How to perform microbubble generation?

Introduction to microbubble generation

Foams are composed of microbubbles dispersed in a liquid continuous phase. Foam properties rely on microbubble generation properties and performances. Due to the interplay between several length scales, foam properties are complex and require a fine tuning to facilitate their study. Consequently, microfluidics has been widely used in the last two decades to selectively study different aspects of microbubble generation.

Advantages

• Fine control of the gas & liquid inlet pressures.
• Highly monodisperse bubbles (CV < 2%).
• Gravity neglected

Applications

Microfluidics has also supported this field of research for a wide range of applications, from food industry (Laporte et al., 2016) and the fabrication of biocompatible scaffold (Chung et al., 2009; Costantini et al., 2015; Andrieux et al., 2017) to the improvement of foam recovery efficiency in Enhanced Oil Recovery (EOR) (Quennouz et al., 2014).

Leslie Labarre presents the basics of bubble generation via microfluidics and some of its main applications from single bubble generation study to scaffold generation via liquid foam templating.

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Principles of microbubble generation

Video 1: Microbubble generation performed by flow-focusing microfluidics with the OB1 Elveflow pressure-driven flow controller.

Researchers like Vuong and Anna (2012) established geometrical models to predict microbubble patterns for specific geometries.

Other researchers studied all the different foam structures/microbubble patterns available for a fixed geometry by varying the liquid and gas inlet flow-rates, or pressures (Coleman and Garimella, 1999; Liu and Sur, 2009; Zhao and Middelberg, 2011). The equivalent of a microbubble phase diagram was drawn per specific geometry which is called “foam regime map”. Different foam patterns were reported in the literature: they start with a single row of bubbles called “hex-one”, moving on to double-rows of bubbles called “hex-two” and so on, up to four rows of bubbles as reported in (Garstecki and Whitesides, 2006). The microbubble patterns available are function of the geometry and of the gas and liquid inlet pressures.

Later, scientists studied how the properties of the continuous phase, such as surfactant type and concentration (Liu and Sur, 2009; Micheau et al., 2016), viscosity (Lu et al., 2014), or elasticity (Olivieri et al., 2011) were affecting single bubble generation.

To discover more tips and tricks about droplets & bubble generation, please check our droplet Userguide!

Microbubble generation methods

PDMS microfluidic device

The microfluidic device used consists of a rectangular cross-section microchannel of d = 25 μm depth with a w1 = 50 μm wide gas inlet which meets at a flow-focusing junction with two w2 =100 μm wide liquid inlets as shown in Figure 3. 2. Microbubbles are formed and studied in w3 = 280 μm wide channel.

Two reservoirs of air and liquid are connected via a pressure controller (OB1 MK3, Elveflow) to the two inlets via PTFE tubings to accurately control the gas and liquid inlet pressures.

The pressure named Pgas for the gas and Pliq for the foaming solution are upstream applied pressures above atmospheric pressure.

Device Channel wall surface treatment

In order to assure a homogeneous foam formation in the channel, the device is surface treated to become hydrophilic via a layer-by-layer technique which implies alternately flowing  segments of poly(allylamine hydrochloride) (PAH, Sigma Aldrich) and poly(sodium 4-styrenesulfonate) (PSS, Sigma Aldrich) solutions (both 0.1% w/v in 0.5 M aqueous NaCl solution) with aqueous NaCl washing solution (0.1 M) segments in between (Bauer et al., 2010). Indeed, the flow pattern depends strongly on the wetting properties of the fluid-wall interface. To get ordered patterns, it is required to get a complete wetting of the continuous phase on the wall of the channel. Shao et al., (2009) showed that the lack of affinity to the channel wall would favour the formation of bubbles. Besides, Cubaud et al., (2006) demonstrated that in hydrophobic channel, the liquid does not lubricate the wall; the hysteresis and friction effects make the flow axisymmetric with respect to the axial direction.

Setup for Microbubble generation

Fig 4. Schematic representation of the setup diagram employed in this application note comprising: a pressure-driven flow controller (OB1 Mk3, Elveflow), tubing, falcon reservoirs, fittings and a flow-focusing microfluidic chip.

List of components

Results & discussion

In this study based on the work of Labarre L. and Vigolo D. (2019), microbubbles were generated via microfluidics flow-focusing geometry for a broad range of gas and liquid inlet pressures detailed in Fig 5. The effect of both inlet pressures and formulations was investigated. Several solutions were employed. the SDS (anionic surfactant, Sodium Dodecyl Sulfate) concentration is kept at five times the CMC to ensure a constant concentration throughout the whole duration of the experiment. The combination of SDS with glycerol is selected to study the effect of increased  viscosity on the foam recovery properties after a gradual deformation at two concentrations (20% and 40% wt.). Then, a third formulation made of SDS and DOH (Dodecanol) is chosen to observe the effect of surface elasticity on foam hysteresis. Thirdly, they study the effect on microbubble generation of the addition of Xantham Gum (XG) and a small quantity of glycerol (5 % wt.) added to facilitate the dissolution in the reference solution. In this case, the focus is given to the impact of the shear-thinning property of the continuous phase deriving from the thickener nature of XG.

Microbubble generation regime map

A microbubble generation regime map, which gives a detailed view of all the different microbubble patterns available, was generated for each solution by changing the gas and inlet pressures between 200 and 1400 mbar. Thus, areas of steady patterns of bamboo and two-row foam could be identified and selected for our study. The “bamboo” pattern or “hex-one” (Garstecki and Whitesides, 2006) is described as a single layer of bubbles containing only one bubble in the full width of the channel. The “two-row” pattern or “hex-two” (Garstecki and Whitesides, 2006) can be depicted as a single layer containing two rows of bubbles in the width of the channel. The microbubble generation regime maps obtained for each formulation are presented in Fig 5. Spherical and monodisperse microbubbles size ranging from 10 to 270 microns were obtained.

In this work, the use of pressure-driven flow controlled microfluidics to generate bubbles in various solutions allowed for an accurate control of the resulting bubbles size, monodispersity and shape for a broad range of pressure regimes. If you’re interested in reproducing this setup, feel free to contact our team of experts!

References