In this review the phenomenon of ultrasonic cavitation and associated sonochemistry is presented through system parameters. Primary parameters are defined and considered, namely; pressure amplitude, frequency and reactor design; including transducer type, signal type, vessel-transducer ratio, liquid flow, liquid height, liquid temperature and the presence of a reflective plate. Secondary parameters are similarly characterised and involve the use of gas and liquid additives to influence the chemical and physical environments. Each of the parameters are considered in terms of their effect on bubble characteristics and subsequent impact on sonochemical activity. Evidence suggests that via parametric variation, the reaction products and efficiency may be controlled. This is hypothesised to occur through manipulation of the structural stability of the bubble.
Advances made in recent years have allowed the application of colorants obtained from natural sources into textile dyeing. The use of ultrasound in the dyeing method is reported to increase dye uptake and decrease dyeing times. The aim of this work is to further extend the knowledge of natural hair dyes considering the use of ultrasound in the dyeing method with commercially available herbal dyes and using goat hair as a model for human hair. Optimal ultrasonic parameters were selected by considering the effects of sonication times (5, 10 and 15 min), frequencies (44, 400 and 1000 kHz) and total dyeing times (30, 60 and 120 min) in the morphology of the dyed hair and the colour intensity. Damage to the hair surface was evaluated by scanning electron microscopy (SEM) images, differences in colour of the dyed hair was obtained by ImageJ analysis and quantification of dye uptake was determined by UV-visible spectroscopy. The evidence from this study suggests an increase in goat hair coloration with the use of ultrasonic energy. Optimal dyeing conditions in consideration of colouration efficacy without hair damage were identified as sonication at 400 kHz for 10 min with a total dyeing time of 60 min.