Wetting–dewetting films: The role of structural forces

The liquid wetting and dewetting of solids are ubiquitous phenomena that occur in everyday life. Understanding the nature of these phenomena is beneficial for research and technological applications. However, despite their importance, the phenomena are still not well understood because of the nature of the substrate's surface energy non-ideality and dynamics. This paper illustrates the mechanisms and applications of liquid wetting and dewetting on hydrophilic and hydrophobic substrates. We discuss the classical understanding and application of wetting and film stability criteria based on the Frumkin–Derjaguin disjoining pressure model. The roles of the film critical thickness and capillary pressure on the film instability based on the disjoining pressure isotherm are elucidated, as are the criteria for stable and unstable wet films. We consider the film area in the model for the film stability and the applicable experiments. This paper also addresses the two classic film instability mechanisms for suspended liquid films based on the conditions of the free energy criteria originally proposed by de Vries (nucleation hole formation) and Vrij–Scheludko (capillary waves vs. van der Waals forces) that were later adapted to explain dewetting. We include a discussion of the mechanisms of nanofilm wetting and dewetting on a solid substrate based on nanoparticles' tendency to form a 2D layer and 2D inlayer in the film under the wetting film's surface confinement. We also present our view on the future of wetting–dewetting modeling and its applications in developing emerging technologies. We believe the review and analysis presented here will benefit the current and future understanding of the wetting–dewetting phenomena, as well as aid in the development of novel products and technologies.     

Direct visualization of dewetting of molecularly thin liquid films on solid surfaces

The effect of the surface energy gamma, disjoining pressure, Pi, and roughness on the dewetting of molecularly thin liquid lubricant films on magnetic disks, which have sub-nanometer surface topography, has been investigated by visualizing the dewetting process directly using ellipsometric microscopy. The dewetting process of thin liquids on the rough surface is determined not only by the well-known instability of films, which is determined by the sign of dPi/dh, but also by the sign of Pi and the surface topography of the substrate even if its roughness is of the sub-nanometer order. The dewetting film formed small droplets, which were not along the surface topography of the substrate, when Pi < 0. On the other hand, it formed grooves along the surface topography with a sub-nanometer roughness when Pi > 0. Moreover, the sub-nanometer roughness initiated the dewetting of the metastable liquid thin films.     

Autophobic dewetting of Z-tetraol perfluoropolyether lubricant films on the amorphous nitrogenated carbon surface

The thermodynamic stability of thin films of the perfluoropolyether (PFPE) Z-Tetraol, as a function of molecular weight, on amorphous nitrogenated carbon, CNx, is investigated. An optical surface analyzer is used to image the autophobic dewetting of the Z-Tetraol films. Film dewetting results when the PFPE film thickness applied to the CNx surface exceeds a critical value. This critical dewetting thickness is identified as the monolayer thickness of the adsorbed PFPE film via measurements of the changes in the surface energy as a function of lubricant film thickness. The observed dewetting coincides with the film thickness at which the disjoining pressure goes to zero. The critical dewetting thickness is dependent on the PFPE molecular weight.     

Dynamic contact angle in rim instability of dewetting holes

The effects of dynamic contact angle (thetad), between a substrate and the melt of a dewetting polymer thin film, on the evolution of rim instabilities of dewetting holes were reported. Various thetad's were achieved by covering SiOx surfaces with different coverage of octadecyltrichlorosilane. On each surface, the morphology of the dewetting holes was examined in detail as the hole grew to a certain size. Rim instabilities, in terms of undulations in both r and z directions, became more pronounced as thetad increased, under which condition, narrower and higher rims were also observed. Experimentally, atomic force microscopic scans of the rim were used to obtain the rim profile, which was predicted using thetad. The predicted rim profile was used, in combination with the analysis of Rayleigh instability of a cylindrical fluid, to interpret the rim instability. The model captures the basic trend of the rim instability dependency on thetad. The study demonstrates the importance of the substrate properties on the rim instability and the destabilization of polymer thin films during hole growth.     

Dewetting Revisited: New Asymptotics of the Film Stability Diagram and the Metastable Regime of Nucleation and Growth of Dry Zones

Stability properties of a nonwetting film are discussed. Assuming a general form of the disjoining pressure, accurate asymptotic formulas for the upper thickness range of the film instability/metastability are derived. This analysis is applied to two particular cases: a nonionic liquid film with the (m, n) power form of the disjoining pressure and an ionic liquid film with an exponentially decaying electrostatic part of the disjoining pressure. The metastable regime of dewetting is considered, and an expression for the critical radius of a hole is derived. A new Fokker-Planck kinetic model of metastable dewetting, applicable at early stages of the process, is developed. It yields a relationship between the number of viable holes (per unit area and unit time) moving in steady-state regime to the supercritical part of the "embryo size space" and the equilibrium number of "critical" holes determined from thermodynamics. The dynamics of metastable dewetting is quantitatively described in terms of the surface fraction of holes in the film. Continuous dynamic models of the metastable dewetting applicable in the entire range of times have to include the thermal noise, as proposed by V. S. Mitlin (1994, Colloids Surf. A 89, 97). Copyright 2000 Academic Press.     

