We investigate the influence of surfactants on capillary rise and corner flow in angular pores. We therefore study capillary rise for simple fluids and surfactant solutions, comparing square to cylindrical capillaries. We show that fingers start to form in the corners of the square capillaries when the capillary rise slows down before reaching the equilibrium height. The corner flow scales as t 1/3 and its quantitative understanding necessitates that the surface wettability is taken into account in terms of the liquid’s advancing contact angle on the capillary walls inside the corner. Adding surfactants to water greatly influences the corner flow in square capillaries: depending on the nature of the surfactant, the corner flow can be either suppressed completely due to autophobic effects or enhanced due to the presence of Marangoni stresses caused by a surface tension gradient inside the liquid fingers.

Deliquescence is a first-order phase transition, happening when a salt absorbs water vapor. This has a major impact on the stability of crystalline powders that are important for example in pharmacology, food science and for our environment and climate. Here we show that during deliquescence, the abundant salt sodium sulfate decahydrate, mirabilite (Na2SO4·10H2O), behaves differently than anhydrous salts. Using various microscopy techniques combined with Raman spectroscopy, we show that mirabilite crystals not only lose their facets but also become soft and deformable. As a result, microcrystals of mirabilite simultaneously behave crystalline-like in the core bulk and liquid-like at the surface. Defects at the surface can heal at a speed much faster than the deliquescence rate by the mechanism of visco-capillary flow over the surface. While magnesium sulfate hexahydrate (MgSO4⋅6H2O) behaves similarly during deliquescence, a soft and deformable state is completely absent for the anhydrous salts sodium chloride (NaCl) and sodium sulfate thenardite (Na2SO4). The results highlight the effect of crystalline water, and its mobility in the crystalline structure on the observed softness during deliquescence. Controlled hydrated salts have potential applications such as thermal energy storage, where the key parameter is relative humidity rather than temperature.

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The crystalline structure of minerals due to the highly ordered assembly of its constituent atoms, ions or molecules confers a considerable hardness and brittleness to the materials. As a result, they are generally subject to fracture. Here we report that microcrystals of natural inorganic salt hydrates such as sodium sulfate decahydrate (Na2SO4 • 10H2O) and magnesium sulfate hexahydrate (MgSO4 • 6H2O) can behave remarkably differently: instead of having a defined faceted geometrical shape and being hard or brittle, they lose their facets and become soft and deformable when in contact with their saturated salt solution at their deliquescence point. As a result, the hydrated crystals simultaneously behaves as crystalline and liquid-like. We show that the observed elasticity is a consequence of a trade-off between the excess capillary energy introduced by the deformation from the equilibrium shape and viscous flow over the surface of the microcrystals. This surprising, unusual mechanical properties reported here reveals the role of the water molecules present in the crystalline structure. Although many compounds can incorporate water molecules in their crystalline frameworks, the relationship between the different hydrates and anhydrate crystal forms are still poorly investigated. Our results on the floppy behaviour of such crystalline structure reveals some unexplored properties of water molecules entrapped in the crystalline structure and can open novel routes for their application in various fields such as pharmaceutical sciences , thermal energy storage and even the traceability of water on Mars