Forming Three-Dimensional Colloidal Structures Using Holographic Optical   Tweezers
  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. 
C. R. Knutson, J. Plewa