Angela Pitenis

Angela Pitenis is an Associate Professor in the Materials Department at the University of California, Santa Barbara. She leads the Interfacial Engineering Laboratory, an interdisciplinary team of materials scientists, mechanicians, chemical engineers, and bioengineers, and her group researches friction, deformation, adhesion, and wear across soft, biological, and bio-inspired material interfaces.

Title: Slippery Physics at the Critical Edge: Neutron Reflectometry and Operando Tribology

Abstract: Quality of life and mobility critically hinge upon the ability of articular cartilage to sustain extremely low friction over a lifetime; however, the fundamental mechanisms underpinning its lubricity remain elusive, in part due to its compositionally-graded structure. Owing to their similar water content and network structure, hydrogels with depth-wise polymer density gradients (surface gel layers) serve as tractable analogues of cartilage-like stress modulation; yet even in these simplified, water-rich networks, tribological behavior is highly sensitive to subtle buried interfacial structure that can reorganize under load and sliding. In an effort to link nanoscale interfacial reorganization to macroscale tribological responses of hydrogels, we conducted operando neutron reflectometry with a custom-built linear reciprocating tribometer at the Spallation Neutron Source at Oak Ridge National Laboratory. Neutron reflectometry across hydrogel-silicon interfaces submerged in D2O revealed load-driven near-surface network densification, consistent with mesoscale simulations. Our results point to polymer density gradients and network mobility as key design parameters for engineering tissue-like, stress-modulating hydrogels and tuning friction across soft interfaces.