  Soft elastic composite materials can serve as actuators when they transform
changes in external fields into mechanical deformation. Here, we address the
corresponding deformational behavior of magnetic gels and elastomers,
consisting of magnetizable colloidal particles in a soft polymeric matrix and
exposed to external magnetic fields. Since many practical realizations of such
materials involve particulate inclusions of polydisperse size distributions, we
concentrate on the effect that mixed particle sizes have on the overall
deformational response. To perform a systematic study, our focus is on binary
size distributions. We systematically vary the fraction of larger particles
relative to smaller ones and characterize the resulting magnetostrictive
behavior. The consequences for systems of various different spatial particle
arrangements and different degrees of compressibility of the elastic matrix are
evaluated. In parts, we observe a qualitative change in the overall response
for selected systems of mixed particle sizes. Specifically, overall changes in
volume and relative elongations or contractions in response to an induced
magnetization can be reversed into the opposite types of behavior. Our results
should apply to the characteristics of other soft elastic composite materials
like electrorheological gels and elastomers when exposed to external electric
fields as well. Overall, we hope to stimulate the further investigation on the
purposeful use of mixed particle sizes as a means to design tailored requested
material behavior.
