Controlled acoustic nozzle-free droplet generation of glycerol-water mixtures and TEMPO-oxidised cellulose nanofibre suspensions

Ultrasound acoustic generators are a practical and inexpensive technology for the ejection and deposition of viscous droplets controlling a wide range of droplet sizes. However, the interplay of acoustic ejectors and liquid properties remains largely unexplored. In this work, we manufacture two acoustic droplet ejectors of the Fresnel zone plate type (2.3 MHz and 4.0 MHz). Glycerol-water (GW) Newtonian mixtures and 2,2,6,6-tetramethylpiperidine-1-oxyl radical (TEMPO)-oxidised cellulose nanofibre (TOCNF) suspensions with remarkable rheological shear-thinning and extensional properties are prepared. We study the formation of multiple and single acoustic-generated droplets of GW mixtures with 42% and 54% v/v and TOCNF suspensions by varying their solid content from 0.1% to 0.6% w/v at a constant pH of 7.8. The analysis of droplet ejection reveals that the GW mixture with the lowest viscosity (42% v/v) and TOCNF suspensions with a relatively low concentration (0.1%-0.4% w/v) generate either multiple or single droplet ejection. In contrast, single droplets are more frequently generated for GW at 54% v/v, while single droplets are persistently generated for TOCNF at higher solid contents (0.5%-0.6% w/v). We find that the highest concentrated TOCNF suspensions constantly generate single droplets due to their shear-thinning behaviour and without inducing elastic effects by traditional polymer modification.