Detergency in a tube

We study the motion of a drop of oil inside a capillary tube induced by a gradient of surfactant concentration. We first show that the wetting condition selects whether the drop moves towards or away from the surfactant. We then focus on the non-wetting case, for which the drop eventually escapes from the tube, and explore the different dynamics of propulsion. On a long time scale, we observe two regimes: either the drop reaches a constant velocity, or it moves intermittently, by successive jumps and stops. As a paradoxical consequence, viscous drops can be as mobile or even faster than inviscid ones.