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Evaporation from Porous Media

Evaporation from porous media is of significant importance in many fields ranging from hydrology and agriculture, to food sciences and engineering applications. Evaporation involves coupled heat and mass flow and depends on the transport properties of the medium and the external atmospheric conditions. We have analyzed the dynamics of the evaporation from porous media and described the effects of several parameters such as the particle size distribution, wettability, structure of porous media, and the atmospheric conditions on the evaporation from porous media.

Within this context, we are currently studying a) fractal and scaling behavior of the drying front in three dimensional porous media, b) the effects of the depth of water table on the evaporation rate, c) the dynamics of the transition from stage-1 to stage-2 evaporation, d) the partial wettability effects on the dynamics and morphology of the drying front, and e) the effects of the presence of wettability interfaces on the evaporation.


Fig. 1. Liquid phase distribution during evaporation from a sand column obtained by synchrotron x-ray tomography. The arrangement of sand grains and liquid phase within the first scanned block (3.3 x 3.3 x 1.7 mm3) is shown for five time steps. Isolated liquid clusters were filtered out to highlight continuous liquid phase only.


Fig. 2. Studying the dynamics of the drying front displacement during evaporation from a sand column initially saturated with water. The experiment was conducted by using neutron radiography technique. Top row shows four gray level images (obtained by neutron transmission technique) segmented into saturated (black) and partially air-filled (white) zones. The numbers at the top denote the elapsed time since beginning of the drying experiment.


Fig. 3. Experimental setup for studying water evaporation from sand columns in the presence of a water table: 1, Plexiglas cylindrical columns with 600 mm in height and 70 mm in diameter; 2, the HygroClip sensor measuring the ambient relative humidity and temperature above the sand columns; 3, a fan which was used to boost the evaporation; 4, Mariotte siphons maintaining fixed water tables at desired depths below the sand surface; 5, Mettler Toledo balances to measure evaporative water losses; and 6, PC for the data acquisition and processing.

Dynamics of Salt Distribution in Porous Media

Understanding of the dynamics of salt distribution in porous media is of crucial concern in various environmental and hydrological applications such as soil salinization, rock weathering, terrestrial ecosystem functioning, microbiological activities and biodiversity in vadose zone. Vegetation, plant growth and soil organisms can be severely limited in salt-affected land. Therefore it is important to understand fundamentals of the processes that govern dynamics of the upward and downward salt transport in soil.

Normally, dissolved salt is transported by convection induced by capillary liquid flow toward the surface (due to water evaporation) where it accumulates, whereas diffusion (Brownian motion) tends to spread the salt and homogenize concentrations in space. The resulting interplay between convection and diffusion affects the dynamics of salt distribution in porous media. We are currently investigating the dynamics of salt distribution affected by the transport properties of porous media and the external atmospheric conditions.


Fig. 1. Experimental setup for studying the effects of salt concentration on the drying of porous media. The white color at the surface of the columns indicates the precipitated salt. The patterns and dynamics of salt deposition are significantly affected by the salt concentration.


Fig. 2. Gray value histogram of a typical imagetaken from surface of the sand column illustrated as colorimage in the inset. The white color indicates the area coveredby salt corresponding to the last peak in the histogram. Thevertical line indicates the threshold used to segment the grayvalue image to black and white. The area covered by salt inthe segmented image (i.e., the white color) was used to calculatethe rate of surface coverage by precipitated salt duringevaporation.


Fig. 3. Pore scale images of evaporating sand surface at different times illustrating the phase distribution at surface obtained by synchrotron x-ray tomography. Black, light gray and dark gray on top row correspond to air, solid and liquid phases, respectively (bright color indicate high salt concentrations in the liquid phase). On bottom row, the black color corresponds to both air and solid phases and other colors indicate regions differing in salt concentrations in liquid phase (red color marks high concentration). This figure shows that salt is preferentially deposited in finer water-filled pores connected to evaporating surface.

Hydrate Patterns and Distribution in Porous Media

Gas hydrates are solids composed of water molecules surrounding a gas molecule. They are found mainly in permafrost regions and continental margins. They are important in various environmental and engineering applications such as geo-hazards, climate change, and petroleum industry. The enormous amount of methane stored in hydrates is the main factor that makes the gas hydrates attractive as a future potential energy resource that can supply energy for hundreds years. In order to explain hydrate formation, distribution and dissociation in geological sediments, it is essential to understand accurately the interaction between the physical properties of porous media and hydrate.


Fig. 1. (a) Distribution of organic carbon in the earth system (adapted from K.A. Kvenvolden (1988), Chemical Geology, 71, 41-51). (b) worldwide location of the gas hydrates (map courtesy of USGS).

We are studying the effects of the physical and chemical characteristics of porous media on the formation, distribution and dissociation of hydrates at micro- and macro-scale. We apply the acoustic emission technique to map the hydrate distribution in porous media. Our main goal here is to investigate the relationship between the spatial acoustic wave amplitude and the patterns and spatial distribution of hydrate in sediment. Analysis of wave propagation through porous media provides an excellent opportunity to study the effects of sediment heterogeneity on hydrate formation and distribution. Our goal is to develop a model based on the wave equation to characterize precisely the propagation of acoustic waves in porous media as affected by the presence of hydrate.

Acoustical Signature of the Drying Front Displacement

Displacement of fluid interface in porous media produces acoustical crackling noise. An acoustic hit (noise) is generated when a fluid interface jumps across pores in porous media. Our preliminary results show that the movement of the drying front (defined as the interface between the saturated and unsaturated zone) can be "heard" by using the suitable acoustic sensors! We are currently investigating the acoustical signature of the drying front displacement during evaporation from various porous media differing in texture. Our ultimate goal is to estimate the evaporation rate, the water content in a porous medium and its texture just by "hearing" the drying front displacement through the medium.


Fig. 1. The experimental setup used to study the acoustic signatures of the propagating drying front in porous media.

Fracture Patterns and Dynamics in Desiccating Clay

Fractures occur when tensile stresses created by external conditions overcome the intrinsic strength of a material. When moisture loss and shrinkage of desiccating clay are differential, the created stresses become high enough for crack formation. These crack characteristics can vary significantly under different environmental and mechanical conditions, and its final patterns have a self-similar nature. Cracking phenomena has a presence in a wide range of fields, including geo-environmental engineering, agronomy and manufacturing and materials science, due to its general nature. With this in mind, our research focuses on establishing general mathematical and physical processes that govern this cracking behavior by analyzing crack pattern and dynamics relationships in desiccating clay under varying conditions.



Fig. 2. Final crack patterns resulted from desiccation of a thin layer of clay after 2 days.
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