A multichannel flow cell is used to laminarly flow different chemical solutions, including one made up of small polystyrene beads attached to individual DNA molecules, side by side with little mixing. An optical trap is used to pull single DNA molecules via their attached polystyrene beads into each of the different chemical solutions or species sequentially, and the resultant change in the structure of the DNA molecule can be observed using fluorescence microscopy. The technique can be used with molecules other than DNA. Examples of different chemical species include condensing agents such as protamine, enzymes, polymerases, and fluorescent probes and tages.
 The United States Government has rights in this invention pursuant to Contract No. W-7405-ENG-48 between the United States Department of Energy and the University of California for the operation of Lawrence Livermore National Laboratory.