Title: Designing and understanding friction in soft matter.

Abstract: Energy losses arise from viscous and solid dissipation as two contacting surfaces move against each other. As technologies such as wearable electronics, functional textiles, and soft robotics emerge, the need to engineer friction at the microscale becomes increasingly important. In this talk, I discuss ways in which our group studies soft matter mechanics using high speed confocal imaging and rheometry. The first part of the talk will focus on the role of particle roughness in the rheology and dynamics of concentrated colloidal suspensions. A series of experiments and simulations demonstrates that surface roughness is key to shear thickening, jamming, and changes in rotational dynamics within colloidal suspensions. Transient creep experiments show that rough particles exhibit stochastic strain jumps that are reminiscent of large-scale mudslides and avalanches. The second part of the talk will discuss the role of microtexturing on the lubrication properties of soft elastomeric substrates. While emerging applications such as synthetic cartilage, finger-touchscreen contacts, and soft robotics have sought design principles from biology, a general framework is lacking because these soft interfaces experience a complex multiphysics coupling between solid deformation and fluid dissipation. We investigate the elastohydrodynamic sliding of more than fifty patterned 2D sliding pairs comprising elastomers, thermoplastics, and hydrogels, and discover that texturing universally induces a jump in the lubrication film thickness that leads to a critical transition in the macroscopic friction coefficient. A simple scaling framework that combines lubrication theory and elastic deformation is able to capture this localized transition in the friction coefficient. Our predictions provide physical insights for which the critical elastohydrodynamic friction in a broad class of soft materials can be engineered using pattern geometry, material elasticity, and fluid properties.

Bio: Lilian Hsiao earned her B.S. in Chemical Engineering from the University of Wisconsin-Madison (2007) and her Ph.D. in Chemical Engineering from the University of Michigan (2014). She received the Rackham Predoctoral Fellowship for her doctoral dissertation on understanding colloidal gels and suspensions in flowing systems. She went on to Massachusetts Institute of Technology (Chemical Engineering) to pursue postdoctoral work in functional nanoemulsions and hydrogels from 2014-2016. She started as an assistant professor at NC State (Chemical and Biomolecular Engineering) in August 2016. She recently received the AAAS Marion Milligan Mason Award (2019-2020) for top early career female scientists in the chemical sciences.