## 2 Principles of Cavitation-Based Extraction  
If the cavitation event occurs, due to the passage of ultrasound waves in the liquid medium, then it is termed as acoustic cavitation, whereas if it occurs, due to the pressure variations in the flowing liquid concerning the change in the geometry of constriction, then it is called as hydrodynamic cavitation (HC). Introduction of ultrasonic waves of certain frequencies (20-1000 kHz) can create bubbles followed by the collapse events [4]. In case of HC, when the liquid passes through the constriction, the velocity of the liquid increases at the expense of local pressure and the pressure around the point of vena contracta falls below the threshold pressure (usually the vapor pressure of the medium at the operating temperature), resulting in the formation of cavities. At the downstream of the constriction, as the liquid jet expands, the pressure recovers which results in the collapse of cavities [5]. NPC is also considered as a part of hydrodynamic cavitation, where the creation of negative pressure governs the cavitation phenomena via a vacuum pump and a continuous air-flow maintains the turbulence through the reactor. Figure 1 shows the types of cavitation-based extraction, where figure (a) represents the ultrasound-assisted extraction (UAE), figure (b) represents negative pressure cavitation (NPC) extraction and figure (c) demonstrates venturi type hydrodynamic cavitation (HC) extraction. Considering UAE reactor, an ultrasonic generator and a probe have been implemented to create cavitation phenomena. NPC device consists of an extraction pot, a collection pot, a heating system, a vacuum pump and a condenser, whereas the venturi type hydrodynamic cavitation (HC) reactor ideally consists of a feed tank, a plunger pump for circulating the solution through the reactor, a pressure gauge and the cavitation chamber consisting of an orifice or a venturi tube as the constriction to generate bubbles. The collapse of cavities can generate temperatures of up to 5000 K inside the bubble core along with the production of highly reactive free radicals owing to the homolytic cleavage of water molecules and dissolved gases, such as oxygen [6]. The cavitation events generate transient bubbles, and their successive collapse generates several physical effects, such as turbulence,shear forces, shock waves, and microjets. The overall mechanism behind the enhancements as observed in the extraction of natural products via cavitational effects can be summarized below:  
1. Increased mass transfer rate and enhanced solvent penetration into the cells, due to the temperature and pressure generated during bubble collapse events resulting in thinning of membranes and disruption of cells.
2. Enhanced diffusion caused by microscopic level turbulence, intense inter-particle collision, and agitation in microporous particles of the matrix, due to the implosion of cavitating bubbles.
3. Enhanced diffusion of solvent into the matrix, due to hydration and swelling of the matrix with the enlargement of pores.
4. Generation of highly reactive free radicals and the associated radical driven cell disruption.
5. The increased surface area of matrix following disintegration by shock waves and microjets.  
The already reported results indicated the formation of micro-fissures, microchannel and the generation of pores on the matrix surface leading to improved permeability. Enhanced accessibility of the solvent to the internal structure of cells facilitates the release of target compounds and their diffusion from the matrix to the solvent increases to manifold [7].