Solubility effects on the propulsion of the Marangoni boats

We study the propulsion at the surface of water of small objects (2cm) called the Marangoni boats. Several studies have been conducted on similar swimmers the camphor boats, a non soluble surfactant. However, it has been shown that one can generate a controlled Marangoni flow thanks to soluble surfactants such as TTAB. The solubility adds a complexity to the Marangoni flow, hence in this study we try to characterise the displacement of the swimmers by probing the effects of the solubility and the concentration of the surfactant solution. The boats are composed of two parts. The first is the floater cut out of a transparent plastic sheet. And the motor is a filter paper soaked with a souble surfactant solution sticked to the floater with nail polish. When the boat is deposited at the surface of a water layer, the surfactant spreads at the surface decreasing the surface tension at the back of the boat. It creates a gradient of surface tension between the bow and the stern of the boat, thus a difference in capillary force appears resulting in the propulsion of the boat forward. We record and track the motion of the boat with a camera and the trackpy python algorithm. From the trajectory we can measure the velocity of the boat while it swims. We perform the experiments with four different surfactants (HTAC, TTAB, DoTAB, DeTAB) with different Critical micellar concentrations  (respectively 1.6 mM, 4mM, 15 mM and 65mM). In addition we propose a simple model to predict the initial velocity based on the balance between the capillary force as the driving force and the skin friction force opposed to the movement of the boat. The agreement between the experimental data and the model seems good. We observed that the initial velocity of the boat and the evolution of the velocity versus the time changes with the concentration but also the solubility of the surfactant. The more soluble the surfactant is the less efficient it is to propel the boat .