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

该奖项支持有关致密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：玉米淀粉是此类系统的一个很好的例子。类似的高密度悬浮物包括水泥、陶瓷和泥浆。这些悬浮液通常充满颗粒，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，这种行为缺乏预测理论。一个障碍是弄清楚两个粒子之间的相互作用如何导致许多粒子放在一起时所看到的行为。该奖项支持的研究重点是使用理论，数值模拟和实验数据分析相结合来创建理解。由于悬浮液的行为很容易在实验和数值模拟中获得，并且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。该小组致力于教育工作的外联和多样性：一种通过舞台戏剧化展示基础科学的新方法将与纽约市的合作者一起进行，而两位研究者将指导本科研究生，优先从代表性不足的群体中招募。该奖项支持研究和教育，以发展对密集，剪切增稠悬浮液。使用离散颗粒模拟方法，将颗粒摩擦接触在应力高于阈值水平，不连续剪切增稠（DST）和剪切堵塞（SJ）过渡的行为进行了探讨。 重点将是测量动态或运动变量的相关性，如波动速度和旋转运动的空间和时间相关性。 这将通过分析接近堵塞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，当行为受到两个奇点的影响时特别有用：这在当前系统中可以看出因为它经历两种形式的干扰，一种是由于在应力为零时几何确定的无摩擦接触，另一种是由于在大应力时应力引起的摩擦接触。悬浮液流动的连续描述在一个新的方向，通过扩展最近的方法来描述堵塞的无定形填料的涌现弹性也将进行研究。这个方向的工作是基于这样一个假设，即在逐渐变大的尺度上的刚性是由应力的增加引起的，这在足够密集的悬浮液中导致刚性和堵塞的渗透;连续统理论是基于通过模拟研究运动相关性。为了探索更复杂的运动学，模拟空间变化的流动模拟活动微观流变学，其中探针颗粒被拉过一个稠密的悬浮液，将承担。这个奖项反映了NSF的法定使命，并已被认为是值得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的支持。

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.