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Ewoldt and team offer new insight into design of responsive soft materials


Randy Ewoldt
Randy Ewoldt

Associate Professor and Kritzer Faculty Scholar Randy Ewoldt, along with his former MechSE doctoral students Dr. Gaurav Chaudhary (now a postdoctoral researcher at Harvard) and Dr. N. Ashwin Bharadwaj (now a research scientist at Nike) and MatSE Professor Paul Braun, has published a new paradigm for the design of magnetorheological materials—that is, soft materials that stiffen in response to magnetic fields. The team constructed a new microscopic scaling theory that provides key insights into the underlying physics and provides guidance for future design of materials.

In their paper, “Exploiting Nonlinear Elasticity for Anomalous Magnetoresponsive Stiffening,” in ACS Macro Letters, they experimentally demonstrated magnetic-stiffening that exceeds the prior theoretical maximum sensitivity for this class of materials. Their novel idea included making the elastic solid not from standard linear polymers, but rather from semi-flexible strain-stiffening polymers.

Using a model system of a ‘fibrin’ network (the biopolymer that solidifies blood into a clot) embedded with micron-sized carbonyl iron particles, they demonstrated that even at a modest particle volume fraction (0.5−4%), a coupling between the magnetically interacting dipoles and a strain-stiffening polymer mesh provides previously unexplored opportunities for material design. Their experiments indicated that confined particles within the fibrin network internally stiffen the polymer mesh when an external field is applied, resulting in a field-dependent stiffening response from the polymer mesh that superposes with the magnetic interparticle interactions.

“To experimentally surpass a theoretical maximum is exciting but required great care in testing our hypothesis of how and why this could occur. The students truly experienced both engineering and science: creating a material with new mechanical properties, but also new knowledge of what is possible and how it works,” Ewoldt said.

This exciting advancement could impact responsive soft material systems in soft robotics, vibrational energy harvesting, and biomedical research.