Energy Landscapes of Small, Fixed Numbers of DNA-coated Colloids

We experimentally investigate clustering of small numbers of colloidal spheres with short-range attraction via surface-grafted DNA-DNA molecular recognition. We coat 1 micron diameter spheres with 20nm long DNA strands; particles with different surface-bound DNA sequences represent different particle ``types.'' Pairwise interactions between some types are attractive but interactions between other types are purely repulsive. In a typical experiment, 6 total particles of 3 different types explore a volume less than 100 picoliters and assemble into equilibrium clusters. We optically characterize the structures and compare ensemble averages with statistical mechanical predictions.