(6ev) Structure, Rheology and Processing of Complex Fluids Towards Scalable Manufacturing of Functional Devices

There is growing interest in solution-processed manufacturing of functional devices through scalable (i.e. roll-to-roll) coating/printing technologies to new sectors such as energy, electronics and medicine, due to many potential benefits, including huge cost savings. Critical to the functionality of the manufactured device in many of these applications is achieving a desired pattern (1D, 2D or 3D) or a certain structure. Precise control of the microstructure of formulations and their processing behavior (in coating/printing and during processing) are critical to efficient fabrication of targeted pattern/structures. Material formulations are often complex in structure as they often contain several functional additives (i.e. particles, polymers and surfactants) and can exhibit a complex flow behavior. Current challenges in solution-processed manufacturing of many applications are the lack of clear understanding of the structure of functional formulations and the processing behavior of rheologically complex materials in various roll-to-roll coating/printing processes.

My PhD research focused on addressing fundamental problems in structure-rheology-processing relationships for roll-to-roll processing of complex fluids, with a focus on shear-thickening colloidal dispersions. The flow behavior of the shear-thickening particle dispersions in large amplitude oscillatory shear and extensional flows, and their dynamics in gravure printing and slot-die coating were studied. The studies provided new insights on the flow behavior and structure of shear-thickening dispersions, and that the fluid rheology can significantly influence the ink transfer and the coating stability in gravure printing and slot-die coating process, respectively.

My postdoctoral research at National Renewable Energy Laboratory (NREL) has been focused on investigating the structure and rheology of catalyst inks for solution-processed manufacturing of catalyst layers (CL) for proton-exchange-membrane fuel cells and electrolyzers. The catalyst ink formulations are a colloidal mixture of catalyst particles and ionomer in water-alcohol mixtures. The structure of the CL is found to strongly influence the electrochemical performance, however, how the ink microstructure and processing affects the evolution of CL structure is currently less clear. My research focused on understanding how the formulation components (ionomer-particle-solvent) interact and the impact the catalyst ink microstructure primarily using rheological tools, that has been little utilized, and USAXS in collaboration with Argonne National Laboratory (ANL). The studies revealed insights on the influence of ionomer and solvent (alcohol-ratio) on the agglomerated structure of different catalysts (based on structure and surface chemistry), and the implications on the processing and the evolution of CL microstructure.

My future research is motivated towards addressing the fundamental gaps in soft matter and non-Newtonian fluids dynamics that enable solution-processed manufacturing of functional devices. In the short-term, I hope to focus on fundamental problems that are critical for understanding the microstructure and processing behavior (in coating and drying processes) of catalyst ink formulations in the fabrication of catalyst layers for fuels cells, electrolyzers and other electrochemical technologies. The research problems span topics of the structure and rheology of colloid-polymer mixtures, and the dynamics of non-Newtonian fluids in coating and drying processes.

Teaching Interests:

Fluid Mechanics, Transport Phenomena, Rheology of Complex Fluids

Publications:

1. S Khandavalli, JP Rothstein Extensional rheology of shear-thickening fumed silica nanoparticles dispersed in an aqueous polyethylene oxide solution J. Rheol., 2014, 58 (2), 411-431
2. S Khandavalli, JA Lee, M Pasquali, JP Rothstein The effect of shear-thickening on liquid transfer from an idealized gravure cell J. Non-Newtonian Fluid Mech., 2015, 221, 55-65
3. S Khandavalli, JP Rothstein Large amplitude oscillatory shear rheology of three different shear-thickening particle dispersions Rheol. Acta, 2015, 54 (7), 601-618
4. S Khandavalli, JP Rothstein The effect of shear‐thickening on the stability of slot‐die coating AIChE J., 2016 62 (12), 4536-4547
5. S Khandavalli, J Hendricks, C Clasen, JP Rothstein A comparison of linear and branched wormlike micelles using large amplitude oscillatory shear and orthogonal superposition rheology J. Rheol., 2016, 60 (6), 1331-1346
6. S Khandavalli, JP Rothstein Ink transfer of non-Newtonian fluids from an idealized gravure cell: The effect of shear and extensional deformation J. Non-Newtonian Fluid Mech., 2017, 243, 16-26
7. S Khandavalli, P Rogers, JP Rothstein Roll-to-roll fabrication of hierarchical superhydrophobic surfaces Appl. Phys. Lett., 2018, 113 (4), 041601
8. S Khandavalli, JH Park, NN Kariuki, DJ Myers, JJ Stickel, K Hurst, KC Neyerlin, M Ulsh, SA Mauger Rheological Investigation on the Microstructure of Fuel Cell Catalyst Inks ACS Appl. Mater. Interfaces, 2018, 10 (50), 43610-43622
9. S Khandavalli, JH Park, NN Kariuki, S Zaccarine, S Pylypenko, DJ Myers, M Ulsh, SA Mauger Structure and Rheology of Catalyst Inks for Low-Temperature Proton Exchange Membrane Electrolyzers (in preparation)