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

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 ultrasound process on a cellulose nanocrystal (CNC) suspension. Dedicated to in situ Small-Angle X-ray Scattering (SAXS) and Particle Image Velocimetry (PIV) measurements, channel-type filtration cells coupled with US have been 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 pressure demonstrated a change in CNCs orientation as a function of the distance from the membrane surface to the vibrating blade, with the appearance of an orthotropic organization: CNCs are vertically oriented near the vibrating blade, then have an isotropic organization in the middle and exhibited horizontal orientations near the membrane surface. In this work it has been emphasis that this orthotropic organization appear above a threshold in transmembrane pressure of about 0.6 × 10⁵ Pa. Concurrently, in situ micro-PIV measurements revealed the formation of Rayleigh acoustic streaming in CNCs suspension, for the same threshold in transmembrane pressure and same US conditions, highlighting the origin of the orthotropic organization. It has been proposed that this threshold allows for sufficient accumulation of CNCs near the membrane surface, thus enabling confined flow, to generate the acoustic streaming. This work highlights the interplay between acoustic streaming and CNC particle orientations, advancing understanding of liquid crystal-like suspensions manipulation in microfluidic applications.