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

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。人们已经确定，降雨会在最近烧毁的山坡上引发泥石流。野火过后，表层燃烧的土壤是防水或疏水的，阻止雨水渗透，导致突然和快速的泥石流。野火后的重力驱动的泥石流是不可预测的，突然发生，并迅速向下移动，变成泥石流并移动大型和重型巨石。2018年1月，在加州的蒙特西托，一场持续15分钟的强烈爆发演变成了一场毁灭性的泥石流，造成21人死亡，4.21亿美元的损失，并关闭了主要的交通走廊。这些实验将研究疏水土壤颗粒与气泡的附着如何导致聚集体的形成，这些聚集体可能会引起在泥石流中观察到的异常流动行为。颗粒-空气-水混合物形成有趣的结构（气泡、管道和簇），其形状主要由重力与气泡和防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模型系统，该系统由通过化学处理变得疏水的沙粒、空气和水组成。混合后，材料将流过有机玻璃通道，颗粒运动和聚集体的演变将被成像，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，将了解重力在聚集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。该研究小组将包括学生，特别是那些来自代表性不足的群体，该项目将支持当地社区高中生的教育活动。 该项目的目标是在地球上和微重力条件下进行实验，以便在微观力学层面上将泥流组成与流动和运输特征联系起来。了解重力对流动的空气-水-颗粒混合物中微观结构变化的作用，特别是对颗粒-气泡团聚体的形成的作用，对于预测泥石流的流变行为至关重要。实验将集中在如何泥流剪切行为取决于相对数量的水，被困的空气，和各种大小的颗粒。在地球上进行的实验将确定混合物的成分如何影响流动行为，并将划定在国际空间站上进行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运输的影响。结果将被用来推导控制方程，可以描述的混合物的流动行为，包括对流动的混合物流变学的影响。了解泥石流的发生过程与降雨强度和持续时间的关系，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知识价值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.