Anisotropic de Gennes narrowing in confined fluids

The collective diffusion of dense fluids in spatial confinement was studied by combining high-energy (21 keV) x-ray photon correlation spectroscopy and small-angle x-ray scattering from colloid-filled microfluidic channels. We found the structural relaxation in confinement to be slower compared to bulk. The collective dynamics is wave vector dependent, akin to de Gennes narrowing typically observed in bulk fluids. However, in stark contrast to bulk, the structure factor and de Gennes narrowing in confinement are anisotropic. These experimental observations are essential in order to develop a microscopic theoretical description of collective diffusion of dense fluids in confined geometries.