The Ohio State University

SURFACE CHARACTERIZATION, ADHESION, AND FRICTION PROPERTIES OF HYDROPHOBIC LEAF SURFACES AND NANOPATTERNED POLYMERS FOR SUPERHYDROPHOBIC SURFACES

Zachary T. Burton
M.S. Thesis,
Department of Mechanical Engineering, June 2005

ABSTRACT

Super-hydrophobic surfaces as well as low adhesion and friction are desirable for various industrial applications. Certain plant leaves are known to be hydrophobic in nature. These leaves are hydrophobic due to the presence of microbumps and a thin wax film on the surface of the leaf. The purpose of this study is to fully characterize the leaf surface and to separate out the effects of the microbumps and the wax on the hydrophobicity. Furthermore, the adhesion and friction properties of the leaves, with and without wax, are studied. Using an optical profiler and an atomic/friction force microscope (AFM/FFM), measurements on the hydrophobic leaves, both with and without wax, were made to fully characterize the leaf surface. Using a model that predicts contact angle as a function of roughness, the roughness factor for the hydrophobic leaves has been calculated, which is used to calculate the contact angle for a flat leaf surface. It is shown that both the microbumps and the wax play an equally important role in the hydrophobic nature as well as adhesion and friction of the leaf. This study will be useful in developing super-hydrophobic surfaces.

The next logical step in realizing hydrophobic surfaces that can be produced in the lab is to learn from the hydrophobic leaves and design surfaces based off of that learning. This study looks at polymer surfaces that have a nanopattern present on the surface that mimics the surface found on hydrophobic leaves, such as lotus. This study explores the effect of nanopatterning on hydrophobicity, adhesion and friction for two different hydrophilic polymers, poly(methyl methacrylate) (PMMA) and polyurethane acrylate (MINS) with two types of patterned asperities, low aspect ratio and high aspect ratio. In addition to the polymers, a hydrophobic coating was deposited on the surface of the patterned PMMA to study the effect of roughness on the contact angle, along with adhesion and friction. Relative contribution due to change in contact angle and real area of contact are explored. Scale dependence on adhesion and friction was also studied using AFM tips of various radii. Since applications of these surfaces will require operation in varying environments, the effect of relative humidity is investigated.

Tribological issues in micro/nanoelectromechanical systems (MEMS/NEMS) and BioMEMS/NEMS are one of the concerns for the reliability of these devices. Silicon based components for MEMS/NEMS devices have been the standard but new materials, such as polymers, are gaining use due to their advantages over silicon in the area of BioMEMS. Adhesion and friction properties of two polymers of interest, poly(methyl methacrylate) (PMMA) and polydimethylsiloxane (PDMS), along with a single-crystal silicon substrate, Si (100), are investigated by using a novel microtriboapparatus capable of adopting BioMEMS components used in microfluidic devices. The adhesion and friction properties of PMMA and PDMS are dependent upon the surrounding environment and to study these issues, three different liquids are applied to the interface of the contacts in two different forms, a drop and a film. Other environmental effects are also investigated in this study by looking at relative humidity, temperature, rest time and different gaseous environments such as nitrogen gas and reduced pressure.