No abstract

No abstract

No abstract

International audienceWe show that the drying rate of plaster pastes is significantly lower than that expected for a pure liquid evaporating from a simple homogeneous porous medium. This effect is enhanced by the air flow velocity and the initial solid/water ratio. Further tests under various conditions and with the help of additional techniques (MRI, ESEM, Microtomography) for measuring the drying rate and local characteristics (water content, porosity) prove that this effect is due to the crystallization of gypsum ions below the sample free surface, which creates a dry region and decreases the drying rate by increasing the length of the path the vapor has to follow before reaching the free surface

We study the impact on the drying rate, of the presence of suspended elements, such as calcium sulfate ions with a low solubility, in the interstitial fluid of a porous medium. In order to single out this process in the complexity of a porous medium, we study it through drying in a simple capillary exhibiting characteristics such that it reproduces some critical aspects of drying of porous media. Another specificity of our work is that we focus on the evaporation of initially ion saturated solutions. We first show that in such a capillary the drying process varies depending on the wettability characteristics. Typically the drying rate is much smaller with hydrophobic surfaces because of the airliquid interface which tends to withdraw inside the medium, while for hydrophilic surfaces there remains a continuous liquid film up to the entrance. Then it appears that an ionic solution dries slower than a pure liquid, because the crystals formed along the capillary walls tend to induce a dewetting of the capillary entrance, pushing inwards the first liquid-air interface from which most of the evaporation occurs. An experiment with a model colloidal suspension further illustrates this mechanism: the accumulation of solid particles along the wall forms a deposit which pushes inwards the first liquid-air interface from which evaporation takes place. Finally we look at the impact, on the drying characteristics, of the presence of different additives in the ionic solution.

We have observed a narrowing of the linear cyclotron resonance linewidth with an increase in the density of surface electrons on liquid helium in the electron-vapor atom scattering regime. The effect changes sign at densities n s . 1.7 3 10 8 cm 22 . The data are interpreted as a Coulombic effect on the Landau level width produced by a strong fluctuating electric field. [S0031-9007(99) PACS numbers: 73.20. Dx, 73.25. + i, 76.40. + b Cyclotron resonance (CR) studies in two-dimensional (2D) electron systems such as inversion layers on semiconductor surfaces and electrons bound at the free surface of liquid helium have been of great interest for many years [1,2]. Cyclotron resonance serves as a powerful probe for the effects of quantization of both out-of-plane and in-plane (orbital) motions on electron transport phenomena [3,4]. Two-dimensional electrons on a liquid helium surface are usually under extremely strong coupling conditions with respect to mutual Coulomb interaction: e 2 p pn s ͞k B T ¿ 1 (here n s is the electron density). At such conditions a strong influence of electron-electron interaction on the CR linewidth and line shape is expected. In the ultraquantum limit for short-ranged scatterers, according to [1] , the linewidth is determined by the Landau level width itself. Therefore the CR may also serve as a probe for Coulombic effects on the Landau level width.Experimental studies of the Coulomb effect on the static magnetoconductivity of surface electrons (SE) on liquid helium extensively carried out over the years [5,6] were shown to be in agreement with the theoretical concept of a quasiuniform many-electron fluctuating field E f . However, the only direct study of the many-electron effect on the CR of SE [7] showed mysteriously a density dependence of the linewidth that is opposite to the theoretical predictions [8,9], putting in question the applicability of this concept.The intriguing decrease of the CR linewidth with the electron density was predicted nearly two decades ago [8]. This effect caused by the many-electron fluctuating electric field E f is expected to be quite universal: From a theoretical point of view, it does not depend much on a particular scattering mechanism and remains valid for the Wigner solid [9]. The CR data reported by that time [4] showed a slight decrease of the CR linewidth with the increase of the holding electric field E Ќ 2pen s at low temperatures T , 0.8 K, where electrons are scattered by capillary wave quanta (ripplons). Under these conditions the electron-ripplon coupling is also dependent on E Ќ which interferes with the many-electron effect. At T ϳ 1.3 K, where the main scatterers are helium vapor atoms, the opposite behavior was reported: The linewidth increases linearly with E Ќ [4]. Later, a more detailed exploration of the low temperature regime with the electron density n s being varied independently of the holding electric field E Ќ showed no sign of the linewidth narrowing with n s for the Wigner solid phase [7]. Additionally, the onset of the ele...

A novel high-frequency electron spin resonance (ESR) spectrometer is described. The features are high sensitivity, a continuously tunable operating frequency of 40–60 GHz, a cylindrical cavity with a novel coupling scheme, and simultaneous detection of absorption and dispersion. A radio frequency coil can be integrated into the cavity for electron nuclear double resonance (ENDOR) experiments. ESR and ENDOR spectra of a test sample are presented in order to demonstrate in particular the scientific potential of a variable-frequency spectrometer.

The dynamics of plaster drying and the impact of subflorescence on the process are described through Magnetic Resonance Imaging and X-Ray Microtomography measurements. It is shown that crystals deposit around the air-liquid interface the closest to the sample free surface, which induces a recession of this interface within the sample at a rate only depending on the current saturation (water to pore volume ratio). Thus the distribution of crystals deposited during evaporation essentially depends on the history of saturation. The drying dynamics then results from vapor diffusion through the less porous layers of crystal accumulation below the sample free surface. This in particular makes it possible to predict the dramatic decrease of the drying rate after successive imbibition-drying cycles.