Department of Mechanical and Process Engineering, ETH Zürich, 8092 Zürich, Switzerland.
Department of Chemical Engineering, Columbia University, New York, NY 10027.
Proc Natl Acad Sci U S A. 2019 May 7;116(19):9263-9268. doi: 10.1073/pnas.1820820116. Epub 2019 Apr 22.
The motion and mixing of granular media are observed in several contexts in nature, often displaying striking similarities to liquids. Granular dynamics occur in geological phenomena and also enable technologies ranging from pharmaceuticals production to carbon capture. Here, we report the discovery of a family of gravitational instabilities in granular particle mixtures subject to vertical vibration and upward gas flow, including a Rayleigh-Taylor (RT)-like instability in which lighter grains rise through heavier grains in the form of "fingers" and "granular bubbles." We demonstrate that this RT-like instability arises due to a competition between upward drag force increased locally by gas channeling and downward contact forces, and thus the physical mechanism is entirely different from that found in liquids. This gas channeling mechanism also generates other gravitational instabilities: the rise of a granular bubble which leaves a trail of particles behind it and the cascading branching of a descending granular droplet. These instabilities suggest opportunities for patterning within granular mixtures.
在自然界的几个环境中可以观察到颗粒物质的运动和混合，这些现象通常与液体有惊人的相似之处。颗粒动力学存在于地质现象中，也为从药品生产到碳捕获等技术提供了支持。在这里，我们报告了在垂直振动和向上气流作用下的颗粒混合物中存在一类引力不稳定性，包括一种类似于瑞利-泰勒（Rayleigh-Taylor，RT）的不稳定性，其中较轻的颗粒以“手指”和“颗粒气泡”的形式穿过较重的颗粒上升。我们证明，这种类似于 RT 的不稳定性是由于局部气体通道引起的上曳力与向下的接触力之间的竞争产生的，因此物理机制与在液体中发现的完全不同。这种气体通道机制还会产生其他引力不稳定性：颗粒气泡的上升会在其后面留下一条颗粒轨迹，以及下降的颗粒液滴的分支级联。这些不稳定性为颗粒混合物中的图案形成提供了机会。
