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Purely-elastic flow instabilities and transition to elastic turbulence in microscale flows of complex fluids

Final Report Summary - ELASTIC-TURBULENCE (Purely-elastic flow instabilities and transition to elastic turbulence in microscale flows of complex fluids)

Most synthetic fluids used in our daily life and many biological fluids (synovial fluid, DNA solutions, blood, etc.) exhibit non-Newtonian rheological behaviour. Due to their nonlinearity, flows of viscoelastic liquids often exhibit complex and counterintuitive behaviour and, above critical conditions, generate instabilities even under low Reynolds number flow conditions which are not observed in the equivalent Newtonian fluid flows.

The remarkable properties of complex fluids arise from the interaction between their molecular structure and the flow field. The local flow conditions induce a molecular rearrangement, with the polymer molecules being stretched and oriented with the flow. This non-equilibrium configuration of the molecules generates large and anisotropic normal stresses, which influence the flow field leading to the occurrence of complex phenomena which were investigated in this project.

The main goal of the project was to expand the frontiers of current knowledge regarding the mechanisms that lead to the development of purely-elastic flow instabilities, and to investigate the transition to so-called “elastic turbulence”, a turbulent-like phenomenon which can arise for complex fluids even under inertialess flow conditions.

The following main outcomes were achieved in the project:

- Development of stable high-order parallel viscoelastic flow solvers for GPU or CPU based computations. Pressure-driven flows and electro-osmotic flows can be simulated with the numerical codes developed. The OpenFOAM opensource software has also been used to simulate numerically single and multi-phase flows of viscoelastic fluids using the developed opensource rheoTool library;

- Use of serpentine microchannels as micro-rheometers to measure relaxation times of dilute viscoelastic fluids and to investigate purely-elastic flow instabilities. Numerical simulations and experiments showed a good agreement and scaling laws for the onset of elastic instabilities in serpentine channels were determined;

- Development of a device to measure the relaxation time of ultra-dilute polymer solutions in extensional flow;

- Design of optimised microfluidic devices for generation of strong extensional flows with controlled flow kinematics, with relevant applications in extensional rheometry and controlled deformation of macromolecules and soft materials under homogeneous extensional flow;

- Experimental investigation of complex spatiotemporal flow instabilities of wormlike micellar solutions in long rectangular straight microchannels;

- Experimental and numerical analysis of the onset of purely-elastic flow instabilities and transition to elastic turbulence in microfluidic channels, such as contraction/expansion geometries, serpentine microchannels, cross slots and flow focusing microfluidic devices;

- Numerical simulation of enhanced mixing and chaotic flow in the elastic turbulence regime;

- Experimental and numerical investigation of purely-elastic instabilities and chaotic flow of complex fluids in electro-osmotic flows.

The ELASTIC-TURBULENCE project allowed the creation of a dedicated research team focusing on the investigation of flow instabilities and elastic turbulence in complex fluid flows at the microscale.