Nanoscale measurement of apparent slip velocity near a moving contact line

We report the nanoscale flow measurements within tens of microns from a moving contact line on hydrophobic substrates. A moving contact line was generated using a liquid bridge instability induced by retreating syringe. Contact line speeds ranging from 0.15 to 3 mm/s were recorded. The motions of tracer nanoparticles were measured using two independent experimental techniques: multi-layer flood illumination and Total Internal Reflection Fluorescence Microscopy. The flow field was derived using a novel probabilistic particle tracking velocimetry, which allows the accurate estimation of the rapidly changing flow field near a contact line without bias due to binning or fitting. The results confirm that for distances larger than a few microns from the contact line, the velocity field scales with the instantaneous contact line speed and agrees well with the corner flow solution predicted by the biharmonic equation. A significant slip velocity is shown to exist close to the contact line, decaying rapidly within a few microns.