Capillary skimming of hydrophilic ratchet by surface tension*

A meniscus formed on a hydrophilic surface can skim a light object or fluid by use of surface tension force of water. This capillary adhesion, however, is insufficient to skim heavier objects or other immiscible fluids. It is found that secure capillary adhesion by symmetric meniscus can occur more easily when the solid surface and water surface form a more obtuse angle. In the present work, we propose a hydrophilic ratchet that is more advantageous for skimming these materials by employing a periodically changing slope of solid surface of the ratchet in contact with the water surface. The optimal size of the ratchet is comparable to the capillary length, surface tension and inertia both play important roles. This ratchet structure is also found to be beneficial in detaching viscous materials stuck on a ratchet surface by inducing capillary flow via ratchet geometry. These advantages are demonstrated to provide an oil skimmer with anti-oil-fouling characteristics, which is especially required for cleaning oil spills of highly viscous oils.