Thin lubrication film around moving bubbles measured in square microchannels

Abstract

The thin lubrication film around bubbles moving in square microchannels was measured using an optical interference method down to the scale of 0.1–10 μm in thickness, and the three-dimensional profile of the lubrication film around the bubble was obtained experimentally. The lubrication film of the elongated bubble translating along the microchannels presented a three-dimensional saddle shape, and the thinnest locations were located at the sides where the rupture of the lubrication film was found to occur. The minimum film thickness of the axial section in the downstream direction was found to obey the relationship of hmin ∼ Ca2/3 as predicted by the Bretherton equation, where Ca is the capillary number. Also, the minimum film thickness in the across-stream direction gradually deviated from this relationship and eventually obeyed hmin ∼ Ca1 as the film thickness was decreasing, which matches well with the existing theoretical results. Therefore, the optical interference method is effective to reveal the dynamics of the thin lubrication film in droplet microfluidics three-dimensionally.