Evaporation From Confined Porous Media Due to Controlled IR Heating From Above

Investigating the origin of laser-induced fluorescence in mannan-rich Phytelephas macrocarpa seeds before and after thermal aging

Natural polysaccharides, e.g., starch, cellulose and sodium alginate have been highlighted as unconventional chromophores owing to their chain structures containing clustered electron-rich groups and the rigidification imposed by inter/intramolecular interactions. On account of the abundant hydroxyl groups and dense packing of low-substituted (< 5 %) mannan chains, we have investigated the laser-induced fluorescence of mannan-rich vegetable ivory seeds (Phytelephas macrocarpa), both in the native state and after thermal aging. The untreated material emitted fluorescence at 580 nm (yellow-orange) when excited at 532 nm (green). This luminescence is intrinsic to the polysaccharide matrix abundant in crystalline homomannan, as demonstrated by lignocellulosic analyses, fluorescence microscopy, NMR, Raman, FTIR and XRD. Thermal aging at 140 °C and above intensified the yellow-orange fluorescence and caused the material to fluoresce when excited by a near-infrared laser (785 nm). In view of the clustering-triggered emission mechanism, the fluorescence of the untreated material can be attributed to hydroxyl clusters and the conformational rigidification in mannan I crystals. On the other hand, thermal aging caused dehydration and oxidative degradation of mannan chains, inducing the substitution of hydroxyl groups by carbonyls. These physicochemical changes may have affected cluster formation and increased conformational rigidification, enhancing fluorescence emission.

Evaporation rate from surfaces of various granular rocks: Comparison of measured and calculated values

Knowledge of the evaporation rate from rock surfaces is critical for obtaining the water flux in the rock-atmosphere interphase, for understanding moisture distribution, and for quantification of damage from salt crystallization within the rock. Evaporation from rocks is a poorly understood, yet important process. We present a study on evaporation from 10 lithologies, including sedimentary, igneous, and metamorphic granular rocks. The evaporation rate was measured from rock cores with a set vaporization plane depth in a humid temperate continental climate during at least eight observation periods for eight months. The measured evaporation rate varied over four orders of magnitude (0.4-2447 mm/year), being dependent on the vaporization plane depth, lithology, and climate seasonality at the site. The evaporation rate from the rock cores was calculated based on Fick's law. The calculations reasonably followed the measured values. Using contrasting, yet field-realistic values in the calculation, virtual time series of the seasonal evaporation rate from natural rock outcrops in three different climates (arid, semi-arid, humid) were constructed. This revealed possible annual evaporative losses from the rock outcrops (0.1 mm-896 mm). Within the range of observed values, the evaporation rate was mostly influenced by the vaporization plane depth (by up to 2.2 orders of magnitude), which was followed by: lithology (up to 1.1 order of magnitude), local climate (up to 1.0 order of magnitude), and climate seasonality (up to 0.8 order of magnitude). Thus, our study shows the key role of the vaporization plane depth in the evaporation rate. This approach can find employment in a large number of investigations such as in the evaporation estimates and hydrologic balance in rock landforms and rocky slopes, hydrologic processes in the shallow rock subsurface, living conditions of endolithic and epilithic organisms, weathering processes, and in the protection of carved or rock constructed cultural heritage.

Formation of a hard surface layer during drying of a heated porous media

We report surface hardening or crust formation, like caking, during evaporation when a porous medium was heated from above using IR radiation. These crusts had higher strength than their closest counterparts such as sandcastles and mud-peels which essentially are clusters of a partially wet porous medium. Observed higher strength of the crusts was mostly due to surface tension between the solid particles, which are connected by liquid bridges (connate water). Qualitative (FTIR) and quantitative (TGA) measurements confirmed the presence of trapped water within the crust. Based on the weight measurements, the amount of water trapped in the crusts was ~1.5%; trapped water was also seen as liquid bridges in the SEM images. Further, in the fixed particle sizes case, the crust thickness varied slightly (only 10–20 particle diameters for cases with external heating) while with the natural sand whole porous column was crusted; surprisingly, the crust was also found with the hydrophobic glass beads. Fluorescein dye visualization technique was used to determine the crust thickness. We give a power-law relation between the crust thickness and the incident heat flux for various particle sizes. The strength of the crust decreased drastically with increasing hydrophilic spheres diameter while it increased with higher surface temperature.