Mechanics and Physics of Meniscus Instability due to Drying of Granular Media

  • Chen, Ruo Yu (Duke University)
  • Mielniczuk, Boleslaw (Duke University)
  • Guevel, Alexendre (Duke University)
  • Veveakis, Manolis (Duke University)
  • Hueckel, Tomasz (Duke University)

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Changes in capillary water morphology and dynamics in clusters of wet grains affecting capillary forces evolving during drying are reported. The work focuses on a stable motion of menisci (air/liquid interfaces) and their unstable (Haines) jump phase. The clusters are either of spherical or cylindrical grains. The meniscus instabilities were shown in the past to be a trigger of soil drying-cracking. The forces considered are Laplace pressure and surface tension resultants deduced from the menisci image-processed every 10 s. The Laplace pressure for cylindrical clusters depends on a single curvature of the meniscus. Several known criteria the capillary body instability are examined using the experimental data. An extended Gibbs criterion points to a source of the instability and allows for a prediction of the instability. The stability is lost when the meniscus approaches the throat between the grains and when the balance is lost between the changes of forces of the Laplace pressure and of the surface tension acting at the meniscus. The presence of the surface tension forces in the instability considerations in addition to the Laplace pressure distinguishes Gibbs criterion from other criteria. Gibbs meniscus energy estimate also provides an inertia force arising to maintain the linear momentum balance for the meniscus, and a duration of the jump. Despite the presence of jumps, and the associated energy releases at a grain scale, the average rate of the progress of air entry at meso- and macro-scale is dominated by a slow evaporation controlled meniscus displacement, rendering the rate of ensuing drying-cracking process its ductile character.