Abstract
When a micro/nano-textured superhydrophobic surface is submerged underwater, a thin layer of gas, known as a plastron, can be trapped between the solid surface and the liquid. This trapped gas layer enables a wide range of applications, including drag reduction, anti-biofouling, anti-icing, and enhanced boiling heat transfer. However, the plastron can gradually dissolve into the surrounding liquid due to gas diffusion, ultimately leading to the failure of the superhydrophobic surface. Although significant progress has been made over the past two decades in understanding gas diffusion on superhydrophobic surfaces, the effect of surface gas permeability on plastron longevity remains insufficiently explored. This thesis aims to address this knowledge gap through a combination of experimental and numerical investigations. Since gas-permeable materials, such as polydimethylsiloxane (PDMS), are among the most commonly used base materials for fabricating superhydrophobic surfaces, this work is important for the design and manufacture of long-lasting superhydrophobic surfaces for real-world applications. In experimental work, superhydrophobic surfaces with different gas permeabilities were fabricated using PDMS and polyepoxide epoxy as base materials. PDMS exhibited high gas permeability due to the relatively large free volume between its polymer chains, whereas epoxy was treated as a non-gas-permeable material. The surface texture consisted of an array of micro holes with a diameter of 60 µm, a depth of 61 µm, and a pitch of 100 µm, fabricated using a soft lithography technique. The samples were fully submerged in a long tube filled with undersaturated water, allowing the air trapped on the samples to dissolve into the surrounding liquid. Plastron longevity was measured using a nonintrusive optical method. In the numerical study, the two-dimensional diffusion equation was solved using COMSOL Multiphysics finite element solver (COMSOL). Appropriate initial and boundary conditions were applied to initiate gas transfer. The mass flux and plastron longevity were calculated based on the resolved gas concentration gradient near the air–water interface and Fick's first law. Two opposite trends were observed, depending on how gas diffusion was triggered. When diffusion was induced by increasing the liquid pressure, PDMS showed shorter plastron longevity than epoxy. This reduction was primarily caused by gas transfer from the plastron into the PDMS, which accelerated the wetting process. In contrast, when diffusion was induced by lowering the gas concentration in the liquid, PDMS showed longer plastron longevity than epoxy. This improvement resulted from gas release from the PDMS into the plastron, which effectively increased the plastron volume. Finally, this work demonstrates that plastron longevity can be extended by injecting gas through the gas-permeable material.