Soft Matter and Biophysics Seminar: Friction, Relaxation and Recovery in Hydrogel Interfaces

Join Justin Burton, associate professor of physics at Emory University, for this informative seminar.

Abstract: Hydrogels are cross-linked polymer networks that can absorb and retain a large fraction of water, often up to 90 percent by weight. They are widely used in many engineering applications as well as agriculture industries due to their ultralow friction, biocompatibility, and chemical transport capabilities. Previous research in our lab has identified and characterized three distinct regimes of friction in polyacrylamide (PAAm), polyacrylic acid (PAA, and agarose hydrogels on smooth surfaces. Most striking, near a critical velocity, the friction coefficient decreases by an order of magnitude and displays relaxation over multiple timescales. Here we examine this regime in closer detail for PAA hydrogels. We find that the frictional relaxation time decreases exponentially with sliding velocity. Additionally, any pre-shearing of the hydrogel will induce changes in the relaxation time prior to experiments, and if left to rest, the hydrogel friction will return to its original state. This behavior is qualitatively consistent with a model of two superimposed relaxation processes: a velocity-dependent shearing and extension of entangled polymer chains, and a constant relaxation timescale from thermal fluctuations. Finally, we show that the sliding interface can retain a “memory” of its initial sliding direction, a memory that persists over 24 hours of continuous experiments.

Justin Burton is an associate professor of physics at Emory University. He has conducted several influential studies surrounding fluid singularities, liquid droplet coalescence and jammed matter and its relation to the mechanics of glacier-calving events. He received his Ph.D. in physics from UC Irvine in 2006 and went on to become an ICAM Postdoctoral Fellow at the University of Chicago.

Professor Burton took part in the 2010 Arthur H. Compton Lecture Series, and worked with the Emory-Tibet Science Initiative to bring physics education to three major monastic universities in India. He is the recipient of an NSF CAREER Award, and a Sustainability Innovator Award at Emory.

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Image courtesy of Emory University.