Liquid-Infused Nanostructured Surfaces with Extreme Anti-Ice and Anti-Frost Performance
Original entry by Bryan Weinstein, Fall 2012
Authors: P. Kim, T. Wong, J. Alvarenga et al.
Keywords: Wetting, Anti-Ice, Contact Angle Hysteresis, Thin Film, Coating, Porous Surface, Textured Surface, Intruding fluid, Lubrication, SLIPS
Ice formation on various technologies today poses serious safety risks. For example, if ice forms on the wing of an airplane before it takes off, the plane's lift will be greatly reduced and it can crash. Despite the need for anti-ice coatings, most state-of-the-art coatings prevent freezing only under "moderate conditions" (for example, a temperature less than <math>-5</math> degrees Celsius and a relative humidity greater than 50%).
There are two main approaches to creating icephobic materials. One approach is to create an extremely smooth surface with small contact angle hysteresis and low wettability. In practice, however, this is quite difficult; it is nearly impossible to eliminate all defects and inhomogeneities on a surface. Another strategy is to create a “highly textured surface” that will decrease the surface’s ice-forming ability.
Dr. Aizenberg’s lab has created a new type of anti-ice coating by saturating a porous substrate with a lubricating liquid in order to create an ultra-smooth lubricating layer. This liquid layer is both defect free and molecularly flat; its contact angle hysteresis is consequently extremely small. As a result, it repels almost all immiscible materials and consequently helps to prevent the formation of ice. The design of the “slippery, liquid-infused porous surfaces” (SLIPS) were motivated by the slippery surface of the pitcher plant. An artists illustration of SLIPS can be seen in Figure 1.
In order for SLIPS to work, three criteria must be fulfilled, according to the paper:
- "The lubricating and repellent fluid have to be immiscible"
- "The chemical affinity between the lubricating fluid and the solid should be higher than the affinity between the repellent fluid and solid"
- "The solid surface should have a porous structure to provide increased surface area for the adhesion of the lubricating fluid."
The Aizenberg group developed a method to pattern SLIPS on aluminum as aluminum is widely used in industry. They deposited highly textured polypyrrole (PPy) via electrodeposition on aluminum; this created the porous surface. They found that this method could be scaled-up to create porous surfaces on arbitrarily large pieces of aluminum, an important find for industry applications. To complete the SLIPS, they fluorinated the structured with trichlorosilane and infiltrated the material with prefluorinated Krytox 100 (the lubricating liquid).
The Aizenberg group then wanted to quantitatively measure the performance of the anti-ice layer. Essentially, the smaller the contact angle hysteresis of a material, the smaller the drops are when they fall off of the material due to an incline. Therefore, to quantify the material’s anti-ice performance, the Aizenberg group measured the material’s contact angle hysteresis. The SLIPS were found to have a hysteresis of <math>\delta\theta = 2 \pm 1</math>°. In comparison, untreated aluminum had a contact angle hysteresis of <math>\delta\theta = 41 \pm 4</math>°. Consequently, droplets fell off of SLIPS at diameters approximately 8 times smaller than that of untreated Aluminum.
To test that the smaller droplet diameter helped to prevent water from freezing on the SLIPS, the Aizenberg group built a humidity-controlled chamber where they could carry out frosting/defrosting experiments. They carried out all of their experiments at 60% relative humidity. As expected, the SLIPS displayed much better anti-icing performance than the untreated aluminum. Ice took much longer to form on the SLIPS and when it did, it had a much less stable structure. Ice tended to occur in large, isolated patches while ice on the untreated aluminum tended to consist of densely packed sheets. The average ice adhesion strength on SLIPS was about 15.6 kPa while the adhesion strength on the untreated aluminum was 1359 kPa. Before SLIPS, state-of-the-art ice-repelling materials had an adhesion strength of approximately 165 kPa. Since the adhesion force between ice and the SLIPS is so small, SLIPS would work particularly well on objects that are exposed to weak shear forces or tilt; the ice would simply fall off. SLIPS clearly have superior performance to other anti-ice coatings.
SLIPS are clearly a brilliant piece of technology that will be used by many industries in the future. Their ability to repel ice is astonishing; its ice-repelling behavior is over an order of magnitude better than other leading coatings. Its utility, however, will be limited by how easy it is to pattern on various surfaces and how expensive it is to do so. This is an engineering problem that the Aizenberg lab is working on now.
 Kim, P. et al. Liquid-infused nanostructured surfaces with extreme anti-ice and anti-frost performance. ACS Nano 6, 6569–77 (2012).