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Müller, Xavier; Kinoshita, T.; Dupont‐Roc, J.

doi:10.1023/a:1022542620167

Cited by 8 publications

(4 citation statements)

References 11 publications

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Section: Effect Of Substrate Roughness On Wetting and Absorptionmentioning

confidence: 66%

“…In a porous medium, hysteretic capillary condensation can be observed even in a complete wetting situation [5,7]. Our hypothesis is strengthened by the observation that a receding meniscus leaves a connected superfluid film [8].WK also argue that their model explains an "extremely" asymmetric pinning of the contact line (advancing is easy, and receding is difficult). Experimental evidence for this is lacking, since the true equilibrium contact angle u eq cannot be measured on rough substrates.…”

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Effect of Substrate Roughness on Wetting and Absorption

et al. 2001

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Effect of Substrate Roughness on Wetting and AbsorptionIn a recent Letter, Wyatt and Klier (WK) [1] claim that surface roughness provides an explanation for (i) the extreme metastability of wetting films, and (ii) the "memory" the Cs substrates have after having been in contact with liquid He. We show here that their arguments are partly erroneous and that other explanations are more plausible.For (i) it has been shown theoretically and experimentally that homogeneous thick films, undercooled from above to below the prewetting line have a diverging lifetime near bulk coexistence. This is a generic feature of first-order wetting transitions unrelated to any disorder in the system [2,3]. This point is stressed by the experimental observation of long-lived metastable thick films for wetting on liquid "substrates" for which disorder is absent [3]. At bulk coexistence and below the wetting temperature, a macroscopic wetting layer can coexist with nonwet surface regions; consequently, a temperature change will not limit the lifetime of a thick film. When (as in some but not all of the He͞Cs experiments) part of the substrate is not covered by a thick film, the thermal nucleation barrier is less relevant; hysteresis then likely involves a pinning of the contact line.The memory effect, (ii), is explained in [1] by the formation of "micropuddles" of liquid He in small defects of the Cs substrate, excluding capillary condensation. However, this is in conflict with their use of the Laplace pressure p 2 p 0 2s͞R below Eq. (1). For capillary condensation, in thermodynamic equilibrium equilibration can take place through the vapor [4] and no memory effect should be expected. Because of their semicircular shape the micropuddles will not fill continuously, while, e.g., for square cavities there is no energy barrier for capillary condensation and hence no hysteresis [5]. This shows that the scenario is sensitively dependent on details of the surface topography. We believe it is more reasonable to assume the surface as a spongelike porous medium made of connected channels (a microscopic version of the disordered substrates of [6]). In a porous medium, hysteretic capillary condensation can be observed even in a complete wetting situation [5,7]. Our hypothesis is strengthened by the observation that a receding meniscus leaves a connected superfluid film [8].WK also argue that their model explains an "extremely" asymmetric pinning of the contact line (advancing is easy, and receding is difficult). Experimental evidence for this is lacking, since the true equilibrium contact angle u eq cannot be measured on rough substrates. The relevant physical parameter is the force per unit length F acting on the triple line, F s LV ͑cosu eq 2 cosu͒. The experimental data on contact angle hysteresis are indeed compatible with a symmetric pinning. At low temperature, the advancing angle is about 25 ± [9]. In some cases, a nonzero receding angle [9] has been observed. One can then assume that the contact line is close to receding when u ϳ 0. Then, ...

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Section: Effect Of Substrate Roughness On Wetting and Absorptionmentioning

confidence: 66%

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confidence: 66%

Effect of Substrate Roughness on Wetting and Absorption

et al. 2001

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“…A stationary contact line due to strong pinning is observed on a particularly disordered substrate. 17,27 The pinned contact line holds a saturated film on the Cs. We note that the helium films on the Cs and the wetted substrate will be in equilibrium.…”

Section: A Strong Pinningmentioning

confidence: 99%

Wetting behavior of liquidHe4on rough Cs films: Pinning, memory effect, and micropuddles

et al. 2005

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The liquid 4 He-cesium system is a nearly ideal one for studying wetting phenomena. However, it can show nonideal behavior such as an extreme wetting hysteresis and a memory of being in contact with liquid 4 He. We believe that this is caused by the roughness of the Cs surface. We review the wetting characteristics of Cs surfaces produced by various methods, and we qualitatively classify Cs surfaces according to the strength of pinning of the contact line. New data are presented on quench-condensed Cs surfaces that show that the pinning can be weak and the contact line can move freely to dewet this Cs. We discuss how micropuddles can form on strong-pinning surfaces, and we discuss how this leads to the memory effect and changes the effective pinning. These phenomena should be generally relevant to the wetting behavior of rough surfaces.

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“…Cesium thickness and helium coverage are measured through an improved Nomarski microscope adapted to low temperatures [35][36][37]. It provides a map of the local gradient along a given direction of the local surface height.…”

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confidence: 99%

Prewetting transition on a weakly disordered substrate: Evidence for a creeping film dynamics

Müller¹,

Dupont‐Roc²

2001

Europhys. Lett.

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PACS. 68.10.Gw -Interface activity, spreading. PACS. 68.15.+e -Liquid thin films. PACS. 68.35.Rh -Phase transitions and critical phenomena.Abstract. -We present the first microscopic images of the prewetting transition of a liquid film on a solid surface. Pictures of the local coverage map of a helium film on a cesium metal surface are taken while the temperature is raised through the transition. The film edge is found to advance at constant temperature by successive avalanches in a creep motion with a macroscopic correlation length. The creep velocity varies strongly in a narrow temperature range. The retreat motion is obtained only at much lower temperature, conforming to the strong hysteresis observed for prewetting transition on a disordered surface. Prewetting transition on such disordered surfaces appears to give rise to dynamical phenomena similar to what is observed for domain wall motions in 2D magnets.Introduction. -Over the past twenty years, prewetting transition has attracted a lasting interest since its first consideration by Ebner, . Considering an equilibrium between two fluid phases, the prewetting transition on a surface is simply the continuation off coexistence of the wetting transition. The surface coverage corresponding to the wetted state is simply finite rather than infinite. Apart from very special matching conditions [4,5] for which critical wetting transition has indeed been observed [6], first order transitions are expected, resulting from long range Van der Waals forces between the surface and the various fluid components [2][3][4]. In the simple case of a liquid/vapor phase equilibrium in presence of a surface, the surface coverage changes from a thin coverage to a thick liquid film. Experimental observations of these transitions on various systems have been made over the last decade [7][8][9][10][11][12][13][14][15][16][17][18][19][20][21][22]. A particular goal of experimentalists has been to check the first order nature of the transition, to determine the prewetting line in the T -∆µ plane and to locate the prewetting critical point. As a matter of fact, experiments have raised further questions about hysteresis phenomena, about the consequences of surface disorder, about the nucleation of the new phase at the transition and its subsequent growth. Presently there is no unique interpretation of the various experimental observations of prewetting transitions, especially at low temperatures. To our best knowledge, nobody has ever provided a clear

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Cited by 8 publications

References 11 publications

Effect of Substrate Roughness on Wetting and Absorption

Effect of Substrate Roughness on Wetting and Absorption

Wetting behavior of liquidHe4on rough Cs films: Pinning, memory effect, and micropuddles

Prewetting transition on a weakly disordered substrate: Evidence for a creeping film dynamics

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