Electric field control of propulsion force of a boat driven by chemical Marangoni effect

Abstract

Chemical Marangoni propulsion (CMP) is a driving on fluid by surface tension imbalance around object caused by anisotropic distribution of surfactant or by the reaction of Marangoni convection caused by the surface tension gradient. The purpose of the study is a remote control of the propulsion force in CMP. Electrostatic interaction of ionic surfactants as propellant with external electric field (EF) is expected to modify the surfactant distribution and modulate the surface tension gradient. We selected surface active imidazolium-based ionic liquids (ILs) as propellants. Effects of the application of the EF (667 kV/m) to the IL layer emitted from the object on the propulsion force and the accompanying Marangoni convection were evaluated on aqueous surface. When hydrophobic ILs with [PF6] anion were used, the application of the EF decreased the propulsion force and also decreased the flow velocity and flow width of Marangoni convection. The decreasing rate of the propulsion force increased with the alkyl-chain length of the IL cations. The maximum decreasing rate was 33 %. We considered that the main reason for the EF-induced reduction in the propulsion force is the decrease in the surface tension gradient due to the increases in the surface tension of the emitted IL layers caused by the rearrangement of the surface active IL molecules on aqueous surface. This is the first report about the EF control of the propulsion force in CMP.