We revisit the classical problem of diffusion-limited ion transport to a membrane (or electrode) by considering the effects of charged side walls. Using simple mathematical models and numerical simulations, we identify three basic mechanisms for over-limiting current in a microchannel: (i) surface conduction carried by excess counterions, which dominates for very thin channels, (ii) convection by electro-osmotic flow on the side walls, which dominates for thicker channels and transitions to (iii) electro-osmotic instability on the membrane end in very thick channels. These intriguing electrokinetic phenomena may find applications in biological separations, water desalination, and electrochemical energy storage.

Most electrochemical processes, such as electrodialysis, are limited by diffusion, but in porous media, surface conduction and electroosmotic flow also contribute to ionic flux. In this article, we report experimental evidence for surface-driven overlimiting current (faster than diffusion) and deionization shocks (propagating salt removal) in a porous medium. The apparatus consists of a silica glass frit (1 mm thick with a 500 nm mean pore size) in an aqueous electrolyte (CuSO4 or AgNO3) passing ionic current from a reservoir to a cation-selective membrane (Nafion). The current-voltage relation of the whole system is consistent with a proposed theory based on the electroosmotic flow mechanism over a broad range of reservoir salt concentrations (0.1 mM to 1.0 M) after accounting for (Cu) electrode polarization and pH-regulated silica charge. Above the limiting current, deionized water (≈10 μM) can be continuously extracted from the frit, which implies the existence of a stable shock propagating against the flow, bordering a depleted region that extends more than 0.5 mm across the outlet. The results suggest the feasibility of shock electrodialysis as a new approach to water desalination and other electrochemical separations.

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, 10(8) 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.

The development of energy and infrastructure efficient water purification systems are among the most critical engineering challenges facing our society. Water purification is often a multistep process involving filtration, desalination, and disinfection of a feedstream. Shock electrodialysis (shock ED) is a newly developed technique for water desalination, leveraging the formation of ion concentration polarization (ICP) zones and deionization shock waves in microscale pores near to an ion selective element. While shock ED has been demonstrated as an effective water desalination tool, we here present evidence of other simultaneous functionalities. We show that, unlike electrodialysis, shock ED can thoroughly filter micron-scale particles and aggregates of nanoparticles present in the feedwater. We also demonstrate that shock ED can enable disinfection of feedwaters, as approximately 99% of viable bacteria (here E. coli) in the inflow were killed or removed by our prototype. Shock ED also separates positive from negative particles, contrary to claims that ICP acts as a virtual barrier for all charged particles. By combining these functionalities (filtration, separation and disinfection) with deionization, shock ED has the potential to enable more compact and efficient water purification systems.2

One-dimensional nanostructures with high aspect-ratios and nanometer cross-sectional dimensions have been the focus of recent studies in the persistent drive to miniaturize devices. Conventional bottom-up methods such as vapor-liquid-solid growth have been widely applied for the fabrication of uniform and high quality nanowires. Two challenges toward nanoelectronics and other applications remain: on the singlenanowire level, precisely manipulating an individual nanowire for the sophisticated functionalities, and on the multiple-nanowire level, integrating nanowires into designed architecture at large scale. Thus, an alternative approach with the capacity to achieve ordered and extended nanowires is highly desirable.In this thesis, we observe an intriguing phenomenon that a cylindrical shell upon reaching a characteristic thickness breaks up into filament arrays during optical-fiber thermal drawing. This structural evolution occurs exclusively in the cross-sectional plane, while the uniformity along the axial direction remains intact. We demonstrate crystalline semiconductor nanowires by post-drawing annealing procedure and characterize their electrical and optoelectric properties for the devices such as optical switch. This top-down thermal drawing approach provides new opportunities for nanostructure fabrication with high throughput and at low cost, and offers promising applications in renewable energy and data storage.In order to understand the stability (or instability) of thin shells and filaments, we explore a physical mechanism during the complicated thermal drawing. A perspective of capillary instability from fluid mechanics is focused. Axial stability of continuous filaments is consistent with capillary instability. Axial stability of a thicker cylindrical shell arises from large radius and high viscosity. These results provide theoretical guidance in the understanding of attainable feature sizes and in materials selection to expand the potential functionalities of devices in microstructured 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.

We report on the fabrication and characterization of globally ordered crystalline selenium filaments with diameters about 200 nm and aspect ratios upwards of 10 5 . Amorphous Se filaments are fabricated by a recently developed approach in which a thin film evolves into an ordered array of filaments in fiber. Single-crystal and polycrystalline filaments are attained with a postdrawing annealing procedure. Arrays of two-cm-long crystalline nanowires, electrically contacted to external circuitry through the fiber end facets, exhibit a two-orders-of-magnitude change in conductivity between dark and illuminated states. These results hold promise for the fabrication of filament-detector arrays that may be integrated with large-area electronics.

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.