Background
Ultrasound is generally applied to process liquids through the use of a high frequency vibrating solid surface in intimate contact with the liquid. This generates compression waves through the liquid and as well as setting up liquid streams directed away from the vibrating surface.
Above a certain threshold of intensity the compression waves passing through can interact with liquid, usually through contact with tiny bubbles inside the liquid, to generate cavitation.
This occurs when a succession of waves act to grow the tiny bubble to a size at which it becomes unstable and suddenly collapses in an event that generates an intense release of energy. These localised cavitation events are the main work horse of mechanical ultrasonic processing and can be used to stimulate nucleation as well as providing a mechanism to mill down, de-agglomerate or round existing particles within the liquid.
The power used to drive ultrasound into the liquid is ultimately dissipated as heat within the liquid through the friction generated by the liquid motion and cavitation events. Power input to the liquid is controlled by several parameters, in particular the frequency, amplitude and area of the vibrating surface. For mechanical ultrasound, lower frequencies are more efficient at directing power into desirable cavitation events. In particular 20 kHz is the usual frequency applied and lies at the bottom end of the ultrasonic range. In general the geometry of the equipment used to generate ultrasound is designed to suit a set frequency and will have a fixed vibrating surface area. As such power input is usually controlled by altering the amplitude of the vibration and the frequency only fine tuned to minimise transmission losses.
The optimal choice of power used for processing depends on several factors. At low amplitudes most of the power is dissipated in the movement of the liquid, without generating cavitation events and as such can provide some mixing but little of the mechanical processing required for nucleation or milling. As amplitude increases cavitation events start to occur within the liquid and power is directed into these events to provide the mechanical processing benefits. However, above a certain threshold, liquid cannot maintain contact with the vibrating surface and starts to decouple. At this point power transmission into the bulk of the liquid can no longer increase and instead additional power is focused at the liquid solid interface. This decoupling of the liquid and solid surfaces leads to the generation of local mechanical erosion through cavitation close to the vibrating surface, a problem commonly observed with ultrasonic probe based technology. See images of eroded probes above.
Benefits of the ProsonitronTM Technology
In conventional probe technology power is supplied by the use of a vibrating cylindrical device. The relatively small surface area and high amplitude of the probe systems generates high surface intensity that easily decouples and promotes cavitation close to the tip of the probe with little penetration into the bulk of the liquid. This leads to rapid erosion of the probe and the shedding of probe material into the process liquid - see eroded probes images above. The localised cavitation can also produce problems in achieving even processing of the liquid, with different fluid elements getting different processing experiences.
In contrast the Prosonitron technology provides ultrasonic power input through an array of bonded transducers mounted on the outside of a cylindrical 
pipe. This spreads the power input over the whole surface of the vessel allowing significant power input for a low local surface intensity. The first benefit of this is to greatly reduce surface erosion and any associated contamination. The second and more striking benefit is through the geometry of the vessel itself. As the waves travel radially into the liquid the intensity of the waves increase and carries the cavitation deep into the bulk of the liquid, and in fact results in the strongest cavitation occurring at the centre of the pipe. The image to the left shows acoustic cavitation measurements made with the National Physical Laboratory (UK) broadband acoustic sensor. Compared with cavitation occurring only at the tip of a probe this provides an environment for highly uniform ultrasonic processing whilst minimising the negative effects such as surface erosion.
