Bubbles in Microchannels
Thomas Cubaud and Chih-Ming Ho
Work supported by DARPA/MTO Micro Power Generation Program

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Multiphase flow occurs in many operations in the chemical, petroleum, and power generation industries (such as nuclear power plants and micro fuel cells). Unlike large-scale systems, gas bubbles can present significant problems in microfluidic systems by disturbing and eventually blocking the flow. Understanding how bubbles affect the flow resistance in microchannels is, beside its fundamental aspect, a concern of determining the pumping or energy requirement for portable microfluidic devices where two-phase flow is involved such as in a micro direct methanol fuel cell (μDMFC).

Flow patterns in 525 μm square channel (play movie ~ 7 Mb)

Channels are made with glass and silicon using microfabrication techniques. Channel masks are printed in positive with a high-resolution printer on transparent paper for lithography. Photoresist is spin coated on a silicon wafer. The wafer is then exposed to UV light through the mask. Channels are etched at different depths using Deep Reactive Ion Etching (DRIE). The sealing is made with Pyrex glass using anodic bonding, providing optical access for flow analysis.

Two-phase flow microchannel module

On-chip cross-shaped mixing section

Properly introducing gas bubbles is an important step in producing a two-phase flow pattern. We micromachined an effective small mixing section on-chip. Well-defined bubbles in a wide range of sizes can be produced as a function of liquid and gas flow rates. Because of the square channel symmetries, bubbles flow “naturally” in the center of the channel. The cross-shaped was chosen to produce a steady uniform flow. The minimal size of bubbles is about the size of the channel. In these experiments, this mixing system is used as an effective way to produce two-phase flows in microdevices.

Flow patterns

Bubbly flow (play movie ~ 5 Mb) Wedging flow (play movie ~ 5 Mb)

Air/water flow map

Slug flow

Annular flow

Dry flow

Pressure drop and resistance to flow

Two-phase flow pressure drop The two-phase pressure drop ΔP2-phase was scaled by the single liquid flow pressure drop ΔPL associated to the liquid flow rate QL in the channel (ΔPL = RLQL). As can be seen in the figure, data collapse more or less on a single master curve with two distinct regimes. The bubbly and the wedging flows are depicted on one side and the slug, annular and dry flows are on the other side.

Two-phase flow resistance As αL changes from 1 to 0, the system goes from a single liquid flow to a single gas flow. The fluidic resistance R = ΔP/Q is an interesting parameter because it is calculable from direct measurements without assuming any correlation.

Surface modifications