Fig 1.
Variation of the evaporation rate versus time for the (hydrophilic) glass beads with 0.78 mm diameter.
Curves for the other cases are seen in the inset. Heat flux received by the top surface in all the cases was ~1000 W/m2. ‘Sand’ was slightly more porous (~43% porosity) compared to the other cases with the (hydrophilic) glass beads (35–37% porosity).
Fig 2.
Rate of evaporation versus the saturation (S) for the case of 0.78 mm diameter (hydrophilic) glass beads.
The experiment is the same as in Fig 1. IR images corresponding to four important instants are also seen. White curved lines in the IR images represent the boundary between the completely wet (inner) and the completely dry regions on the surface of the porous medium. Also mentioned are the temperature scales for the IR images.
Fig 3.
Images at the end of the experiments with different (hydrophilic) samples.
Crusts, formed within a few top layers, in the case of (a) 0.13 mm and (b) 0.78 mm diameter glass spheres respectively. Clumps, similar to sandcastles, rather than the crusts are seen (c) at the deeper locations away from the top exposed surface. The crust in case of the natural sand (d) is not limited to a few layers near the top but covers the entire column height. Heat flux incident on the porous media top surface in all the above experiments was ~1000 W/m2.
Fig 4.
Microscopic images of the crust showing (a) two liquid bridges between 0.13 mm diameter glass beads– 100x magnification and (b) liquid bridges at multiple locations between 0.78 mm diameter (hydrophilic) glass beads– 10x magnification. Traces of water in between the glass beads are clearly seen in these images. Under low pressure conditions in SEM, these liquid bridges either led to solidification or particle deposition as seen in (c) and (d); particles used in these images are 0.13 mm diameter (hydrophilic) glass beads. The control image (showing no liquid bridges) of the dry 0.13 mm diameter glass beads are seen in (e).
Fig 5.
FTIR spectrum showing transmittance values versus the wavenumber of two samples.
A strong peak at ~3300 cm-1 in (a) indicates the presence of water in the crust. Such a peak was absent (b) in case of completely dry (unused) glass beads.
Fig 6.
Variations, with the TGA/DTA chamber temperature (horizontal axis), of the percentage (primary vertical axis) and rate of (secondary vertical axis) mass lost from the crusted sample.
The results with the unused sample are not included here.
Fig 7.
Experiment with fluorescein dye and 0.13 mm diameter (hydrophilic) glass beads in a small Teflon container.
The porous medium is green throughout initially (a). The crusted thin upper layer is clearly seen in (b) at the end of the experiment. The lower regions of the porous medium do not show any significant deposition.
Fig 8.
Snapshots showing the condition of samples of crusts for different incident heat fluxes.
Different (hydrophilic) glass bead sizes were used for this study as well. The initial setup in (a) ensures different heating loads, for the four small containers (consisting of DI water, fluorescein dye, and 0.13 mm diameter hydrophilic glass beads), as the distances from its surface to the IR heater was different. Image (b) showing the conditions at the end of the experiment. The image (c) shows the removed hardened upper crusted layers for the different heat load experiments. The end conditions are seen for hydrophilic 0.45 mm diameter glass beads (d) and 0.78 mm diameter glass beads (e).
Table 1.
Experimental parameters in the present study.
Experiments with sample number ‘1–6’ and ‘8’ were conducted in small Teflon boxes, ‘9’ in a medium-sized acrylic container, and ‘7’ and ‘10–15’ in large glass beakers. Hard layer thickness for the corresponding experiments is also mentioned. Average surface temperatures in stage 1 of evaporation are mentioned as a guide. The particles were hydrophilic in all the 15 cases mentioned here.
Fig 9.
Variation of the thickness of the near-surface hard layer formed in different experiments as a function of the particle sizes and the incident heat fluxes.
The data shown here is only for the hydrophilic particles.