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.     

Dynamic instability of a sol-gel-derived thin film

We present a study on the dynamic instability of a sol-gel-derived (SG) thin film on a nonwettable substrate. Because of the structural instability accompanied by syneresis stress in a film deposited on the substrate, there exists a regular distribution of dewetting patterns required to relieve the in-plane stress, such as holes in the earlier stages, and droplets accompanying a regular polygonal distribution in the later stages of the dynamic instability. The characteristic length scales in each stage scaled linearly with the film thickness during the duration of dewetting. For the formation of holes during the earlier stages of rupture of the film, the dewetting velocity was analyzed with a viscous sintering theory of a SG thin film. In the earlier stages of the dynamic instability, the dewetting velocity decreases with increasing dewetting time and increases with increasing the initial film thickness, which indicates that the SG thin film behaves partially like a slipping polymer thin film. In the final times of the film rupture, the radius of the hole has a linear relationship with the film thickness, and the growth rate of the hole (dewetting velocity) is nearly constant, regardless of the film thickness. These dewetting behaviors indicate that the SG thin film in the final times of the rupture is somewhat similar to nonslipping film. From these observations, we found that the dewetting behavior of a SG thin film has ambivalent dewetting characteristics of slipping and nonslipping films and that a SG thin film is not a purely viscous film.     

A thin film analog of the corneal mucus layer of the tear film: an enigmatic long range non-classical DLVO interaction in the breakup of thin polymer films

We present experimental results on the instability and dewetting of thin liquid polydimethylsiloxane (PDMS) films intercalated between an aqueous medium and a silicon wafer grafted with PDMS ‘brushes’. This is a thin film analog of the precorneal thin mucus coating sandwiched between the aqueous tear film and the glycocalyx carrying corneal epithelial surface. Lowering of the PDMS–water interfacial tension by a surfactant results in dewetting even of micrometer thick films within a few minutes. The instability appears to be induced by a long range non-classical DLVO force which has the same decay behavior as the nonretarded van der Waals force, but a magnitude which is about 2–3 orders higher. Implications for the breakup of the precorneal mucus layer and the tear film are discussed.     

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.     

Instability and dewetting of ultrathin solid viscoelastic films on homogeneous and heterogeneous substrates

Instability and dewetting engendered by the van der Waals force in soft thin (<100 nm) linear viscoelastic solid (e.g., elastomeric gel) films on uniform and patterned surfaces are explored. Linear stability analysis shows that, although the elasticity of the film controls the onset of instability and the corresponding critical wavelength, the dominant length-scale remains invariant with the elastic modulus of the film. The unstable modes are found to be long-wave, for which a nonlinear long-wave analysis and simulations are performed to uncover the dynamics and morphology of dewetting. The stored elastic energy slows down the temporal growth of instability significantly. The simulations also show that a thermodynamically stable film with zero-frequency elasticity can be made unstable in the presence of physico-chemical defects on the substrate and can follow an entirely different pathway with far fewer holes as compared to the viscous films. Further, the elastic restoring force can retard the growth of a depression adjacent to the hole-rim and thus suppress the formation of satellite holes bordering the primary holes. These findings are in contrast to the dewetting of viscoelastic liquid films where nonzero frequency elasticity accelerates the film rupture and promotes the secondary instabilities. Thus, the zero-frequency elasticity can play a major role in imposing a better-defined long-range order to the dewetted structures by arresting the secondary instabilities.     

Spinodal dewetting of thin films with large interfacial slip: implications from the dispersion relation

We compare the dispersion relations for spinodally dewetting thin liquid films for increasing magnitude of interfacial slip length in the lubrication limit. While the shape of the dispersion relation, in particular the position of the maximum, are equal for no-slip up to moderate-slip lengths, the position of the maximum shifts to much larger wavelengths for large slip lengths. Here, we discuss the implications of this fact for recently developed methods to assess the disjoining pressure in spinodally unstable thin films by measuring the shape of the roughness power spectrum. For polystyrene (PS) films on octadecyltrichlorosilane (OTS) covered Si wafers (with slip length b approximately 1 microm), we predict a 20% shift of the position of the maximum of the power spectrum which should be detectable in experiments.     

Dynamics of dewetting at the nanoscale using molecular dynamics

Large-scale molecular dynamics simulations are used to model the dewetting of solid surfaces by partially wetting thin liquid films. Two levels of solid-liquid interaction are considered that give rise to large equilibrium contact angles. The initial length and thickness of the films are varied over a wide range at the nanoscale. Spontaneous dewetting is initiated by removing a band of molecules either from each end of the film or from its center. As observed experimentally and in previous simulations, the films recede at an initially constant speed, creating a growing rim of liquid with a constant receding dynamic contact angle. Consistent with the current understanding of wetting dynamics, film recession is faster on the more poorly wetted surface to an extent that cannot be explained solely by the increase in the surface tension driving force. In addition, the rates of recession of the thinnest films are found to increase with decreasing film thickness. These new results imply not only that the mobility of the liquid molecules adjacent to the solid increases with decreasing solid-liquid interactions, but also that the mobility adjacent to the free surface of the film is higher than in the bulk, so that the effective viscosity of the film decreases with thickness.     

