Stimuli-driven jamming in dense suspensions: training and memory

Shear-induced jamming and its impact-induced solidification of dense suspensions is an important behavior that can allow access to stress mitigating functional materials. We have designed responsive particles whose crosslink density can be reversibly tuned in the presence of external stimuli. Upon exposure of these particles to either ultraviolet light or an electric field, a significant impact on the jamming transition is observed. We found that the exposure of such external stimulus to a dense suspension of these particles triggered jamming and solidification and that the material relaxes back to a fluid upon removal of the stimulus. It is also shown that the suspension can be “trained” by oscillatory shear in which structural memory is introduced, and thus the yield strain may be tailored. The structural memory from the training persists even after the external stimulus is removed. The trained suspension can unjam and lose its memory when the applied strain exceeds the training strain under no external stimuli. Introduction of shear memory enhances the functionality of impact-resisting suspensions that one can load with erasable structural memory by switching on and off the external stimulus.