(140h) Multifunctional Soft Colloidal Particles through Hierarchical Assembly of Nanoparticles inside Polymeric Droplets

Magnetic field-directed colloidal interactions offer facile tools for assembly of hierarchical functional structures, which we use to engineer stimuli responsive particles. These interactions are directional in nature and lead to the formation of out-of-equilibrium magnetic nanoparticle superstructures. Our goal is the fabrication of new classes of colloidal polymeric composites, where a nonmagnetic material such as hydrogel or elastomer serves as a host for magnetic nanoparticle assemblies. Here, we will specifically focus on the engineering of a novel class of soft responsive polymeric microbeads with embedded ferromagnetic nanoparticle (MNP) assemblies. The synthesis is based on shear-driven formation of magnetically responsive microdroplets from polydimethylsiloxane precursor containing iron-oxide nanoparticles. Subsequently, these droplets are placed in the presence of a uniform magnetic field while they are cured simultaneously. This facilitates the formation of soft microbeads wherein the MNPs undergo dipolar interactions, forming assemblies within. We investigated how the microstates of the assemblies change as we vary the MNP concentrations inside the droplets. We discovered that the spatial confinement of the droplets governed the assembly process of the MNPs. We further observed a sequence of structures ranging from linear chains at a low MNP loading, transitioning to a combination of chains and bundles, to solely 3D bundles at a high MNP loading. These experimental results were analyzed with the aid of COMSOL simulations where we calculated the potential energy to find out the favorable assembly conformation. The chains at higher MNP loading minimize their energy by aggregating laterally to form bundles with their MNP dipoles out-of-registry. Our calculations can therefore accurately predict the phase transitions of the assemblies inside the confined microdroplets. The resulting different classes of beads constitute an interaction toolbox with different magnetic macroscale responses. These responses are governed by the magnetic characteristics and structural anisotropy of the assemblies inside the microbeads. Thanks to their magnetic response by field-induced torque under an applied rotating magnetic field, these beads were able to rotate in-plane and out-of-plane and behave as autonomous entities. This functionality of the microbeads can be used in applications such as optical modulators, active colloidal rollers, and magneto-rheological gels.