DNA molecule dynamics in converging-diverging microchannels.

The conformation and dynamics of double-stranded DNA molecules in a pressure-driven flow-through sharp/gradual converging (contraction half)-diverging (expansion half) square microchannels with a contraction/expansion ratio of 4:1/1:4 were examined using fluorescently labelled lambda-phage DNA at 2.2< or =De< or =30.7 (De is Deborah number, a dimensionless number used in rheology to characterize how fluid a material is). Both the diffusion and stretching of individual DNA molecules and their local flow velocity were visualized, recorded and measured with using MPIV (microparticle image velocimetry) and CLSM (confocal laser-scanning microscopy). The onset of the coil-stretch transition in DNA molecules and their time-dependent velocity in the elongational flow were determined at the micro scale. Velocity distribution was measured via MPIV through orange fluorescent particles and carboylate-modified microspheres of 1 mum diameter. The channel geometry effect in terms of the channel contraction/expansion angle, the mass flow rate and the solution viscosity on the conformation and diffusion were also studied and discussed.