Fabrication and testing of a self-propelled, miniaturized PDMS flotilla

In this work, we developed a miniaturized PDMS flotilla, which was capable of moving on a water surface. The flotilla consisted of five mm-scaled PDMS boats that were linked together through μm-scaled PDMS bars. We explored its design, fabrication, propulsion, and motions in straight and circular channels. The flotilla was fabricated using a molding approach with two-layer SU-8 structures as the mold, and actuated by the difference between fore-and-aft surface tensions. It was tested in straight and circular channels at water levels of 2.0, 3.0, 6.0 and 9.0 mm, respectively. No collisions were observed between neighboring boats in all the tests. Travel distances ranged from 385 to 1,128 mm. Average speeds varied from 38.5 to 102.5 mm/s, and increased with the increase in water depth. Dramatic squat and trim phenomena were directly observed in the two types of tests conducted in the straight channel. They were mainly induced by the difference between surface tensions of water and flotilla propellant. Simple models were further developed to interpret driving mechanisms of the flotilla, and to extract its resistance coefficients in different motions. These coefficients ranged from 0.1 to 0.7 in the 2-, 3-, 6- and 9-mm tests performed in the straight channel. The two ends of the flotilla had direct contact with the outer sidewall of the circular channel during a circular motion. The supporting forces of the outer sidewall at these contact points provided the centripetal force needed for the circular motions. In the four tests conducted in either the straight or the circular channel, water resistances increased with the decrease in water depth. Furthermore, the flotilla suffered much larger resistances in the 2-mm test than in another three, implying that, to avoid the effect of shallow water, water is preferred to be deeper than 2 mm in the applications of the miniaturized flotilla. The results presented in this work indicate that the water flotilla was capable of traveling reasonably long distances in straight and circular channels at fast speeds. Due to its mobility on a liquid surface, such a miniaturized flotilla would have the capability of delivering different materials and sensors on demand in small-scale fluidic applications.