Instabilities in Complex Fluid Flows: understanding, exploitation and cure {ICOMPFF}

Lead Research Organisation: University of Liverpool
Department Name: Mech, Materials & Aerospace Engineering


Complex fluid flows are ubiquitous in both the natural and man-made worlds. From the pulsatile flow of blood through our bodies, to the pumping of personal products such as shampoos or conditioners through complex piping networks as they are processed. For such complex fluids the underlying microstructure can give rise to flow instabilities which are often totally absent in "simple" Newtonian fluids such as water or air. For example, many wormlike micellar surfactant ("soap/detergent") systems are known to exhibit shear-banding where the homogenous solution splits into two (or more) bands of fluid: such flows are often unstable to even infinitesimally small perturbations. At higher pump speeds the flows can develop chaotic motion caused by the elastic normal-stresses developed in flow. Such "elastic turbulence" can also develop for other flowing complex fluids, such as polymer solutions and melts, and give rise to new phenomena. Often such instabilities are unwelcome, for example in rheometric devices when the aim is to measure material properties or in simple pumping operations when they can give rise to unacceptably large pressure drops and prevent pumping. In other cases they can give rise to enhanced mixing of heat and mass which would otherwise be difficult to achieve (e.g. microfluidics applications).

Planned Impact

The Impact Plan for the proposed project can be divided into four broad categories: (i) Impact by dissemination (ii) Impact through collaboration, (iii) Impact through capability development and knowledge exchange, and (iv) Impact by exploitation and application of project outcomes

Impact by dissemination- The project outcomes will be disseminated through participation in international academic conferences and publication in scientific journals. A proposed Technical and Managerial Steering committee, meeting yearly, will also provide a route to early dissemination of the latest results and, given the strong industrial involvement in this committee, provide an excellent opportunity to disseminate the results to key industrial stakeholders in a timely fashion. The Fellow would look to host one BSR midwinter meeting - towards the end of Year 4 - around the theme of strand #1 purely-elastic instabilities. At the end of the Fellowship an additional workshop would be proposed on "Instabilities in Flowing Wormlike Surfactants".

Impact through collaboration- The Fellowship represents an excellent opportunity to support existing, and build entirely new, national and primarily international collaborations (as highlighted in Academic Beneficiaries section). The direct involvement of P&G in providing materials and guidance on micellar fluid systems of industrial interest will also provide enhanced impact and make the results of wide interest. The data-set produced during the Fellowship, using rigs of industrial scale, will be of direct relevance to P&G. The direct involvement of the Fellow with Unilever through the offer of visitor status, including work space, at their Port Sunlight site as stated in their supporting statement will provide excellent real-time engagement and feedback from industry. Not only will this embed knowledge of the Fellowship results directly within Unilever but it will also expose the Fellow to challenges faced by end users and provide excellent opportunities to maximise impact and build an impact case study.

Impact through capability development and knowledge exchange- In the course of the project, the PDRAs will receive extensive training and, given the collaborative nature of the project, they will also be exposed to international collaboration with world-leading groups. Knowledge exchange will be delivered directly to Industry. In addition to the extended visits to P&G sites as highlighted in the attached SoS and the visitor status offered by Unilever, the PI would apply to the Royal Academy of Engineering's Industrial Secondment at the end of the Fellowship to better enable direct knowledge exchange with an additional new Industrial Sector and enhance impact. These interactions will also enable the Fellow to "view the landscape" and access opportunities for future research challenges such as post-Fellow joint TSB/EU/EPSRC proposals or new research questions and or opportunities for consulting. To disseminate the results to a wider audience and enhance knowledge exchange to areas outside of the scientific community the Fellow will develop interactive movies which will highlight the flow instabilities. The Fellow will undertake a series of school visits, supported by the University's Widening Participation Team, to highlight the outcomes of the research and these videos will be placed on the University website and on file-sharing sites.

Impact by exploitation and application of project outcomes- It is anticipated that the fundamental understanding gained from this project will lead to potentially exploitable devices or processes. For example, the work packages in strand #1 will lead to ways in which elastic-instabilities can be delayed or removed completely and thus should open the way to better designs for microfluidic rheometers. An additional benefit of the increased understanding that this proposal will produce is that it may offer different routes for the pumping of these fluids


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Description If a fluid has any internal "structure" - such as a polymer in solution or the suspension of various cells which make up our blood - they can exhibit counter-intuitive "non-Newtonian" properties. Measurement of these properties for such "complex" fluids would ideally be done with a very small amount of fluid in what scientists call a "microfluidic" cell where the characteristic length scale is on the order of a few hundred microns. One may also wish to create miniature "labs" in such tiny devices to mix fluids or undertake many other chemical processes. Unfortunately for such complex fluids these small length scales can give rise to unstable flows due to so-called "purely-elastic" instabilities. We have found ways to control these instabilities such that we can delay them to enable property measurements to take place and also to encourage them to make the devices more efficient from a mixing perspective. In particular for one such microfluidic set-up, two intersecting channels called a "cross slot", we have been able to answer a long-standing question, via a simple geometric modification (the addition of a cylinder at the centre of the domain) about the underlying mechanism for this instability. In so doing we are now using this understanding to develop ways of controlling the instability. We have demonstrated this using both numerical modelling - by using well-studied existing models but also testing more novel ones - and then also via physical experiment.

An additional problem for those who try to model such polymer solutions at these small scales is that the generally-used models do not accurately predict the pressure drop required to drive the flow if the geometry in the device is in any "complex" (i.e. not just a straight channel). We implemented and tested a new model which was proposed to overcome this issue - the so-called Adaptive Length Scale Model - in a sophisticated computational model (Computational Fluid Dynamics) we have developed in our group. In so doing we found that this model is also not capable of quantitative predictions even for the simplest "dilute" polymer solutions.
Exploitation Route The control strategies for the cross-slot device may be used by those wishing to create devices to study complex fluids.
Sectors Agriculture, Food and Drink,Chemicals,Healthcare,Manufacturing, including Industrial Biotechology,Pharmaceuticals and Medical Biotechnology