Drying of Salt Solutions from Porous Media: Effect of Surfactants

Transport in Porous Media - The evaporation of salt (NaCl) solutions from porous media is studied in the presence of surfactants, because surfactants are often used as cleaning agents for...     

Delayed fracture in porous media

The fracture of porous media subjected to a constant load is studied. Contrary to homogeneous solids in which fracture usually happens instantaneously at a well-defined breaking strength, the fracture of a porous medium can occur with a delay, allowing us to quantify the average lifetime of the unbroken material. We show that the average fracture probability, a key property for risk analysis in civil engineering, is given by the probability of crack nucleation. The nucleation process can be understood qualitatively by calculating the activation energy for crack nucleation, taking into account the porosity of the medium.     

Evaluation of acid-base properties of ammonia plasma-treated polypropylene by means of XPS

Acid-base properties of ammonia plasma-treated polypropylene (APTPP) were characterized by X-ray photoelectron spectroscopy (XPS) in conjunction with the molecular probe technique and using chloroform (TCM) as a reference Lewis acid. It is shown that TCM is retained by the basic surfaces of APTPP but not by the untreated PP. The retention of TCM is shown to be entirely due to the formation of TCM:APTPP acid-base complexes. This is supported by the C12p^3/2 binding energy (BE) and the shape of the Cl2p peak from the adsorbed TCM. ΔH^AB, the heat of TCM:APTPP acid-base interaction was found to be in the range of 3.1–4.3 kcal/mol using a published Cl2p^3/2 BE−ΔH^AB correlation. This ΔH^AB compares remarkably well with the values of 4.3–4.9 kcal/mol determined for TCM:amine complexes, and shows indeed that TCM is complexed by nitrogen containing basic groups grafted at the surface. However, the TCM/N ratio has an optimal value up to 1 second of plasma treatment and then decreases sharply, showing that less specific interaction sites are accessible at the surface for longer treatment times. This parallels previous findings about the metallization of APTPP by aluminium which was found to be optimal for treatment times lower than 1 second in our experimental conditions. This work shows that XPS can now indeed be used to quantitatively assess the acid-base properties of modified polymer surfaces.     

Effect of Wetting on Gravity Drainage in Porous Media

A series of benchmark experiments on the effect of the wetting state on the flow properties in porous media were performed, allowing us to relate the wetting properties at the pore scale to the macroscale hydrodynamics. Drainage of n-alkanes (oils) displaced by air in a model porous medium consisting of water-wet sand was studied using gamma-ray densitometry and weight measurements. The enormous advantage of our system is that we know and control the wetting properties perfectly: we can tune the wetting properties by changing the salinity of the water. This allows us to perform porous medium flow experiments for the different wetting states without changing the transport properties (viscosity and density) of the oil. Drainage is found to be more efficient, and consequently oil recovery more important for partial wetting.     

Effect of Wetting on the Dynamics of Drainage in Porous Media

We examine the consequences of the wettability properties on the dynamics of gravity drainage in porous media. The relation between the wetting properties at the pore scale and the macroscale hydrodynamics is studied. Model porous media consisting of hydrophilic and hydrophobic glass beads or sand with well defined wetting properties, are prepared for this study. Gravity drainage experiments with air displacing water (two-phase flow), are performed for different Bond numbers, and using different techniques such as gamma-ray densitometry, magnetic resonance imaging (MRI) and weight measurements. The dynamics of drainage is found to be different for hydrophilic and hydrophobic porous media in the transition zone (funicular regime). Moreover, for hydrophilic (water-wet) porous media, MRI experiments reveal the importance of drainage through the continuous water film, which leads to an increase of the residual quantity of water in the transition zone with time.     

Salt crystallization during evaporation: impact of interfacial properties

Salt damage in stone results in part from crystallization of salts during drying. We study the evaporation of aqueous salt solutions and the crystallization growth for sodium sulfate and sodium chloride in model situations: evaporating droplets and evaporation from square capillaries. The results show that the interfacial properties are of key importance for where and how the crystals form. The consequences for the different forms of salt crystallization observed in practice are discussed.