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. 2023 Mar:94:106323.
doi: 10.1016/j.ultsonch.2023.106323. Epub 2023 Feb 8.

Emulsification mechanism in an ultrasonic microreactor: Influence of surface roughness and ultrasound frequency

Aniket Pradip Udepurkar  1 , Christian Clasen  2 , Simon Kuhn  3
Affiliations

Emulsification mechanism in an ultrasonic microreactor: Influence of surface roughness and ultrasound frequency

Aniket Pradip Udepurkar et al. Ultrason Sonochem. 2023 Mar.

Abstract

An ultrasonic microreactor with rough microchannels is presented in this study for oil-in-water (O/W) emulsion generation. Previous accounts have shown that surface pits or imperfections localize and enhance cavitation activity. In this study cavitation bubbles are localized on the rough microchannels of a borosilicate glass microreactor. The cavitation bubbles in the microchannel are primarily responsible for emulsification in the ultrasonic microreactor. We investigate the emulsification mechanism in the rough microchannels employing high-speed imaging to reveal the different emulsification modes influenced by the size and oscillation intensity of the cavitation bubbles. The effect of emulsification modes on the O/W emulsion droplet size distribution for different surface roughness and frequency is demonstrated. The positive effect of the frequency on minimizing the droplet size utilizing a reactor with large pits is presented. We also demonstrate microreactor systems for a successful generation of miniemulsions with high dispersed phase volume fractions up to 20%. The observed emulsification mechanism in the rough microchannel offers new insights into the utility and scale-up of ultrasonic microreactors for emulsification.

Keywords: Cavitation; Emulsification; O/W emulsion; Pits; Ultrasonic microreactors; Ultrasound frequency.

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Conflict of interest statement

Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Figures

Fig. 1
Fig. 1
(a) Borosilicate glass reactor with two inlet ports and one outlet port bonded onto the top layer. (b) Sketch of the glass reactor with dimensions in mm. (c) Schematic of the setup to acquire high-speed images and videos for the emulsification mechanism study in the glass microreactors.
Fig. 2
Fig. 2
(a) Top view of the waterjet cut microreactor, with large pits visible at the top end of the side wall. (b) Side view of the waterjet cut microreactor, with small pits visible in the middle region of the side wall (Ra ∼ 4 μm). (c) Cavitation bubble situated in a pit on the channel side wall. The ultrasound frequency is 48 kHz, and the load power is 10 W. (d) Microbubble ejection from the cavitation bubble at t = 1 ms. The outline of the microchannel is shown with a dashed line in (c) and (d).
Fig. 3
Fig. 3
(a) Channel side wall of the laser etched microreactor (LCR). The top view (left) shows pits with depths of 10–20 μm and the side view (right) shows pits with length and breadth of 30–60 μm. (b) Extracted frames from the high-speed video of the transient cavitation on the channel wall at 48 kHz. The time step between each frame is 6.25 μs. Scale bar = 100 μm. The image is enhanced by changing the brightness to better visualize the cavitation bubbles.
Fig. 4
Fig. 4
Emulsification of the sunflower oil slug at the entrance of the (a) WJR and (b) LCR at 48 kHz. The slug is emulsified to minuscle oil droplets (encircled in green), also defined as fine dispersion, and large droplets (encircled in red).
Fig. 5
Fig. 5
Emulsification modes in the WJR. The timestamp in milliseconds (ms) is indicated in the top left corner (a) Fine dispersion generation on the channel wall. The cavitation bubble on the channel wall oscillates and is encapsulated by an oil layer at 20 ms. The violent oscillation generates a fine dispersion. (b) A cavitation bubble situated in a pit (white dashed rectangle) ejects microbubbles which enter the oil slug (white dashed region on the oil slug). The microbubbles cluster (white dashed region at 10 ms) and oscillate violently to generate a fine dispersion (green dashed region at 50 ms). (c) An oil-encapsulated cavitation bubble (white dashed line at 0 ms) undergoes weak oscillation to generate large droplets (droplets inside green dashed line at 20 ms). (d) An oil-encapsulated cavitation bubble (white dashed line at 0 ms) ejects large oil droplets into the aqueous phase (droplets inside green dashed line at 1 ms).
Fig. 6
Fig. 6
(a) The large decane droplets (D > 60 μm) close to the microstreaming of the transient bubble cloud (in the green dashed region), bunch together and undergo breakup into smaller droplets. The timestamp in milliseconds (ms) is indicated in the top left corner. (b) The sunflower oil droplet (D = 21 μm, encircled in red dashed circle) close to the transient cavitation bubble cloud (inside green dashed line) moves rapidly through the bubble cloud and retains its size. The timestamp in milliseconds (ms) is indicated in the top left corner. (c) The decane droplet (D = 32 μm, encircled in red dashed line) is in the close vicinity of a collapsing cavitation. The droplet does not undergo any visible breakup. The timestamp in milliseconds (ms) is indicated in the top right corner.
Fig. 7
Fig. 7
Droplet size distribution of the sunflower oil-in-water (O-W), hexadecane-in-water (H-W), and decane-in-water (D-W) emulsions in (a) waterjet cut microreactor (WJR) and (b) laser etched microreactor (LCR). The emulsion is generated at a load power of 20 W and frequency of 48 kHz. The continuous phase (water + Tween 20(3 wt%)) flow rate is 0.2 ml/min, and the dispersed phase flow rate is 0.05 ml/min. The residence time is 4 min.
Fig. 8
Fig. 8
Schematic diagram of the different emulsification modes observed in the WJR and LCR. (a) Generation of a fine dispersion due to chaotic oscillation of a cavitation bubble on the channel wall in the WJR. (b) Generation of a fine dispersion due to a microbubble cluster situated on the channel wall in the WJR and the LCR. (c) Generation of a fine dispersion due to a transient cavitation bubble cloud close to the oil interface in the LCR. (d) Generation of large droplets due to the oscillation of an oil-encapsulated cavitation bubble. (e) Generation of large droplets due to a transient cavitation bubble cloud situated on the channel wall in the LCR. (f) Droplet breakup in a microreactor when the cavitation bubble is smaller than the oil droplet (RCB < Rd), and no droplet breakup when the cavitation bubble is larger than the oil droplet (RCB > Rd).
Fig. 9
Fig. 9
Droplet size distribution for the decane-in-water (a), hexadecane-in-water (b), and sunflower oil-in-water (c) emulsion at frequencies of 48 kHz, 142 kHz, 310 kHz, and 525 kHz. The aqueous phase flow rate is 0.2 ml/min, the dispersed phase flow rate is 0.05 ml/min, and the applied load power is 20 W. The sunflower oil did not undergo emulsification at 525 kHz and thus the droplet size distribution is not shown in the figure.
Fig. 10
Fig. 10
The breakup of a decane slug in the microchannel at 525 kHz and a load power of 10 W. Upon actuation the decane slug undergoes deformation (1 s) and eventually breaks into large droplets of few hundred micrometres (2 s).
Fig. 11
Fig. 11
(a) The fluorescent polystyrene particles spread evenly at the bottom of the channel. On the actuation of ultrasound at 525 kHz and 1 W, the particles start migrating towards the pressure nodes (40 s) and eventually concentrate at the pressure nodes in the microchannel (80 s). The approximate interval between the two pressure nodes (120 s) is roughly equal to the ultrasound wavelength in the microchannel. (b) The overview of the microreactor with the bright spots corresponding to the particle concentration at the pressure nodes in the microchannel. The area in red in (c) is the pictured microreactor region in (b).
Fig. 12
Fig. 12
The breakup of a decane slug in a microchannel section with 2 pressure nodes. The top left picture depicts the position of the pressure nodes in the channel. The position of the pressure nodes (and the particle position) is depicted by the dashed red circle in the channel. The slug is deformed as it approaches the first pressure node (0 s) and moves around the pressure node. As it is located between the two pressure nodes (1 s), it undergoes deformation and oscillation. The slug eventually breaks into large droplets (2 s), which occupy the channel wall.
Fig. 13
Fig. 13
Decane emulsification in the aqueous phase at 525 kHz. In the first section of the reactor (region in blue), the decane slugs undergo breakup and are trapped in the channel. The gas bubbles in the reactor coalesce in the channel to form a gas slug. The slug pushes the droplets not emulsified (region in red) as well as the miniemulsion (region in green and white emulsion in red region).
Fig. 14
Fig. 14
(a) D-W miniemulsion droplet size distribution for the WJR (decane vol% 7.5 %), Micromixer and WJR in series (decane vol% 17 %), and two WJR in series (decane vol% 20 %). The total flow rate is 0.25 ml/min, and the power is 20 W. (b) D-W miniemulsion droplet size distribution for two WJR in series in 2 different configurations in comparison with the WJR operated at a single frequency. The first is 48 kHz WJR followed by 525 kHz WJR and the second is 525 kHz WJR followed by 48 kHz WJR. The single WJR is operated at a total flow rate of 0.25 ml/min, a power of 20 W and a decane volume fraction of 20 %. The WJR in series are operated at a total flow rate of 0.5 ml/min, a power of 10 W for each reactor and a decane volume fraction of 20 %.
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