As record-high temperatures in triple digits bake much of the U.S. Midwest and parts of the East Coast this summer, most people are consumed with figuring out how to beat the heat. But not Azar Alizadeh. Her concern is keeping ice away. It’s not that Alizadeh has an ice phobia. She’s trying to create surfaces with one.
As a senior materials scientist and research group leader at GE Global Research, in Niskayuna, N.Y., Alizadeh is one of several researchers working to understand the processes involved in water and ice accumulation on surfaces. The scientists’ aim is to devise methods for making materials that are water phobic and ice repellent.
“Ice buildup on surfaces is a serious problem in many areas of technology,” Alizadeh says. The added weight can interfere with the operation of aircraft wings, wind turbine blades, and electrical and telecommunications equipment. Accumulated ice in those cases can reduce the equipment’s performance efficiency, leading to economic loss and compromising safety. And removing the ice after it accumulates, for example by scraping or spraying warm deicing fluids, is time and energy intensive and costly.
The adverse effects of ice storms—especially in the areas of transportation and power transmission—are well-known throughout ice-prone regions, such as the northeastern U.S. But those problems are not limited to cold climates or the winter.
Even in warm climates, as airplanes pass through clouds on takeoff and landing, they can strike ice particles and cloud-borne water droplets that can be transformed into ice, Alizadeh points out. And on the ground, air conditioners and refrigerators can quickly lose their cooling capacity, especially on hot and humid days, if the units’ cooling and evaporation coils ice up.
During the past 50 years, researchers have tried to prevent those kinds of problems by preparing materials with various types of long-lasting ice-phobic or low-ice-adhesion coatings, says Ali Dhinojwala, a professor of polymer science at the University of Akron. He adds that the limited level of success with that approach until now underscores scientists’ incomplete understanding of the fundamentals of water-surface interactions.
For example, the detailed process through which water cools when in contact with a surface remains unclear, he says. Also, the onset of ice nucleation and the detailed nature of water layers immediately adjacent to a cold surface are poorly understood. A more thorough understanding of those processes may lead to the desired ice-repellent surfaces.
To begin addressing those issues, Alizadeh, Dhinojwala, and others teamed up to study the dynamics of droplet impact and ice nucleation over a range of temperatures and surface types. The nature of the surfaces ranged from hydrophilic to superhydrophobic. Those characterizations come from classic studies in this field that quantify surface wettability.
On hydrophilic (easily wetted) surfaces, individual droplets spread out, losing their spherical shape and adopting a low surface-contact angle—the angle at which liquid droplets meet solid surfaces, a measure of surface wettability. On hydrophobic and superhydrophobic surfaces, droplets bead up and maintain large or very large (>150°) contact angles.
To probe and quantify the temporal evolution of wetting processes, the team developed a high-speed photography technique and imaged droplets as they impinged, spread, retracted, and finally stabilized on a variety of model surfaces. The list of models includes a flat silicon surface treated with polyethylene glycol (a hydrophilic surface) and a photolithographically textured (roughened) silicon surface treated with a highly fluorinated compound (a superhydrophobic surface).
The key finding in that study is that droplet retraction, especially on hydrophilic surfaces, depends strongly on surface temperature. The lower the temperature, the less retraction (Appl. Phys. Lett., DOI: 10.1063/1.3692598). Reduced retraction means droplets maintain large contact area as they impinge on a cold surface. That condition favors heat transfer to the surface, which, if it’s cold enough, should lead to rapid cooling and ice formation.
To learn more about that low-temperature transformation and ultimately how to avoid it, the group added infrared thermometry capabilities to their experimental setup and probed ice nucleation dynamics. It turns out, as expected, that droplets cool more slowly on hydrophobic than hydrophilic surfaces as a result of the reduced contact area. But on superhydrophobic surfaces, even after droplets equilibrated with the –20 °C surface, there was an unexpectedly long delay—on the order of a minute—before the onset of ice nucleation (Langmuir, DOI: 10.1021/la2045256).
The study shows that heat transfer isn’t the only important parameter affected by contact area. Reducing the area also strongly influences the rate of ice nucleation by lowering the probability of nucleation events at the interface between the droplet and surface, Dhinojwala emphasizes. He adds that the studies provide a basis for understanding the roles of surface structure (roughness) and surface chemistry (hydrophobicity) in forming anti-icing surfaces.
Those two parameters are the focus of other research efforts in this area. At Harvard University, for example, a research group led by Joanna Aizenberg, a professor of materials science and chemistry, reported last year on so-called slippery liquid-infused porous surfaces, SLIPS, which repel a large variety of liquids and solids. The group made the surfaces by impregnating a porous nanostructured material with a water- and oil-repellent (omniphobic) lubricating liquid (C&EN, Sept. 26, 2011, page 5). Now, Aizenberg, postdoc Philseok Kim, and coworkers have applied that design strategy to anti-icing surfaces.
Aizenberg explains that inspiration for the SLIPS design came from the slippery pitcher plant, which is endowed with a microscopically rough surface that locks in a smooth lubricating coating of water. Other bioinspired materials, especially lotus-leaf-style superhydrophobic ones, have also been the focus of ice-resistance studies in recent years. Those materials effectively repel ice under conditions of low humidity and mild temperatures. But under more extreme conditions, the textured high-surface-area materials can induce ice nucleation even faster than smooth ones.
In a just-published study, Aizenberg and coworkers demonstrated a SLIPS-based method for endowing aluminum, one of the most commonly used metals in industrial applications, with water and ice resistance that stands up to high humidity and cold temperatures (ACS Nano, DOI: 10.1021/nn302310q). To form the coating, the group electrodeposited a nanotextured film of polypyrrole on large aluminum sheets, fluorinated the polymer, and infiltrated and coated the film with a smooth polyfluorinated lubricant. They note that the chemical functionalization step fixes the lubricant in place.
Tests comparing SLIPS-treated aluminum to the bare metal show that at high humidity, water droplets readily roll off the treated surface even during rapid cooling, leaving it ice-free during most of the test. Under those conditions, the bare metal rapidly ices over. During the melting phase of the tests, ice and water quickly clear from the treated aluminum but cling to the bare metal.
Ice buildup grounds airplanes, downs power lines, and can compromise performance and safety in numerous applications, even on hot summer days. Ongoing investigations in anti-icing strategies will lead to ways to help keep the frosty stuff from harming equipment, leaving it instead to chill drinks.