Giovedì 19 Ottobre 2023 alle ore 10 Aula Pacinotti
Gareth H. McKinley FRS
with J. Soulages, T. K. Ober, C. J. Pipe, and Dr. Simon Haward
Hatsopoulos Microfluids Laboratory, Department of Mechanical Engineering
Massachusetts Institute of Technology (MIT), Cambridge, USA
The development and growth of microfluidics has stimulated interest in the behavior of complex liquids in microscale geometries and provided a rich platform for rheometric investigations of non-Newtonian material phenomena at small scales.
Microfluidic techniques present the rheologist with new opportunities for measurement of fluid properties, and enable the systematic investigation of strong elastic effects at very high deformation rates without the complications of fluid inertia. In this presentation we provide an overview of the use of microfluidic devices to measure bulk rheology and onset of viscoelastic flow instabilities in both shear and extensional flows, using a combination of local velocimetric imaging, mechanical measurements of pressure drop and full-field optical probes of flow-induced birefringence.
Steady and time-dependent flows of a range of dilute polymer solutions and wormlike micellar fluids are considered. The ability to rapidly and precisely fabricate complex flow geometries also enables us to exploit the predictions of computational optimization and design, from first principles, an optimized shape cross-slot extensional rheometer (or OSCER) that achieves homogeneous planar extensional kinematics and large fluid strains. Local birefringence measurements along the stagnation streamlines, combined with bulk measurements of the excess pressure drop across the device, provide self-consistent estimates of the extensional viscosity over a wide range of deformation rates up to 2000 s -1 . The results are also in close agreement with numerical simulations based on a finitely extensible non-linear elastic dumbbell model.
As the imposed extension rate in the microfluidic rheometer is increased the homogeneous planar elongational flow ultimately becomes unstable. High-frame rate video-imaging of the birefringence field is used to construct space-time diagrams of the evolution in the flow for a range of different polymer solutions and to construct the first stability diagram for planar extensional flows in cross-slot devices.
The mode of instability is found to depend on the elasticity number (El = Wi/Re) of the fluid, with a steady symmetry-breaking purely-elastic bifurcation observed at high El >> 1, and time-dependent three- dimensional inertio-elastic instabilities dominant for El < 1.
BIO
Gareth Huw McKinley FRS is Professor of Teaching Innovation in the Department of Mechanical Engineering at Massachusetts Institute of Technology (MIT).
Prof. McKinley's work focuses on understanding the rheology of complex fluids such as surfactants, gels and polymers, which are ubiquitous in foods and consumer products. His research interests include non-Newtonian fluid dynamics, microfluidics, extensional rheology, field-responsive materials, super- hydrophobicity and the wetting of nanostructured surfaces.
Prof. McKinley served as director of MIT's program in polymer science & Technology (PPST) from 2004-2009. McKinley is also co-founder of Cambridge Polymer Group, a Boston-based company employing 20 people and specializing in bespoke instrumentation, materials consulting and orthopedic polymeric materials.
Prof. McKinley was awarded the 2013 Bingham Medal from the Society of Rheology and the 2014 Gold Medal of the British Society of Rheology. He served as editor of the Journal of Non- Newtonian Fluid Mechanics (JNNFM) from 1999 to 2009. He was elected a member of the National Academy of Engineering of the United States in 2019.
