Randall M. Erb

Composites Reinforced in Three Dimensions by Using Low Magnetic Fields

Dispersal in 3D The fabrication of composites containing small proportions of nanoparticles is limited by the ability to disperse the particles uniformly in all three dimensions. Erb et al. (p. 199; see the Perspective by Fratzl) describe a process for creating nanoparticle composites in which a magnetic field is used to align the nanoparticles. Surprisingly, the magnetic alignment of iron-oxide functionalized nanorods and discs was enabled using very small magnetic fields and low-volume fractions of magnetic nanoparticles, which allowed control of the orientation of the nanorods and discs three-dimensionally. Iron oxide−coated rods and platelets can reinforce a polymer composite through alignment with magnetic fields. The orientation and distribution of reinforcing particles in artificial composites are key to enable effective reinforcement of the material in mechanically loaded directions, but remain poor if compared to the distinctive architectures present in natural structural composites such as teeth, bone, and seashells. We show that micrometer-sized reinforcing particles coated with minimal concentrations of superparamagnetic nanoparticles (0.01 to 1 volume percent) can be controlled by using ultralow magnetic fields (1 to 10 milliteslas) to produce synthetic composites with tuned three-dimensional orientation and distribution of reinforcements. A variety of structures can be achieved with this simple method, leading to composites with tailored local reinforcement, wear resistance, and shape memory effects.

Magnetic assembly of colloidal superstructures with multipole symmetry

The assembly of complex structures out of simple colloidal building blocks is of practical interest for building materials with unique optical properties (for example photonic crystals and DNA biosensors) and is of fundamental importance in improving our understanding of self-assembly processes occurring on molecular to macroscopic length scales. Here we demonstrate a self-assembly principle that is capable of organizing a diverse set of colloidal particles into highly reproducible, rotationally symmetric arrangements. The structures are assembled using the magnetostatic interaction between effectively diamagnetic and paramagnetic particles within a magnetized ferrofluid. The resulting multipolar geometries resemble electrostatic charge configurations such as axial quadrupoles (‘Saturn rings’), axial octupoles (‘flowers’), linear quadrupoles (poles) and mixed multipole arrangements (‘two tone’), which represent just a few examples of the type of structure that can be built using this technique.

Self-shaping composites with programmable bioinspired microstructures

Shape change is a prevalent function apparent in a diverse set of natural structures, including seed dispersal units, climbing plants and carnivorous plants. Many of these natural materials change shape by using cellulose microfibrils at specific orientations to anisotropically restrict the swelling/shrinkage of their organic matrices upon external stimuli. This is in contrast to the material-specific mechanisms found in synthetic shape-memory systems. Here we propose a robust and universal method to replicate this unusual shape-changing mechanism of natural systems in artificial bioinspired composites. The technique is based upon the remote control of the orientation of reinforcing inorganic particles within the composite using a weak external magnetic field. Combining this reinforcement orientational control with swellable/shrinkable polymer matrices enables the creation of composites whose shape change can be programmed into the materials microstructure rather than externally imposed. Such bioinspired approach can generate composites with unusual reversibility, twisting effects and site-specific programmable shape changes.

Stretchable heterogeneous composites with extreme mechanical gradients

Heterogeneous composite materials with variable local stiffness are widespread in nature, but are far less explored in engineering structural applications. The development of heterogeneous synthetic composites with locally tuned elastic properties would allow us to extend the lifetime of functional devices with mechanically incompatible interfaces, and to create new enabling materials for applications ranging from flexible electronics to regenerative medicine. Here we show that heterogeneous composites with local elastic moduli tunable over five orders of magnitude can be prepared through the site-specific reinforcement of an entangled elastomeric matrix at progressively larger length scales. Using such a hierarchical reinforcement approach, we designed and produced composites exhibiting regions with extreme soft-to-hard transitions, while still being reversibly stretchable up to 350%. The implementation of the proposed methodology in a mechanically challenging application is illustrated here with the development of locally stiff and globally stretchable substrates for flexible electronics.

