The maximum spreading of drops impacting on smooth and rough surfaces is measured from low to high impact velocity for liquids with different surface tensions and viscosities. We demonstrate that dynamic wetting plays an important role in the spreading at low velocity, characterized by the dynamic contact angle at maximum spreading. In the energy balance, we account for the dynamic wettability by introducing the capillary energy at zero impact velocity, which relates to the spreading ratio at zero impact velocity. Correcting the measured spreading ratio by the spreading ratio at zero velocity, we find a correct scaling behaviour for low and high impact velocity and, by interpolation between the two, we find a universal scaling curve. The influence of the liquid as well as the nature and roughness of the surface are taken into account properly by rescaling with the spreading ratio at zero velocity, which, as demonstrated, is equivalent to accounting for the dynamic contact angle.

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The study of the behavior of sessile droplets on solid substrates is not only associated with common everyday phenomena, such as the coffee stain effect, limescale deposits on our bathroom walls , but also very important in many applications such as purification of pharmaceuticals, de-icing of airplanes, inkjet printing and coating applications. In many of these processes, a phase change happens within the drop because of solvent evaporation, temperature changes or chemical reactions, which consequently lead to liquid to solid transitions in the droplets. Here we show that crystallization patterns of evaporating of water drops containing dissolved salts are different from the stains reported for evaporating colloidal suspensions. This happens because during the solvent evaporation, the salts crystallize and grow during the drying. Our results show that the patterns of the resulting salt crystal stains are mainly governed by wetting properties of the emerging crystal as well as the pathway of nucleation and growth, and are independent of the evaporation rate and thermal conductivity of the substrates.

ABSTRACT-We study the spontaneous nucleation and growth of sodium chloride crystals induced by controlled evaporation in confined geometries (microcapillaries) spanning several orders of magnitude in volume. In all experiments, the nucleation happens reproducibly at a very high supersaturation S~1.6 and is independent of the size, shape and surface properties of the microcapillary. We show from classical nucleation theory that this is expected: S~1.6 corresponds to the point where nucleation first becomes observable on experimental time scales.A consequence of the high supersaturations reached at the onset of nucleation is the very rapid growth of a single skeletal (Hopper) crystal. Experiments on porous media reveal also the

Salt crystallization is a major cause of weathering of rocks, artworks and monuments. Damage can only occur if crystals continue to grow in confinement, i.e. within the pore space of these materials, thus generating mechanical stress. We report the direct measurement, at the microscale, of the force exerted by growing alkali halide salt crystals while visualizing their spontaneous nucleation and growth. The experiments reveal the crucial role of the wetting films between the growing crystal and the confining walls for the development of the pressure. Our results suggest that the measured force originates from repulsion between the similarly charged confining wall and the salt crystal separated by a ~1.5 nm liquid film. Indeed, if the walls are made hydrophobic, no film is observed and no repulsive forces are detected. We also show that the magnitude of the induced pressure is system specific explaining why different salts lead to different amounts of damage to porous materials.

We report an ellipsometry study of the wetting of hexane on water. By adding salt to the water, we are able to tune the Hamaker constant of this system. This allows us to demonstrate, for the first time, that two rather than one wetting transitions can exist in a single system. Upon increasing the temperature, a discontinuous (first-order) transition from a microscopic film to a mesoscopic film occurs, followed by a continuous (critical) wetting transition that leads to a thick adsorbed film. The latter is due to the Hamaker constant which changes sign with temperature. The first-order transition temperature changes by the same amount as the critical wetting temperature upon changing the Hamaker constant.[S0031-9007(98)06032-3] PACS numbers: 68.45.Gd If one considers a liquid droplet on a substrate, in general one distinguishes two possible situations. If the sum of the liquid-substrate and the liquid-vapor interfacial tension is larger than the substrate-vapor interfacial tension, the droplet will have a contact angle between 0 ± and 180 ± , a situation called partial wetting. On the other hand, the situation may arise that the sum of the liquid-substrate and the liquid-vapor surface tension equals the substratevapor surface tension. The contact angle will then be zero, and the droplet will form a uniform film that covers the whole substrate surface: The liquid completely wets the substrate. The transition between these states, say as a function of temperature, is believed to be a first-order (discontinuous) surface phase transition, as there is a discontinuity in the first derivative of the surface free energy with respect to the temperature [1].The last few years have seen tremendous progress in the study of these phenomena. For the first time, evidence for the existence of the prewetting line has been obtained in a few systems [2]. Several observations of metastable surface states, such as the discovery of hysteresis in wetting transitions, also underline the generic first-order (discontinuous) character of the wetting transition [2]. On the other hand, Ragil et al. recently demonstrated the existence of a counterexample [3]. Studying the wetting behavior of pentane on water, a transition from a mesoscopic ͑ഠ50 Å͒ to a very thick film was found that was completely continuous. The possible existence of such a continuous (or critical) wetting transition had been disputed for a long time [4]. The conclusion was that it might occur in real systems with long-range interactions, but that these interactions, quantified by the Hamaker constant, should change sign at the critical wetting temperature [4]. It was demonstrated that this was indeed the case for the pentanewater system, and, moreover, it was shown that this is a necessary but not a sufficient condition for critical wetting to occur. Additionally, the system has to be in a state that would show complete wetting in the absence of long-range forces [3].Ragil et al. thus proposed a new scenario for wetting transitions: Instead of two possible surface state...

