Influence of Surfactant in the Motion of an Elongated Bubble under Electric Field*

In order to reverse the motion of an object, it is natural to think of changing the direction of the external force. However, for some objects driven by an electric field, the addition of surfactants will fulfill the task. We actuate here a bubble confined in a cylindrical capillary by an electric field in the presence of an anionic surfactant, sodium dodecyl sulfate (SDS), to modify the adsorption and the corresponding zeta potential of the bubble. First, we found that the bubble velocity, which shows linearity at low voltage, becomes non-linear above a critical threshold voltage. We also discovered, more unexpectedly, that varying the surfactant concentration results not only in the magnitude of the bubble motion but also in its moving direction. We will present here these experimental results together with an interpretation of these peculiar behaviors.