Joshua J. Kaufman

Structured spheres generated by an in-fibre fluid instability

From drug delivery to chemical and biological catalysis and cosmetics, the need for efficient fabrication pathways for particles over a wide range of sizes, from a variety of materials, and in many different structures has been well established. Here we harness the inherent scalability of fibre production and an in-fibre Plateau–Rayleigh capillary instability for the fabrication of uniformly sized, structured spherical particles spanning an exceptionally wide range of sizes: from 2 mm down to 20 nm. Thermal processing of a multimaterial fibre controllably induces the instability, resulting in a well-ordered, oriented emulsion in three dimensions. The fibre core and cladding correspond to the dispersed and continuous phases, respectively, and are both frozen in situ on cooling, after which the particles are released when needed. By arranging a variety of structures and materials in a macroscopic scaled-up model of the fibre, we produce composite, structured, spherical particles, such as core–shell particles, two-compartment ‘Janus’ particles, and multi-sectioned ‘beach ball’ particles. Moreover, producing fibres with a high density of cores allows for an unprecedented level of parallelization. In principle, 108 50-nm cores may be embedded in metres-long, 1-mm-diameter fibre, which can be induced to break up simultaneously throughout its length, into uniformly sized, structured spheres.

Multimaterial preform coextrusion for robust chalcogenide optical fibers and tapers.

The development of robust infrared fibers is crucial for harnessing the capabilities of new mid-infrared lasers. We present a novel approach to the fabrication of chalcogenide glass fiber preforms: one-step multimaterial extrusion. The preform consists of a glass core and cladding surrounded by a built-in, thermally compatible, polymer jacket for mechanical support. Using this approach we extrude several preform structures and draw them into robust composite fibers. Furthermore, the polymer cladding allows us to produce robust tapers with submicrometer core diameter.

Thermal drawing of high-density macroscopic arrays of well-ordered sub-5-nm-diameter nanowires.

We investigate the lower limit of nanowire diameters stably produced by the process of thermal fiber drawing and fiber tapering. A centimeter-scale macroscopic cylindrical preform containing the nanowire material in the core encased in a polymer scaffold cladding is thermally drawn in the viscous state to a fiber. By cascading several iterations of the process, continuous reduction of the diameter of an amorphous semiconducting chalcogenide glass is demonstrated. Starting from a 10-mm-diameter rod we thermally draw hundreds of meters of continuous sub-5-nm-diameter nanowires. Using this approach, we produce macroscopic lengths of high-density, well-ordered, globally oriented nanowire arrays.

Observation of the Plateau-Rayleigh capillary instability in multi-material optical fibers

We report the observation of the Plateau-Rayleigh capillary instability during the tapering of a multi-material optical fiber. The fiber core is a glass, and the cladding is an amorphous polymer. The instability is manifested in the breakup of the core into a periodic string of size-tunable micro-scale droplets embedded along the fiber axis. The particle diameters may be tuned in the 1–20 μm range through control of the tapering speed and temperature. Extending this approach to the fabrication of polymer and glass nanoparticles appears feasible.

Controlled fragmentation of multimaterial fibres and films via polymer cold-drawing

Polymer cold-drawing is a process in which tensile stress reduces the diameter of a drawn fibre (or thickness of a drawn film) and orients the polymeric chains. Cold-drawing has long been used in industrial applications, including the production of flexible fibres with high tensile strength such as polyester and nylon. However, cold-drawing of a composite structure has been less studied. Here we show that in a multimaterial fibre composed of a brittle core embedded in a ductile polymer cladding, cold-drawing results in a surprising phenomenon: controllable and sequential fragmentation of the core to produce uniformly sized rods along metres of fibre, rather than the expected random or chaotic fragmentation. These embedded structures arise from mechanical–geometric instabilities associated with ‘neck’ propagation. Embedded, structured multimaterial threads with complex transverse geometry are thus fragmented into a periodic train of rods held stationary in the polymer cladding. These rods can then be easily extracted via selective dissolution of the cladding, or can self-heal by thermal restoration to re-form the brittle thread. Our method is also applicable to composites with flat rather than cylindrical geometries, in which case cold-drawing leads to the break-up of an embedded or coated brittle film into narrow parallel strips that are aligned normally to the drawing axis. A range of materials was explored to establish the universality of this effect, including silicon, germanium, gold, glasses, silk, polystyrene, biodegradable polymers and ice. We observe, and verify through nonlinear finite-element simulations, a linear relationship between the smallest transverse scale and the longitudinal break-up period. These results may lead to the development of dynamical and thermoreversible camouflaging via a nanoscale Venetian-blind effect, and the fabrication of large-area structured surfaces that facilitate high-sensitivity bio-detection.

