Seminar - Lubrication by Polyelectrolytes – Ionic Strength, Valency and Geometry
Date: March 8, 2013
Time: 4:00 PM
Location: Papadakis Integrated Sciences Building, Room: 120
Lin Han, PhD
School of Biomedical Engineering, Science and Health Systems
Polyelectrolytes effectively reduce surface friction by decreasing surface contact adhesion, and creating a fluid-like hydration sheath surrounding charged groups. Utilizing lateral force microcopy, we investigated the mechanisms and environmental factors that can quantitatively control the magnitude of polyelectrolyte surface friction coefficient, μ, at nano- to micrometer deformation length scales. In the first model system, with a microspherical tip, lateral force microscopy was utilized to measure the friction of an end-attached monolayer of the comb-like, negatively charged, biomacromolecules extracted from articular cartilage, aggrecan. Values of μ were characterized in aqueous solutions at different ionic strengths and divalent [Ca2+] to demonstrate the importance of electrostatic repulsion. Increasing ionic strength and divalent [Ca2+] both decrease the degree of electrostatic repulsion, and thus, significantly increase the values of μ. In a second model system, we studied the layer-by-layer assembled poly(allylamine hydrochloride)/poly(acrylic acid) (PAH/PAA) in various microscale geometries (planar film versus tube forest). The increase in μ upon pH-induced neutralization of the PEM further elucidates the importance of electrostatic repulsion in surface lubrication. In addition, we found that the tube forest geometry introduces additional assymetric bending/buckling and substrate constraints mechanisms that contribute to the net friction. Understanding these environmental factors provides a platform to design dynamic substrates with quantitatively controllable μ at nm- to μm-scales. This work thus holds great potential for fabricating dynamic substrates with tunable surface friction and energy dissipation capabilities for use in applications sensitive to surface properties, such as biosensors, cell adhesion and tissue engineering.
Lin Han obtained his B.E. degree from Tsinghua University in Beijing, P.R. of China, and his PhD degree from the Massachusetts Institute of Technology in the area of Bio- and Polymeric Materials. After graduation, he worked as a quantitative analyst in Aristeia Capital, LLC from 2007 to 2009, where he was responsible for developing and testing statistical models. He also worked as a post-doctoral associate in the Department of Materials Science and Engineering and the Center for Biomedical Engineering at MIT. Currently, he is an Assistant Professor in the School of Biomedical Engineering, Science & Health Systems at Drexel University. His research interests focus on exploring the nanoscale structure-property relationships of biomaterials, which aim to provide important insights into the application of disease diagnostics, tissue regeneration and bio-inspired material design.
The Papadakis Integrated Sciences Building is located on the northeast corner of 33rd and Chestnut Streets.