Breakdown of Stokes-Einstein: probing size-dependent viscosities in liquids

We propose to measure the viscosity of liquids by means of microrheology-based X-ray photon correlation spectroscopy (XPCS). In contrast to techniques using visible light, XPCS is not limited by the particle size. This allows measuring particle dynamics down to molecular length scales where many studies found indications for a violation of the Stokes-Einstein law in water. Thanks to recent advances in beamline and detector technology, and most importantly the synthesis of tracer particles with very narrow size distribution and tunable radius, precise measurements of viscosity will become possible with the required spatial and temporal resolution. Therefore, we plan to measure the dynamics of nm-sized particles in (1) water, (2) polymer melts and (3) liquid porphyrins.