Measurement of the Pressure induced by salt crystallization in confinement

Salt crystallization is a major cause of weathering of artworks, monuments and rocks. Damage will occur if crystals continue to grow in confinement, i.e. within the pore space of these materials generating mechanical stresses. We report on a novel method that allows to directly measure, at the microscale, the resulting pressure while visualizing the spontaneous nucleation and growth of alkali halide salts. The experiments reveal the crucial role of the wetting films between the growing crystal and the confining walls for the development of the pressure. The results suggest that the pressure originates from a charge repulsion between the similarly charged wall and the crystal separated by a ~1.5 nm salt solution film. Consequently, if the walls are made hydrophobic, no film and no crystallization pressure are detected. The magnitude of the pressure is system-specific and explains how a growing crystal exerts stresses at the scale of individual grains in porous materials

Evaporation of aqueous salt solutions in sandstone during dissolution/crystallization cycles

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Metastability limit for the nucleation of NaCl crystals in confinement

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 formation of Hopper crystals in the entrapped liquid pockets in the porous network and consequently underline the fact that sodium chloride can easily reach high supersaturations, in spite of what is commonly assumed for this salt.Comment: 16 pages, 6 figure

Damage in porous media due to salt crystallization

We investigate the origins of salt damage in sandstones for the two most common salts: sodium chloride and sulfate. The results show that the observed difference in damage between the two salts is directly related to the kinetics of crystallization and the interfacial properties of the salt solutions and crystals with respect to the stone. We show that, for sodium sulfate, the existence of hydrated and anhydrous crystals and specifically their dissolution and crystallization kinetics are responsible for the damage. Using magnetic resonance imaging and optical microscopy we show that when water imbibes sodium sulfate contaminated sandstones, followed by drying at room temperature, large damage occurs in regions where pores are fully filled with salts. After partial dissolution, anhydrous sodium sulfate salt present in these regions gives rise to a very rapid growth of the hydrated phase of sulfate in the form of clusters that form on or close to the remaining anhydrous microcrystals. The rapid growth of these clusters generates stresses in excess of the tensile strength of the stone leading to the damage. Sodium chloride only forms anhydrous crystals that consequently do not cause damage in the experiments

Impact of recrystallization cycles on drying of salt contaminated porous media

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Single layer porous media with entrapped minerals for microscale studies of multiphase flow

The behaviour of minerals (i.e. salts) such as sodium chloride and calcite in porous media is very important in various applications such as weathering of artworks, oil recovery and CO2 sequestration. We report a novel method for manufacturing single layer porous media in which minerals can be entrapped in a controlled way in order to study their dissolution and recrystallization. In addition, our manufacturing method is a versatile tool for creating monomodal, bimodal or multimodal pore size microporous media with controlled porosity ranging from 25% to 50%. These micromodels allow multiphase flows to be quantitatively studied with different microscopy techniques and can serve to validate numerical models that can subsequently be extended to the 3D situation where visualization is experimentally difficult. As an example of their use, deliquescence (dissolution by moisture absorption) of entrapped NaCl crystals is studied; our results show that the invasion of the resulting salt solution is controlled by the capillary pressure within the porous network. For hydrophilic porous media, the liquid preferentially invades the small pores whereas in a hydrophobic network the large pores are filled. Consequently, after several deliquescence/drying cycles in the hydrophilic system, the salt is transported towards the outside of the porous network via small pores; in hydrophobic micromodels, no salt migration is observed. Numerical simulations based on the characteristics of our single layer pore network agree very well with the experimental results and give more insight into the dynamics of salt transport through porous media

Salt crystallization dynamics in building rocks: a 4D study using laboratory X-ray micro-CT

We employ laboratory X-ray micro-computed tomography (μCT) during climate-controlled salt weathering experiments to acquire data on the kinetics of drying and salt precipitation and the distribution of crystals within the pore space of Mšené sandstone. For that purpose, a custom-designed setup was built at the UGCT’s scanners of the Ghent University Centre for X-ray Tomography (UGCT) that allows to acquire 4D scans while drying. Samples were initially capillary saturated with a saturated NaCl-solution and subsequently dried at 20% RH and at 50% RH, at room temperature. These RH-values are representative for winter and summer conditions for the salt NaCl, which is not temperature sensitive. Different salt precipitation dynamics result in different drying kinetics at the two RH’s. These crystallization and transport dynamics can be directly linked as revealed by the 4D X-ray μCT datasets

Salt deposition patterns from evaporating drops compared to the ‘coffee ring’ effect.

The “coffee-ring effect” [1] is an appealing problem as it is a frequent everyday observation, but the deposition of colloidal particles can also influence for instance printing and coating applications. Previous studies indicates that the deposition pattern of colloidal suspensions depends on contact line pinning [1], the presence of Marangoni flows [2], the thermal conductivity of the substrate [3], and the shape of the colloids [4]. Besides the coffee ring, in everyday life we are also very familiar with crystal deposits resulting from evaporation of drops of salt solutions such as calcium deposits on bathroom walls. Although the problem has been much studied for colloidal suspensions, for salts there are much less results. We experimentally study the effect of the combined wetting properties and thermal conductivity of the substrate on deposition of salt crystals from solutions. We use both Sodium Chloride and Calcium Sulfate solutions, and image the crystallization process and end deposits on various hydrophobic and hydrophilic surfaces. We observe for both salt solutions that the final deposition pattern is mostly very different from the coffee ring, and depends strongly on the wetting properties of the substrate rather than the thermal conductivity. [1] Deegan et al. Nature 1997, 389, 827-829 [2] H. Hu and R.G. Larson, J. Phys. Chem. B 2006, 110, 7090-7094 [3] Ristenpart et al. PRL 2007, 99, 234502 [4] Yunker et al. Nature 2011, 476, 308-31

Evaporation of water:Evaporation rate and collective effects

We study the evaporation rate from single drops as well as collections of drops on a solid substrate, both experimentally and theoretically. For a single isolated drop of water, in general the evaporative flux is limited by diffusion of water through the air, leading to an evaporation rate that is proportional to the linear dimension of the drop. Here, we test the limitations of this scaling law for several small drops and for very large drops. We find that both for simple arrangements of drops, as well as for complex drop size distributions found in sprays, cooperative effects between drops are significant. For large drops, we find that the onset of convection introduces a length scale of approximately 20 mm in radius, below which linear scaling is found. Above this length scale, the evaporation rate is proportional to the surface area