September 24, 2012
Low-friction, anti-corrosive research at Stevens promises durability and improved hydrodynamics for naval vessels
Dr. Chang-Hwan Choi
Natural surfaces of plants such as lotus leaves, the skin of marine animals, and the wings of insects have fascinated people throughout history because of their ability to repel water and stay clean in wet, muddy environments. Sometimes referred to as the Lotus Effect, this phenomenon means that water droplets on these surfaces tend not to compress or deform, instead remaining upright in the shape of a bead. Due to a complicated surface structure that minimizes adhesion, the droplets can easily roll off when the surface is tilted, often taking dirt particles with them.
The property of being extremely hard to wet, to the extent that the angle at which a water droplet meets the surface exceeds 150 degrees and the angle of tilt at which the droplet will roll off the surface is less than 10 degrees, is called superhydrophobicity. In recent years, materials scientists and engineers have taken inspiration from these natural surfaces and sought to understand and develop superhydrophobicity for use in marine vessels and military vehicles that are susceptible to the long-term effects of exposure to water. Dr. Chang-Hwan Choi of the Department of Mechanical Engineering at Stevens Institute of Technology has previously been awarded a Young Investigator Award grant from the Office of Naval Research (ONR) to develop superhydrophobic surfaces that avoid corrosion, and he has recently been awarded a Defense University Research Instrumentation Program (DURIP) grant from the ONR to take his research further, maximizing the long-term stability of the surface air layer entrapped on superhydrophobic surfaces, which will significantly enhance the hydrodynamic and anti-corrosive efficiency of marine vessel surfaces.
(a-b) Scanning electron microscope images of multi-level superhydrophobic surface structures of a lotus leaf (a: coarse-scale microstructures of ~10-20 µm, b: finer nanostructures of ~100-500 nm covering the coarse microstructures). (c-d) Contact angle and roll-off angle of a water droplet placed on the lotus leaf surface.
“This grant affirms the expanding role of Stevens in engineering next-generation nanoscale surfaces,” says Dr. Michael Bruno, Dean of the Charles V. Schaefer, Jr. School of Engineering and Science. “Dr. Choi’s work is transforming our understanding of the potential applications of nano-engineered surfaces, and maximizing the practical benefit of that knowledge to US naval capabilities.”
Dr. Choi’s previous work on the subject used rough surface structures at extremely small scales to maximize superhydrophobic properties. His latest innovation involves surfaces patterned at the nanoscale to emit and hold air bubbles, creating a layer of gas that reduces the friction of the surface in the same way that air holes on an air-hockey table allow the puck to move freely. Because resistance depends strongly on the viscosity of the fluid through which an object moves, and gases are far less viscous than water, a low viscosity gas layer between a vessel and water would provide much less resistance. Previous attempts show that the air bubbles gradually dissolve or dissipate due to the flow of water against the surface. Researchers have attempted to use microscale patterning and bubbles, which are relatively easy to develop and analyze using common apparatus, such as a microscope. However, Dr. Choi discovered that x-ray spectroscopy can visualize the shape, morphology and surface dynamics of air bubbles at the nanoscale, where he will develop structures that prevent the loss of gas bubbles in water. The DURIP grant will allow Stevens to acquire small-angle x-ray spectroscopic equipment in order to deliver structural information, such as average particle size, shape, distribution, and surface-to-volume ratio, on macromolecules that are as small as 5 nanometers (a nanometer is a billionth of a meter). At this scale, Dr. Choi will be able to develop a stable and robust superhydrophobic surface that retains a low-friction gas layer, reducing drag and allowing a vessel to move more efficiently through water.
In addition to providing low friction, Dr. Choi’s breakthrough surfaces will allow marine vessels to avoid excessive contact with seawater, which contains chlorine ions that corrode the aluminum-based surfaces of modern naval vessels. Over 3% of the nation’s gross domestic product is spent combating corrosion. The Department of Defense spends about $23 billion a year to mitigate its effects, and the US Navy alone spends 10-12 billion dollars on corrosion protection annually. Consequently, surfaces that avoid or slow corrosive processes in a more cost-effective manner would have a substantial financial impact for the military and the general population. Dr. Choi will also assess the potential benefits of the gas layer in terms of the dynamics of temperature and pressure, thus establishing multiple facets of advantage for the technology.
“As Dr. Choi broadens our understanding the mechanisms behind superhydrophobicity and enables more intelligent engineering of these surfaces, he is remarkably widening the already vast scope of applications of the technology,” says Dr. Constantin Chassapis, Deputy Dean of the School of Engineering and Science, and Director of the Department of Mechanical Engineering.
Dr. Choi's previously received a DURIP grant in 2011 to fund an environmental scanning electron microscope (ESEM) that enhanced his study of the functionalities of novel prototypes of nanostructured materials. The ESEM allows his lab to study and observe the wetting dynamics of water—phenomena such as condensation and evaporation—on nano-patterned superhydrophobic surfaces.
In 2010, he also received a DURIP grant to fund a state-of-the-art thin film deposition system. This instrumentation allows Dr. Choi to deposit layers of light metals such as aluminum, which is commonly used in naval applications, with engineered nanocharacteristics that create a water-repelling surface.