Ultrasound Emulsification in Microreactors: Effects of Channel Material, Surfactant Nature, and Ultrasound Parameters

Ultrasound emulsification, including the emulsification
mechanism
as well as the stability and size distribution of the prepared emulsion,
was systematically investigated in laboratory-built ultrasonic microreactors.
The respective cavitation-induced emulsification processes in hydrophilic
and hydrophobic microchannels were first analyzed, after which a bubble
shuttling and dispersing mechanism was proposed. The type of prepared
emulsion was found to be strongly associated with the hydrophilic–lipophilic
balance (HLB) value and water/oil ratio of the emulsified system,
but was hardly affected by the ultrasound parameters. The size distribution
of the prepared emulsion was characterized using both microscopy and
dynamic scattering technique, and was measured to be in the range
of 700–1800 nm. It was also found that higher ultrasound intensity,
lower frequency, and larger residence time are more favorable to producing
fine emulsion. Finally, an operation strategy was proposed on the
basis of the experimental results, expecting to provide recommendations
for the application of ultrasonic microreactors for continuous emulsification.