Dramatic squat and trim phenomena of mm-scaled SU-8 boats induced by Marangoni effect

Abstract

Surface tension gradients may induce the motions of a floating solid fragment. This mechanism has been employed to drive miniaturized objects on a water surface, which could potentially serve as rotators, mixers, locomotives, and so on. To properly apply these objects, it is important to understand their motions. The previous consideration mainly focused on the horizontal motions of these small objects. In this work, we considered translational and rotational motions of a mm-scaled SU-8 boat in the vertical plane, which were so-called squat and trim phenomena, respectively, in the case of a macroboat. Through two types of tests, we demonstrated that dramatic squat and trim phenomena observed in the motions of the SU-8 boat were mainly induced by the surface tension difference between water and isopropyl alcohol (boat propellant), instead of horizontal movements of the miniaturized boat. This difference created a hollow spot behind the stern and made the boat sink and tilt along the sidewall of the hollow spot. In addition, we found that the motion of a SU-8 boat in a channel included two stages. In the first stage, it was propelled due to the difference between fore-and-aft surface tensions. In addition to moving forward, it also had dramatic squat and trim movements. However, in the second stage, in which no more isopropyl alcohol exited the boat, it was driven by a water wave generated in the first stage and did not show any visible squat and trim phenomena. The results of this work provided a better understanding of motions of the SU-8 boats. These results might also apply to surface tension-driven motions of other miniaturized objects.