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ICS-3 Group Seminar:

Nanoscopic Thickness Transitions and Variations in Stratifying, Micellar Foam Films

Vivek Sharma
Chemical Engineering, University of Illinois at Chicago, IL.

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01 Jul 2019 14:00
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01 Jul 2019 15:00
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Seminarraum A1-3, Building 04.06, Entrance E1

Abstract:

We pursue an understanding of molecular principles underlying that are critical for understanding and controlling the three desirable attributes – stability, lifetime and rheology— of foams (and emulsions). Foam films typically consist of fluid sandwiched between two surfactant-laden surfaces that are ~ 5 nm - 10 microns apart, and the drainage in films occurs under the influence of viscous, interfacial and intermolecular forces, including disjoining pressure. Drainage in foam film formed by surfactant concentrations above the critical micelle concentration proceeds in a non-monotonic, step-wise fashion called stratification in contrast to the monotonic thinning exhibited by films containing no micelles. In reflected light microscopy, stratifying films display regions with distinct shades of grey implying that domains and nanostructures with varied thickness coexist in the thinning film. Understanding and analyzing such nanoscopic thickness transitions and variations have been long-standing experimental challenge due to the lack of technique with the requisite spatio-temporal resolution, and theoretical challenge due to the absence of models for describing hydrodynamics in stratified thin films. Using interferometry, digital imaging and optical microscopy (IDIOM) protocols we developed recently, we show that the nanoscopic thickness variations in stratifying films can be visualized and analyzed with an unprecedented spatial (thickness ~ 1 nm, lateral ~500 nm) and temporal resolution (< 1 ms). Stratification proceeds by formation of thinner domains that grow at the expense of surrounding films. Using the exquisite thickness maps created using IDIOM protocols, we provide the first visualization of nanoridges as well as mesas that form at the moving front around expanding domains, and report the discovery of spinodal stratification. We measure the non-DLVO supramolecular oscillatory surface force contributionto disjoining pressure as a function of thickness. Most significantly, we develop a self-consistent theoretical framework, a nonlinear thin film equation model that explicitly accounts for the influence of supramolecular oscillatory surface forces (used expressions developed as a part of this study), as well as the physicochemical properties of surfactants including critical micellar concentration, micellar size and interactions, Debye length, and surface tension. We utilize combination of thin film studies and small angle x-ray scattering studies to characterize change in micelle size/shape and intermicellar interactions. We show the complex spatio-temporal evolution of domains, nanoridges, nanoridge-to-mesa instability and mesas, both before and after 2D coalescence, in stratifying foam films can be modeled quantitatively, and we elucidate how surfactant-dependent hydrodynamic and thermodynamic parameters can be manipulated and controlled for the molecular engineering of foams.

Short-Bio:

Dr. Vivek Sharma is an Assistant Professor of Chemical Engineering at the University of Illinois Chicago. Before joining UIC in November 2012, he worked as a post-doctoral research associate in Mechanical Engineering at Massachusetts Institute of Technology. He received his Ph. D. (Polymers/MSE, 2008) and M. S. (Chemical Engineering, 2006) from Georgia Tech., an M. S. (Polymer Science, 2003) from the University of Akron, and a bachelor's degree from IIT Delhi. Dr. Sharma's research interests broadly lie in optics, dynamics, elasticity, and self-assembly (ODES) of complex fluids and soft materials. At UIC, Dr. Sharma's Soft Matter ODES-lab combines experiments and theory to pursue the understanding of, and control over interfacial and nonlinear flows, focused on the interplay of (a) viscoelasticity and capillarity for printing applications and extensional rheometry, and (b) interfacial thermodynamics and hydrodynamics in fizzics (the science of bubbles, drops, thin films, jets, fibers, emulsions and foams). Dr. Sharma was selected as the Distinguished Young Rheologist by TA Instruments in 2015, won the 2017 College of Engineering Teaching Award at UIC, and was awarded 3M Non-Tenured Faculty Award, 2019.

Contact

Prof. Jan K.G. Dhont
Phone: +49 2461 61-2160
Fax: +49 2461 61-2765
email: j.k.g.dhont@fz-juelich.de

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