Investigation of the isotherms of the disjoining pressure of wetting films of binary nonionic solutions by the ellipsometric method

The paper deals with an experimental investigation into the influence of the second component on the thicknesses of the wetting films of a nonionic solvent. A technique has been developed for the production of pure, smooth, thin glass substrates for wetting liquid films.

The use of these glass substrates enabled us to exclude the influence on the experimental results of such noncontrollable factors as roughness and pollution of the substrate surface. The isotherms of the disjoining pressure of wetting films of a number of two-component mixtures of nonionic liquids on glass substrates were experimentally determined. The film thicknesses were measured by an ellipsometric method; the disjoining pressure for the film was preset by adjusting the pressure of solvent vapours. The results obtained demonstrate a qualitative agreement with the theory of the adsorption component of disjoining pressure developed by Derjaguin and Churaev.

It is also shown that even very small additions of a polar substance to a nonpolar solvent may cause a marked change in the thickness of films. In addition to adopting the theory of the adsorption component of disjoining pressure, certain assumptions are made about the formation of the structural component resulting from the addition of a polar component to quantitatively describe the results obtained. The contribution of the adsorption and structural components of disjoining pressure to the stability of films of solution is estimated.     

On the Theory of the Stability of Dipole Molecule Solution Interlayers in Apolar Solvents: 4. A Localized Adsorption at the Dipole Orientation Parallel to the Interface

Image forces and their contribution to the disjoining pressure of solution films at the orientation of dipole molecules of adsorbed component parallel to the film interfaces are analyzed. Expressions for the calculation of the correlation attraction of adsorbed monolayers under various conditions of adsorption on opposite film boundaries are obtained. Relation for calculating the contribution of interaction between the adsorbed monolayers and their images to the film disjoining pressure is derived. Numerical estimates of the disjoining pressure arising due to aforementioned interactions are made.     

Can contact-angle measurements determine the disjoining pressure in liquid nanofilms on rigid substrates?

The disjoining pressure of lubricant nanofilms used in the magnetic recording industry controls the equilibrium wetting, the dynamics of film restoration, and the evaporation kinetics of the film. It has been claimed that by measuring the contact angle of nonpolar and polar liquids on lubricant films, the disjoining pressure can be extracted using the method of Girafalco and Good, and such analyses have appeared in the literature. The approximations underlying the method have been discussed before in the literature. In view of the importance of measuring disjoining pressure in nanofilms of lubricants, it is timely to revisit these assumptions to understand the validity of the contact-angle method presently in use. We re-derive the relevant equations using a thermodynamic-interaction-energy approach which is free of the problems inherent in the original derivation and make explicit the assumptions which must be made in the derivation. General interaction energy arguments are then invoked to demonstrate that it does not appear possible to obtain the disjoining pressure in the film from contact-angle measurements in an unambiguous manner.     

Homogeneous nucleation at high supersaturation and heterogeneous nucleation on microscopic wettable particles: A hybrid thermodynamic/density-functional theory

Homogeneous nucleation at high supersaturation of vapor and heterogeneous nucleation on microscopic wettable particles are studied on the basis of Lennard-Jones model system. A hybrid classical thermodynamics and density-functional theory (DFT) approach is undertaken to treat the nucleation problems. Local-density approximation and weighted-density approximation are employed within the framework of DFT. Special attention is given to the disjoining pressure of small liquid droplets, which is dependent on the thickness of wetting film and radius of the wettable particle. Different contributions to the disjoining pressure are examined using both analytical estimations and numerical DFT calculation. It is shown that van der Waals interaction results in negative contribution to the disjoining pressure. The presence of wettable particles results in positive contribution to the disjoining pressure, which plays the key role in the heterogeneous nucleation. Several definitions of the surface tension of liquid droplets are discussed. Curvature dependence of the surface tension of small liquid droplets is computed. The important characteristics of nucleation, including the formation free energy of the droplet and nucleation barrier height, are obtained.     

Flow of nonfreezing water interlayers and frost damage in porous solids

On the basis of the thermodynamics of irreversible processes, a solution has been obtained for the problem of mass transfer between nonfreezing interlayers in a slit model of a pore filled with ice, and also between the thawed and frozen zones of a porous solid. The disjoining pressure plays a role in determining the equilibrium thickneses of the interlayers and in the phenomena of frost damage and heaving of soils. The theory that is developed in this work is supported by experimental findings, and guidelines are formulated for future research in the field of the theory of moisture movement in porous solids during the freezing process.     

