Morphological clues to wet granular pile stability

Abstract

When a granular material such as sand is mixed with a certain amount of liquid, the surface tension of the latter bestows considerable stiffness to the material, which enables, for example, sand castles to be sculpted. The geometry of the liquid interface within the granular pile is of extraordinary complexity and strongly varies with the liquid content. Surprisingly, the mechanical properties of the pile are largely independent of the amount of liquid over a wide range. We resolve this puzzle with the help of X-ray microtomography, showing that the remarkable insensitivity of the mechanical properties to the liquid content is due to the particular organization of the liquid in the pile into open structures. For spherical grains, a simple geometric rule is established, which relates the macroscopic properties to the internal liquid morphologies. We present evidence that this concept is also valid for systems with non-spherical grains. Hence, our results provide new insight towards understanding the complex physics of a large variety of wet granular systems including land slides, as well as mixing and agglomeration problems.

Nanoscale Morphology of Water in Silica Colloidal Crystals

“…29,30 Regarding the cohesion of the assemblies, the sphere arrangements will gradually improve their stability with the sphere coordination number on account of the increasing number of capillary bridges per particle. 7 In the case of the opaline fcc arrangement, this figure is maximum. By increasing the disorder in the assembly, the number of water necks per sphere will decrease in average, so the cohesion of the arrangement is expected to diminish.…”

“…25,26 Factors such as the ambient conditions (temperature and humidity) or the silica hydrophilicity will drastically change the amount of water and its morphology in the opal. 7,17 The influence of ambient conditions was readily demonstrated by subjecting the opal to different temperature and RH in the reactor and, subsequently, to one-pulse CVD. Water necks exhibit an obvious size rise at higher humidity (e.g., R = 90 ± 8 nm at 60% RH; Figure 2a) as the water contained in the pore network increases.…”

Colloidal crystals and water: Perspectives on liquid–solid nanoscale phenomena in wet particulate media

“…The large neck size leads to merging in the presence of a further neighboring sphere (Figure 9c). Interestingly, water does not flood the interstice between the spheres (which contradicts observations and predictions elsewhere [47,78,79,91]), which is a relevant aspect because menisci need of the gaseous phase to exert a capillary force between the spheres [33,36].…”

“…A minimum amount is needed to, first, surpass the rough surface of grains, and, second, form a meniscus between non-touching neighbors, so the number of necks is critically affected at low liquid volumes. Thus, for increasing contents of wetting liquid, powders [171], granular materials [79][80][81]172,173] and capillary suspensions [30,31] show a rapid rise from vanishing yield stress to a stable value. This value corresponds to full pendular bridging (all particles contribute to the capillary network [31]) and remains constant along the pendular state, which indicates that further increase of the neck volume does not change the cohesion between the grains.…”

“…An obvious goal would be to directly image the water in the particulate medium, which is actually a very difficult task, pursued in different systems since decades. For instance, in granular materials the gasliquid phase has not been resolved below the micrometer range, even with exigent techniques like X-ray microtomography [78,79]. The use of optical microscopy requires the immersion in index-matching fluids [80,81], thus departing strongly from the original water-air configuration.…”