S.E.C.R.E.T. : Shear Extension Combined Rheology Experimental Techniques

你熟悉用管道在蛋糕上撒糖霜吗？当你放下珠子时，我们对许多流动过程的理解实际上并没有被完全理解，你会不会感到惊讶？无论流体在哪里被“拉紧”--滑动、挤压或改变形状，它都会用力或“压力”来响应。研究流体对应变的响应被称为“流变学”。这些力影响了附近其他流体的运动方式，使其成为计算模拟流体流动问题的重要信息：模型告诉我们将熔融的塑料加工成日常物品，或者我们对蜘蛛如何旋转它的丝的理解。流变学通俗地描述流体的“稠度”，但流体的行为可以有很大的不同，这一切都取决于流体中发生的微观相互作用。通过管道喷嘴的流动和应变是非常复杂的。在喷嘴壁附近，结冰主要经历一种“剪切”流动，即流体层之间相互滑动--这种流动类型在实验室中是很容易理解和测量的。在中心附近，流体正在经历“伸展”，流体包在流动方向上被拉伸，在其他方向上被挤压。喷嘴逐渐变细导致了这一点。这种延伸性流动较少被理解或测量，但在过去的50年里，我们的理解有所提高，这主要是因为塑料行业。在壁面位置和流动中心之间，同时存在剪切和伸展--我们称之为“运动混合”流动。不是搅拌的，而是混合在一起的，就像在不止一种类型的过滤中一样。到目前为止，我们在该地区验证模型的唯一方法是测量流体速度(例如)，并查看我们的数学模型预测是否符合-基于纯剪切或拉伸流动数据的模型。到目前为止，还没有一种方法可以明确地隔离和测量这种流动中间的单独应力，这取决于流体中的微观相互作用，同时取决于剪切和拉伸。让情况变得更加复杂的是，结冰是“悬浮”的一个例子，这是一种表现出所谓“屈服”应力的流体--只有当施加的应力超过某个阈值时，它才会流动。这使得结冰可以在管袋被挤压时流动，但这意味着它在重力下沉积在蛋糕上后可以抵抗流动。目前人们对悬浮液在拉伸下的行为知之甚少，与我们所知的塑料相比，更不用说它们在运动混合流动中的行为了。受影响的不仅仅是冰激凌蛋糕。3D打印水泥建造新房在概念上是相同的过程，扩大了规模，必须在不流动的情况下应对更大的压力。在电子制造中沉积焊膏有相似之处，将石墨烯纤维加工成下一代高性能材料也是如此。塑料加工是一种混合流动，人们对此并不完全了解，甚至发动机轴承中的润滑油流动也是混合的。事实上，很少有纯粹的剪切或伸展流动，缺乏一种方法来直接看到流体应力在这些混合流动下的响应，这不利于能够准确地模拟和预测它们。这将影响我们围绕它设计工业流程的能力，也许在未来，使用它来设计具有特定应用的精确流动响应的新材料。该奖学金将开发一种新的实验技术，使我们能够通过使用磁共振成像-与医院使用的相同技术-来测量在运动混合流动中发生的剪应力，关键是，使流体是透明还是不透明变得不重要。随着模型界成员对该项目感兴趣，并计划召开“圆桌会议”，将进行基准实验，为新流体模型的开发提供信息，从而促进各种下一代材料和制造工艺的发展。

Are you familiar with piping icing onto cakes? Would you be surprised to know that our understanding of many of the flow processes taking place whilst you lay down beads are not actually fully understood?Wherever a fluid is "strained" - slid, squashed, or changed in shape, it responds with a force, or "stress". Studying fluid response to straining is known as "rheology". These forces influence how the rest of the fluid nearby moves, making it vital information to computationally model fluid flow problems: models that inform processing molten plastic into everyday objects, or our understanding of how a spider spins it's silk. Colloquially, rheology describes how "thick" a fluid is, but fluids can have hugely varying behaviours, all dependent on microscopic interactions occurring in the fluid. The flow and straining occurring through a piping nozzle is quite complicated. Near the nozzle walls, icing is mainly undergoing a "shearing" flow, where fluid layers slide over one another - this flow type is well understood and measurable in a lab. Near the centre, the fluid is experiencing "extension", where fluid packets are stretched in the flow direction and squashed in other directions. The nozzle tapering causes this. This extensional flow is less well understood or measureable, but in the last 50 years our understanding has improved, mainly because of the plastics industry. Between the location of the wall and the centre of the flow, simultaneous shear and extension exists - we call this a "kinematically mixed" flow. Not stirred, but mixed as in more than one type of straining present. To date, our only approach to validate models in this region has been to measure fluid velocity (for example) and see if our mathematical model predictions agree - models based on data from pure shear or extensional flows. Until now there hasn't been a way to unambiguously isolate and measure separate stresses within the middle of such flows, something that depends, via microscopic interactions in the fluid, on both shear and extension together. Making the situation even more complex, icing is an example of a "suspension", a class of fluids that display what is called a "yield" stress - it only flows when an applied stress exceeds some threshold. This allows icing to flow when the piping bag is squeezed, but means it resists flow under gravity after being deposited on a cake.The behaviour of suspensions under extension is particularly poorly understood at this time, versus what we know for plastics, let alone their behaviour under kinematically mixed flows. Not just icing cakes is affected. 3D printing cement to build novel houses is conceptually the same process, scaled up, and must handle much more stress without flowing. Depositing solder paste in electronics manufacture has similarities, as does processing graphene fibres into next-gen high performance materials. Plastics processing, a mixed flow, is not perfectly understood, and even lubricant flow in engine bearings is mixed. In fact, few flows are purely shear or extensional, and lacking a method to directly see how fluid stresses are responding under these mixed flows is detrimental to being able to accurately model and predict them. This impacts our ability to design industrial processes around it, and perhaps in the future, to use it to engineer new materials with exacting flow responses for specific applications.This fellowship will develop a new experimental technique that allows us to measure what shearing stress is occurring throughout a kinematically mixed flow by using magnetic resonance imaging - the same technology used in hospitals - and critically, makes whether a fluid is clear or opaque unimportant. With members of the modelling community interested in the project and a "round table" planned, benchmark experiments will be conducted to inform new fluid model development, and thereby facilitate a wide range of next generation materials and manufacturing processes.