Microfluidic mobility of single (DNA-grafted) colloids in dilute DNA suspensions

By employing single particle tracking on the basis of fast video microscopy, we directly measure the equilibrium short and long time diffusion coefficients of blank and DNA-grafted (molecular weight of 250, 1000 and 4000 base pairs (bp)) tracer colloids in dilute λ-DNA solutions. We compare these results to recently reported microfluidic mobilities under non-equilibrium steady state flow and to the predictions of quantitative theoretical models. A pronounced discrepancy between the measured equilibrium and non-equilibrium mobilities is observed for colloids grafted with 4000 bp DNA molecules, revealing the inapplicability of the Stokes–Einstein relation under these conditions. We demonstrate that this deviation may be interpreted as a strong micro-shear-thinning effect that is caused by the formation of entanglements between the grafted DNA brush and λ-DNA coils from the suspension.