Stop-Flow Lithography of Colloidal, Glass, and Silicon Microcomponents

The assembly of oxide and non-oxide microcomponents from colloidal building blocks is central to a broad array of applications, including sensors, optical devices, and microelectromechanical systems (MEMS), as well as to fundamental studies of granular materials. Progress in these areas has been hindered by the availability of colloidal microcomponents of precisely tailored size, shape, and composition. Hence, there is tremendous interest in developing new patterning methods for creating precisely tailored microcomponents composed of colloidal building blocks, amorphous or polycrystalline oxides, and silicon. For example, colloidal-based microcomponents produced in simple non-spherical shapes, such as discoid, triangular, cuboid, and rectangular, may serve as novel granular feedstock for ceramics, optical display technologies and pharmaceuticals. Traditional methods for producing colloidal granules, such as fluid bed granulation, high shear mixer granulation, and spray drying, do not provide adequate control over granule size, shape, or composition. Equally important is the need to create porous and dense oxide and non-oxide microcomponents for functional devices ranging from micro-mixers and heat exchangers to MEMS. Although several fabrication methods have been recently introduced, including lithography, electroplating and molding (LIGA), micro-extrusion, microinjection molding, micro-stereolithography, and microelectro-discharge machining, each lacks the materials flexibility or rapid assembly times desired for many applications. Microfluidic assembly techniques provide a new platform for creating novel polymer particles from photopolymerizable resins and hydrogels as well as colloidal granules. In most cases, the particles (or granules) are produced by co-flowing immiscible liquids through a microfluidic device that induces droplet break off yielding one particle at a time. Due to surface-tension effects, only spherical shapes or deformations thereof are readily produced. Another technique for the production of polymeric microparticles, forms polystyrene microbeads into higher order assemblies via microfluidic patterning and thermal fusion. By contrast, stop-flow lithography (SFL) enables a rich array of simple and complex shapes to be produced, in parallel, at production rates in excess of 10 min!1. SFL employs microscope projection photolithography to create patterned structures within a microfluidic device, eliminating the need for clean room conditions. To date, SFL has been used for applications such as biomolecular analysis, assembly of Janus particles and interference lithography. Here, for the first time, we report the assembly of colloidal granules and microcomponents in the form of microgear, triangular, discoid, cuboid, and rectangular shapes using SFL as well as demonstrate pathways by which they can be transformed into both porous and dense oxide and non-oxide structures. We demonstrate this novel assembly method by first designing a model colloidal suspension capable of being rapidly polymerized via projection lithography within a microfluidic device. The system is composed of silica microspheres suspended in a mixture of dimethyl sulfoxide (DMSO) and water at a volume fraction, fsilica, of 0.5. The suspension also contains acrylamide monomer (facrylamide1⁄4 0.08), a cross-linking agent (monomer:crosslinking agent ratio of 4:1 by weight), and photoinitiator (finitiator1⁄4 0.03). This photopolymerizable colloidal suspension must exhibit limited scattering and absorption of the incident ultraviolet light to ensure high resolution of the as-patterned features. Aqueous silica suspensions are opaque due to the refractive index difference between silica (n1⁄4 1.46) and water (n1⁄4 1.33). By adding an appropriate amount of DMSO (n1⁄4 1.48), the colloid and fluid phases are index-matched thereby minimizing scattering from the suspended particles. The SFL setup utilized in the present work is illustrated in Figure 1a. Patterned microcomponent(s) are formed by projecting ultraviolet light through a photomask inserted into C O M M U N IC A IO N