Increasing water resistance of rough surfaces

Dr. Neil Canter, Contributing Editor | TLT Tech Beat December 2013

A one-atom layer thick graphene coating significantly improves water resistance.

KEY CONCEPTS
• A one-atomic layer of graphene known as a nanodrape acts to greatly improve the water repellency of a rough metal surface.
• This nanodrape dramatically reduces the friction holding a water droplet to the metal surface.
• A further reduction in friction is achieved by increasing the number of graphene layers from one to three.

WATER’S ROLE AS A CONTAMINANT in lubricant systems is well known. The presence of water can lead to such problems as corrosion and microbial contamination.

Researchers are continuing to develop surfaces that are very effective at repelling water. Such surfaces exhibit a property known as hydrophobicity.

In a previous TLT article, the development of a material known as SLIPS (slippery liquid-infused porous surfaces) was described (1). This porous network of polytetrafluoroethylene fibers exhibits the ability to repel both water and mineral oil and can form omniphobic surfaces.

Graphene is a single, two-dimensional layer of carbon atoms organized into hexagonal structures. A good deal of research is underway to better understand graphene’s properties. As an example, a previous TLT article discussed the use of graphene as a potential corrosion inhibitor on copper and nickel (2). Application of graphene through the use of chemical vapor deposition produced metal-coated surfaces that demonstrated superior corrosion protection compared to bare metal surfaces in tests such as cyclic voltammetry.

Due to graphene’s hydrophobic nature, this material has been looked at to repel water on flat surfaces. But research evaluating the potential for developing a protective coating for rough surfaces has not been fully explored.

Dr. Nikhil Koratkar, the John A. Clark and Edward T. Crossan Professor of Engineering at Rensselaer Polytechnic Institute in Troy, N.Y., says, “There are many applications where improving water repellency of a rough surface can lead to improved performance. Window panes on buildings and cars are impacted by water that can cause staining when combined with contaminants that are already present. The optical transparency of graphite may be able to facilitate water removal which will minimize staining.”

Rough surfaces present a bigger challenge than smooth surfaces in repelling water. In particular, water can displace entrapped air pockets on a rough surface and adhere quite strongly to that surface, making removal extremely difficult. Koratkar says, “When water initially is placed on a rough surface, it will literally flow on top of the surface in a low-friction state known as the Cassie state. But if water is impacted onto a surface, it will displace air and transition into a sticky, high-friction state known as the Wenzel state.”

Graphene has the potential to act as a coating to prevent water droplets from displacing air pockets and facilitating their removal from rough surfaces. Research now demonstrates the benefits of using graphene in this capacity.

NANODRAPES
Koratkar and his fellow researchers have demonstrated that a thin layer of graphene can act as a coating to dramatically improve the water repellency of a rough metal surface. He says, “We applied graphene at a thickness of one-atomic layer to a rough surface prepared from copper nanorods and found a significantly lower level of friction compared to the baseline copper nanorod surface.”

This coating is less than a nanometer thick and can be described as a nanodrape that is chemically inert and protects the properties of the metal substrate. The copper nanorods were applied onto a flat silicon wafer.

Koratkar says, “Copper nanorods were chosen because they are hydrophilic in nature and form a well-defined nanoscale surface roughness profile. However, we expect the results to be equally valid for other metal surfaces.”

Water droplets were then applied to the grapheme-draped metal surface and the baseline metal surface at a velocity of 60 centimeters per second. Images of the interaction of the water droplet with each surface were recorded and contact angles measured as the water droplet was advanced and retracted along the surface.

The friction between the water droplet and the surface was measured by determining the contact angle hysteresis, the difference between the contact angle for an advancing and retracting water droplet. Koratkar says, “We found that the graphene drape strongly influences the receding contact angle. Without a graphene coating, the receding contact angle is nearly zero degrees, which indicates that the water droplet strongly adheres to the surface. In the presence of the graphene coating, the receding contact angle increases to approximately 60 degrees.”

The result is that the contact angle hysteresis or friction holding the water droplet to the surface dramatically declines in the presence of the graphene drape. Figure 1 shows this effect as the water droplet’s movement on the graphene- draped surface is recorded in real-time.


Figure 1. Snapshots show a water droplet traveling at a velocity of approximately 60 centimeters per second, striking the surface of graphene-draped copper nanorods. The time listed in each picture is the time after the water droplet strikes the surface. (Courtesy of Rensselaer Polytechnic Institute)

Koratkar says, “In our dynamic contact angle study, we found from our measurements that the graphenedraped surface exhibited over an order of magnitude less frictional energy dissipation as compared to the baseline metal nanorods surface.”

The researchers found that increasing the number of graphene layers from one to three leads to a further reduction in friction. Koratkar says, “We believe that the higher friction seen with one layer is due to defects in the graphene that occur during processing. Once a second graphene layer is applied to the nanodrape, the preceding graphene layers provide a cushioning effect that minimizes damage to the topmost graphene layer.”

Future work will involve finding potential applications for graphene nanodrapes. Koratkar says, “We are examining how graphene nanodrapes can be used on windows to improve their performance through minimization of staining. As part of this effort, we are working with the RPI School of Architecture.”

Graphene nanodrapes have now been shown to repel water, reject penetration of other contaminants and not affect the properties of the substrate. This means graphene nanodrapes may impart lubricity, as well as act as a protective coating on metal surfaces. As a result, this type of self-lubricating coating may find use in a number of lubricant applications.

Further information can be found in a recent article (3) or by contacting Koratkar at koratn@rpi.edu.

REFERENCES
1. Canter, N. (2012), “Self-Lubricating Omniphobic Surface,” TLT, 68 (1), pp. 10-11.
2. Canter, N. (2012), “Graphene: Potential Corrosion Inhibitor,” TLT, 68 (7), pp. 12-13.
3. Singh, E., Thomas, A., Mukherjee, R., Mi, X., Houshmand, F., Peles, Y., Shi, Y. and Koratkar, N. (2013), “Graphene Drape Minimizes the Pinning and Hysteresis of Water Drops on Nanotextured Rough Surfaces,” ACS Nano, 7 (4), pp. 3512-3521.


Neil Canter heads his own consulting company, Chemical Solutions, in Willow Grove, Pa. Ideas for Tech Beat items can be sent to him at neilcanter@comcast.net.