Wetting phenomena are ubiquitous in nature and technology. A solid substrate exposed to the environment is almost invariably covered by a layer of fluid material. In this review, the surface forces that lead to wetting are considered, and the equilibrium surface coverage of a substrate in contact with a drop of liquid. Depending on the nature of the surface forces involved, different scenarios for wetting phase transitions are possible; recent progress allows us to relate the critical exponents directly to the nature of the surface forces which lead to the different wetting scenarios. Thermal fluctuation effects, which can be greatly enhanced for wetting of geometrically or chemically structured substrates, and are much stronger in colloidal suspensions, modify the adsorption singularities. Macroscopic descriptions and microscopic theories have been developed to understand and predict wetting behavior relevant to microfluidics and nanofluidics applications. Then the dynamics of wetting is examined. A drop, placed on a substrate which it wets, spreads out to form a film. Conversely, a nonwetted substrate previously covered by a film dewets upon an appropriate change of system parameters. The hydrodynamics of both wetting and dewetting is influenced by the presence of the three-phase contact line separating "wet" regions from those that are either dry or covered by a microscopic film only. Recent theoretical, experimental, and numerical progress in the description of moving contact line dynamics are reviewed, and its relation to the thermodynamics of wetting is explored. In addition, recent progress on rough surfaces is surveyed. The anchoring of contact lines and contact angle hysteresis are explored resulting from surface inhomogeneities. Further, new ways to mold wetting characteristics according to technological constraints are discussed, for example, the use of patterned surfaces, surfactants, or complex fluids.

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Wetting and Spreading

Intermolecular And Surface Forces

Jets, ie collimated streams of matter, occur from the microscale up to the large-scale structure of the universe Our focus will be mostly on surface tension effects, which result from the cohesive properties of liquids Paradoxically, cohesive forces promote the breakup of jets, widely encountered in nature, technology and basic science, for example in nuclear fission, DNA sampling, medical diagnostics, sprays, agricultural irrigation and jet engine technology Liquid jets thus serve as a paradigm for free-surface motion, hydrodynamic instability and singularity formation leading to drop breakup In addition to their practical usefulness, jets are an ideal probe for liquid properties, such as surface tension, viscosity or non-Newtonian rheology They also arise from the last but one topology change of liquid masses bursting into sprays Jet dynamics are sensitive to the turbulent or thermal excitation of the fluid, as well as to the surrounding gas or fluid medium The aim of this review is to provide a unified description of the fundamental and the technological aspects of these subjects

Physics of liquid jets

Oil spills and industrial organic pollutants have induced severe water pollution and threatened every species in the ecological system. To deal with oily water, special wettability stimulated materials have been developed over the past decade to separate oil-and-water mixtures. Basically, synergy between the surface chemical composition and surface topography are commonly known as the key factors to realize the opposite wettability to oils and water and dominate the selective wetting or absorption of oils/water. In this review, we mainly focus on the development of materials with either super-lyophobicity or super-lyophilicity properties in oil/water separation applications where they can be classified into four kinds as follows (in terms of the surface wettability of water and oils): (i) superhydrophobic and superoleophilic materials, (ii) superhydrophilic and under water superoleophobic materials, (iii) superhydrophilic and superoleophobic materials, and (iv) smart oil/water separation materials with switchable wettability. These materials have already been applied to the separation of oil-and-water mixtures: from simple oil/water layered mixtures to oil/water emulsions (including oil-in-water emulsions and water-in-oil emulsions), and from non-intelligent materials to intelligent materials. Moreover, they also exhibit high absorption capacity or separation efficiency and selectivity, simple and fast separation/absorption ability, excellent recyclability, economical efficiency and outstanding durability under harsh conditions. Then, related theories are proposed to understand the physical mechanisms that occur during the oil/water separation process. Finally, some challenges and promising breakthroughs in this field are also discussed. It is expected that special wettability stimulated oil/water separation materials can achieve industrial scale production and be put into use for oil spills and industrial oily wastewater treatment in the near future.

Biomimetic super-lyophobic and super-lyophilic materials applied for oil/water separation: a new strategy beyond nature

The dynamics and stability of thin liquid films have fascinated scientists over many decades: the observations of regular wave patterns in film flows down a windowpane or along guttering, the patterning of dewetting droplets, and the fingering of viscous flows down a slope are all examples that are familiar in daily life. Thin film flows occur over a wide range of length scales and are central to numerous areas of engineering, geophysics, and biophysics; these include nanofluidics and microfluidics, coating flows, intensive processing, lava flows, dynamics of continental ice sheets, tear-film rupture, and surfactant replacement therapy. These flows have attracted considerable attention in the literature, which have resulted in many significant developments in experimental, analytical, and numerical research in this area. These include advances in understanding dewetting, thermocapillary- and surfactant-driven films, falling films and films flowing over structured, compliant, and rapidly rotating substrates, and evaporating films as well as those manipulated via use of electric fields to produce nanoscale patterns. These developments are reviewed in this paper and open problems and exciting research avenues in this thriving area of fluid mechanics are also highlighted.

