Suspension Rheology at Constant Pressure

1337097 PI：缓速胶体悬浮液广泛用于工业、医学和自然环境中，包括牙膏、油漆、电池内部和可喷涂太阳能电池板等多种系统。了解悬浮液的流变特性对其加工、点胶、耐用性和性能至关重要。大多数悬浮液流变学的研究都是在固定体积（或固定体积分数）下进行的。虽然这对于许多应用可能是足够的，但通常悬浮液流不是在固定体积下，而是在固定应力（或固定压力或压降）下。在固定体积和固定压力下，流动行为是否相同？如果悬浮颗粒的体积分数足够低，则应该可以将一种测量转换为另一种测量。但是当接近最大流动分数时，不再清楚这两个条件将导致相同的流动行为。胶体悬浮液在固定的压力，允许系统的扩张或收缩和体积分数波动的模拟研究，提出了必要的。加速斯托克斯动力学模拟方法将适用于允许模拟体积改变，并用于研究布朗硬球悬浮液的流动行为，因为与热布朗力相比，剪切力的强度在很宽的范围内变化。完整的微观细节可从模拟中获得，包括粒子分布函数、序参数、短时间和长时间粒子位移等，并将观察到的宏观行为与底层粒子动力学联系起来。特别注意将集中在流动行为的最大流动分数接近和缩放的流动特性附近的这一点。了解悬浮液流变学是一个重要的主题，在其本身的权利，但检查的流动行为的最大流动分数接近可能有重要的影响玻璃和堵塞系统。已知静止的胶态分散体以接近0.58的体积分数形成玻璃，远低于无规紧密堆积（对于单分散球体为0.64）。在固定压力和剪切应力下对快速颗粒流和粘性非布朗悬浮液的实验显示出非常相似的行为：剪切与法向应力的比率-摩擦系数-在两个系统中是相同的，最大流动体积分数也是如此，尽管微尺度物理学非常不同-惯性动力学与粘性力。布朗胶体分散体很可能会表现出类似的行为，这将在堵塞的颗粒介质和胶体玻璃之间建立重要的联系。如果得到证实，这种联系将改变我们对玻璃和堵塞系统的理解，并可能提供对堵塞的普遍理解。这项研究将能够在颗粒尺度上设计胶体分散体，以满足油漆和涂料行业等特定应用的流动要求，从而减少能源消耗和产品浪费。有助于理解玻璃和玻璃形成系统，特别是它们的动态特性，将对从基础物理学和化学到生物学的各个学科产生广泛的影响-蛋白质和蛋白质复合物在拥挤的细胞内部的运动与胶体玻璃中的受阻和异质运动有很强的相似性。最后，由本研究支持的研究生将在连续介质和统计力学，胶体物理学和计算科学方面接受良好的培训，并将加入国家的科学劳动力队伍。

1337097PI: BradyColloidal suspensions are widely used in industry, medicine and in natural environments, and encompass systems as diverse as toothpaste, paints, the interior of a cell and sprayable solar panels. Understanding the rheological properties of suspensions is critical to their processing, dispensing, durability and performance. Most studies of suspension rheology have been at fixed volume (or fixed volume fraction). While this may be adequate for many applications, often suspension flows are not at fixed volume but rather at fixed stress (or fixed pressure or pressure drop). Is the flow behavior the same at fixed volume and fixed pressure? If the volume fraction of suspended particles is low enough it should be possible to covert one measurement into the other. But as the maximum flowing fraction is approached, it is no longer clear that the two conditions will lead to the same flow behavior. A simulation study of colloidal suspensions at fixed pressure, allowing the system to dilate or contract and the volume fraction fluctuate as necessary, is proposed. The Accelerated Stokesian dynamics simulation methodology will be adapted to permit the simulation volume to change and used to study the flow behavior of Brownian hard-sphere suspensions as the strength of the shearing forces compared to thermal Brownian forces is varied over a wide range. Complete microscale detail is available from simulation, including particle distribution functions, order parameters, short- and long-time particle displacements, etc., and will connect the observed macroscopic behavior to the underlying particle dynamics. Particular attention will be focused on the flow behavior as the maximum flowing fraction is approached and the scaling of the flow properties near this point.Understanding suspension rheology is an important subject in its own right, but examining the flow behavior as the maximum flowing fraction is approached may have important implications for glassy and jammed systems. Colloidal dispersions at rest are known to form a glass at volume fractions near 0.58, well below random close packing (0.64 for monodisperse spheres). Experiment on both rapid granular flows and viscous non-Brownian suspensions at fixed pressure and shear stress have shown very similar behaviors: the ratio of shear to normal stress - the friction coefficient - is the same in the two systems, as is the maximum flowing volume fraction, despite the very different microscale physics - inertial dynamics versus viscous forces. It is quite possible that Brownian colloidal dispersions will display a similar behavior, which would then make an important link between jammed granular media and colloidal glasses. If demonstrated, such a connection would transform our understanding of glasses and jammed systems, and possibly provide a universal understanding of jamming.This research will enable the design, at the particle scale, of colloidal dispersions to meet the flow requirements of specific applications in, for example, the paints and coatings industry, thus reducing energy consumption and product waste. Contributing to the understanding of glasses and glass-forming systems, and particular their dynamic properties, would have broad impact across disciplines from fundamental physics and chemistry to biology - the motion of proteins and protein complexes in the crowded interior of a cell has strong similarities with the hindered and heterogeneous motion in colloidal glasses. Finally, the graduate student supported by this research will be well-trained in continuum and statistical mechanics, colloidal physics and computational science, and will join the scientific workforce of the nation.