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

We report surface hardening or crust formation, unlike caking, during drying when a confined porous medium was heated from above using IR radiation. These crusts have higher strength than their closest counterparts such as sandcastles and mud-peels which essentially are clusters of partially wet porous medium. Observed higher strength of the crusts is mostly due to surface tension between the solid particles which are connected by liquid bridges (connate water). Qualitative (FTIR) and quantitative (TGA) measurements confirm the presence of trapped water within the crust. Amount of the trapped water was ~1.5% (this is about 10 times higher than in the samples with caking) which was confirmed using SEM images. Further, in the fixed particle sizes case, the crust thickness varied slightly (10-20 particle diameters only for cases with external heating) while with the natural sand whole porous column was crusted; surprisingly, 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 decreases drastically with increasing hydrophilic spheres diameter while it increases with higher surface temperature.Comment: 17 pages, 7 figures, 1 table Information regarding 'Supplementary Information File' is mentioned in the main tex

Probing atomic level interactions in Ni nanorods and AFM cantilever using atomic force microscopy based F–D spectroscopy

Atomic force microscopy based force-displacement spectroscopy is used to quantify magnetic interaction force between sample and magnetic cantilever. AFM based F–D spectroscopy is used widely to understand various surface-surface interaction at small scale. Here we have studied the interaction between a magnetic nanocomposite and AFM cantilevers. Two different AFM cantilever with same stiffness but with and without magnetic coating is used to obtain F–D spectra in AFM. The composite used has magnetic Ni nanophase distributed uniformly in an Alumina matrix. Retrace curves obtained using both the cantilevers on magnetic composite and sapphire substrate are compared. It is found for magnetic sample cantilever comes out of contact after traveling 100 nm distance from the actual point of contact. We have also used MFM imaging at various lift height and found that beyond 100nm lift height magnetic contrast is lost for our composite sample, which further confirms our F–D observation

Influence of surface roughness on the scatter in hardness measurements - A numerical study

Indentation hardness is usefully applied in the field of rock comminution, where high contact stresses are commonly encountered. Hardness measurements pro- vide a means for estimating the discontinuity wall strength which is required to understand the shear strength of the discontinuity. The hardness value is also used to estimate compressive strength of rocks with known density