Colloquium

A balance of repulsive and attractive molecular forces drives natural assembly of molecules into complex organic and inorganic assemblies with dimensions from nanometers to microns. Our current nascent efforts to mimic nature involve harnessing these same forces to produce synthetic assemblies of the same colloidal-scale dimensions, while at the same time using external fields to bias the assembly. Common fields are gravity and surface tension as well as alternating electric fields, which easily move and guide colloidal particles by dielectrophoresis. The range of possibilities for engineering materials will be illustrated with several examples. First, colloidal crystals are formed and used as templates for synthesis of nanoporous metals. Second, electric fields between planar electrodes on either side of a gap are used to form wires and crystals. The wires are made of metallic nanoparticles and have micrometer diameter and millimeter length. The crystals feature single domains of specific orientation and areas larger than 25 mm2. Finally, surface tension is used to shape objects with structure on nanometer, micrometer and millimeter scales.