A depletion layer in polymer solutions at an interface oscillating at the subnano- to submicrometer scale

The mobility of segments of the polymer mesh in a solution determines the dynamic response of the depletion layer (DL) to mechanical stimuli. This phenomenon can be used to vastly decrease the local viscosity experienced by any device performing periodic motion at the nano- and microscale in complex liquids. We refined the vibrating quartz tuning fork (QTF) method to probe the viscosity of model aqueous solutions of polyethylene glycol, covering a broad range of molecular weights (3 kDa to 1 MDa) and QTF oscillation amplitudes (50 pm to 100 nm). For semidilute solutions of PEGs of high molecular weight, we found a drop of local viscosity, up to two orders of magnitude below the bulk value. We propose a simple explanation based on the motion of the depletion layer, strongly supported by rheometry and dynamic light scattering results. We show that it is possible to directly probe the viscosity of the DL and increase its thickness far above the equilibrium value. The key role is played by the rate of relaxation of the entangled system. The relevance of this paradigm ranges from the basic research on dynamics of entangled systems to design of energy-efficient nanomachines operating in a crowded environment.