  Systems driven far from equilibrium often retain structural memories of their
processing history. This memory has, in some cases, been shown to dramatically
alter the material response. For example, work hardening in crystalline metals
can alter the hardness, yield strength, and tensile strength to prevent
catastrophic failure. Whether memory of processing history can be similarly
exploited in flowing systems, where significantly larger changes in structure
should be possible, remains poorly understood. Here, we demonstrate a promising
route to embedding such useful memories. We build on work showing that exposing
a sheared dense suspension to acoustic perturbations of different power allows
for dramatically tuning the sheared suspension viscosity and underlying
structure. We find that, for sufficiently dense suspensions, upon removing the
acoustic perturbations, the suspension shear jams with shear stress
contributions from the maximum compressive and maximum extensive axes that
reflect the acoustic training. Because the contributions from these two
orthogonal axes to the total shear stress are antagonistic, it is possible to
tune the resulting suspension response in surprising ways. For example, we show
that differently trained sheared suspensions exhibit: 1) different
susceptibility to the same acoustic perturbation; 2) orders of magnitude
changes in their instantaneous viscosities upon shear reversal; and 3) even a
shear stress that increases in magnitude upon shear cessation. To further
illustrate the power of this approach for controlling suspension properties, we
demonstrate that flowing states well below the shear jamming threshold can be
shear jammed via acoustic training. Collectively, our work paves the way for
using acoustically induced memory in dense suspensions to generate rapidly and
widely tunable materials.
