Sonic disruption of wood pulp fibres aided by hydrophobic cavitation nuclei

For paper manufacturing and biofuel production, the controlled deformation of wood pulp is of interest, provided that the integrity of the fibre structure remains intact. Conventional ultrasonic pretreatment in the near-audible range has been observed to cause uncontrolled inertial cavitation damage in the wood pulp fibres. To prevent internal damage, we proposed to subject wood pulp mixed with hydrophobic particles to 1-MHz short pulses above the nucleation threshold of the particles but below the Blake threshold, and to observe the interaction of pulsating cavities and wood pulp fibres assisted by high-speed photography. Our 1-MHz results showed the interaction of a collapsing bubble with a wood pulp fibre wall to form a liquid jet hitting the fibre, without apparent destruction of the structure, whilst our 20-kHz controls confirmed previously observed structural destruction. This study shows the feasibility of controlled wood fibre deformation at a high ultrasound frequency.