2.2. Atomization of nanofluids

When nanofluids are applied to NMQLC grinding, studies on their atomization modes are mainly divided into three types: pneumatic atomization, electrostatic atomization, and ultrasonic atomization.

Pneumatic atomization is a process in which the liquid is sprayed in the machining region after small liquid drops are formed through high-pressure gas atomization and it is the most commonly used method for the atomization of nanofluids. Influenced by external forces like aerodynamic force, the liquid will be split while surface tension of the liquid will make the liquid form spherical liquid drops which are also influenced by viscosity. When external force born by the liquid is greater than liquid surface tension and viscous resistance, force balance of the liquid will be broken and the liquid will be split into fine liquid drops which will form nanofluid drop shapes when they are small enough [28].

Besides high-pressure gases are used for atomization of nanofluids, researchers have also put forward electrostatic atomization and ultrasonic atomization. The formers refer to atomizing the liquid drops under electric charge through high-voltage electrostatic field after nanofluids are sprayed out from the nozzle so as to form electrically charged liquid drop flocks, which will be controllably and orderly transported onto workpiece surface under the effect of electric field force. For ultrasonic atomization, micro shock waves are generated during ultrasonic cavitation and bubble closing process in the liquid to damage the interaction between liquid molecules and then liquid particles are dragged out from liquid surface to form liquid drops [29].
