Suspension Rheology at Constant Pressure

1337097PI：刹车胶体悬浮液广泛应用于工业、医药和自然环境中，涵盖了牙膏、油漆、电池内部和可喷涂太阳能电池板等各种系统。了解悬浮液的流变性对其加工、分配、耐久性和性能至关重要。悬浮液流变学的研究大多是在固定体积(或固定体积分数)下进行的。虽然这对许多应用来说可能是足够的，但通常悬浮流量不是固定体积，而是固定应力(或固定压力或压降)。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，就应该可以将一种测量方法转换为另一种测量方法。但随着最大流动分数的接近，不再清楚这两种条件是否会导致相同的流动行为。提出了一种固定压力下胶体悬浮液的模拟研究，允许系统膨胀或收缩，体积分数根据需要波动。采用加速的Stokesian动力学模拟方法，允许模拟体积的变化，并用于研究当剪切力相对于热布朗力的强度在很大范围内变化时，布朗硬球悬浮液的流动行为。从模拟中可以获得完整的微观尺度细节，包括粒子分布函数、序参数、短期和长期粒子位移等，并将观察到的宏观行为与潜在的粒子动力学联系起来。当接近最大流动分数时，将特别关注流动行为以及这一点附近流动特性的标度。理解悬浮流变学本身是一门重要的学科，但考察接近最大流动分数时的流动行为可能对玻璃体和堵塞系统具有重要意义。已知静止的胶体分散体形成体积分数接近0.58的玻璃，远低于随机紧密堆积(单分散球体为0.64)。对快速颗粒流和粘性非布朗悬浮液在恒压和剪切应力下的实验都显示出非常相似的行为：尽管微观物理-惯性动力学与粘性力非常不同，但两个系统的剪切与法向应力之比-摩擦系数-相同，最大流动体积分数也是相同的。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。如果证明，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。这项研究将使胶体分散体的设计能够在颗粒尺度上满足涂料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，特别是它们的动力学性质，将产生从基础物理和化学到生物学的广泛影响--蛋白质和蛋白质复合体在细胞拥挤的内部的运动与胶体玻璃中的受阻和不均匀运动有很大的相似之处。最后，这项研究支持的研究生将在连续介质和统计力学、胶体物理和计算科学方面接受良好的培训，并将加入国家的科学大军。

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.