Rayleigh streaming phenomena at the physical origin of cellulose nanocrystals orientations during combined ultrasound and ultrafiltration processes

Abstract

Rayleigh acoustic streaming, a phenomenon resulting from the interaction of ultrasound (US) with a fluid, was revealed for the first time during simultaneous frontal filtration and US processes on a cellulose nanocrystal (CNC) suspension. According to in situ small-angle X-ray scattering (SAXS) and particle image velocimetry (PIV) measurements, channel-type filtration cells coupled with US were developed to simultaneously generate a vertical acoustic force via an ultrasonic vibrating blade at the top and to concentrate the CNCs under a transmembrane pressure force at the bottom. SAXS measurements under different transmembrane pressures demonstrated a change in CNCs orientation as a function of the distance from the membrane surface to the vibrating blade. These measurements led to the appearance of an orthotropic organization: CNCs were vertically oriented near the vibrating blade, then had an isotropic organization in the middle, and exhibited horizontal orientations near the membrane surface. This orthotropic organization appeared above a threshold in transmembrane pressure of ∼0.6 × 10⁵ Pa. Concurrently, in situ micro-PIV measurements revealed the formation of Rayleigh acoustic streaming in the CNCs suspension, for the same threshold in transmembrane pressure and same US conditions, thereby highlighting the origin of the orthotropic organization. We propose that this threshold allows for sufficient accumulation of CNCs near the membrane surface, thus enabling confined flow, to generate acoustic streaming. This work highlights the interplay between acoustic streaming and orientations of CNC particles, thereby advancing understanding of the manipulation of liquid crystal-like suspensions in microfluidic applications.