The Interactions Of Modified Surfaces: From Oil Spills To Gecko Adhesion
Surface Modification; Pickering Emulsion; Dry Adhesive Abstract: The flow of fluids, the interaction of solid surfaces, and the presence of molecules (or particles) at interfaces has implications in almost every scientific topic. This work encompasses the diverse topics of oil spill remediation and dry adhesives. In both cases, materials are shaped both physically and chemically in order to accomplish a specific goal, often requiring novel methods and tests to find. The first topic of oil spill remediation through Pickering emulsions is broken into two sections. The first is a fundamental study into ways in which oil interacts with surfaces that it may contact during an oil spill. Contact angle measurements represent surface structure and energy, and a universal testing machine (tribometer) allows for well-controlled force measurements during the formation of oil bridges between the submerged control and test surfaces. Flat, unstructured surfaces with water/air contact angles of 0o, 43 o, 66 o, 87 o, 96 o, and 108 o were tested first to obtain base readings, after which photolithography was used to introduce structured surfaces representative of biological systems. It was shown that the more hydrophilic a surface is, the less prone to oil contamination it is and also that the Cassie-Baxter approximation holds up for submerged oil in water systems. Additionally, the addition of surface structure, even on strongly hydrophobic (oleophilic) surfaces, greatly reduced (≈75% reduction in Fadhesion) a surface’s affinity for oil, thereby preventing oil adhesion. As a continuation of this project, monodispersed hard carbon spheres were coated with poly(styrenesulfonate) to increase their hydrophilicity and force them to sit at a predetermined depth at an oil-water interface. Using the tribometer test previously developed, oil drops coated with these spheres at the interface were pushed into hydrophobic surfaces to which they previously bridged. The presence of the spheres at the interface, however, caused enough steric hindrance to where the drop did not recognize the surface and never spread. In addition, calculations show the spheres to be very stable at the interface and able to stabilize small oil drops at concentrations as low as 0.04 wt. % by water. Successful tests against other drops and a rough metal screw show that the coating to be very stable and able to withstand relatively large forces that may be encountered in a spill. While outside the scope of this research, these spheres could work synergistically with surfactants in order to create a unique solution to oil spills in which drops are small, stable, and non-adhering to surfaces in general (allowing them to stay in the water and be consumed). The second topic of this research revolves around the increase in abilities of dry adhesives, or gecko pad mimics. The Pesika lab has created a working prismatic gecko mimic that creates anisotropic forces in a “gripping” or “releasing” direction. These mimics are effective, cheap, and easy to create yet are limited in use to smooth surfaces. A system is shown to strengthen the adhesion and friction forces of the pads by remedying their ability to deal with larger (mm-scale) and smaller (nm-scale) roughness. In an effort to deal with larger scale roughness, a method of creating “thin” gecko pads that can be molded onto polymer foams is described and an argument is made for use of closed-cell foams as a solution. Going in the other direction, a method of growing zinc oxide (ZnO) nanorods off of the pillars is introduced, and a method to replicate the prismatic pillars with the ZnO nanorods is introduced.