Soft Matter and Complex Systems Seminar

DNA is a semi-flexible polyelectrolyte with a persistence length of about 50 nm. At equilibrium, entropic forces cause the individual molecules to form compact spherical coils, which can be easily stretched by a weak shear flow into a cigar-like conformation that is not, on average, aligned with the flow direction. Because the electrophorectic mobility of the sheared polyelectrolyte is no longer isotropic, DNA can be driven to the walls of a confining channel by an electric field that pushes the DNA in the opposite direction to the flow. The migration velocity is sufficient to keep the DNA in a thin layer next to the wall. With a suitable choice of field strengths, the DNA can be driven against the flow (since it is next to the channel walls), while all the other components of a cell lysate are flushed in the opposite direection by the flow. This is a much stronger separation than in typical microfluidic processes, which rely on mobility contrast between the species. A simple microfludic device, assembled from acrylic sheets for less than $1, can provide a chemical-free purification of DNA. In this talk I will outline the physics underpinning the separation and describe experiments that purify DNA in sufficient quantities (up to 40 ng) for PCR amplification and gel electrophoresis.