Binary colloidal suspensions are assembled into planar superlattices using ac electric fields. Either triangular or square-packed arrays form, depending on the frequency and relative particle concentrations. The frequency dependence is striking since superlattices develop at low and high frequencies but not at intermediate frequencies. We explain the low frequency behavior <3 kHz in terms of induced-dipole repulsion balanced by attraction resulting from electrohydrodynamic (EHD) flow. At high frequencies (20–200 kHz), EHD flow is negligible but aggregation occurs since dipole-dipole interactions become attractive.
In order to achieve the revolutionary new defense capabilities offered by materials science and engineering, innovative management to reduce the risks associated with translating research results will be needed along with the R&D. While payoff is expected to be high from the promising areas of materials research, many of the benefits are likely to be evolutionary. Nevertheless, failure to invest in more speculative areas of research could lead to undesired technological surprises. Basic research in physics, chemistry, biology, and materials science will provide the seeds for potentially revolutionary technologies later in the 21st century.
Micropatterning of Pb(Zr0.52Ti0.48)O3 (PZT) thin films with line features as small as 350 nm was demonstrated through capillary molding of organometallic solutions within the continuous channels of an elastomeric mold. Despite the large stresses that develop during the evaporation of the solvent, pyrolysis of the organics, and the densification and crystallization of the inorganic gel, the patterned crystalline PZT films were crack-free and mechanically robust. Flawless regions as large as 1 cm2 were obtained. The cross-sectional shape of the patterned PZT lines was trapezoidlike. Single perovskite PZT grains that formed during annealing at 600–700 °C completely filled the cross-sectional area of the patterned lines. Lead acetate, zirconium propoxide, and titanium isopropoxide were used as the starting materials. Substrates used included silver tape, stainless steel plate, silicon wafer, and platinum-coated silicon wafer.
Asimple method is described for controlling the organization of proteins on surfaces using two-dimensional arrays of micron-sized colloidal particles. Suspensions of colloids functionalized with proteins are deposited onto coverslips coated with gold using a combination of gravitational settling and applied electrical fields. Varying settling time and particle concentration controls the density of particles on the substrate. Surface coverage ranged from an essentially continuous coating of protein on close-packed arrays to domains of protein separated by distances as large as 16 ím. Colloidal particle arrays were also patterned into 500 µm islands on substrates using elastomeric lift-off membranes. The applicability of this approach to the promotion of fibroblast cell adhesion and spreading was demonstrated using particles coated with the cell adhesion protein fibronectin. Behavior of adherent cells varied with particle density. This method provides a general strategy for controlling the organization of functional proteins at surfaces on three length scales: the size of individual colloidal particles, the spacing between particles, and the organization of particles in patterned arrays.
The patterning of sol-gel-derived thin films by micromolding in capillaries can produce unintended topographical deviations from the shape of the original mold that may limit the utility of the technique in potential applications. During drying and heat treatment, nonuniform shrinkage across the film due to the densification of the gel matrix results in “double-peak” film topographies whereby the film thickness is greater at the lateral edges than in the middle. Using the same framework used to understand the imbibition and wetting of the sol-gel in the capillary channels, we developed a mechanism to explain the formation of the double-peak profile. As a model system, patterned Pb(Zr0.52Ti0.48)O3 thin films were studied. Atomic force microscopic characterization was used to quantify the effect of the rate of gelation on the topography of the patterned thin films. Modifications to the channel mold design eliminate the peak formation, producing more homogeneous patterns that better replicate the features of the mold.