Virtual seminar"Stretched Polymer Physics, Extensional Rheology and Free Surface Flows"Vivek Sharma


Monday, July 19, 2021 - 09:00




Micro/Bio/Nanofluidics (Shen) Unit would like to invite you to the virtual seminar by Prof. Vivek Sharma on July 19 (Monday).
Date: Monday, July 19 2021
Time: 09:00-10:00
**Zoom session

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Meeting ID: 854 9859 5789
Passcode: 702994


Prof. Vivek Sharma
Department of Chemical Engineering
University of Illinois, USA


Stretched Polymer Physics, Extensional Rheology and Free Surface Flows


Liquid transfer and drop formation/deposition processes associated with printing, spraying,
atomization and coating flows involve complex free-surface flows including the formation of
columnar necks that undergo spontaneous capillary-driven instability, thinning and pinch-off.
For simple (Newtonian and inelastic) fluids, a complex interplay of capillary, inertial and viscous
stresses determines the neck thinning dynamics. In rheologically complex fluids, extra elastic
stresses as well as non-Newtonian shear and extensional viscosities dramatically alter the pinchoff
dynamics. Stream-wise velocity gradients that arise within the thinning columnar neck create
an extensional flow field, and many complex fluids exhibit a much larger resistance to
elongational flows than Newtonian fluids with similar shear viscosity. Characterization of
extensional viscosity, extensional relaxation time and finite extensibility effects, as well as
macromolecular properties that determine pinch-off dynamics are beyond capabilities of
conventional shear and extensional rheology techniques in which free surface flows are absent.
Here we show that dripping-onto-substrate (DoS) rheometry protocols we developed recently
can be used for measuring extensional viscosity and extensional relaxation time of polymeric
complex fluids, including low viscosity printing inks and polymer solutions that are beyond the
measurable range of commercially-available capillary break-up extensional rheometer (CaBER).
Using DoS rheometry protocols that involve visualization and analysis of capillary-driven
thinning and pinch-off dynamics of a columnar neck formed between a nozzle and a sessile drop,
we elucidate the stretched polymer hydrodynamics underlying observed rheological response and
processing behavior of polymeric complex fluids. We discover that our pursuit involves myriad,
intertwined quests and insights into conformation-dependent hydrodynamic and excluded
volume interactions, Pincus’ tension blobs, finite extensibility effects, as well as coil-stretch
transition and hysteresis. We find that the extensional relaxation times exhibit concentration
dependence distinct from shear rheology response or anticipated by blob models developed for
relaxation of weakly perturbed chains. We show that the influence of molecular weight and
chemistry can be evaluated a priori, using three macromolecular parameters: flexibility,
extensibility and segmental dissymmetry for neutral polymers. Finally, we characterize and
analyze the pinch-off dynamics of charged polymers and ionic surfactant–neutral polymer
complexes in the presence of varying salt concentration, to elucidate the influence of complex
interplay of electrostatic and hydrodynamic stretching of macromolecules on processability.


Prof. Amy Shen

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