A new device for the generation of microbubbles

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Jose ́ M. Gordillo, Zhengdong Cheng, Alfonso M. Ganan-Calvo, M. Ma ́rquez and D. A. Weitz Physics of Fluids 16 2828-2834 (2004).

Soft Matter Keywords

Microbubbles, Reynolds number, Atomizer

Summary

The authors present a new method for the production of bubble-liquid suspensions (BLS) composed of micron-sized bubbles and with gas to liquid volume ratios larger than unity. They flow gas through one hole, and liquid through two lower holes into a chamber, and the bubbles emerge from the bottom. By changing the pressure of the gas in the chamber relative to atmosphere, they can control the generation of microbubbles. When the gas reaches a critical pressure, it generates a spray of liquid rather than bubbles. The authors note that, unlike in a foam where bubbles tend to become polyhedral, the bubbles maintain a spherical shape throughout the process.

Experiment Details

Liquid was supplied into the chamber through syringe pumps connected to two holes. The use of these kind of pumps permitted the authors to control and fix to a given value the liquid flow rate, Ql. The authors controlled and measured the increment of gas pressure inside the chamber with respect to the atmospheric one. The BLS produced discharged directly into the atmosphere and was collected in a calibrated syringe.

Figure 1 - Sketch of the operating priciples of the device in regions (I), (II) and (III)

Three operating regimes were used:

  • I - At low values of gas pressure, most of the liquid injected through the two ports exited the chamber through the bottom, leading to the malfunctioning of the device. When the pressure was increased all the liquid abandoned the chamber through the exit channel steadily and filled it completely and no bubbles were generated. This regime is reached for a single value of the gas pressure.
  • II - By further increasing in gas pressure, bubbles were formed within the exit channel, which is now only partially filled with liquid. The Reynolds number of the liquid now increases, and the resulting product is a BLS and thus it constitutes the desired operating condition for the device. Pictures of the bubbles generated are shown in Figure 2. The fact that consecutive events are found in such a long period of time proves that the bubble formation process is periodic and that gas flow rate does not vary in time for fixed values of gas flow.
  • III - For high gas pressures the liquid flows attached to the exit channel walls whereas gas flows continuously through the channel core. In this case the device operates as a very fine atomizer.


Results

The authors were able to produce uniformly sized bubbles in water:glycerine (1:1) for many hours in the micro size range. The size of the bubbles was controlled by changing the chamber height.

Figure 2 - BLS obtained with the device. The bubble size is ~46 um. The working fluid is a mixture 50% – 50% water – glycerine.


Conclusions and Soft Matter Discussion

Production of bubbles and foams is an area of major importance in chemical and food industry, and the production of uniformly sized bubbles is a significant challenge. Microbubbles are used for various industrial applications, such as water treatment and fisheries cultivated shells. They also have good properties for physiological and physiochemical purposes. The use of phospholipid-coated microbubbles for medical applications is gaining considerable attention. However, the preparation of lipid-coated microbubble suspensions containing the ideal size and size distribution of bubbles still represents a considerable challenge. Additionally, microbubbles are used to reduce the skin friction in ships. This type of drag reduction is particularly important for maritime transportation applications, since 80% of total drag in a large ship is due to skin friction. Moreover, this technique of reducing skin friction is an environmentally friendly method, compared to some another methods, such as polymer drag reduction [4].


Microbubbles can be produced in many ways. One common method is to place the needle through which the gas is injected in a coaxial liquid co-flow. This has a disadvantage that the diameter of the bubbles formed scale with the needle injection diameter. Bubbles with sizes much less than the injection needle diameter can be obtained only if flow focusing geometry is employed. This method is however, unsuitable for the mass generation of microbubbles in industrial applications. The authors also not that multiplexing of this kind of devices is straightforward, since a horizontal array of the device shown in Fig. 1 would permit mass foam production and its use in industrial applications.