Paramagnetic colloids: Chaotic routes to clusters and molecules
We present computer simulations and experiments on dilute suspensions of superparamagnetic particles subject to rotating magnetic fields. We focus on chains of four particles and their decay routes to stable structures. At low rates, the chains track the external field. At intermediate rates, the chains break up but perform a periodic (albeit complex) motion. At sufficiently high rates, the chains generally undergo chaotic motion at short times and decay to either closely packed clusters or more dispersed, colloidal molecules at long times. We show that the transition out of the chaotic states can be described as a Poisson process in both simulation and experiment.
Assembling particle clusters with incoherent 3D magnetic fields
Here, we investigate a family of incoherent 3D magnetic fields that are capable of creating controlled and tunable particle assemblies such as dimers, trimers, and quadramers. These field functions can be tuned to assemble monodisperse clusters with long term stability and can quickly switch the clusters between different states. This subset of three-dimensional field functions that we have studied demonstrates the rich phase space available to tune colloidal suspensions with magnetic fields.
Direct-write 3D printing of composite materials with magnetically aligned discontinuous reinforcement
This paper describes a new direct-write style 3D printing system that incorporates magnetic fields to actively control the orientation of reinforcing fibers during the printing of fiber reinforced composites. Such a manufacturing system is fraught with complications from the high shear dominated alignment experienced by the fibers during extrusion to the slow magnetic alignment dynamics of fibers in viscous media. Here we characterize these issues and suggest effective operating windows in which magnetic alignment is a viable approach to orienting reinforcing particles during direct-write 3D printing.
High-performance battery electrodes via magnetic templating
Here we show that magnetic control of sacrificial features enables the creation of directional pore arrays in lithium-ion electrodes. The directional pores result in faster charge transport kinetics and enable electrodes with more than threefold higher area capacity at practical charge–discharge rates. We demonstrate these capabilities in laboratory cells under various test conditions, including an electric vehicle model drive cycle.
We review significant developments in manipulating soft matter systems through the use of magnetic torque. Magnetic torque enables the orientation, assembly, and manipulation of thermally fluctuating systems in broad material fields including biomaterials, ceramic and composite precursor suspensions, polymer solutions, fluids, foams, and gels.
Understanding and overcoming shear alignment of fibers during extrusion
We have quantified the magnetic and shear energy of fibers in an extrusion flow. In certain regimes magnetic energy can dominate shear energy. In most regimes, magnetic energy can be used to turn off Jeffrey orbits.
A book chapter about natural composites that utilize functional gradients to tune material properites at the nano- and micro-scale. These natural composites will inspire the next generation of engineered functional materials.
Self-shaping composites with programmable bioinspired microstructures
We have designed composites with bioinspired architectures that change shape like pinecones, wheat awns, and orchid tree seedpods. To do this, we have recreated the basic microcelluose fibril architectures of the natural seedpods with engineered alumina architectures within synthetic polymers. We obtain programmable shape change from bending to twisting.
Mechanics of platelet-reinforced composites assembled using mechanical and magnetic stimuli
This paper investigates alumina composite microstructures with high volume fractions of discontinuous ceramic particles (up to 30 vol%). Above 20 vol% local nematic ordering can be observed with vibrational sources. Combining magnetic and vibrational sources can create globally ordered alumina composite microstructures well beyond 20 vol%.