We are excited to share our latest work on magnetic hydrogels, where we take a closer look at why embedded nanoparticles show hindered rotational dynamics - even without covalent bonding to the polymer network. By combining experiments with simulations, we show that neither hydrodynamics nor simple interactions explain the effect; instead, subtle preferential polymer attachment to heterogeneous nanoparticle surfaces plays a key role. These insights help clarify the microscopic mechanisms governing the dynamic response of magnetic soft materials.
You can find the full paper here: https://doi.org/10.1063/5.0323088
Abstract:
Magnetic gels are soft hydrogels with incorporated magnetic nanoparticles, combining viscoelastic properties with responsiveness to magnetic fields. In many experimentally relevant systems, the nanoparticles are not covalently attached to the polymer network but are instead physically trapped within its meshes. Despite this weak mechanical coupling, experiments reveal signatures of hindered rotational dynamics. Here, we investigate the microscopic origin of this behavior by combining experiments on polyacrylamide hydrogels loaded with cobalt ferrite nanoparticles with simulations that explicitly account for polymers, nanoparticles, and hydrodynamic interactions. We probe the nanoparticle–polymer coupling by exploring the systems’ magnetic AC susceptibility—a quantity that is accessible both by experiment and in simulations, and that is sensitive to rotational dynamics. Experimentally, we observe a reduced low-frequency susceptibility, indicating partial orientational blocking of the nanoparticles, even in the absence of covalent bonding. Simulations show that this behavior cannot be explained by hydrodynamic coupling or isotropic van der Waals-like interactions alone. Instead, our results demonstrate that a degree of preferential attachment of polymers to spots on the nanoparticle surface—arising from chemical or topographical heterogeneity—is essential to reproduce the experimentally observed response.