Mass transfer in frozen porous bodies

The theory of moisture transport in frozen porous bodies under the action of temperature gradient along unfreezing communications represented by water in thin pores and in interlayers between the pore surface and the ice was elaborated. It was shown that most of the flow in the pores filled with water is directed toward a cold side and can be calculated using the disjoining pressure isotherms of unfreezing interlayers. To obtain isotherms, we used the data of previous measurements of the thickness of unfreezing interlayers in micron-sized quartz capillaries as a function of pressure and temperature. The viscosity of unfreezing interlayers in quartz capillaries was estimated based on the measurements of the displacement velocity of ice columns in the quartz capillaries. Calculated flow rates of unfreezing moisture were consistent with the experimental data for the model porous bodies and frozen grounds.Translated from Kolloidnyi Zhurnal, Vol. 66, No. 6, 2004, pp. 835–839.Original Russian Text Copyright © 2004 by Churaev.     

Disjoining pressure and capillarity in the constrained vapor bubble heat transfer system

Using the disjoining pressure concept in a seminal paper, Derjaguin, Nerpin and Churaev demonstrated that isothermal liquid flow in a very thin film on the walls of a capillary tube enhances the rate of evaporation of moisture by several times. The objective of this review is to present the evolution of the use of Churaev's seminal research in the development of the Constrained Vapor Bubble (CVB) heat transfer system. In this non-isothermal "wickless heat pipe", liquid and vapor flow results from gradients in the intermolecular force field, which depend on the disjoining pressure, capillarity and temperature. A Kelvin-Clapeyron model allowed the use of the disjoining pressure to be expanded to describe non-isothermal heat, mass and momentum transport processes. The intermolecular force field described by the convenient disjoining pressure model is the boundary condition for "suction" and stability at the leading edge of the evaporating curved flow field. As demonstrated by the non-isothermal results, applications that depend on the characteristics of the evaporating meniscus are legion.     

Some new aspects of and conclusions on theory of stability of colloids and their experimental verification

This paper considers three aspects of colloid stability theory: phase stability, the stability of disperse composition and aggregative stability in relation to the merits of a direct operational determination of the disjoining pressure. Four components of the disjoining pressure are treated: the dispersion, ion/electrostatic, adsorption and structural terms. The simplest and most general way to derive the second and third components of disjoining pressure is to use the Gibbs-Duhem equation generalized by inclusion of terms corresponding to the electric work of charging the particle interface. The theory of the adsorption component of disjoining pressure explains the stability of free films of some binary solutions demonstrated experimentally by Sheludko and Ekerova.

With regard to other aspects of colloid stability we note that flow-ultramicroscopic measurements of the concentration of colloid particles are free from the shortcomings of other methods. This makes accessible the kinetics of slow coagulation of sols (e.g., gold sols) and reveals the role of disaggregation here and in the establishment of aggregative quasi-equilibrium. The measurements of molecular attraction between crossed metal wires as a function of gap width are presented.     

Disjoining pressure in a plane-parallel asymmetric slit with finite sizes

An approach has been developed to calculation of the Irving-Kirkwood pressure tensor inside a circular plane-parallel empty slit between bodies with dispersion forces. The disjoining pressure is determined as the normal component of the pressure tensor on a slit wall and is found as a function of both the width and radius of the slit. It has been shown that, at a preset slit width, the disjoining pressure acquires a value almost equal to that inherent in an infinitely long slit already at a circular slit radius five times larger than the slit width.     

Disjoining pressure of thin layers of binary solutions

Theory of dispersion forces and thermodynamics was used for the calculation of the adsorption and disjoining pressure of a binary solution interlayer between two identical and two dissimilar plates. Disjoining pressure isotherms were obtained for liquid interlayers between solids, for wetting and free films of solutions. The conditions were determined under which the overlapping of the diffuse adsorption layers of the solute can provide the main contribution to the interaction and make the disjoining pressure positive. It has been shown by numerical methods that the disjoining pressure isotherms of thin interlayers of solutions may intersect the thickness axis, and that the repulsion forces appear at small distances. Thus, disperse systems may be stabilized in the presence of binary, nonionic solutions.     

Disjoining pressure measurements using a microfabricated groove for a molecularly thin polymer liquid film on a solid surface

A method for measuring disjoining pressure of a molecularly thin liquid film on a solid surface by using a microfabricated groove has been developed. The shape of the meniscus of a thin film in the microgroove was measured with an atomic force microscope, and the disjoining pressure was obtained from the capillary pressure obtained from the measured curvature of the meniscus. Our method is applicable to a film with a thickness greater than the diameter of gyration in the polymer molecule. Moreover, the method can detect the changes in the disjoining pressure caused by ultraviolet light irradiation, and it is effective in investigating the intermolecular interaction between a thin film and a solid surface.     

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.     

Modern state of the investigation of long-range surface forces

Development of the concept of surface long-range forces and, in particular, the equilibrium disjoining pressure of liquid and gaseous interlayers has been set forth. Considered are the molecular, adsorption, electrical, structural, and electronic components of disjoining pressure. The contribution of the disjoining pressure to the hydrodynamics of thin layers is considered. The first theory of the frost heaving of soils has been formulated. Stated are the investigations of surface forces, in particular, in the processes of the formation of new interfaces and arising phenomena of the emission of electrons, ions, photons, and neutrons.