Formation of Ordered Cellular Structures in Suspension via Label-Free Negative Magnetophoresis

The creation of ordered cellular structures is important for tissue engineering research. Here, we present a novel strategy for the assembly of cells into linear arrangements by negative magnetophoresis using inert, cytocompatible magnetic nanoparticles. In this approach, magnetic nanoparticles dictate the cellular assembly without relying on cell binding or uptake. The linear cell structures are stable and can be further cultured without the magnetic field or nanoparticles, making this an attractive tool for tissue engineering.

Traveling wave magnetophoresis for high resolution chip based separations

A new mode of magnetophoresis is described that is capable of separating micron-sized superparamagnetic beads from complex mixtures with high sensitivity to their size and magnetic moment. This separation technique employs a translating periodic potential energy landscape to transport magnetic beads horizontally across a substrate. The potential energy landscape is created by superimposing an external, rotating magnetic field on top of the local fixed magnetic field distribution near a periodic arrangement of micro-magnets. At low driving frequencies of the external field rotation, the beads become locked into the potential energy landscape and move at the same velocity as the traveling magnetic field wave. At frequencies above a critical threshold, defined by the beads hydrodynamic drag and magnetic moment, the motion of a specific population of magnetic beads becomes uncoupled from the potential energy landscape and its magnetophoretic mobility is dramatically reduced. By exploiting this frequency dependence, highly efficient separation of magnetic beads has been achieved, based on fractional differences in bead diameter and/or their specific attachment to two microorganisms, i.e., B. globigii and S. cerevisiae.

Towards holonomic control of Janus particles in optomagnetic traps.

Janus particles generally refer to a class of colloids with two dissimilar faces having unique material properties. The spherical asymmetry associated with Janus particles is the key to realizing many commercial applications, including electrophoretic displays, nanosviscometers, and self-propelling micromachines. These diverse functionalities were accomplished by using an external electric or magnetic field to control the particle orientation, and in the process, modulate its reflectivity, hydrodynamic mobility, or direction of motion, respectively. However, these same asymmetries can interfere with optical trapping techniques that are used to control the translational degrees of freedom of a particle. Optical fields present an effective method for controlling the three translational degrees of freedom for particles ranging from tens of nanometers to micrometers in size. Previously, optical fields have been used in combination with magnetic fields to control four degrees of freedom of an asymmetric particle or particle aggregate. To achieve five or more degrees of freedom, magnetic Janus particles can theoretically be used; however, none so far have been stable in an optical trap. Controlling all six degrees of freedom of Janus particles, including three translational and three rotational, would open up new applications not only in biophysical force and torsion measurements, but also in microfluidics and material selfassembly. Here we report on a new type of spherical Janus that can be manipulated by a combination of optical and magnetic fields. We demonstrate the ability to directly control five degrees of freedom of the particle’s motion (three translational and two orientational) while constraining the final sixth degree of freedom. Ultimately, this demonstration represents the most control ever achieved over freely suspended spherical colloidal particles and opens up many exciting applications; the most obvious being the exertion of torsional and linear forces on biomolecules. The main achievement reported here was to develop a method of synthesizing magnetically anisotropic Janus particles that are also compatible with conventional optical trapping systems. We developed a novel lithographic technique for forming so-called ‘‘dot’’ Janus particles, which have a metallic coating covering <20% of their surface area. The advantage of this approach is that the dot Janus particles behave more like normal dielectric particles in an optical trap, while also responding to magnetic forces and torques produced by an external magnetic field. Purely dielectric and metallic Mie and Rayleigh particles have been optically trappedusing a variety of techniques. Bothdielectric microparticles and nanoparticles can be trapped in three dimensions with a high degree of spatial control. Metallic nanoparticles can also be trapped in three dimensions because scattering frommetallic and dielectric particles are similar in this size regime. However, metallic microparticles can only be controlled in two dimensions, due to considerations previously documented by others. For anisotropic Janus particles, such as dielectric particles that are partially covered by metal, the trapping stability in a focused optical beam depends to a great extent on the degree of metal coverage of the particle surface. Here we propose a general explanation for why optical trapping is more easily accomplished with dot Janus particles than with half-coated Janus particles. In the Mie size regime, where the particle diameter is large compared with the trapping wavelength, l, the momentum imparted by a focused optical beam can be described using geometric ray optics following Ashkin’s line of reasoning. In brief, each light ray refracts and reflects at the particle/fluid interface according to Snell’s law, and the momentum change between the incident ray and the refracted/reflected ray is summed over all incident rays to determine the net force on the particle. Typically, the net force is artificially divided into a gradient force, arising from refraction through the particle, and a scattering force, arising from reflection at the particle surface. The gradient force tends to pull the particle towards the beam focus, whereas the scattering force tends to push the particle away from the emission source. Figure 1 illustrates the incident light rays a and b refracted through the particle and the gradient forces ~Fa and ~Fb imparted on the particle due to each light ray. The ray optics approach reveals the importance of the symmetry of conjugate light rays in an optical trap. As long as the gradient force balances the scattering force, ~Fs, the trap will remain stable. For particles partially coated by reflective metal, the symmetry of this process may be broken, leading to unbalanced torques and forces that will depend on the position and orientation of the particle. As illustrated in Figure 1b, the metal coating inhibits light

