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The motion of droplets and bubbles inside a channel flow is an important aspect of many microfluidic applications and can, in a broader context, also serve as a lab-scale model system for transport processes that occur in large-scale reaction columns in process engineering. In a series of experiments, we demonstrate and explain the unique and counter-intuitive behavior of droplets and bubbles in a channel flow with a specific flow configuration: a co-axial flow of (partially) miscible components. For studying droplets in this context, we inject a mixture of ethanol and oil into a sheath flow of water and leverage the ouzo effect to create the droplets [1]. For studying bubbles, we inject ethanol that is oversaturated with CO2 into water, resulting in the creation of bubbles inside the channel. Depending on the composition, these co-axial flow configurations can obviously result in large concentration gradients in the radial direction, which, however, inevitably also create concentration gradients in the axial direction. This is due to the combination of the flow in axial direction and the species diffusion in radial direction. As a result, a droplet (or bubble) in this flow configuration experiences solutal Marangoni forces in axial direction, as well as in radial direction if the droplet is not centered inside the channel. The axial Marangoni force can easily dominate over the drag and buoyancy forces, resulting in droplets traveling upstream inside the channel and bubbles descending against buoyancy. At the same time, the radial concentration gradients effectively keep the droplets (or bubbles) centered, conveniently avoiding wetting of the channel walls. Besides the aesthetically pleasing dance the droplets and bubbles perform inside the channel, there are also practical implications for possible microfluidic applications, like a novel type of pipette that uses Marangoni forces to withdraw tiny droplets or bubbles from a container for further processing.
[1] Bisswanger, S., Rocha, D., Dehe, S., Diddens, C., Baier, T., Lohse, D. and Hardt, S. Upstream motion of oil droplets in co-axial Ouzo flow due to Marangoni forces.
Soft Matter (2026), Advance Article.