Coarsening in the electrohydrodynamic patterning of thin polymer films.

Periodic pillarlike microstructures can be created from initially flat polymer films via the electrohydrodynamic instabilities. Those patterns, however, are metastable. Our experimental observations show that the average pillar size increases slowly after linear growth.

Self-formation of sub-60-nm half-pitch gratings with large areas through fracturing.

Periodic micro- and nanostructures (gratings) have many significant applications in electronic, optical, magnetic, chemical and biological devices and materials. Traditional methods for fabricating gratings by writing with electrons, ions or a mechanical tip are limited to very small areas and suffer from extremely low throughput. Interference lithography can achieve relatively large fabrication areas, but has a low yield for small-period gratings.

Direct measurements of critical stresses and cracking in thin films of colloid dispersions.

Useful films can be formed by drying colloidal dispersions, but the negative capillary pressure generated often promotes cracks. Complex lateral flows during drying compromised previous measurements of the pressure required for cracking. Here we report data for the onset of cracking, and the additional cracks that appear at higher pressures, from high-pressure ultrafiltration experiments on homogeneously compressed films.

Generalized Hertzian model for the deformation and cracking of colloidal packings saturated with liquid.

The process of drying colloidal dispersions generally produces particulate solids under stress as a result of capillary or interparticle forces. The derivation of a constitutive relation on the basis of Hertzian contact mechanics between spheres provides a model for quantitatively predicting the conditions under which close-packed colloidal layers form continuous void-free films or homogeneous porous films or crack under tensile stresses.

Charge driven, electrohydrodynamic patterning of thin films.

In electrohydrodynamic patterning, electrical forces and surface tension acting at the interface between two fluids sandwiched between silicon wafers compete to set the period of pillar arrays, gratings, and concentric rings. Shrinking the period to deep submicron lengths requires a precise understanding of the source of the electric field. Previous modeling efforts have assumed that applied voltages, contact potentials, and static charge drive the flow.

Electric-field-induced patterns in thin polymer films: weakly nonlinear and fully nonlinear evolution.

A thin polymer melt on a substrate can be unstable to an electric field normal to the interface, a phenomenon that can be harnessed as a patterning technique with a range of potential applications. Motivated by the variety of patterns observed in experiments for polymers under both unpatterned and patterned masks, we describe here, from theoretical and numerical analyses, how nonlinear effects govern the growth of the instability and determine the final patterns. In particular, we discuss the nonlinear growth in terms of interactions among different Fourier modes and show that the second- and third-order nonlinearities favor the growth of hexagonal patterns under a featureless mask, in agreement with experimental observations.

Effect of hydrophobically modified polymers on shear-induced multilamellar vesicles.

The effects of polymer concentration, polymer molecular weight, and hydrophobe substitution level of modified poly(acrylic acid) polymers on the formation, size, and viscoelastic properties of shear-induced multilamellar vesicles (onions) are studied by rheology and light diffraction. The onions are close-packed, space-filling vesicles formed by shearing aqueous lamellar phases of C12E5 surfactant to produce phases with sufficient order and size uniformity (O(1-3 microm)) to diffract light. The addition of hydrophobically modified polymers enhances the rate of formation, uniformity, and stability independent of hydrophobe substitution level.

Experiments on random packings of ellipsoids.

Recent simulations indicate that ellipsoids can pack randomly more densely than spheres and, remarkably, for axes ratios near 1.25:1:0.8 can approach the densest crystal packing (fcc) of spheres, with a packing fraction of 74%.

Cracking in drying latex films.

Thin films of latex dispersions containing particles of high glass transition temperature generally crack while drying under ambient conditions. Experiments with particles of varying radii focused on conditions for which capillary stresses normal to the film deform the particles elastically and generate tensile stresses in the plane of the film. Irrespective of the particle size, the drying film contained, simultaneously, domains consisting of a fluid dispersion, a fully dried packing of deformed spheres, and a close packed array saturated with water.

Role of capillary stresses in film formation.

Stresses generated during film formation were deduced from the deflection of a copper cantilever coated with a drying latex. Experiments with particles of varying radii and glass transition temperatures (Tg) focused on conditions for which capillary stresses normal to the film deform the particles to close the voids. Soft particles (low Tg) formed continuous films, but hard ones (high Tg) produced fascinating arrays of cracks.

Limitations on length scales for electrostatically induced submicrometer pillars and holes.

Thin leaky and perfect dielectric films can be driven electrically to form well-ordered patterns, typically of pillar arrays. While the technique appears to promise nanometer scale features, this paper begins to examine some of the limitations. The process, sometimes referred to as lithographically induced self-assembly, begins by spin coating a polymer onto a silicon wafer generating an initially featureless film and then overlaying a mask, which may be patterned, leaving a small gap.

Reversible self-assembly and directed assembly of DNA-linked micrometer-sized colloids.

We present a technique for the directed assembly and self-assembly of micrometer-scale structures based on the control of specific DNA linkages between colloidal particles. The use of DNA links combined with polymer brushes provides an effective way to regulate the range and magnitude of addressable forces between pairs (and further combinations) of different particles. We demonstrate that the autoassembly of alternate microbeads as well as their directed assembly, by using laser tweezers, is reversible.

Cylindrically symmetric electrohydrodynamic patterning.

Cylindrically symmetric structures such as concentric rings and rosettes arise out of thin polymeric films subjected to strong electric fields. Experiments that formed concentric rings and theory capable of explaining these and other cylindrical structures are presented. These rings represent an additional member of a class of structures, including pillars and holes, formed by electrohydrodynamic patterning of thin films, occasionally referred to as lithographically induced self-assembly.

Nature of the divergence in low shear viscosity of colloidal hard-sphere dispersions.

Measurements of the low-shear viscosity eta(o) with a Zimm-Crothers viscometer for dispersions of colloidal hard spheres are reported as a function of volume fraction phi up to 0.56. Nonequilibrium theories based on solutions to the two-particle Smoluchoski equation or ideal mode coupling approximations do not capture the divergence.