Driving mechanisms of CM-scaled PDMS boats of respective close and open reservoirs

A floating solid object may have a spontaneous motion on a water surface when a liquid, which has a surface tension lower than water, exits the solid object and has contact with water. This motion may be induced due to the difference between surface tensions of water and the liquid. In order to interpret this motion, it is important to know how the liquid exits the solid object. In this article, we reported in situ observation of exchange processes of water and isopropyl alcohol (IPA) inside both close and open reservoirs of cm-scaled polydimethylsiloxane (PDMS) boats. Based on this observation and force analysis, we interpreted driving mechanisms of these boats. We found that the exchange processes may be different in the PDMS boats of respective close and open reservoirs. The PDMS boat of a close reservoir might have a bubble trapped in its reservoir during the motion. This bubble slowed down the exiting process of the IPA from the reservoir, made the motion last longer, and enabled the boat to have a longer travel distance. Also, such a boat had gurgling-like motions (i.e., approximately periodic motions of deceleration and acceleration) after the majority of the IPA had exited the reservoir. The speeds of this boat had an order of 1 cm/s. On the other hand, neither bubbles nor gurgling-like phenomena were found in the motions of the PDMS boat of an open reservoir. The speeds of this boat were in the order of 1 cm/s as well. In addition, based on observed exiting processes of the IPA and experimentally determined speed-time relationships, we also set up a simple model to find force–time relationships. Furthermore, we investigated the exiting process when the open reservoir of the cm-scaled PDMS boat was shallow. We found that, depending on the values of the height difference, h, between the bottom of the reservoir and the water surface outside the reservoir, three different phenomena might appear: (1) if h ≤ 1.33 mm, then the reservoir was empty at the end of the test and no water flowed into the reservoir during the test; (2) if 1.3 mm < h ≤ 2.3 mm, then the reservoir was empty at the end of the test while water flowed into the reservoir during the test; and (3) if 2.3 mm < h ≤ 3.0 mm, then the reservoir was filled by water at the end of the test. The difference in the filling results was induced by the interplay among Marangoni effect, free convection, and recovery of a hollow spot by surrounding water. The corresponding findings interpreted the exiting process of the IPA observed in the open reservoir of a mm-scaled SU-8 boat that we have previously developed, and also explained the filling result in the open reservoir of a PDMS boat.