Evaporative destabilization of a salt crust with branched pattern formation.

The impact of salt crust formation over porous media on water evaporation is an important issue in relation with the water cycle, agriculture, building sciences and more. The salt crust is not a simple accumulation of salt crystals at the porous medium surface but undergoes complex dynamics with possible air gap formation between the crust and the porous medium surface. We report on experiments that allow to identify various crust evolution regimes depending on the competition between evaporation and vapor condensation. The various regimes are summarized in a diagram. We focus on the regime where dissolution-precipitation processes lead to the upward displacement of the salt crust and the generation of a branched pattern. It is shown that the branched pattern results from the crust upper surface destabilization whereas the crust lower surface remains essentially flat. We show that the resulting branched efflorescence salt crust is heterogeneous with a greater porosity in the salt fingers. This leads to the preferential drying of the salt fingers followed by a period in which the crust morphology change only occurs in the salt crust lower region. The salt crust eventually tends toward a frozen state where no visible change occurs in the salt crust morphology, but without blocking the evaporation. These findings provide in-depth insights into the salt crust dynamics and pave the way for the better understanding of the impact of efflorescence salt crusts on evaporation and the development of predictive models.

Detachment mechanism and reduced evaporation of an evaporative NaCl salt crust.

Salt crusts forming at the surface of a porous medium are commonly observed in nature as well as on building materials and pieces of our cultural heritage where they represent a risk for the supporting substrate integrity. Previous research indicates that the salt crust can detach from the porous substrate and severely reduces the evaporation. However, the current understanding of the detachment mechanisms and the reduced evaporation is very limited. In the present experiment, we evidence dissolution-precipitation processes as key mechanisms in the detachment process. We also show that the crust remains wet and the observed reduced evaporation is explained by the formation of tiny pores in the nanometer range and the Kelvin effect. The resulting crust permeability is very low. Combined with previous results, this shows that the crust permeability is highly dependent on the crust formation conditions. More generally, salt structures in a water vapor concentration gradient are shown to be self-propelled systems capable to carry small objects such as, for instance, soil particles. Our study has significance for understanding the impact of salt crusts on evaporation and the associated important phenomena, such as soil salinization and porous material degradation inherent to salt crystallization.

Dissolution-precipitation-driven upward migration of a salt crust.

Salt crusts forming at the surface of a porous medium can dynamically evolve with crust displacements leading to the formations of domes and blisters or simply to the upward migration of the crust. However, the mechanisms explaining the displacements are unclear. It has been conjectured that they could be related to dissolution-precipitation phenomena and/or to mechanical effects associated with the concept of crystallization pressure. We present a simple experiment where the crust upward migration is significant and can be entirely explained from the consideration of dissolution-precipitation phenomena. Equations governing the crust displacement are derived, leading to quite good agreement with the experimental results.

Quantitative 3D comparison of biofilm imaged by X-ray micro-tomography and two-photon laser scanning microscopy.

Optical imaging techniques for biofilm observation, like laser scanning microscopy, are not applicable when investigating biofilm formation in opaque porous media. X-ray micro-tomography (X-ray CMT) might be an alternative but it finds limitations in similarity of X-ray absorption coefficients for the biofilm and aqueous phases. To overcome this difficulty, barium sulphate was used in Davit et al. (2011) to enable high-resolution 3D imaging of biofilm via X-ray CMT. However, this approach lacks comparison with well-established imaging methods, which are known to capture the fine structures of biofilms, as well as uncertainty quantification. Here, we compare two-photon laser scanning microscopy (TPLSM) images of Pseudomonas Aeruginosa biofilm grown in glass capillaries against X-ray CMT using an improved protocol where barium sulphate is combined with low-gelling temperature agarose to avoid sedimentation. Calibrated phantoms consisting of mono-dispersed fluorescent and X-ray absorbent beads were used to evaluate the uncertainty associated with our protocol along with three different segmentation techniques, namely hysteresis, watershed and region growing, to determine the bias relative to image binarization. Metrics such as volume, 3D surface area and thickness were measured and comparison of both imaging modalities shows that X-ray CMT of biofilm using our protocol yields an accuracy that is comparable and even better in certain respects than TPLSM, even in a nonporous system that is largely favourable to TPLSM.

Slow invasion of a nonwetting fluid from multiple inlet sources in a thin porous layer.

We numerically study the process of quasistatic invasion of a nonwetting fluid in 2D and 3D porous layers from multiple inlet injection sources and show that a porous layer acts as a two-phase filter as a result of the repeated convergence of flow paths: The probability for a pore at the outlet to be a breakthrough point is significantly lower than the fraction of active injection points at the inlet owing to the merging within the porous layer of liquid paths originating from different inlet injection points. The study of the breakthrough point statistics indicates that the number of breakthrough points diminishes with the system thickness and that the behavior of thin layers, defined here as systems of typical thicknesses of less than 15 lattice spacing units (≈15 pore or grain mean sizes), is distinct from thicker layers. For thicker systems, it is found that the probability of an outlet pore to be a breakthrough pore scales as l(1-d) where l is the system thickness and d is the space dimensionality, whereas, a power law behavior is not obtained with a thin system. Other properties, such as the invading phase occupancy profiles are studied. We also described a kinetic algorithm that allowed us to compute the occurrence times of breakthrough points. The distribution of these times is markedly different in 2D and in 3D.

Colloidal surface interactions and membrane fouling: investigations at pore scale.

In this paper, we examine the contributions of colloidal surface interaction in filtration processes. In a first part, we describe the way surface interactions affect the transport of colloidal particles or macromolecules towards a membrane, and its theoretical description. The concept of critical flux is introduced and linked to particle-membrane wall and particle-particle surface interactions. From this review, it seems important to consider how surface interactions occur at pore scale and control the development of fouling layers. In this context, we report in a second part experiments where the capture of micron-sized particles is observed in a poly-dimethylsiloxane (PDMS) microfluidic filtration device. Direct observations of the filtering part by video-microscopy allow to investigate the way the fouling of the microchannels by the particles is taking place. The experimental results underline the important role played by the particle-wall interactions on the way particles are captured during filtration. A small change in surface properties of the PDMS has important consequences in the way pore clogging occurs: in more hydrophobic conditions the particles first form arches at the microchannels entrance, then leading to the growth of a filtration cake, whereas in more hydrophilic conditions the particles are captured on the walls between the microchannels, then leading to the progressive formation of dendrites. To conclude, both experimental and theoretical approaches show the important role played by surface interactions in filtration processes. The complex interplay between multi-body surface interactions and hydrodynamics at nanometric scale leads to clogging phenomena observed experimentally in microfluidic systems that have not been predicted by numerical simulations. In the future, the two way coupling between simulation and experimental approaches at the pore scale have to progress in order to reach a full understanding of the contribution of colloid science in membrane processes.

Three periods of drying of a single square capillary tube.

The drying kinetics of a porous medium is classically described in three main periods, which depend on the interplay between the external and internal mass transfers during evaporation. The first period is described as essentially depending on the external mass transfer, whereas the third period is dominated by the internal mass transfer. The second period is a crossover period. We show experimentally that a similar drying kinetics can be obtained from a much simpler system owing to the effect of corner liquid films: a capillary tube of square cross section.