Magnetic torque-driven deformation of Ni-nanorod/hydrogel nanocomposites

Abstract

Nickel (Ni) nanorods were prepared by the anodized aluminum oxide (AAO) template method and dispersed in poly(acrylamide) (PAM) hydrogels. The deformation of the magnetoresponsive composites was studied with particular attention to the consequences of finite magnetic shape anisotropy as compared to rigid dipoles on the field-dependent torque. For comparison with experiments, the composite was described as an elastic continuum with a local magnetic torque density, applied by discrete particles and determined by the local orientation of their magnetic anisotropy axis with respect to the magnetic field. The mean magnetic moment of the single domain particles m and their volume density in the composite φvol were derived from the static field-dependent optical transmission (SFOT) of linear polarized light. The mechanical coupling between the particles and their viscoelastic environment was retrieved from the rotational dynamics of the nanorods using oscillating field-dependent optical transmission (OFOT) measurements. Field- and orientation-dependent magnetization measurements were analyzed using the Stoner–Wohlfarth (SW) model and a valid parameter range was identified by introducing an effective anisotropy constant KA as a new empirical model parameter. This adapted SW-model for quantitative description of the field- and orientation dependence of the magnetic torque was validated by measuring the local rotation of nanorods in a soft elastic hydrogel. Finally, torsional and bending deformation of thin magnetically textured composite filaments were computed and compared with experiments.

Field-induced deformation of ferromagnetic soft nanocomposites

“…The rotation increased with field and with a larger angle between texture axis and field direction. When plotting the rotation angle as a function of the calculated torque, the measurements at different axis orientations created a common master curve with the linear relationship ϑ = T/(K v G) [53]. The local shear modulus derived from the slope of the field-induced rotation agreed with the macroscopic modulus of the pure PAM hydrogel matrix, indicating that the crosslinking reaction was not significantly different near the nanoparticles and that the macroscopic modulus was also not affected by the nanoparticles due to their very small volume fraction of ϕ vol ⩽ 1.6 × 10 −4 .…”