  A method for forming permanent three dimensional structures from colloidal
particles using holographic optical trapping is described. Holographic optical
tweezers (HOT) are used to selectively position charge stabilized colloidal
particles within a flow cell. Once the particles are in the desired location an
electrolyte solution is pumped into the cell which reduces the Debye length and
induces aggregation caused by the van der Waals attraction. This technique
allows for the formation of three dimensional structures both on and away from
the substrate that can be removed from solution without the aid of critical
point drying. This technique is inexpensive, fast, and versatile as it relies
on forces acting on almost all colloidal suspensions.
