team called “declumping” the fibrillation of the bulk cellulose diven by by both AC and HC (Figure 1). Indeed, ultrasonically processed cellulose was completely fibrillated into individually separated fibrils less than 100 nm thick due to the more intense cavitational collapse and absence of fluid flow of acoustic cavitation, whereas hydrodynamic cavitation resulted into full cellulose fiber-fiber detachment with some fibers at nano level and most of the fibers of micrometer thickness.
Both forms of cavitation reduced cellulose crystallinity from 87% to 38%. Such dominant presence of amorphous domains enhances the flexibility and plasticity of the material, and lowers both stiffness and elasticity.42 The particle size decreased to 1.36 µm and 0.3 µm for the hydrodynamically and ultrasonically processed cellulose samples, respectively. whose thermal stability was significantly higher of the newly obtained nanocelluloses due to decrystallization.
We briefly remind that the cavitation bubbles generated in water via acoustic or hydrodynamic cavitation upon collapse locally release short-lived (µs duration) shockwaves of extreme pressures (1000-2000 atm) and temperatures (5000 K) that are ideally suited for the extraction of natural products.43
Two years later, Paquin and co-workers in Quebec reported that by carrying out the aminoxyl-radical mediation oxidation of a diluted suspension (1 wt%) of bleached hardwood kraft pulp in a 45 L flow-through sonoreactor, a 87.5% decrease in energy consumption (in comparison to the process in batch reactor) and a 95% higher production rate of oxidized fibers (compared to reaction without any ultrasound in batch mode) are achieved. These results, the team concluded, suggest the possibility of scaling up the process on industrial scale directly in continuous mode.44
The original discovery of Pandit and Pinjari using no added chemicals to fibrillate crystalline cellulose, however, remained without further investigation until the teams of Meneguzzo and Pagliaro demonstrated the process on semi-industrial scale by processing via HC more than 30 kg of citrus (lemon and grapefruit) processing waste in 120 L water.45