Memory effect and anisotropy of particle arrangements in granular paste

Abstract

It is known that pastes of fine powder, for example those of clay, retain memory of shaking applied early in a drying process. This memory results in the appearance of anisotropic patterns of desiccation cracks after drying. In this work, we find a similar behavior in pastes consisting of large granular particles, specifically cornstarch and Lycopodium spores. Because of the large particle size, we were able to observe particle arrangements in Lycopodium paste with micro-focus X-ray computerized tomography ( μ CT). We prepared pastes consisting of Lycopodium particles and water. Agar was added to the paste in order to allow for solidification during a drying process. In these samples, we found statistical anisotropy induced by shaking applied early in the drying process. This anisotropy possesses a feature that was predicted on the basis of results obtained in previous experimental and theoretical studies.

X-ray computerized tomography observation of Lycopodium paste incorporating memory of shaking

“…We found that, in addition to the short-range anisotropy in the arrangement of particles reported in Ref. [9], shaking induces anisotropic fluctuations in the density of particles forming interstices. We investigated the differences among the types of anisotropy and how these depend on the conditions and the height within the sample.…”

“…In situations that paste exhibits this effect, after horizontal, unidirectional oscillation is applied to a uniform layer of paste, parallel cracks first appear during desiccation in the direction perpendicular to the initial shaking. This effect was first found in clay-like paste consisting of calcium carbonate [3] and then also observed in wet granular materials, such as mixtures of starch powder and water [9]. As these experimental studies revealed, parallel cracks form only when the shaking is sufficiently strong to exert shear stresses larger than the yield stress on the paste.…”

“…What anisotropic structures are formed microscopically by shaking, and how do they influence the fracture properties of paste? In a previous study [9], we observed the arrangements of particles using µCT and found that shaking induces short-range anisotropy in irregular particle arrangements. However, we were not able to investigate the details of the anisotropy and their relation to the resultant cracks.…”

“…The µCT facility at SPring-8 has a field of view that is twice as large as that used in Ref. [9], and we were able to make measurements with many samples. We found that, in addition to the short-range anisotropy in the arrangement of particles reported in Ref.…”

Mechanism of memory effect of paste which dominates desiccation crack patterns

“…This anisotropy depends on the direction of the horizontal vibration in such a manner that the number of neighbouring particles increases in the direction perpendicular to the shaking [25], as is schematically drawn in figure 8. It is also shown by measuring the time evolution of stress in paste during the drying process that the stress difference induced by the memory of vibration, such as σ xx − σ yy , is not so large just after the vibration stops, but the difference gradually increases as the paste dries and finally it plays an important role in inducing which direction cracks have to propagate [26]. More challenges are required to incorporate these effects into theoretical models based on residual tension theories.…”