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Adhesion-force micro-scale study of desiccating granular material
Géotechnique ( IF 5.8 ) Pub Date : 2020-11-17 , DOI: 10.1680/jgeot.18.p.298
Tomasz Hueckel 1 , Boleslaw Mielniczuk 1, 2, 3 , Moulay Said El Youssoufi 2, 3
Affiliation  

Experiments on five-, four-, three- and two-wet-hydrophilic-grain clusters were performed to investigate evolution of adhesion of granular media during drying on the micro-scale. The experiments show that the adhesion-force of a cluster initially grows at most to three times the original value before decreasing to zero by the end of evaporation. The adhesion-force is composed of capillary pressure force acting over the liquid/solid contact surface area, and surface tension forces acting over the three-phase contact perimeter length. This is in contrast with most macro-scale phenomenological models, in which the only desaturation process variables affecting strength are suction and saturation. Both the contact surface area and contact perimeter length are reduced to zero upon complete liquid evaporation. The morphology of an evaporating water body evolves through slow flow controlled by evaporation rate, interrupted by various modes of fast air entry, which are non-equilibrium jumps of liquid/gas interfaces (Haines jumps). The instabilities involve large adhesion force discontinuities and substantial water mass reconfiguration with water flow in an extremely short time, which makes the process transient. The reconfigurations can reduce the original multi-grain water clusters to four-, three- and two-grain clusters by way of three different instability modes: of thin-sheet instability, or meniscus snap-through instability, depending on the sign of the Gauss curvature of the liquid surface, or finally, for two-grain bridges only, a liquid wire pinch-off. For larger meso-scale assemblies, however, the global adhesion-force evolution is little affected by the jumps. The air entries are potential sites for drying cracks. The (approximately) calculated capillary pressure for two- and three-grain clusters, in no cases is seen to reach high values, predicted from water retention curves.

中文翻译:

干燥粒状材料的附着力微观研究

进行了五个,四个,三个和两个湿亲水颗粒簇的实验,以研究干燥过程中颗粒介质在微观尺度上的粘附演变。实验表明,团簇的附着力最初最多增长到原始值的三倍,然后在蒸发结束时降低到零。粘附力由作用在液/固接触表面积上的毛细压力和作用在三相接触周长上的表面张力组成。这与大多数宏观现象学模型相反,在大多数宏观现象学模型中,影响强度的唯一脱饱和过程变量是吸力和饱和度。液体完全蒸发后,接触表面积和接触周长都减小为零。蒸发水体的形态通过受蒸发速率控制的缓慢流动而演变,并被各种快速空气进入模式中断,这些模式是液/气界面的非平衡跃迁(海因斯跃迁)。这种不稳定性包括很大的粘附力不连续性,以及在极短的时间内随着水流的大量水质重新配置,这使过程变得短暂。重新配置可以通过三种不同的不稳定模式将原始的多粒水团簇减少为四粒,三粒和两粒簇:薄层不稳定或弯月面捕捉不稳定,具体取决于高斯的符号液体表面的曲率,或者最后,仅对于两粒桥而言,液体丝会被夹断。但是,对于较大的中规模装配体,全球粘附力的变化几乎不受跳跃的影响。空气进入是干燥裂纹的潜在场所。根据保水曲线预测,两粒和三粒簇的(近似)计算毛细压力在任何情况下都不会达到很高的值。
更新日期:2020-11-17
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