In-fiber production of polymeric particles for biosensing and encapsulation

Significance A scalable, chemistry-independent, fluid-instability–mediated in-fiber route for fabricating uniformly sized spherical polymeric particles over a wide span of diameters is developed targeting biomedical applications. Both surface functionalization of solid biocompatible polymer particles for protein–protein interactions and volume encapsulation of a biological material in spherical hollow polymer shells are confirmed, in addition to combining both surface and volumetric functionalities in the same polymeric particle. Polymeric micro- and nanoparticles are becoming a mainstay in biomedicine, medical diagnostics, and therapeutics, where they are used in implementing sensing mechanisms, as imaging contrast agents, and in drug delivery. Current approaches to the fabrication of such particles are typically finely tuned to specific monomer or polymer species, size ranges, and structures. We present a general scalable methodology for fabricating uniformly sized spherical polymeric particles from a wide range of polymers produced with complex internal architectures and continuously tunable diameters extending from the millimeter scale down to 50 nm. Controllable access to such a wide range of sizes enables broad applications in cancer treatment, immunology, and vaccines. Our approach harnesses thermally induced, predictable fluid instabilities in composite core/cladding polymer fibers drawn from a macroscopic scaled-up model called a “preform.” Through a stack-and-draw process, we produce fibers containing a multiplicity of identical cylindrical cores made of the polymers of choice embedded in a polymer cladding. The instability leads to the breakup of the initially intact cores, independent of the polymer chemistry, into necklaces of spherical particles held in isolation within the cladding matrix along the entire fiber length. We demonstrate here surface functionalization of the extracted particles for biodetection through specific protein–protein interactions, volumetric encapsulation of a biomaterial in spherical polymeric shells, and the combination of both surface and volumetric functionalities in the same particle. These particles used in distinct modalities may be produced from the desired biocompatible polymer by changing only the geometry of the macroscopic preform from which the fiber is drawn.

Dispersion characterization of chalcogenide bulk glass, composite fibers, and robust nanotapers

We report the results of systematic measurements of the group velocity dispersion (GVD) in chalcogenide glass (ChG) bulk samples, composite ChG fibers, and robust high-index-contrast nanotapers. The composite ChG-polymer fibers are drawn from an extruded multimaterial preform incorporating a thick built-in polymer jacket that is thermally compatible with the ChG used, and the nanotapers are then produced without removing the polymer. We isolate the contributions of material and waveguide GVD to the total dispersion in the nanotapers and support the results with finite-element simulations. These results indicate many possibilities for dispersion engineering and nonlinearity enhancement in all-solid index-guiding ChG fibers stemming from the flexibility of this fiber fabrication methodology.

NanoRunner: a very small wireless robot with three piezoactuated legs suited for design experimentations and validations through preprogrammed behaviors

The NanoRunner is designed to be primarily used as an experimental wireless robot in order ot quickly test and validate several hardware/software issues and ideas prior to being implemented on the more expensive and complex wireless instrumented NanoWalker robot. As such, the NanoRunner, Like the NanoWalker is based on three piezo- actuated legs forming a pyramid with the apex pointing upward. Unlike the NanoWlaker, the NanoRunner has much simpler embedded electronics and is not capable of an accuracy and computational throughput comparable to the NanoWalker. Because of its lighter weight, it can move or run much faster. Furthermore, the NanoRunner does not have a fast infrared communication infrastructure for downloading executable code. Instead the NanoRunner is first pre-programmed with a specific behavior suitable for the tasks to be performed. Nonetheless, the NanoRunner has all the required electronics to be fully autonomous while performing its experimentation tasks. Although not as sophisticated as the NanoWalker, the NanoRunner offers a smaller and simpler robot implementation for less demanding tasks. Another major motivation for the NanoRunner is to validate various ideas in order to decrease the overall size of the robot. The size is critical since our goal is to allow more robots to work within the same area. In this paper, the NanoRunner is described. Aspects such as construction, assembly, and the method used for downloading executable code in order to pre-program the robots behavior are also covered.

Digital design of multimaterial photonic particles

Significance We present an approach for the facile fabrication of dielectric particles having the size of an optical wavelength yet endowed with a complex multimaterial internal nanoscale architecture. This methodology amounts to “digitally designing” the particle by precisely allocating the desired material at prescribed coordinates within the 3D volume of the particle. The digital design of such a photonic particle enables sophisticated strategies for controlling light scattering. As an example, without changing the size of a core–shell particle, its optical scattering strength can be tuned above or below that afforded by its constitutive materials by changing the core–shell diameter ratio. This work may lead to the development of new optical coatings and paints with exotic functionality. Scattering of light from dielectric particles whose size is on the order of an optical wavelength underlies a plethora of visual phenomena in nature and is a foundation for optical coatings and paints. Tailoring the internal nanoscale geometry of such “photonic particles” allows tuning their optical scattering characteristics beyond those afforded by their constitutive materials—however, flexible yet scalable processing approaches to produce such particles are lacking. Here, we show that a thermally induced in-fiber fluid instability permits the “digital design” of multimaterial photonic particles: the precise allocation of high refractive-index contrast materials at independently addressable radial and azimuthal coordinates within its 3D architecture. Exploiting this unique capability in all-dielectric systems, we tune the scattering cross-section of equisized particles via radial structuring and induce polarization-sensitive scattering from spherical particles with broken internal rotational symmetry. The scalability of this fabrication strategy promises a generation of optical coatings in which sophisticated functionality is realized at the level of the individual particles.

In-fiber fabrication of size-controllable structured particles

We present an approach for fabricating single-material and multi-material structured spherical particles in the size range 1 millimeter to 50 nanometers that makes use of the Plateau-Rayleigh capillary instability in a multi-material fiber.