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Comments In a recent paper, J. Moon et al. (Langmuir 2004, 20, 402)1 claim that “while many partial wetting systems with nonzero contact angles and no films have been studied, experimental examples of pseudopartial wetting have not been explicitly recognized in the literature”. Subsequently, they claim to have found this situation by studying the wetting of Pb, Bi, and a Pb-Bi alloy on Cu in two different types of experiments: the wetting of a liquid phase on a solid support and the wetting of a solid phase on a solid support. We disagree with these two claims for the following reasons. In wetting at equilibrium, there are only two fundamentally different types of wetting behavior. For complete wetting, a macroscopically thick wetting layer forms and the equilibrium spreading coefficient S ) γSV - γSL - γLV is zero (γSV, γSL, and γLV are the substrate/vapor, the substrate/liquid, and the liquid/vapor surface tensions, respectively). For solid films in equilibrium on a solid substrate this is called Frank-van der Merwe behavior. It is clearly not the state observed in the systems studied by Moon et al., neither for liquid films nor for solid films. Partial wetting is an equilibrium state where a drop of liquid on a substrate does not spread. This means that the equilibrium spreading coefficient S ) γSV - γSL - γLV is negative. This definition of partial wetting, which is used in the large majority of the literature on wetting, does not rule out the presence of a film of microscopic thickness on the substrate, around the drop. Indeed, in many experiments, the presence of such a microscopic adsorbed film in the partial wetting state is revealed by measurement of the surface tension of the substrate, γSV. This surface tension is usually significantly lower than that measured in the absence of the liquid drop and its vapor, indicating the presence of an adsorbed layer (the surface tension of the substrate is easy to measure when the substrate is a liquid phase). The thickness of this microscopic layer varies from system to system and depends on the distance to the bulk critical point: its thickness is generally of the order of the bulk correlation length in the wetting liquid phase. As an explicit example, Bonn et al. have measured the film thickness of methanol adsorbed on cyclohexane and find a series of discrete layering transitions with up to five layers formed upon approaching the bulk critical point.2 As they measured the wetting transition in the same experiment, there is no doubt that these layers correspond to the partial wetting state. A drop of liquid on a substrate surrounded by a thin film of liquid has been described several times in the literature. Is this partial or pseudopartial wetting? The usual Cahn theory of wetting describes the free energy of the film of the liquid on the substrate as a function of the film thickness for short-range forces. Far
from the critical point, this free energy has two minima, one at small thickness (a microscopic thickness, on the order of the bulk correlation length) and one at infinite thickness. Depending on whether the equilibrium spreading coefficient S is zero or negative, the absolute minimum is for an infinite thickness (complete wetting) or for the microscopic thickness (partial wetting), respectively. In this model, in which only short-range forces intervene, partial wetting is therefore an equilibrium state in which the drop is surrounded by a microscopic film. In the presence of van der Waals forces, the two fundamentally different types of wetting behavior are still the same: complete wetting or partial wetting. “Pseudopartial wetting” is a term with a double meaning. In a publication by de Gennes et al.,3 pseudopartial wetting refers to the specific spreading behavior that nonvolatile liquids can show when there is not enough adsorbate to cover the substrate surface. If such a volume constraint is present, the liquid drop may spread into a thin pancake surrounded by dry solid, thus resembling complete wetting by a nonvolatile liquid, hence the term “pseudopartial”. This definition does not correspond to the situation described by Moon et al., since they have enough adsorbate and do claim to observe droplets. The meaning of the term “pseudopartial” in the situation described by Moon et al. is rather the one that appears in a paper by Brochard et al.4 There, “pseudopartial” wetting is defined as a state in which the drop is surrounded by a wet surface (a surface covered by a film of the liquid). Specifically, to define pseudopartial wetting Brochard et al. add the long-range van der Waals forces to the short-range forces. If the net effect of the van der Waals forces is an effective attraction between the two interfaces, the minimum at infinite thickness of the free energy shifts to a finite thickness in their model. Brochard et al. therefore define pseudopartial wetting as an equilibrium state where the short-range forces alone favor complete wetting but van der Waals forces maintain the film thickness at a finite value. In pseudopartial wetting, a thin film of the wetting liquid surrounds a drop on the substrate; consequently the equilibrium spreading coefficient is negative as for partial wetting. Therefore, we hold that pseudopartial wetting is a particular case of partial wetting. Unfortunately, Brochard et al. define partial wetting as yet another state in which the drop is surrounded by a “dry” substrate, i.e., a two-dimensionnal gas, instead of a liquid film. This is confusing because first, drops means partial wetting, regardless of what surrounds the drops and, second, it is impossible to make an unambiguous distinction between a two-dimensional gas and a liquid film. Has this pseudopartial wetting state been observed? From an experimental point of view, pseudopartial wetting appears very difficult to distinguish from other cases of partial wetting. In both cases the equilibrium spreading coefficient is negative and a thin liquid film
(1) Moon, J.; Garoff, S.; Wynblatt, P.; Suter, R. Langmuir 2004, 20, 402. (2) Bonn, D.; Kellay, H.; Wegdam, G. H. J. Chem. Phys. 1993, 99, 7115.
