Multiphase flows in confined geometries exhibit a variety of intriguing morphologies. At the small scale, the unique balance of forces produces flow patterns that are typically governed by viscous and capillary effects. While surface tension tends to minimize liquid-gas interfacial areas with bubbles having spherical shape, viscous creeping flows can also strongly deform bubbles in velocity fields set with the channel geometry. Here, the deformation of capillary surfaces is accentuated with the use of a highly viscous carrier fluid (silicone oil having a viscosity ν = 103 or 104 cS) and the presence of circular cavities along a square microchannel (h = 250 μm) for smoothly modulating flow velocity. Although the size and concentration of air bubbles are usually determined through pressure and microfluidic flow rates of injection, the use of carbon dioxide bubbles allows us to manipulate mass transfer processes1–3 (i.e., dissolution and stripping mechanisms) and generate a variety of multiphase flow patterns (Figs. 1 and 2). For instance, as CO2 bubbles considerably shrink before reaching the field of view, dense arrangements of small bubbles can be produced. In the viscous-dominated regime with a capillary number Ca ∼ O(1), gas bubbles adopt a range of shapes depending on their size and packing. In particular, when passing through a series of extensions and