Generation of bubbles using the RayDrop

Introduction

Microbubble generation is an area of growing interest in the microfluidic community for its potential in diverse applications (industry, life science, medicine, material sciences…). Controlled generation of bubbles in microfluidic devices generates great interests in medicine due to the ability to non-invasively image molecular events with targeted microbubbles. These imagining methods will be increasingly used in the future to characterize pathophysiology as well as to develop and screen new therapeutic strategies in the treatment of cardiovascular disease, cancer and inflammatory diseases for example¹. Other fields can benefit from this technology as material science^2,3. Bubbles can also be used to study gas-liquid physical processes such as dissolution of CO₂ in solvents, CO₂ reaction and sequestration⁴. Closely related to gas bubbles are foams, which are formed by trapping pockets of gas in a liquid or solid.

The formation of air bubbles in a liquid appears very similar to the formation of liquid or oil droplets. It begins with an elongation of the flowing material (oil, water or air), and eventually a thinning and pinch-off of the “neck” connecting the droplet or bubble to the flowing material. That pinch-off then allows the droplet or bubble to collapse into a spherical shape.

The best production mode to obtain stable and monodispersed droplets and/or bubbles follows the dripping regime, with the droplet/bubble detaching from the jet at the junction between the two immiscible phases. In the case of the Raydrop, this regime was numerically and experimentally described for production of simple emulsion of liquid droplets in Dewandre et al⁵.  We show that with the double emulsion RayDrop technique, it is possible to prepare monodisperse microbubbles with a tunable size and a polydispersity index value <1%. We investigated the parameters affecting the microbubble generation process, such the geometry of the nozzle and the continuous-phase flow rate. We found that the low viscous continuous phase has little impact on the bubble size formation so when using the Raydrop Single emulsion, the bubble size is a function of the extraction capillary size. The geometry of the nozzle only impacts the size of the bubble when it is decoupled from the continuous-phase flow rate. The microbubble generation process using the double emulsion RayDrop technique appears more suitable to control the size and the monodispersity of microbubbles.

Experimental procedure

The double emulsion Raydrop (Secoya Technologies) was used to produce microbubbles through the injection of air (core phase) into an aqueous shell phase, further engulfed by the continuous phase (Figure 1). Both the shell and continuous phases are liquid phases made of 1% polyvinyl alcohol solution. Bubbles were formed by pumping the three components with a pressure controller (Flow EZ, Fluigent). Flow rates were controlled using flowmeters (Flow Unit, Fluigent).

Three different geometries were used in this note (Figure 2) to assess the advantages of using double emulsion for accurate bubble size control.

 

Geometry 1: 

• a = 30 µm
• b = 70 µm
• c = 150 µm

Geometry 2:

• a = 90 µm
• b = 160 µm
• c = 450 µm

Geometry 3:

• a = 90 µm
• c = 450 µm

Figure 2: The three geometries used in the study

Results

Geometry 3 and Geometry 2 were chosen to compare the simple emulsion nozzle with the double emulsion nozzle to produce bubbles and results are presented in Figure 3. This clearly shows the influence of the shell stream on the size of the bubbles.

b) Images from human skin (five cycles, 15 of 19 parameters shown). Scale bars: 200 µm (left), 25 µm (insets). Keratin 10 (K10), Keratin 14 (K14).

 

Using single emulsion nozzle (Geometry 3) and a low viscosity liquid, the size of the bubbles is almost totally constrained by the geometrical parameters the lowest size will be limited by the diameter of the extraction capillary (Figure 4).

 

 

 

With the double emulsion nozzle (Geometry 1 and 2), the lowest size achievable is determined by both the geometrical parameters and the stream of the shell phase (Figure 5). The presence of the shell phase stream shifts the achievable size range towards lower values.

 

 

 

With constant shell and core flowrates, the increase of the continuous phase has no effect on the bubbles size (see Figure 6).

 

 

 

The geometry of the nozzle-capillary system is the most influential factor on bubble size. To optimize the system one should make the inside diameter of the capillary equivalent to the targeted bubble size. Local restriction can be obtained by using a second nozzle attached on the entry of the extraction capillary (Figure 9). With this specific geometry, air bubbles of 30 µm diameter were generated in water at hundreds kHz.

 

 

 

Download the application note for more results.

References

1. Lindner, Jonathan R. Innovation: Microbubbles in medical imaging: current applications and future directions (2004).  3(6), 527–533. doi:10.1038/nrd1417
2. J. I. Park, A. Saffari, S. Kumar, A. G ̈unther, E. Kumacheva, Microfluidic synthesis of polymer and inorganic particulate materials. Annu. Rev. Mater Res. 40, 415–443 (2010)
3. E. Amstad et al., Production of amorphous nanoparticles by supersonic spray-drying with a microfluidic nebulator. Science 349, 956–960 (2015)
4. Mikaela D, Haut B, Scheid B. Bubbly flow and gas-liquid mass transfer in square and circular microchannels for stress-free and rigid interfaces: dissolution model. Microfluid Nanofluid (2015) DOI 10.1007/s10404-015-1619-8
5. Dewandre, A., Rivero Rodriguez, J., Vitry, Y., Sobac, B., & Scheid, B. (2020). Microfluidic droplet generation based on non-embedded co-flow-focusing using 3D printed nozzle. Scientific reports, 10(1), 21616. doi:10.1038/s41598-020-77836-y