Collaborative Research: Statistical mechanics of dense suspensions - dynamical correlations and scaling theory

该奖项支持密集悬浮液的研究和教育，特别是悬浮在水等流体中的粗糙颗粒的集合：玉米淀粉就是这样一个系统的一个很好的例子。类似的密集悬浮物包括水泥、陶瓷和泥浆。这些悬浮液通常充满了颗粒，以至于流速的微小变化可能导致它们停止流动并堵塞成固体。令人惊讶的是，缺乏这种行为的预测理论。一个障碍是弄清楚两个粒子之间的相互作用是如何引起许多粒子放在一起时的行为的。该奖项支持的研究重点是通过结合理论、数值模拟和实验数据分析来创造理解。由于悬浮液的行为很容易在实验和数值模拟中获得，并显示出良好的一致性，因此它们是探索材料如何因流动条件而改变性质这一更大问题的理想系统。该团队致力于推广和教育多样性：通过舞台戏剧展示基础科学的新方法将与纽约市的合作者一起进行，同时两位研究人员将指导本科生，优先从代表性不足的群体中招募。该奖项支持研究和教育，以发展对致密剪切增稠悬浮液微观相关性和宏观行为的理解。采用离散颗粒模拟方法，在应力高于阈值水平时结合颗粒摩擦接触，将探索不连续剪切增厚（DST）和剪切干扰（SJ）转变的行为。重点将是测量动态或运动变量的相关性，例如波动速度和旋转运动的空间和时间相关性。这将通过分析在接近干扰时的发散剪应力和正应力来补充；分析将应用交叉标度方法，这是一种重整化群方法，在行为受到两个奇点影响时特别有用：这在当前系统中可以看到，因为它经历了两种形式的干扰，一种是由于几何决定的无摩擦接触在消失应力下，另一种是由于应力引起的摩擦接触在大应力下。通过扩展最近的方法来描述被堵塞的非晶填料的紧急弹性，在一个新的方向上的悬浮流动的连续体描述也将被研究。这个方向的工作是基于这样一个假设：在逐渐增大的尺度上，刚度是由应力的增加引起的，在足够稠密的悬浮液中，应力的增加会导致刚度的渗透和堵塞；连续统理论是基于对运动相关性的模拟研究。为了探索更复杂的运动学，将进行模拟主动微流变的空间变化流的模拟，其中探针颗粒被拉过密集的悬浮液。该奖项反映了美国国家科学基金会的法定使命，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 award supports research and education on dense suspensions, specifically a collection of rough particles suspended in a fluid like water: cornstarch is a good example of such a system. Similar dense suspensions include cement, ceramics, and mud. These suspensions are often so packed with particles that small changes in flow rate may cause them to stop flowing and jam into a solid. Surprisingly, a predictive theory for this behavior is lacking. One obstacle is figuring out how interactions between two particles cause the behavior seen when many particles are put together. The research supported by this award is focused on creating understanding using a combination of theory, numerical simulations, and analysis of experimental data. Since the behavior of suspensions is easily accessible in experiments and numerical simulations that show good agreement, they are an ideal system for exploring the larger question of how materials change properties due to flow conditions. The team is committed to outreach and diversity-in-education efforts: a novel approach to exposing the basic science through on-stage dramatization will be undertaken with collaborators in New York City, while both investigators will mentor undergraduate research students with recruitment from under-represented groups a priority.This award supports research and education to develop understanding of microscopic correlations and macroscopic behavior in dense, shear-thickening suspensions. Using a discrete-particle simulation approach that incorporates particle frictional contact at stress above a threshold level, the behavior of the discontinuous shear thickening (DST) and shear jamming (SJ) transitions will be explored. The focus will be on measuring dynamical, or motion variable, correlations such as the spatial and temporal correlation of fluctuating velocities and rotational motions. This will be complemented by analysis of divergent shear and normal stress as jamming is approached; the analysis will apply methods of crossover scaling, a renormalization group method especially useful when behavior is influenced by two singular points: this is seen in the current system as it undergoes two forms of jamming, one due to geometrically-determined frictionless contacts at vanishing stress and the other due to stress-induced frictional contacts at large stress. The continuum description of suspension flow in a novel direction, by extension of recent approaches to describe the emergent elasticity of jammed amorphous packings will also be investigated. This direction of work is based on a postulate that rigidity on progressively larger scales is induced by increase of the stress, which in a sufficiently dense suspension leads to a percolation of rigidity and jamming; the continuum theory is based in the investigation of motion correlations by simulation. To explore more complex kinematics, simulation of a spatially varying flow mimicking active microrheology, in which a probe particle is pulled through a dense suspension, will be undertaken.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.