Increase buoyancy of a solid fragment using micropillars

When a solid fragment has a higher density than water, it may still float on water surface due to the support of surface tension. However, once the solid fragment is completely immersed inside water, it sinks down to water bottom due to elimination of the surface tension. In this work, we demonstrated that, after the incorporation of micropillars on such a solid fragment, it may still get back to water surface. Air is trapped between micropillars. This increases the space that the solid fragment occupies under water. Accordingly, the buoyancy is increased. The critical values of trapped air, micropillar-covered area, micropillar height and water depth for a micropillar-covered fragment to rise up to water surface were formulated. Four micropillar-incorporated SU-8 fragments were fabricated and tested to validate the derived relationships. Experimentally determined critical values of micropillar-covered areas and trapped air volumes had good match with theoretically predicted ones.In addition, through force analysis, we showed that surface tension at the triple lines between water, micropillars and trapped air retains the volume of the trapped air but does not provide any lifting force to make the fragment rise up to water surface. Since the incorporation of the micropillars may make a completely immersed fragment get back to water surface from under water, the presented results have potential applications in developing miniaturized devices, such as submarines and water robots, that may need to rise up to water surface.