Effects of ultrasound on colloidal organization at nanometer length scale during cross-flow ultrafiltration probed by in-situ SAXS

Effects of ultrasound (US) on the structural organization within concentrated particle layers during cross-flow ultrafiltration of Laponite dispersions have been characterized for the first time by in-situ time-resolved small-angle X-ray scattering (SAXS). A novel ‘SAXS Cross-Flow US-coupled Filtration Cell’ has been developed to, on one hand, apply ultrasonic waves close to the flat membrane by embedding in the feed compartment a thin titanium vibrating blade connected to a 20 kHz ultrasonic generator and on the other hand, to monitor in-situ the colloidal organization of the concentrated layer by SAXS. Thanks to this cell, concentration profiles have been measured as a function of the distance z from the membrane surface with a 20 µm accuracy and simultaneously linked to the permeate flux, cross-flux and transmembrane pressure. In-situ ultrasonication leads to a significant increase of permeate flux arising from the break-up of the concentrated layer. Results also suggest that ultrasonication could be considered as an additional force of an effective range on the order of micrometers or smaller. It is capable to completely remove the particles from the membrane surface when the feed dispersions are dense and aggregated, and is more efficient than classical procedures based on an increase of the cross-flow flux.