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.
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