Probing the role of frustrated interactions in experiments with an amorphous solid

In an amorphous solid that has been mechanically annealed, small plastic deformations cause local groups of particles to reversibly switch between two states. This mechanism supports the generic behavior known as return-point memory, which allows the strain(s) of past deformations to be read out. In general, pairs of rearranging regions may have frustrated (antiferromagnetic) interactions, leading to deviations from return-point memory. However, it has been unclear whether these deviations could be large enough to be identified in experiments. In our experiments, we cyclically shear a soft, frictionless amorphous solid composed of bidisperse colloidal particles at an oil-water interface. We report on tests that use asymmetric shear and amplitude variation to clearly distinguish the effects of frustration from return-point memory and noise. Our results are an estimate of the importance of frustrated interactions in an annealed amorphous solid.