Cavitation usually refers to the generation of cavities and the subsequent dynamic behaviors when a liquid suffers from a sufficient pressure drop. According to the mode of production, cavitation can usually be divided into acoustic cavitation, hydrodynamic cavitation, light cavitation and particle cavitation. It has been widely used in sonochemistry, biomedical, environmental science and many other fields.
Acoustic cavitation is usually high intensive but limited to the throughput, while hydrodynamic cavitation as an alternative method is easy to be scaled up but limited to the intensity.
A bubble initially at rest can grow during the rarefactional half cycles of an applied acoustic field. Through rectified diffusion, gas and vapour are transported into the bubble, until it reaches a critical size and a range of secondary effects that drive processes, and may also be measured.
where p is pressure, P is the density, V is the velocity, h is elevation, and g is the gravitational acceleration.
When the generated vapor cavities meet a zone with higher pressure, such as downstream a nozzle, they undergo fast collapse (implosion), thereby concentrating the kinetic energy of the bulk medium into very small size hot spots. Temperature and pressure inside a collapsing bubble increase dramatically up to 5,000-10,000 K and 300 atm, respectively, due to the work done by the liquid to the shrinking bubble, producing very strong shear forces, micro-jets and pressure shockwaves.
The use of hydrodynamic cavitation as a bottom-up method for continuous creation of foams comprising of air microbubbles in aqueous systems containing surface active ingredients, like proteins or particles. The Aquastaser hydrodynamic cavitation device is created using a converging-diverging nozzle.
Aquastaser Hydrodynamic cavitation can be simply generated by the alterations in the flow field in this high speed/high pressure device and also by passage of the liquid through a constriction such as orifice plate/venturi. Hydrodynamic cavitation results in the formation of local hot spots, release of highly reactive free radicals, and enhanced mass transfer rates due to turbulence generated as a result of liquid circulation currents.
The principle of hydrodynamic cavitation (HC) is based on the passage of the liquid through a constriction (orifice plate, venturi), resulting in an increase in liquid velocity at the expense of local pressure, as well as a fall in pressure around the point of vena contracta below the threshold pressure, which supports the formation of cavities. The subsequent implosion of cavities takes place due to the expansion of liquid jets at the downstream of constriction as the pressure recovers. The generated cavities go through transient collapse events while generating a temperature of up to 10,000 K, as well as high-pressure shock waves and free radicals, which could induce physical and chemical changes to the target matrix.
Due to the industrial revolution, a large number of harmful industrial dyes, pharmaceutical, and organic contaminants are contributing to environmental pollution, which are known for their toxic and bio-accumulative nature. There is mounting evidence on the link between pollutant exposure and human chronic diseases, such as cancer, diabetes, Parkinsons, and Alzheimers [57,58]. The complete elimination of recalcitrant pollutants is vital, but most of the pollutants are resistant towards conventional wastewater treatment technologies, such as biodegradation, adsorption, filtration, sedimentation, and chlorination. Even though membrane-based processes could eﬀectively remove certain persistent pollutants, subsequent incineration is required to destroy the concentrated pollutant in a later phase. Therefore, a cost-eﬀective one-step remediation process that can simultaneously mineralize the organic pollutant and disinfect harmful microorganisms without any additional step is of primary need. Global interest is rising in the implementation of advanced oxidation processes (AOPs), such as sonolysis, ozonation, and electrochemical-based water treatment methods.
However, sonochemical and electrochemical-based treatments are not cost-eﬀective for large-scale water systems. Cavitation-based AOP, such as hydrodynamic cavitation, could be a viable water treatment solution with a high potential impact on the operation cost. The mineralization of the pollutant via cavitation can be possible either by thermal decomposition at the inner or interfacial region of bubble or via radical attack in the bulk solution or at the bubble–liquid interfacial region. Cavitation is known to generate hydroxyl radicals (OH•) following the dissociation of water molecules. HC has been utilized for wastewater remediation, and is, hence receiving unprecedented attention. Numerous reports on the removal of persistent organic pollutants, pharmaceuticals, dyes, cyanobacteria, viruses, bacteria, and microalgae have demonstrated its successful application.