Dense suspensions as trainable rheological metafluids

In materials, the ability to retain the memory of applied stresses or strains opens up new opportunities for enhancing their performance adaptively via training. In dense suspensions, a stress-adaptive response is enabled by non-Newtonian rheology; however, typical suspensions have little memory, which implies rapid cessation of any adapted behavior. Here, we show how multiple adaptive responses can be achieved by designing suspensions where different stress levels trigger different memories. This is achieved through the interplay of particle interactions based on frictional contact and dynamic chemical bridging. These two interactions give rise to stress-activated memories associated with opposite time-dependent trends. As a result, a suspension can be trained to adapt to applied stress either by softening or stiffening, exhibiting targeted viscosity and energy dissipation in response to low-velocity impact. Such behavior, usually associated with mechanical metamaterials, suggests that dense suspensions with multiple memories can be viewed as trainable rheological metafluids.