Suspension Rheology at Constant Pressure

恒压悬浮液流变学

• 批准号: 1337097
• 负责人: John Brady
• 金额: $ 30万
• 依托单位: California Institute of Technology
• 依托单位国家: 美国
• 项目类别: Standard Grant
• 财政年份: 2013
• 资助国家: 美国
• 起止时间: 2013-09-01 至 2016-08-31
• 项目状态: 已结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=1337097&HistoricalAwards=false
• 关键词: Suspension; Rheology; Constant; Pressure

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.

1337097PI：Brady胶体悬浮液广泛应用于工业、医学和自然环境中，涵盖牙膏、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于其加工、分配、耐久性和性能至关重要。大多数悬浮液流变学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来说可能是足够的，但悬浮液流动通常不是固定体积而是固定应力（或固定压力或压降）。固定体积和固定压力下的流动行为是否相同？如果悬浮颗粒的体积分数足够低，则应该可以将一种测量值转换为另一种测量值。但随着接近最大流动分数，这两种条件是否会导致相同的流动行为不再明确。提出了在固定压力下胶体悬浮液的模拟研究，允许系统膨胀或收缩，并且体积分数根据需要波动。加速斯托克斯动力学模拟方法将进行调整，以允许模拟体积发生变化，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，剪切力的强度在很大范围内变化。模拟可以提供完整的微观细节，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。将特别关注接近最大流动分数时的流动行为以及接近该点的流动特性的缩放。了解悬浮液流变学本身就是一个重要的主题，但检查最大流动分数时的流动行为接近可能对玻璃和堵塞系统产生重要影响。已知静态胶体分散体在体积分数接近 0.58 时形成玻璃，远低于无规密堆积（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进行的实验显示出非常相似的行为：剪切与法向应力之比（摩擦系数）在两个系统中是相同的，最大流动体积也是如此尽管微观物理非常不同——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，以满足特定应用的流动要求，例如，油漆和涂料行业，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细胞内部的运动有很强的相似性。胶体玻璃中的受阻和异质运动。最后，这项研究支持的研究生将在连续介质和统计力学、胶体物理学和计算科学方面接受良好的培训，并将加入国家的科学队伍。