175 research outputs found
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
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
The effect of the thermal conductivity of the substrate on droplet evaporation
The evaporation of liquid droplets is of fundamental importance to industry, with a vast number of applications including ink-jet printing, spray cooling and DNA mapping, and has been the subject of considerable theoretical and experimental research in recent years. Significant recent papers include those by Deegan [1], Deegan et al. [2], Hu and Larson [3], Poulard et al. [4], Sultan et al. [5], and Shahidzadeh-Bonn et al. [6]
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
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