(368al) Stimuli Responsive Multifunctional Magnetic Colloidal Particles for Autonomous Propulsion

Magnetic field directed colloidal assembly and propulsion has gained interest to engineer stimuli responsive particles, which can be used as autonomous materials. One recent advance in this area is the fabrication of composite particles made of a nonmagnetic material such as hydrogel or elastomer within which magnetic nanoparticle assemblies are embedded. In the current work, we first present the foundation principles behind engineering novel responsive microbeads embedded with anisotropic magnetic nanoparticle (MNP) assemblies. The synthesis is based on shear-fracturing of polydimethylsiloxane precursor uniformly embedded with iron-oxide nanoparticles, which led to the formation of magnetic microdroplets. Subsequently, these droplets undergo curing in the presence of a magnetic field, facilitating the formation of soft microbeads wherein the MNPs undergo dipolar interactions, forming assemblies within. By varying the MNP concentrations inside the droplets, we discovered structural phase transitions ranging from 2D linear chains to a combination of chains and networked bundles, to solely 3D bundles. These different classes of beads constitute an interaction toolbox of microbeads with different magnetic macro responses and will be governed by the magnetic characteristics and types of the assemblies inside the microbeads. By leveraging their magnetic response due to field-induced torque under an applied out-of-plane rotating magnetic field, we have reported new principles of propulsion of these microbeads in shear-thinning fluids on a substrate. Typically, in a Newtonian fluid, the microbeads propel in the forward direction as they rotate clockwise due to the field-induced torque. However, in shear-thinning fluids, we report a new pattern of dynamic active motion, what we refer to as “moonwalking”, where the microbeads translate backward as opposed to the forward motion in Newtonian fluids. Our results were supported by COMSOL simulations which show that the shear forces acting on the top and the bottom of the particle change due to localized non-uniform shear thinning around the particle. This new phenomenon will enhance our understanding of fundamental principles of autonomous materials and could find applications such as biomedical formulations, drug delivery, optical filters, and sensors.