Novel concept of hydroxyl radical generation in hydrodynamic cavitation on a chip

In this study, the hydrodynamic cavitation on a chip reactor (HC on a chip), facilitated by two configurations: a micro-orifice and a long diaphragm, was utilized to produce the reactive species, particularly hydroxyl radicals (center dot OH). The micro-orifice configuration allowing intense localized pressure gradients promotes intense cavitation events, leading to higher yields of center dot OH per unit energy input (0.6 x 10-6 g/J in comparison to 3.0 x 10- 8 g/J for the long diaphragm configuration in the cavitation inception regime). This is advantageous for applications requiring concentrated center dot OH production, such as advanced oxidation processes. In contrast, the long diaphragm reactor, despite a larger power input (6.4 W at 2.9 MPa compared to 1.5 W for the micro-orifice configuration), provides a more uniform distribution of radicals along its length, based on the temporal trend of I3- formation. This shows a gradual and sustained rate compared to the sharp, early peak in the micro-orifice reactor. The lower reaction in Reactor 2 indicates that radical formation occurs over a wider area rather than being localized. This aligns with the geometry and flow pattern of the diaphragm which allows for a larger zone of cavitation and longer bubble activity. Comparative analysis reveals that microscale reactors demonstrate higher reaction rates and sharper peaks in I3- production than macroscale reactors, which show higher chemical activity. The orifice possesses the highest maximum peak in the microreactors, and the long diaphragm releases more uniformly distributed radicals, with microscale systems overall having higher energy efficiency and lower energy costs.