Flexural Joints for Improved Linear Motion of a Marangoni Propulsion Robot: Design and Experiment

Some aquatic insects can rapidly dash over the water surface by secreting chemical material that lowers the surface tension behind. This locomotion is commonly known as Marangoni propulsion, and we built a non-tethered miniature robot inspired by their mobility. The robot had six circular footpads with equilateral triangular cross section, and weighed 14.8 gram including on-board electronics, a battery, and a servo motor. Although the robot successfully skimmed over the water surface by dripping alcohol (e.g., 3-Methyl-l-butanol), the robot could not maintain a linear motion by itself. Therefore, we designed and attached flexural joints at the hind legs of the robot to compensate its linear motion; the asymmetric force applied to the hind legs subsequently induced another counter moment due to the bending of flexural joints. During the experiments, these joints were effective at reducing undesired lateral deviation more than 3-fold compared to one without flexural joints. Also, the characteristics of the robot's locomotion was similar with the locomotion of aquatic arthropods according to the dimensionless number analysis.