Many natural surfaces repel water, allowing them to stay relatively dry and clean in wet, muddy environments. The most famous example is the lotus leaf, which has a complex surface architecture that prevents water and dirt from adhering, thus allowing it to remain clean in ponds, lakes and marshes. Materials scientists are reproducing this property in the surfaces they fabricate in order to bring self-cleaning, anti-icing, anti-fouling and anti-corrosive properties to the surfaces and structures of modern technology. Dr. Chang-Hwan Choi of the Department of Mechanical Engineering at Stevens Institute of Technology and Dr. Wei Xu, a research associate at Stevens, have published an article in Physical Review Letters which revises the scientific understanding of the stickiness of superhydrophobic surfaces to water droplets.
“Dr. Choi and Dr. Xu’s work is strengthening the foundations of superhydrophobic surface engineering,” says Dr. Michael Bruno, Dean of the Charles V. Schaefer, Jr. School of Engineering and Science. “The publication and highly positive response is indicative of the advanced expertise of Stevens micro- and nanoscale surface researchers.”
Evaporating water droplets on a patterned superhydrophobic surface do not retreat in a regular, even manner, but instead deform based on the geometry of the surface. Researchers previously believed it was the contact area or the apparent contact line between the surface and the droplet that determined the way the droplet retreats, but the Stevens researchers meticulously reassessed the interface between a droplet and a surface and found that neither of the parameters exactly match the depinning force (i.e. the force that determines how an evaporating droplet peels away from the surface). Dr. Choi and Dr. Xu’s work reveals that the actual contact line of a water droplet at its periphery where it meets the extremely water-repellent surface, dynamically altered during evaporation, directly determines this force. They have demonstrated that analyzing the actual contact line of a dynamically moving droplet on a surface will enable accurate prediction of the sticky properties of a surface.
This new insight into superhydrophobic surfaces helps researchers to develop modulated adhesion properties, i.e. “slippery” or “sticky”, for customized applications. The “slippery” superhydrophobic surfaces stay clean and dry, thus increasing durability and reducing maintenance costs for civil infrastructure, biomedical devices, and vehicles. Submarines with such surfaces can move through water with less resistance because of the water repellence. Car windshields with this property would have self-cleaning capabilities, avoiding dirt and water much like a lotus leaf in a pond. Meanwhile, the “sticky” superhydrophobic surfaces still resist complete wetting but remain “sticky”, meaning that a water droplet on the surface has a minimal contact area but rolls off only after a strong force is applied. The “stickiness” allows for better control of the droplets in the applications of liquid transportation and analysis as well as biotechnologies. These surfaces also have promising applications in digital microfluidics, in which researchers are engineering circuits that manipulate liquid droplets, creating a lab-on-a-chip that will enable portable, inexpensive, home medical testing.
Dr. Choi is an expert on superhydrophobicity and its applications, having received a Young Investigator Award grant and three DURIP grants from the Office of Naval Research over the past three years to study the phenomenon.