Evaluation of Fluid Resistance Reduction and Antifouling Function of Scale-Mimicking Surfaces

In recent years, many studies have been conducted to improve the antifouling properties of surfaces in water and to reduce the fluid resistance, and it needs to elucidate the essence of the structural shape and surface composition that provide optimal properties. For example, fish scales are hydrophilic surfaces with riblet structures, and it is known that these structures produce a water layer in water flows to prevent adhesion of dirt and exhibit excellent self-cleaning ability. It is also believed that the microstructures of the scale surface are responsible for further improvement of antifouling properties and reduction of fluid resistance.<br/>The purpose of this study is to evaluate the properties of shark mimetic surfaces that could lead to the elucidation of the unexplained mechanism of shark fluid resistance reduction. Measurement method of dynamic wetting in a liquid atmosphere that established in our laboratory was used to evaluate the antifouling properties and reduction of fluid resistance ability of microstructured surfaces in a water atmosphere.<br/>In the past, surface properties under a water atmosphere were mostly evaluated only from static contact angles. However, surface properties under an air atmosphere are evaluated by combining static and dynamic wettabilities. The reason is dynamic wettability shows functional behavior that cannot be predicted from static wettability. Therefore, we have created a new experimental method to measure the dynamic wettability under a water atmosphere. The dynamic contact angles of air bubbles and oil droplets on the surface with tilting were measured in a water atmosphere.<br/>Experimental substrates were a commodity polymer resin with a riblet structure mimicking shark scales. Hydrophilicity and hydrophobicity of the polymer surfaces were changed by fabricating a self-assembled monolayer after gold-ion sputtering. 10mL of ethanol solutions including hydrophilic and hydrophobic molecules with thiol groups were prepared and the gold-coated polymer substrates were immersed in each ethanol solutions for 3 hours. After immersion, the substrate was washed thoroughly with ethanol and dried naturally. 5 kinds of surface-modified substrates having various ratio of hydrophilic and hydrophobic molecules in outermost surfaces were fabricated and measured by the dynamic wettability analyze system in air or water atmospheres. In an air atmosphere, 15μL of water or 6μL of n-hexadecane was attached and the substrate was tilted. In a water atmosphere, 11μL of an air bubble or 20μL of n-hexadecane was attached and tilted.<br/>As a result, the value of the contact angle when air bubbles were attached in a water atmosphere was larger than that expected from the contact angle when water was attached in an air atmosphere. In addition, the tilt angle required for the bubbles to leave the substrate was smaller. This is thought to be because of product of a water layer on the substrate surface by the shark's microstructure. This result suggests that the shark's scale surface prevents bubbles from attaching to the substrate and reduces fluid resistance by not creating surface irregularities.<br/>Next, the contact angle value when n-hexadecane was attached in a water atmosphere was significantly larger than the value expected from the contact angle when hexadecane was attached in an air atmosphere. However, the tilt angle required for the n-hexadecane to leave the substrate was also large. This is thought to be because of the difference in the height of the microstructure on the shark-mimetic surface. This is thought to this difference in height cause a petal effect, which prevents the oil droplets from leaving the substrate surface even when the substrate is tilted up to 90°. These results strongly emphasize the necessity of measuring dynamic wetting behavior in water atmosphere and indicate the possibility of creating new functional surfaces through further clarification of the functions of organisms existing in water.