The flow resistance of single DNA-grafted colloids as measured by optical tweezers

In rheological experiments, the flow resistance of single blank or DNA-grafted colloids is determined and compared. The length of the double-stranded-DNA varies amongst 1,000, 4,000 and 6,000 base pairs (bp) corresponding to contour lengths varying between 340 and 2,040 nm at a grafting density of 0.03±0.01 μm2/chain. The degree of swelling of the grafted DNA is adjusted by exchanging the ion concentration of the surrounding medium. For all examined flow velocities ranging between 100 and 1,200 μm/s, one observes an interesting deviation from Stokes law which can be traced back to a shear-dependent conformational change of the brush layer. The ratio of the effective hydrodynamic radii of DNA-grafted and blank colloids shows a pronounced dependence on the flow velocity, but as well on the length of the grafted DNA and the ionic strength of the solvent. The experimental findings are in qualitative agreement with hydrodynamic simulations based on an elastically jointed chain model.