ISS: A new paradigm for explaining catastrophic post-wildfire mudflows: transport phenomena and gravity-driven aggregation dynamics of hydrophobic particle-air-water mixtures

这个NSF-CASIS项目将在国际空间站(ISS)和地球上进行一系列实验，以了解重力在泥石流动力学中的作用。众所周知，降雨会在最近烧毁的山坡上引发泥石流。野火过后，地表被烧毁的土壤是防水或疏水的，防止雨水渗透，导致突然而迅速的泥石流。野火后，重力驱动的泥石流不可预测，发生得突然，并迅速向山下移动，变成泥石流，并动员巨大而沉重的巨石。2018年1月，加利福尼亚州蒙特西托，一场15分钟的强烈爆发演变成毁灭性的泥石流，造成21人死亡，造成4.21亿美元的损失，并关闭了关键的过境走廊。这些实验将研究疏水土壤颗粒附着在气泡上如何导致聚集体的形成，从而可能导致在泥石流中观察到的不寻常的流动行为。颗粒-空气-水混合物形成有趣的结构(气泡、管状和团状)，其形状主要由重力和气泡与拒水颗粒之间的吸引力之间的平衡决定。国际空间站和地球上的实验将使用一个由沙粒、空气和水组成的模型系统，沙粒经过化学处理后变得疏水。混合后，材料将通过有机玻璃通道流动，粒子运动和集合体的演变将被成像并与整体流动的特征相关联。通过将国际空间站上的实验与地球上的实验进行比较，我们将了解重力在集合体形成和流动行为中的作用。这项研究的结果将有助于了解泥石流如何受到降雨强度和持续时间的影响，并可能导致更好的预警系统和风险评估。研究小组将包括学生，特别是那些来自代表不足的群体的学生，该项目将支持当地社区针对高中生的教育活动。该项目的目标是在地球上和微重力条件下进行实验，在微观机械水平上将泥石流成分与流动和运输特征联系起来。了解重力对流动的空气-水-颗粒混合物的微观结构变化，特别是对颗粒-气泡团聚形成的作用，对于预测泥石流的流变性至关重要。实验将集中在泥石流剪切行为如何取决于水、滞留空气和不同大小颗粒的相对量。地球上的实验将确定混合物的组成如何影响流动行为，并将描绘出需要在国际空间站上测试的关键参数范围。微重力实验将研究疏水颗粒附着在气泡上的动力学，以及团聚对混合物流动和传输的影响。结果将被用于推导控制方程，该控制方程可以描述混合物的流动行为，包括混合物流变性对流动的影响。了解与降雨强度和持续时间有关的泥石流引发过程，将有助于更准确地预测泥石流的发生和发展，从而减轻灾难性的破坏。该奖项反映了NSF的法定使命，并通过使用基金会的智力优势和更广泛的影响审查标准进行评估，被认为值得支持。

This NSF-CASIS project will conduct a series of experiments on board the International Space Station (ISS) and on Earth to understand the role of gravity in the dynamics of mudflows. It is well established that rainfall triggers mudflows on recently burned slopes. After wildfires, the surficial burned soil is water-repellent or hydrophobic, preventing rain infiltration and leading to sudden and rapid mudflows. Post-wildfire gravity-driven mudflows are unpredictable, occur suddenly, and travel rapidly downhill, turning into debris flows and mobilizing large and heavy boulders. In January 2018 in Montecito, California, an intense 15-minute burst turned into a devastating debris flow which caused 21 deaths, led to $421 million in damages, and closed key transit corridors. The experiments will examine how the attachment of hydrophobic soil particles to air bubbles leads to the formation of aggregates that may give rise to the unusual flow behaviors observed in mudflows. Particle-air-water mixtures form interesting structures (bubbles, pipes and clusters) whose shapes are primarily governed by a balance between gravity and the attractive forces between air bubbles and water-repellent particles. The experiments on the ISS and on Earth will use a model system consisting of sand particles that have been made hydrophobic through a chemical treatment, air and water. After mixing, the material will flow through a plexiglas channel, and particle motions and evolution of aggregates will be imaged and correlated with characteristics of the overall flow. By comparing experiments on the ISS with those on Earth, the role of gravity in aggregate formation and flow behavior will be understood. The results of this study will help understand how mudslides are affected by rainfall intensity and duration and could lead to better early-warning systems and risk evaluation. The research team will include students, especially those from underrepresented groups, and the project will support educational activities to high-school students in local communities. The goal of this project is to run experiments on Earth and in microgravity conditions to correlate mudflow composition with flow and transport characteristics on a micromechanical level. An understanding of the role of gravity on microstructural changes in flowing air-water-particle mixtures and, in particular, on the formation of particle-bubble agglomerates is crucial for predicting the rheological behavior of mudflows. Experiments will focus on how mudflow shear behavior depends on relative amounts of water, trapped air, and particles of various sizes. Experiments on Earth will identify how the mixture composition affect flow behavior and will delineate critical parameter ranges to be tested on board the ISS. Microgravity experiments will study the dynamics of hydrophobic particle attachment to air bubbles and the consequences of agglomeration on mixture flow and transport. Results will be used to derive governing equations that can describe the flow behavior of the mixtures, including effects of mixture rheology on the flow. Understanding the processes of mudslide initiation with respect to rainfall intensity and duration will lead to a more accurate predictive capability for the onset and development of mudslides that could mitigate catastrophic damage.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.