Evaluation of Metal Surfaces with Microscopic Mimetic Structures and New Forming Methods for Metal Substrates

Despite being gill-breathing organisms, Ligia exotica are unable to swim. They breathe to use transported water without external energy caused by using capillary action of microscopic protrusion structures on the surface of their legs. Energy-less liquid-control surfaces not to require the external energy will be possible to create in mimicking of the structure of legs. In previous studies, it has been found that fabrication of microscopic protrusion structures on the polymer surface enables spontaneous liquid transport. However, while polymers are easy to form, there is a fear of deformation and softening above the glass transition temperature. Therefore, the goal of this research is to replace the material of the mimic substrate from polymer to metal. For this purpose, we fabricated a metal film on the surface of a polymer substrate with a mimic structure and checked shapes of droplets on the surface to verify whether the substrate material could be replaced from polymer to metal. Next, a new metallization method was examined, in which a metal substrate is formed by thick plating on a porous polymer mold.<br/>Polymer substrates with mimic structures were fabricated by pouring polydimethylsiloxane (PDMS) into polyethylene (PE) molds. Conical and triangular pyramidal protrusions were fabricated and arranged evenly spaced grid. We also examined whether the behavior of the droplets differed depending on the kinds of metal, such as gold and nickel, and the method of the metal coatings, such as gold vapor deposition and electroless plating. A 1.5 µL drop of water was dropped onto each prepared substrate and spreading of the droplet was observed. As a result, the spaces between the protrusions functioned as flow paths, and the droplets spread vertically and horizontally. Further, the droplet shape on the metal surface was like that on the polymer surface, with the droplet shape being nearly square on each substrate with conical structures, and nearly rectangular on each substrate with triangular pyramids. The aspect ratio was calculated from the wetting shapes of droplet at the point when wetting stopped. The aspect ratio was about 1.0 for cones and about 1.5 for triangular cones. The difference in wetting shape is closely related to the capillary pressure caused by the structure shape. In the case of conical protrusions, the distance between adjacent protrusions is equal from vertical to horizontal, and the strength of capillary pressure does not vary from vertical to horizontal. On the other hand, in the case of triangular pyramidal protrusions, the distance between adjacent protrusions is not constant, which causes a difference in the strength of capillary pressure. Even if the surface is coated with metal, the projection shape without any change, so the capillary pressure gradient is the same as on the polymer surface and the droplets have the same shape.<br/>From the above, it was confirmed that the structure inspired by the legs of ligia exotica can be applied to metal surfaces. As the next step, we tried to fabricate a metal substrate with the mimic structure by the thick plating method. A thin metal film was fabricated on the surface of the mold, and electrolytic plating was utilized to obtain the metal substrate. To facilitate detachment of the mold after molding the metal substrate, the mold was made of flexible PDMS, but PDMS substrates have a very weak durability of the metal thin film fabricated on their surface. Therefore, the PDMS molds were immersed in a polydopamine solution to form a hydrophilic dopamine film on the surface of the molds, and metal films were fabricated on the dopamine film. As a result, the durability of the metal film against peeling increased and electrolytic plating became possible for thicker plating.