Designer Polymer-Based Microcapsules Made Using Microfluidics

Filled microcapsules made from double emulsion templates in microfluidic devices are attractive delivery systems for a variety of applications. The microfluidic approach allows facile tailoring of the microcapsules through a large number of variables, which in turn makes these systems more challenging to predict. To elucidate these dependencies, we start from earlier theoretical predictions for the size of double emulsions and present quantitative design maps that correlate parameters such as fluid flow rates and device geometry with the size and shell thickness of monodisperse polymer-based capsules produced in microcapillary devices. The microcapsules are obtained through in situ photopolymerization of the middle oil phase of water-in-oil-in-water double emulsions. Using polymers with selected glass transition temperatures as the shell material, we show through single capsule compression testing that hollow capsules can be prepared with tunable mechanical properties ranging from elastomeric to brittle. A quantitative statistical analysis of the load at rupture of brittle capsules is also provided to evaluate the variability of the microfluidic route and assist the design of capsules in applications involving mechanically triggered release. Finally, we demonstrate that the permeability and microstructure of the capsule shell can also be tailored through the addition of cross-linkers and silica nanoparticles in the middle phase of the double emulsion templates.

Mechanics of Platelet-Reinforced Composites Assembled Using Mechanical and Magnetic Stimuli

Current fabrication technologies of structural composites based on the infiltration of fiber weaves with a polymeric resin offer good control over the orientation of long reinforcing fibers but remain too cumbersome and slow to enable cost-effective manufacturing. The development of processing routes that allow for fine control of the reinforcement orientation and that are also compatible with fast polymer processing technologies remains a major challenge. In this paper, we show that bulk platelet-reinforced composites with tailored reinforcement architectures and mechanical properties can be fabricated through the directed-assembly of inorganic platelets using combined magnetic and mechanical stimuli. The mechanical performance and fracture behavior of the resulting composites under compression and bending can be deliberately tuned by assembling the platelets into designed microstructures. By combining high alignment degree and volume fractions of reinforcement up to 27 vol %, we fabricated platelet-reinforced composites that can potentially be made with cost-effective polymer processing routes while still exhibiting properties that are comparable to those of state-of-the-art glass-fiber composites.

Non-linear alignment dynamics in suspensions of platelets under rotating magnetic fields

Under rotating magnetic fields, micron-sized platelets suspended in a fluid and decorated with magnetic nanoparticles are found to assume two orientational states. This behavior is very attractive for the development of unusual reinforcement architectures in synthetic composites. However, it is highly dependent on the frequency of the magnetic field and the rheological properties of the fluid. At low frequencies or fluid viscosities, the magnetized platelets continuously rotate in the fluid. At high frequencies and fluid viscosities, a non-linear response is observed in which the platelets align parallel to the plane of the rotating field. In this study we offer a theoretical description and experimental verification of this phenomenon, which can be used to build composites with fully aligned platelet reinforcement.