Visualization of individual DNA molecules in a small-scale coating flow

Individual DNA molecules in an ultradilute solution were observed with a fluorescence microscope as they flow between a scaled-down rotating roll and a stationary glass knife. The roll picks up a thin layer of liquid from a pool and drags it to the knife, establishing a bead delineated by two menisci. At low roll speed the flow is premetered and there is a large recirculation. The DNA experiences nearly rectilinear shear flow at the minimum gap position where there is a zero velocity surface. We report the mean and the distribution of fractional extension of DNA molecules and show that the mean fractional extension asymptotes to 0.5, in agreement with the results of Smith et al. [D. E. Smith et al., Science 283, 1724 (1999)]. Interestingly, no polymer distortion is observed at the two menisci. At high roll speed, capillarity is not strong enough to drive backflow; the big recirculation under the coverslip breaks into two smaller recirculations and two separation surfaces arise upstream and downstream of the location of the minimum gap. At the upstream separation surface, most DNA molecules are extended parallel to the knife as they traverse the field of view. We report the distribution of DNA extension and shape in this flow region. Slow, nodular recirculations are present under the upstream and downstream free surfaces. Notably, most DNA molecules stretch axially as they move in these slow recirculating regions. (C) 2004 The Society of Rheology.