Dry microfoams: Formation and flow in a confined channel
Résumé
We present an experimental investigation of the assembly of microbubbles into a 2D foam and its flow in a rectangular microchannel. Using a flow-focusing method, we produce a foam in situ on a microfluidic chip for a large range of liquid fractions, down to a few percent in liquid. The channel height is 250 μm, resulting in bubbles whose height to diameter aspect ratio ranges between 0.3 and 1. We can monitor the transition from separated bubbles to the desired microfoam, in which bubbles are closely packed and separated by thin films. We find that bubble formation frequency is limited by the liquid flow rate, and that it creates a modulation of the foam flow, rapidly damped along the channel. The average foam flow rate depends non-linearly on the applied pressure, displaying a threshold pressure due to capillarity. Strong discontinuities in the flow rate appear when the number of bubbles in the channel width changes, reflecting the discrete nature of the foam topology. For a given foam structure the pressure drop scales with the flowrate to the power 2/3. We compare this dependance with the case of macroscopic foams pushed without a coflowing liquid. We also produce an ultraflat foam (channel height about 8 μm) reducing the bubble aspect ratio to 0.02; we notice a marked change in bubble shape during the flow.