Dynamics and stability of thin liquid films

We present a quantitative experimental study of the implications of the roughening transition both on the equilibrium shape and the growth kinetics of hcp 4 He crystals. On the one hand, we show that, in close analogy with the Kosterlitz-Thouless transition, the crystal curvature does not vanish continuously at the transition. The corresponding value of the surface stiffness is found in good agreement with recent universal predictions. On the other hand, the growth rate of facets increases near their roughening temperature. For the (0001) facet, the dominant process is the two dimensional nucleation. It allows us to measure the variation of the step energy near the transition. Lastly, the facet mobility also increases when the temperature tends to zero. This shows that the mobility of crystalline steps is limited by their interaction with thermal excitations Etude experimentale de l'effet de la transition rugueuse sur la forme d'equilibre et la cinetique de croissance: la courbure du cristal ne s'annule pas continument a la transition, mais presente un saut analogue a celui rencontre dans les transitions de Kosterlitz-Thouless, avec une tension superficielle en accord avec des previsions universelles recentes. Acceleration de la cinetique de croissance des facettes pres de la transition rugueuse

https://hal.archives-ouvertes.fr/jpa-00210148/document

Crystal growth and crystal curvature near roughening transitions in hcp 4He

We have studied the roughness and the dynamics of the contact line of a viscous liquid on a disordered substrate. We have used photolithographic techniques to obtain a controlled disorder with a correlation length ξ =1 0µm. Liquids with different viscosity were used: water and aqueous glycerol solution. We have found that the roughness W of the contact line depends neither on the viscosity nor ont he velo cityv of the contact line for v in the range 0.2-20 µm/s. W is found to scale with the length L of the line as L ζ with a roughness exponent ζ =0 .51 ± 0.03. This value is similar tothe o ne o btained with superfluid helium. In the present experiment, we have checked that the motion of the contact line is actually overdamped, so that the phenomenological equation first proposed by Ertas and Kardar should be relevant. However, our measurement of ζ is in disagreement with the predicted value ζ =0 .39. We have also analyzed the avalanche-like motion of the contact line. We find that the size distribution does not follow a power law dependence.

Roughness and dynamics of a contact line of a viscous fluid on a disordered substrate

We report on the simultaneous measurements of the speed of sound and the density in liquid water under negative pressure. Application of a focused acoustic wave to the bulk liquid is able to generate negative pressures before nucleation of the vapor phase occurs. A method for time-resolved Brillouin scattering is developed to measure the speed of sound during the passage of a 1 MHz ultrasonic wave. This is coupled with a fiber optic probe hydrophone which allows the determination of the density. Together, these methods give an ambient temperature equation of state of metastable liquid water down to the acoustic cavitation threshold. Empirical equations of state of water are based on experimental data at positive pressure; the validity of their extrapolation to negative pressures had been tested only indirectly or with very weakly metastable liquid. We provide thermodynamic data that prove the fidelity of recent equations of state down to −26 MPa. However, this raises questions regarding the nature of the ...

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Equation of state of water under negative pressure

We measure the center-of-mass fluctuations of the height of a contact line at depinning for two different systems: liquid hydrogen on a rough cesium substrate and isopropanol on a silicon wafer grafted with silanized patches. The contact line is subject to a confining quadratic well, provided by gravity. From the second cumulant of the height fluctuations, we measure the renormalized disorder correlator Δ(u), predicted by Functional RG to attain a fixed point, as soon as the capillary length is large compared to the Larkin length set by the microscopic disorder. The experiments are consistent with the asymptotic form for Δ(u) predicted by Functional RG, including a linear cusp at u=0. The observed small deviations could be used as a probe of the underlying physical processes. The third moment, as well as avalanche-size distributions are measured and compared to predictions from Functional RG.

http://iopscience.iop.org/article/10.1209/0295-5075/87/56001/pdf

Etienne Rolley

Papers

Wetting and Spreading

Crystal growth and crystal curvature near roughening transitions in hcp 4He

Roughness and dynamics of a contact line of a viscous fluid on a disordered substrate

Equation of state of water under negative pressure

Height fluctuations of a contact line: A direct measurement of the renormalized disorder correlator