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Polymers Under Constraint

fd virus is a polymeric virus 1 mm in length and 10 nm in diameter. We bind fluorescently labeled fd to 1 mm diameter polystyrene spheres creating a charged polymer stabilized colloid (hairy bead) and measure the interparticle potential using a double laser trap. We first measure the interaction energy of (a) bare beads and (b) then the hairy beads, seen here in fluorescence microscopy. (c) Electron micrograph of hairy beads. The repulsive energy of hairy beads is large when the beads are close. (d) We implement a double laser trap and employ an algorithm developed for computer simulations to measure the interaction potential as a function of ionic strength.

"Bottom-up" self-assembly is a long term goal of nanotechnology. We are addressing this problem by using genetic engineering techniques to produce rodlike molecules with specific binding sites. The particles we begin with are a filamentous virus, which by itself will self-assemble into liquid crystalline phases consisting of either the smectic phase, where the rods assemble into stacks of layers with the rods parallel to the layer normal, or the nematic phase, where the rods remain as a fluid, but all point in the same direction. By attaching spherical particles to the virus in systematic ways we can alter the phase behavior to form structures consisting of layers of spheres alternating with rods, or phases consisting of the rods assembling into large cylinders, or more exotic structures. These phases will find applications as selective filters, materials with special elastic behavior, solar cells and biosensors, as well as contribute to our knowledge of self-assembly. Quantitative measurements of the pair-potential indicate that the rod-rod repulsion is electrostatic in origin, arising from compression of the double-layer of counter-ions within the overlapping brush.
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