Elastic deformations in semi-dilute Ni nanorod/hydrogel composites

Magnetic nanocomposites were prepared by dispersing uniaxial ferromagnetic Ni nanorods in poly(acrylamide) hydrogels. Field alignment of the nanorods during polymerization resulted in a magnetic texture which was explored for field-induced deformations in the elastic composite. At very low particle volume fraction \(<10^{-6}\), the magnetic torque resulted in a local rotation of the nanorods, measured by optical transmission of linearly polarized light, with a field- and orientation dependence in agreement with the Stoner–Wohlfarth model. The local rotation was virtually unaffected by an increase in the volume fraction to \(\sim 10^{-4}\) which suggested negligible interparticle interactions or mutual compensation of opposing contributions. Elastic interactions, mediated by the deformation of the matrix, were investigated by FEM simulations for nanorods of different aspect ratio and relative spatial positions. Complementary experiments were performed by measuring the rotation of individual nanorods using laser scanning confocal microscopy. The results suggest interparticle interactions to be negligible in textured nanorod composites up to a volume fraction of \(10^{-4}\). Macroscopic deformations of Ni nanorod/hydrogel magnetic actuators in this concentration regime are expected to be solely determined by the intrinsic properties of the nanorods which was demonstrated using the example of a torsion cylinder.