Kelvin-Helmholtz instability as one of the key features for fast and efficient emulsification by hydrodynamic cavitation

Abstract

The paper investigates the oil-water emulsification process inside a micro-venturi channel. More specifically, the possible influence of Kelvin-Helmholtz instability on the emulsification process. High-speed visualizations were conducted inside a square venturi constriction with throat dimensions of 450 µm by 450 µm, both under visible light and X-Rays. We show that cavity shedding caused by the instability results in the formation of several cavity vortices. Their rotation causes the deformation of the oil stream into a distinct wave-like shape, combined with fragmentation into larger drops due to cavitation bubble collapse. Later on, the cavity collapse further disperses the larger drops into a finer emulsion. Thus, it turns out that the Kelvin-Helmholtz instability is similarly characteristic for hydrodynamic cavitation emulsification inside a microchannel as is the Rayleigh-Taylor instability for acoustically driven emulsion formation.

Keywords: Emulsion; Hydrodynamic cavitation; Kelvin-Helmholtz instability; Venturi microchannel.

Graphical abstract

Fig. 1

Ultrasonic emulsification due to Rayleigh-Taylor instability: (a–e) a very much simplified scheme of the process and (f–g) W/O emulsion formation as observed by .

Fig. 2

Experimental setup.

Fig. 3

(Supplemental video 1): Sequence of typical cavity behavior in the venturi microchannel: shedding due to Kelvin-Helmholtz instability, formation of cavity vortices and growth of a new attached cavity (close-up recording under visible light with every 23rd frame shown or 102.7 µs between frames).

Fig. 4

Kelvin-Helmholtz instability formation on the liquid–vapor interface: close-up recording with (a) visible light and (b) X-Rays (note that these images were not taken simultaneously but rather represent 2 different experiments at the same conditions).

Fig. 5

(Supplemental video 2): Emulsification sequence during the cavity growth phase and prior to the onset of Kelvin-Helmholtz instability (recording under visible light with every 15th frame shown or 62.5 µs between frames).

Fig. 6

(Supplemental video 3): Sequence of frames showing the emulsification process induced by Kelvin-Helmholtz instability (recording under visible light with every 8th frame shown or 33.3 µs between frames).

Fig. 7

(Supplemental video 4): Detailed observation of the emulsification process inside the micro-venturi channel: cavitation cloud shedding and oil stream breakup due to Kelvin-Helmholtz instability (close-up recording under visible light with every 8th frame shown or 35.7 µs between frames).

Fig. 8

(Supplemental video 5): (a) Detail on the oil stream breakup and emulsion formation due to the onset of Kelvin-Helmholtz instability with X rays (every 4th frame shown or 14.7 µs between frames) and (b) side-by-side comparison of the first and last frame from the sequence.

Fig. 9

Emulsification driven by Kelvin-Helmholtz (hydrodynamic cavitation) instabilities − a very much simplified scheme of the process.