(3) de Gennes, P. G.; Brochard-Wyart, F.; Que´re´, D. In Gouttes, bulles, perles et ondes; Belin: Paris, 2002; p 89. (4) Brochard-Wyart, F.; di Meglio, J. M.; Que´re´, D.; de Gennes, P. G. Langmuir 1991, 7, 335.
Comment on Pseudopartial Wetting and Precursor Film Growth in Immiscible Metal Systems
10.1021/la040104d CCC: $30.25 © 2005 American Chemical Society Published on Web 03/15/2005
Comments
surrounds the drop. Looking at the wetting of Pb on Cu, Moon et al. observe a monolayer of Pb atoms surrounding the drop of Pb. As mentioned above, a monatomic layer can easily be explained by short-range forces alone and they do not demonstrate that this layer results from a competition between attractive van der Waals forces and repulsive short-range forces. However, in other experiments, a partial wetting state distinctly different from common partial wetting has been observed: in this state a mesoscopic film (typically 50100 Å) is in equilibrium with a drop of liquid.5 The thickness of this film is determined by the van der Waals forces; however, it does not result from an equilibrium between short-range and van der Waals forces. Rather, it is due to a minimum of the free energy due to the van der Waals forces alone: it is much thicker than the range of the short-range forces. In the standard expansion the van der Waals interaction contains a supplementary term compared to Brochard et al.: it can be written A/l2 + B/l3 with A < 0 when the van der Waals forces are attractive and B > 0. The physical interpretation of the term B is that it is due to the fact that the film on the substrate is denser at the liquid/substrate interface than in bulk. This free energy has a minimum for l ≈ B/|A| so that in the experiments a film of this thickness surrounds a droplet. This minimum coexists with the minimum resulting from the short-range forces, as evidenced by the occurrence of a first-order transition between the two states of partial wetting. This partial wetting state is therefore not exactly the pseudopartial wetting state described by Brochard et al. It was called frustrated complete wetting5 to recall that it would be a complete wetting state if A > 0. Fundamentally, it is irrelevant whether there is “no film” or a film of finite thickness. As long as adsorbed droplets have a nonzero contact angle, the state is called (5) (a) Shahidzadeh, N.; Bonn, D.; Ragil, K.; Broseta, D.; Meunier, J. J. Phys. Rev. Lett. 1998, 80, 3992. (b) Bertrand, E.; Dobbs, H.; Broseta, D.; Indekeu, J.; Bonn, D.; Meunier, J. J. Phys. Rev. Lett. 2000, 85, 1282. (c) Rafaı¨, S.; Bonn, D.; Bertrand, E.; Meunier, J.; Weiss, V. C.; Indekeu, J. Phys. Rev. Lett. 2004, 92, 245701.
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partial wetting. However, in the state of equilibrium adsorption of solid films a distinction is made between Volmer-Weber (crystallites but no film) and StranskiKrastanov (crystallites on top of a film of finite thickness) behavior, but both belong to the same general category of partial wetting. The solid wetting films at equilibrium in the systems studied by Moon et al. appear to display Stranski-Krastanov behavior and therefore also correspond to the same general category of partial wetting, just like the liquid films they studied. For heteroepitaxy, relevant to the solid systems reported by Moon et al., elastic forces prevent the formation of a macroscopically thick wetting layer, and partial wetting results. However, this is particular for solids and is not contained in the definition of pseudopartial wetting by Brochard et al., where the finite thickness of the wetting film is due to the van der Waals forces. In conclusion, Moon et al. have not demonstrated that the state of wetting that they observe is pseudopartial wetting. Rather, the atomic thickness of the film is compatible with standard partial wetting and they have not demonstrated that the van der Waals forces play a fundamental role in this system. Moreover, a drop of liquid on a substrate surrounded by a liquid film had already been described in the literature several time. Daniel Bonn and Jacques Meunier*
Laboratoire de Physique Statistique de l’ENS, UMR 8550 du CNRS, associe´ aux Universite´ s Paris VI et Paris VII, 24 rue Lhomond, 75231 Paris Cedex 05, France Joseph Indekeu
Laboratorium voor Vaste-Stoffysica en Magnetisme, Katholieke Universiteit Leuven, B-3001 Leuven, Belgium Received July 23, 2004 In Final Form: December 29, 2004 LA040104D