General rightsIt is not permitted to download or to forward/distribute the text or part of it without the consent of the author(s) and/or copyright holder(s), other than for strictly personal, individual use, unless the work is under an open content license (like Creative Commons). Disclaimer/Complaints regulationsIf you believe that digital publication of certain material infringes any of your rights or (privacy) interests, please let the Library know, stating your reasons. In case of a legitimate complaint, the Library will make the material inaccessible and/or remove it from the website. Please Ask the Library: http://uba.uva.nl/en/contact, or a letter to: Library of the University of Amsterdam, Secretariat, Singel 425, 1012 WP Amsterdam, The Netherlands. You will be contacted as soon as possible. We show experimentally that the sliding friction on sand is greatly reduced by the addition of some-but not too much-water. The formation of capillary water bridges increases the shear modulus of the sand, which facilitates the sliding. Too much water, on the other hand, makes the capillary bridges coalesce, resulting in a decrease of the modulus; in this case, we observe that the friction coefficient increases again. Our results, therefore, show that the friction coefficient is directly related to the shear modulus; this has important repercussions for the transport of granular materials. In addition, the polydispersity of the sand is shown to also have a large effect on the friction coefficient.

Drying of salt contaminated porous media: Effect of primary and secondary nucleationDesarnaud, J.E.; Derluyn, H.; Molari, L.; de Miranda, S.; Cnudde, V.; Shahidzadeh, N.F. Published in:Journal of Applied Physics DOI:10.1063/1.4930292 Link to publication Citation for published version (APA):Desarnaud, J., Derluyn, H., Molari, L., de Miranda, S., Cnudde, V., & Shahidzadeh, N. (2015). Drying of salt contaminated porous media: Effect of primary and secondary nucleation. Journal of Applied Physics, 118(11), [114901]. DOI: 10.1063/1.4930292 General rightsIt is not permitted to download or to forward/distribute the text or part of it without the consent of the author(s) and/or copyright holder(s), other than for strictly personal, individual use, unless the work is under an open content license (like Creative Commons). Disclaimer/Complaints regulationsIf you believe that digital publication of certain material infringes any of your rights or (privacy) interests, please let the Library know, stating your reasons. In case of a legitimate complaint, the Library will make the material inaccessible and/or remove it from the website. Please Ask the Library: http://uba.uva.nl/en/contact, or a letter to: Library of the University of Amsterdam, Secretariat, Singel 425, 1012 WP Amsterdam, The Netherlands. You will be contacted as soon as possible. The drying of porous media is of major importance for civil engineering, geophysics, petrophysics, and the conservation of stone artworks and buildings. More often than not, stones contain salts that can be mobilized by water (e.g., rain) and crystallize during drying. The drying speed is strongly influenced by the crystallization of the salts, but its dynamics remains incompletely understood. Here, we report that the mechanisms of salt precipitation, specifically the primary or secondary nucleation, and the crystal growth are the key factors that determine the drying behaviour of salt contaminated porous materials and the physical weathering generated by salt crystallization. When the same amount of water is used to dissolve the salt present in a stone, depending on whether this is done by a rapid saturation with liquid water or by a slow saturation using water vapor, different evaporation kinetics and salt weathering due to different crystallization pathways are observed.