Collective physics of slowly sedimenting anisotropic objects

非技术摘要固体颗粒在液体中的沉降与许多技术、生物和地球物理过程广泛相关。固体颗粒从液体中沉淀出来是化学提取、提纯和分离、矿石净化以及食品和饮料加工中普遍存在的过程。在自然界中，湖泊、河流和海洋中的沉淀是重要的地球物理过程，岩浆中结晶相的下沉和云中冰晶的沉淀也是如此。海洋碳捕获的一个关键阶段是死亡浮游植物的下沉。大多数相关系统是由非球形(棒、板、片或分枝的丝状结构)组成的，然而，实验室实验和理论在很大程度上解决了球形的情况。在本项目中，利用各种实验技术对非球形形状的沉降进行了一系列的实验研究。除了两名研究生外，还有多名本科生也接受了相关方法的培训。PI通过在马萨诸塞州大学阿默斯特分校组织名为软固体和复杂流体的年度暑期班，并为本科生开办新的暑期班，为本国机构以外的软物质物理学研究生教育做出了贡献。在活动期间，PI与一名同事和两名高中教师一起为初中和高中教师举办了一年一度的暑期计划，题为《围绕着我们的模式》。技术摘要关于球体的沉积悬浮的研究已经表明这是一个令人敬畏的多体问题，然而，关于悬浮中各向异性物体的集体沉积这一实际相关的案例却没有得到太多关注。最近的研究表明，通过粒子形状增加定向自由度会带来定性的新现象，并使答案与更大类别的物理相关系统更相关。在这项研究过程中，将用一系列粒子形状进行实验，以建立一个对称性逐渐降低的物体的词汇表，从轴对称的非极形状(圆盘、棒状、椭球体)开始，轴对称的极形状、双轴形状、手性形状，最后是没有明显对称性的不规则形状。本着从简单系统向复杂系统发展的精神，实验首先研究单颗粒的沉积，然后研究颗粒对动力学，最后研究控制体积分数的悬浮液中的沉积。悬架将从几个几何角度进行研究，以了解约束和硬墙的影响。这些系统将被一些不同的探头，如视频成像、X射线CT和光散射所询问，以测量平均沉降速率、空间梯度以及浓度场、速度场和方向场的波动。这些实验得到了使用斯托克斯莱特表示法的示意性数值计算的支持，该表示法捕捉到了粒子几何形状的对称性。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。

Non-technical abstractThe settling of solid particles in a liquid is of wide relevance to a number of technological, biological, and geophysical processes. Sedimentation of solid particulates out of a liquid phase is a ubiquitous process in chemical extractions, purifications and separations, cleaning of ores, and processing of foods and beverages. In the natural world, settling in lakes, rivers and oceans are important geophysical processes, as are sinking of crystalline phases within magma and the settling of ice crystallites within clouds. A crucial phase in carbon capture in the oceans is the sinking of dead phytoplankton. Most relevant systems are comprised of nonspherical shapes (rods, plates, flakes, or branched filamentary structures) and yet, laboratory experiments and theory have largely addressed the case of spherical shapes. In this project, a series of experiments are executed to study the settling of non-spherical shapes by a variety of experimental techniques. In addition to two graduate students, a number of undergraduate students are also trained in the relevant methods. The PI contributes to graduate education in soft matter physics beyond their home institution by organizing an annual summer school at UMass Amherst entitled Soft Solids and Complex Fluids, and launching a new summer school for undergraduate students. During the activity, the PI with a colleague and two high-school teachers run an annual summer program for middle- and high-school teachers entitled Patterns Around Us.Technical abstractStudies of sedimenting suspensions of spheres have already shown this to be a formidable many-body problem, however, the practically relevant case of the collective sedimentation of anisotropic objects in suspension has received less attention. Recent studies show that adding an orientational degree of freedom via the particle shape leads to qualitatively new phenomena, and makes the answers more relevant to a far larger class of physically relevant systems. In the course of this research, experiments are to be conducted with a range of particle shapes to build up a vocabulary of progressively lower symmetry objects, starting with axially symmetric non-polar shapes (discs, rods, ellipsoids), axially symmetric polar shapes, biaxial shapes, chiral shapes, and finally, irregular shapes with no evident symmetry. In the spirit of progressing from simple to more complex systems, the experiments start by studying single-particle sedimentation, then pair dynamics, and finally sedimentation in suspensions of controlled volume fraction. Suspensions will be studied in a few geometries to understand the effect of confinement and hard walls. These systems will be interrogated by a few different probes such as video imaging, x-ray CT, and light scattering to measure mean settling rate, spatial gradients as well as fluctuations in concentration, velocity and orientation fields. The experiments are supported with schematic numerical calculations using stokeslet representations that capture the shape symmetry of particle geometry.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.