Direct observation of DNA molecules in a convection flow of a drying droplet

The motion of single T4 DNA molecules ( 166 kilo-base pairs) was studied by fluorescence microscopy in drying droplet of polyethylene glycol ( PEG) solution on glass surface. An ordered ow was observed in the vicinity of the droplet contact line. A high stretching of the DNA molecules was found at the droplet periphery, whereas in its central part the DNA molecules were in random coil state. With the increase of PEG concentration, individual DNA molecules show collapse transition directly from the stretched conformation after the formation of nuclei at the ends of the chain. Without PEG all the DNA molecules were in random coil state and slowly di used to the droplet boundary. They were trapped in narrow region near the contact line.