Complex fluid instabilities and flow patterns in shear thickening fluids

剪切增稠液的粘度随着外加应力的增加而增加。在极端情况下，悬浮液的粘度会增加几个数量级，材料会从类液体变为类固体(不连续剪切增厚，DST)。剪切增稠，特别是DST，几十年来一直是复杂流体和流变学领域的难题。然而，最近的突破引起了许多关于DST起源的兴奋和科学辩论，现在解释为响应增加的应力从润滑颗粒接触到摩擦颗粒接触的转变。本项目将研究涉及不连续剪切稠化流体的复杂流型和流体不稳定性，并将观察到的动力学与新的理论模型联系起来。拟议的PHD项目的灵感来自于对玉米淀粉悬浮液的剪切增稠行为引起的新型流动不稳定性的初步观察。具体地说，该项目的目标是研究1)剪切稠化流体沿倾斜表面下的薄膜流动过程中滚波的形成，2)DST材料液体射流中的自由落体平台-瑞利不稳定性，以及3)沿流动方向传播的密度波的潜在形成(类似于交通堵塞冲击波)。主要实验变量包括悬浮密度(目标1、2、3)、重力角(目标1、3)、膜厚(目标1、3)、液流孔径(目标2)和外加压力(目标2)。可观测的包括波长、幅度、时空图、波速、液流“进动”和液滴形成动力学。在实验议程的同时，我们将与西班牙巴斯克应用数学中心的埃莱罗教授密切合作，开发基于平滑粒子流体动力学(SPH)方法的新模拟代码。除了标准的平行板流变学测量外，模拟代码还将以上述丰富的实验结果为基准。该项目的首要目标是揭示颗粒间相互作用如何引起DST行为，而DST行为又引起复杂的流动动力学。该项目的成果将有助于更深入地了解剪切增稠流体流动中如何出现复杂的流动模式，这将使食品和制药部门的加工得到优化。

Shear thickening fluids display an increase in viscosity with applied stress. In the extreme, the suspension viscosity increases by orders of magnitude, and the material goes from liquid-like to solid-like (Discontinuous Shear Thickening, DST). Shear thickening, and DST in particular, has for decades been an intractable problem in the field of complex fluids and rheology. However, recent breakthroughs have generated much excitement and scientific debate as to the origins of DST, now explained as a transition from lubricated to frictional particle contacts in response to increased stress. This project will study complex flow patterning and fluid instabilities involving discontinuous shear thickening fluids and relate the observable dynamics to the new theoretical models. The proposed PhD project is inspired by preliminary observations of novel flow instabilities caused by the shear thickening behaviour of corn starch suspensions. Specifically, the project objectives are to study 1) The formation of roll waves during film flow of shear thickening fluids down inclined surfaces, 2) The free-fall Plateau-Rayleigh instability in liquid jets of DST materials, and 3) The potential formation of density waves travelling against the flow direction (analogous to traffic jam shock waves). The primary experimental variables include suspension density (Objectives 1,2,3), angle of gravity (Objectives 1,3), film thickness (Objectives 1,3), aperture diameter of liquid stream (Objective 2) and imposed pressure (Objective 2). Observables include wavelength, amplitude, space-time diagrams, wave velocities, liquid stream "precession" and droplet formation dynamics. In parallel with the experimental agenda, we will develop a new simulation code based on the Smoothed Particle Hydrodynamics (SPH) method in close collaboration with Prof. Ellero at the Basque Center for Applied Mathematics in Spain. The simulation code will be benchmarked against the rich experimental results described above, in addition to standard parallel plate rheological measurements.The overarching aim of the project is to reveal how the interparticle interactions give rise to DST behaviour which in turn give rise to complex flow dynamics. The project results will contribute to a deeper understanding of how complex flow patterns emerge in shear thickening fluid flows, which will allow optimisation of processing in the food and pharmaceutical sectors.