Born Philip, Schmitz Johannes, Sperl Matthias
Institut für Materialphysik im Weltraum, Deutsches Zentrum für Luft- und Raumfahrt (DLR), 51170 Köln, Germany.
Institut für Theoretische Physik, Universität zu Köln, 50937 Köln, Germany.
NPJ Microgravity. 2017 Nov 3;3:27. doi: 10.1038/s41526-017-0030-z. eCollection 2017.
Handling and transport of granular media are inevitably governed by the settling of particles. Settling into a dense state is one of the defining characteristics of granular media, among dissipation and absence of thermal agitation. Hence, settling complicates the adaptation of microscopic theories from atomic, molecular, or colloidal media to granular media. It is desirable to provide experiments in which selectively one of the granular characteristics is tuned to test suitable adaptation of a theory. Here we show that gas fluidization of granular media in microgravity is a suitable approach to achieve steady states closer to thermally agitated systems free of settling. We use diffusing-wave spectroscopy to compare the spatial homogeneity and the microscopic dynamics of gas-fluidized granular media on the ground and in drop tower flights with increasing packing densities up to full arrest. The gas fluidization on the ground leads to inhomogeneous states as known from fluidized beds, and partial arrest occurs at packing fractions lower than the full arrested packing. The granular medium in microgravity in contrast attains a homogeneous state with complete mobilization even close to full arrest. Fluidized granular media thus can be studied in microgravity with dynamics and packing fractions not achievable on the ground.
颗粒介质的处理和运输不可避免地受颗粒沉降的支配。沉降到致密状态是颗粒介质的定义特征之一，此外还有耗散和不存在热搅动。因此，沉降使从原子、分子或胶体介质的微观理论向颗粒介质的适配变得复杂。希望能提供这样的实验，即选择性地调节颗粒特性之一，以测试理论的合适适配情况。在此我们表明，颗粒介质在微重力环境下的气体流化是一种合适的方法，可实现更接近无沉降热搅动系统的稳态。我们使用扩散波光谱法来比较地面上和落塔飞行中气体流化颗粒介质的空间均匀性和微观动力学，直至堆积密度增加到完全静止。地面上的气体流化会导致如流化床中所知的不均匀状态，并且在低于完全静止堆积分数时会发生部分静止。相比之下，微重力环境下的颗粒介质即使接近完全静止也能达到完全流动的均匀状态。因此，可以在微重力环境下研究流化颗粒介质，其动力学和堆积分数是地面上无法实现的。