Evidence of surface charge at the air/water interface from thin-film studies on polyelectrolyte-coated substrates

The stability of thin water films on silicon substrates coated with cationic and anionic polyelectrolytes was investigated by the thin film pressure balance technique. Depending on the surface charge of the substrate, the water films are either stable (on negatively charged wafers) or rupture rapidly (on positively charged wafers). It is supposed that this behavior is due to a negative surface charge of the free water surface. The underlying assumption that the films' stability is due to electrostatic interactions is supported by measurements of the disjoining pressure on silicon wafers with a native oxide layer, which indicates a decrease of the film thickness, and thus decreasing repulsive interaction between the two film interfaces, with increasing ionic strength.     

Stability of triglyceride liquid films on hydrophilic and hydrophobic glasses

Wetting and dewetting of solid surfaces by oily fluids were investigated in terms of the stability of the liquid film formed between an air bubble and the solid surface. With the objective of understanding how molecules with low polarity but relatively complex molecular structure behave at the solid/liquid interface, three liquid triglycerides with different chain length and saturation were chosen, namely, tributyrin, tricaprylin, and triolein. Tributyrin and tricaprylin exist in milkfat while triolein is present in vegetable oils. The stability of the liquid films may be inferred from the shape of the disjoining pressure isotherms, which represent the dependence of the disjoining pressure on the film thickness. Disjoining pressure isotherms for films of the three triglycerides on hydrophilic and hydrophobic glasses were obtained using a recently developed apparatus, based on the interferometric technique. The experimental curves are compared with the theoretical predictions of London-Hamaker. The deviations between theory and experiment are interpreted in terms of a structural component of the disjoining pressure. All triglycerides form metastable films on both hydrophilic and hydrophobic glasses which means that for disjoining pressures higher than a critical value, pi(c), a wetting transition occurs and the film ruptures. The mechanisms for film rupture are discussed and a correlation between film stability and the apolar (Lifshitz-van der Waals) and the polar components of the spreading coefficient is proposed.     

Pattern formation and dewetting in thin films of liquids showing complete macroscale wetting: from "pancakes"to "swiss cheese"

Based on the complete 3D numerical solutions of the nonlinear thin film equation, we address the problems of surface instability, dynamics, morphological diversity and evolution in unstable thin films of the liquids that display complete macroscale wetting. The twin constraints of complete macroscale wettability and nanoscale instability produce a variety of microscopic morphological phases approximating sharp crystal surfaces with flat tops resembling a mesa or a micro "pancake" or a slice of Swiss cheese. While the maximum thickness of flat regions is found to be independent of the initial film thickness, the precise lateral morphology of microdomains formed depends on the film thickness. As the film thickness is increased, the initial pathway of evolution changes from the formation of small spherical droplets, to long mesas (parapets) and islands, to circular holes, all of which eventually resolve by ripening into a collection of round pancakes at equilibrium. However, beyond a certain transition thickness, a novel metastable honeycombed morphology, resembling a membrane or a slice of Swiss cheese, is uncovered, which is produced by an abrupt "freezing" of the evolution during hole growth. In contrast, the spinodal dewetting in thin films of partially wettable systems always engenders spherical droplets at equilibrium. The equilibrium dewetted area from simulations, as well as from simple mass balance, is shown to decline linearly with the initial film thickness.     

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.     

Competing liquid phase instabilities during pulsed laser induced self-assembly of copper rings into ordered nanoparticle arrays on SiO2

Nanoscale copper rings of different radii, thicknesses, and widths were synthesized on silicon dioxide thin films and were subsequently liquefied via a nanosecond pulse laser treatment. During the nanoscale liquid lifetimes, the rings experience competing retraction dynamics and thin film and/or Rayleigh-Plateau types of instabilities, which lead to arrays of ordered nanodroplets. Surprisingly, the results are significantly different from those of similar experiments carried out on a Si surface. We use hydrodynamic simulations to elucidate how the different liquid/solid interactions control the different instability mechanisms in the present problem.     

Rupture of thin liquid films induced by impinging air-jets

Thin liquid films on partially wetting substrates are subjected to laminar axisymmetric air-jets impinging at normal incidence. We measured the time at which film rupture occurs and dewetting commences as a function of diameter and Reynolds number of the air-jet. We developed numerical models for the air flow as well as the height evolution of the thin liquid film. The experimental results were compared with numerical simulations based on the lubrication approximation and a phenomenological expression for the disjoining pressure. We achieved quantitative agreement for the rupture times. We found that the film thickness profiles were highly sensitive to the presence of minute quantities of surface-active contaminants.     

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.