Development of surface tension-driven microboats and microflotillas

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. These objects could function, for example, as rotators. In the meanwhile, based on such a mechanism, we have recently developed microboats and microflotillas, and explored their movements on water surfaces. Isopropyl alcohol was adopted as the propellant. It had a surface tension much lower than water. Once the IPA exited the reservoir of a microboat or a microflotilla, the force induced by the difference between fore-and-aft surface tensions propelled the corresponding locomotive forwards. This article gives a simple review of the results that we have achieved to date, including exploration of SU-8 microboats, development of SU-8 microflotillas, and determination of driving mechanisms of these microboats and microflotillas. Two types of SU-8 microboats were developed, and their designs, fabrication and propulsion were explored. The first type of microboats did not have side rudders. They were capable of moving along their longitudinal directions. We considered the movements of this type of SU-8 boats in both horizontal and vertical planes. The second type of microboats had side rudders, which enabled these microboats to have radial motions. By varying the lengths of the side rudders, we were able to have a good control of the radial motions. To have more storage space, we further developed microflotillas, which each consisted of five SU-8 microboats that were linked with a Nylon rope. We explored design, fabrication and propulsion of these microflotillas, as well as their motions in both straight and circular channels. The developed boats and flotillas were capable of moving on water surfaces at speeds of order 0.1 m/s. Finally, we performed in situ observation of exchange processes of water and IPA inside the reservoir of a cm-scaled PDMS boat. Based on this observation, we interpreted driving mechanisms of the microboats and microflotillas. Existing approaches mainly use stationary devices for material supply and sensing detection. Compared with these devices, the developed boats and flotillas are mobile. This enables these miniaturized vehicles to actively deliver materials and sensors on demand for microfluidic applications.