Immobilizing a drop of water: fabricating highly hydrophobic surfaces that pin water droplets.

We describe the fabrication of a patterned, hydrophobic silicon substrate that can pin a water droplet despite its large contact angle. Arrays of nm tips in silicon were fabricated by reactive ion etching using polymer masks defined by photolithography. A droplet sitting on one class of these substrates did not fall even after the substrate was turned upside-down. The production allows the fabrication of large arrays of tips with a one-step simple etching process, along with silanization, to achieve a substrate with both very large contact and tilting angles.

Density-based diamagnetic separation: devices for detecting binding events and for collecting unlabeled diamagnetic particles in paramagnetic solutions.

This paper describes the fabrication of a fluidic device for detecting and separating diamagnetic materials that differ in density. The basis for the separation is the balance of the magnetic and gravitational forces on diamagnetic materials suspended in a paramagnetic medium. The paper demonstrates two applications of separations involving particles suspended in static fluids for detecting the following: (i) the binding of streptavidin to solid-supported biotin and (ii) the binding of citrate-capped gold nanoparticles to amine-modified polystyrene spheres. The paper also demonstrates a microfluidic device in which polystyrene particles that differ in their content of CH2Cl groups are continuously separated and collected in a flowing stream of an aqueous solution of GdCl3. The procedures for separation and detection described in this paper require only gadolinium salts, two NdFeB magnets, and simple microfluidic devices fabricated from poly(dimethylsiloxane). This device requires no power, has no moving parts, and may be suitable for use in resource-poor environments.

Ionic electrets: electrostatic charging of surfaces by transferring mobile ions upon contact.

This paper describes the fabrication and characterization of ionic electrets-materials that bear a long-lived electrostatic charge because of an imbalance between the number of cationic and anionic charges in the material. Crosslinked polystyrene microspheres that contain covalently bound ions and mobile counterions transfer some of their mobile ions in air, in the absence of bulk liquid, to another material upon contact. According to the ion-transfer model of contact electrification, this selective transfer of mobile ions yields microspheres that have a net electrostatic charge. A tool that operates on the principle of electrostatic induction measures the charge on individual microspheres (50-450 microm in diameter). Microspheres with a variety of covalently bound ionic functional groups (tetraalkylammonium, alkyltriphenylphosphonium, alkylsulfonate, and arylsulfonate) acquire charges consistent with this ion-transfer mechanism. The charge on a microsphere is proportional to its surface area (ca. 1 elementary charge per 2000 nm2) and close to the theoretical limit imposed by the dielectric breakdown of air. The charge density in an atmosphere of SF6 is more than twice that in an atmosphere of N2. These observations suggest that the charge density of these ionic electret microspheres is limited by the dielectric breakdown of the surrounding gas. Functionalizing the surfaces of glass or silicon with covalently bound ions and mobile counterions generates ionic electrets from these inorganic substrates. Soft lithography can pattern charge on a planar silicon surface (with oxide) and on the surface of 250-mum glass microspheres.

Fabrication and Manipulation of Ionotropic Hydrogels Crosslinked by Paramagnetic Ions.

This manuscript describes the fabrication and manipulation of millimeter-scale spheres fabricated from ionotropic hydrogels that are crosslinked with paramagnetic metal ions (e.g., Ho(3+)). These ionotropic hydrogels experience a force in a magnetic field gradient that correlates with the concentration of the paramagnetic cations crosslinking the polymer. In an externally applied magnetic field, the paramagnetic hydrogel spheres assemble into ordered arrays or confined geometrical structures in the regions of highest magnetic field. These spheres can be separated from heterogeneous mixtures of diamagnetic materials using a simple bar magnet. Two applications using these recoverable hydrogel spheres were demonstrated: i) When prepared with embedded indicator dyes bound to paper, the spheres were used as colorimetric sensors for pH. ii) When prepared with embedded activated carbon powder, they were used to remove organic materials from aqueous solutions.

Biomimetic fabrication of 3D structures by spontaneous folding of tapes.

This paper describes a biomimetic strategy for the fabrication of 3D structures-including an electrically functional light detector-modeled on the folding of biological macromolecules into globular shapes. The process started by fabricating precursors to 3D, millimeter-sized structures using flexible polymer tapes. These tapes were patterned with metal features supporting liquid solder, crimped into strings of 3D corrugations, and attached to flat polymer tapes to generate linear 3D structures. Capillary interactions between droplets of molten solder on adjacent faces of the crimped tapes resulted in folding of the precursors into quasi-3D and truly 3D structures.

Patterning micron-sized features in a cross-linked poly(acrylic acid) film by a wet etching process.

This paper describes a photolithographic method to create sub-micron-scale patterns of cation-cross-linked poly(acrylic acid) (CCL-PAA). PAA can be cross-linked with a wide range of metal cations-including, but not limited to, Ag, Ca, Pd, Al, La, and Ti. Upon patterning a positive photoresist (diazonaphthoquinone-novolac resin) on a film of CCL-PAA, the exposed regions of CCL-PAA were etched by either an aqueous NaOH or EDTA solution. The initial cross-linking cation could be exchanged for a second cation that could not be patterned photolithographically. We used these patterned films of CCL-PAA i) to host and template the reduction of metallic cations to metallic nanoparticles, and ii) to fabricate porous, low- dielectric substrates.

Magnetic self-assembly of three-dimensional surfaces from planar sheets.

This report describes the spontaneous folding of flat elastomeric sheets, patterned with magnetic dipoles, into free-standing, 3D objects that are the topological equivalents of spherical shells. The path of the self-assembly is determined by a competition between mechanical and magnetic interactions. The potential of this strategy for the fabrication of 3D electronic devices is demonstrated by generating a simple electrical circuit surrounding a spherical cavity.

A tool for studying contact electrification in systems comprising metals and insulating polymers.

We describe an analytical system for in situ measurement of the charge that develops by contact electrification when a ferromagnetic sphere rolls on the surface of a polymer. This system makes it possible to survey the ability of polymeric surfaces to charge by contact electrification. Because the measurement of charge using this tool does not require physical contact of the charged sphere with the measuring electrode, it also enables the kinetics of charging to be examined. The research has focused on the contact charging of spheres having a core-and-shell geometry (a common core of ferromagnetic steel, and a variable shell of thin films of metals, or metals with surface oxides) rolling on the surface of polymeric slabs; it has generated an internally consistent set of data that include the polarity and magnitude of charging for a homologous series of polymers that differ chemically in the pendant group on a polyethylene backbone.

Electrostatic self-assembly of macroscopic crystals using contact electrification.

Self-assembly of components larger than molecules into ordered arrays is an efficient way of preparing microstructured materials with interesting mechanical and optical properties. Although crystallization of identical particles or particles of different sizes or shapes can be readily achieved, the repertoire of methods to assemble binary lattices of particles of the same sizes but with different properties is very limited. This paper describes electrostatic self-assembly of two types of macroscopic components of identical dimensions using interactions that are generated by contact electrification. The systems we have examined comprise two kinds of objects (usually spheres) made of different polymeric materials that charge with opposite electrical polarities when agitated on flat, metallic surfaces. The interplay of repulsive interactions between like-charged objects and attractive interactions between unlike-charged ones results in the self-assembly of these objects into highly ordered, closed arrays. Remarkably, some of the assemblies that form are not electroneutral-that is, they possess a net charge. We suggest that the stability of these unusual structures can be explained by accounting for the interactions between electric dipoles that the particles in the aggregates induce in their neighbours.