Collective physics of slowly sedimenting anisotropic objects

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密切相关。从液相中沉淀出固体颗粒是化学提取、纯化和分离、矿石清洗以及食品和饮料加工中普遍存在的过程。在自然界中，沉积在湖泊、河流和海洋中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积也是如此。海洋中碳捕获的一个关键阶段是死亡浮游植物的下沉。大多数相关的系统是由非球形形状（杆，板，薄片，或分支的附属结构），然而，实验室实验和理论在很大程度上解决了球形形状的情况。 本计画利用各种实验技术，进行一系列的实验，以研究非球面形状的沉降。 除了两名研究生外，还对一些本科生进行了有关方法的培训。PI通过在马萨诸塞大学阿默斯特分校组织题为“软固体和复杂流体”的年度暑期学校，并为本科生推出一个新的暑期学校，为软物质物理学的研究生教育做出了贡献。 在活动期间，PI与一位同事和两名高中教师一起为初中和高中教师举办了一个名为“我们周围的模式”的年度暑期项目。技术摘要球体悬浮液沉降研究已经表明这是一个可怕的多体问题，然而，悬浮液中各向异性物体的集体沉降的实际相关情况却受到较少的关注。 最近的研究表明，通过粒子形状添加取向自由度会导致定性的新现象，并使答案与更大的一类物理相关系统更相关。 在这项研究的过程中，实验是进行与一系列的颗粒形状，以建立一个词汇的对称性逐渐降低的对象，开始与轴对称的非极性形状（盘，杆，椭圆），轴对称的极性形状，双轴形状，手性形状，最后，不规则的形状没有明显的对称性。 在从简单到更复杂的系统的进步的精神，实验开始研究单颗粒沉降，然后对动力学，最后沉降在悬浮液中的控制体积分数。 悬浮将在几个几何形状进行研究，以了解限制和硬墙的影响。 这些系统将通过几种不同的探头（如视频成像、X射线CT和光散射）进行询问，以测量平均沉降速率、空间梯度以及浓度、速度和方向场的波动。实验得到了示意性数值计算的支持，该计算使用了捕捉粒子几何形状对称性的stokeslet表示。该奖项反映了NSF的法定使命，并通过使用基金会的智力价值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.