Concentration Profiles of Colloidal Fluids in One-Dimensional Confinement

Abstract

An X-ray diffraction method has been developed to derive concentration profiles of fluids under one-dimensional confinement. A microfluidic array of channels is used as a container for the fluid under investigation. Thereby the ensemble average of many channels is studied rather than individual realizations of the system. Additionally, the X-ray scattering signal is enhanced and the radiation dose per fluid volume reduced. As an example, data are shown for a colloidal solution of silica particles of about 109 nm diameter under confinement in channels ranging from 280 to 615 nm width. The density profiles across the channels have been determined in a model independent way with a resolution in the 10 nm regime. When the electrostatic potential of the colloidal particles is shielded, we observe an oscillatory ordering-disordering behaviour of the colloidal particles as a function of the channel width, while the colloidal solution remains in the liquid state. This phenomenon has been suggested by surface force studies of hard-sphere fluids and also theoretically predicted, but has so far never been seen by direct measurements of the structure for comparable systems. The size of the fluid's constituents and of the confining cavities can be scaled down to below 100 nm, opening up the possibility for the investigation of confinement effects in e.g. protein crystallization.