Effect of Particles Alignment on Giant Reduction in Dynamic Modulus of Hydrogels Containing Needle-Shaped Magnetic Particles

We investigated the giant reduction in the dynamic modulus of magnetic gels with aligned particles. The magnetic gel is consisting of a kappa-carrageenan gel loaded with gamma-Fe2O3 particles with an aspect ratio of similar to 8. The magnetic particles were aligned by a weak magnetic field of 50 mT during gelation. The gels with aligned particles demonstrated giant reductions in the storage Young's modulus on the order of 10(6) Pa due to magnetization; however, no reductions in the storage modulus were observed for the gels with random particles. The storage modulus of gels with aligned particles did not follow the Halpin-Tsai equation above volume fractions of 0.01, indicating the heterogeneous dispersion of the magnetic particles; however, the modulus of the gels with random particles satisfied the equation at all volume fractions, suggesting the random dispersion of the particles. It was noted that the gels with aligned particles demonstrated enhanced nonlinear viscoelasticity and a large value of the loss tangent, while the gels with random particles exhibited weak nonlinear viscoelasticity and a small value of the loss tangent. This indicates that the magnetic particles form a particle network in the gel with aligned particles. It was also found that the magnetic gel with aligned particles did not undergo a marked reduction in the storage modulus when the magnetic field was applied in parallel with the alignment of magnetic particles. This strongly indicates that the giant reduction in dynamic modulus is caused by breaking the particle network developed in the magnetic gel.