In the 1990s, University of Bonn biologist Wilhelm Barthlott noted the uncanny capacity of lotus plants to shed water and dirt despite making their home in murky bogs. This so-called “lotus effect” led he and his research team in Germany to analyze the plant leaves' surfaces, where they found a microstructure that minimized the contact area for water. By developing a similar microstructure in the exterior-grade Lotusan paint for Sto Corp., Barthlott devised a coating with self-cleaning capability.
Since Lotusan was commercialized in 1999, the pursuit of hydrophobicity—translating literally to "water fearing"—in manufactured products has increased. Until now, water repulsion in buildings has primarily been achieved at the macro level, by using homogeneous non-absorptive materials, such as clay tiles, or coatings, such as sealants. Repelling moisture not only helps keep water from penetrating materials but also from degrading them over time, as seen in rusting metal or rotting wood.
Engineering such a property at the micro- and nano-scale, as in the case of Lotusan paint, offers additional functions beyond decay resistance. These include anti-fogging, anti-icing, anti-microbial, oleophobic, self-cleaning, and drag reduction capabilities. Many building materials—including concrete, wood, glass, and even paper—can be enhanced with such properties.
Variations in the surface microstructure of hydrophobic materials can deliver widely differing results. “The hydrophobic properties are extremely dependent on the morphology and the topography of surfaces,” claim scientists Thierry Darmanin and Frédéric Guittard in a 2015 Materials Today article. A fundamental characteristic concerns how quickly a material can repel fluid and is measured by water contact angle. Hydrophobic materials are defined as having a water contact angle—or "wettability"—at 90 degrees or greater. So-called superhydrophobic materials possess a water contact angle exceeding 150 degrees, making it very difficult for water to adhere to the surface.
These surfaces were designed to resemble many natural organisms that, like the lotus, benefit from water and dirt repulsion. “We have looked at about 24,000 different species of plants and animals and many of them are superhydrophobic or at least have superhydrophobic parts,” Barthlott told Chemistry World in April. And scientists continue to analyze the composition of these organisms’ surfaces. A common conception is that nanostructured surfaces’ properties are based on shape, not chemistry; however, the reality is that they result from a combination of the two.
According to Darmanin and Guittard, nature creates most superhydrophobic surfaces from nanostructured waxes. The water-impermeable cuticle of a springtail insect, for example, is composed of several layers of proteins and lipids—an environmentally benign composite. Unfortunately, most commercial hydrophobic coatings today contain an alphabet soup of industrial chemicals. A top coat multi-surface polish, for example, includes ingredients like aliphatic hydrocarbons, hexyl cinnamal, and butylphenyl methylpropional—chemicals with known human health concerns. Conventional waterproofing spray products contain fluoropolymers known to cause respiratory injuries. A 2008 study conducted by the Michigan State University College of Human Medicine prompted the Michigan Department of Community Health to advise that all brands of hydrophobic spray products be handled with “extreme caution.” Contractors are all too familiar with the unsavory and unsafe task of waterproofing building foundations, which Green Builder columnist Jennifer Caughey describes as “a messy, toxic job, done with highly volatile, petroleum-based sealants that leave you dizzy for the rest of the day.”
The good news is that change is underway. An increasing number of products without questionable ingredients are now available. Some penetrating sealers, for example, are composed of water-based polymers without the use of solvents. However, many of these coatings still rely on fossil fuel feedstocks and do not possess the sophistication of naturally occurring superhydrophobicity.
Emerging materials that can mimic this property using nontoxic ingredients hold significant promise for future building products. One example is a new superhydrophobic material made of alumina nanoparticles researched by scientists at Rice University and the University of Swansea, in Wales. Like Lotusan paint, the nanoparticles form a branching microstructure that mimics the texture of the lotus leaf surface. The resulting material has a water contact angle of 155 degrees and is as effective at repelling water as typical commercial coatings containing hazardous materials.
Scientists at the Institute of Space Technology in Islamabad have also used alumina nanoparticles along with PDMS, an organic silicon-based polymer, to create a superhydrophobic coating for wood. The micro-scale features that develop on the wood’s surface upon the coating's application have demonstrated protection advantages, particularly in humid environments. Researchers at the Universidad of Cádiz, in Spain, have used another abiotic, nontoxic substance—silica nanoparticles—to create a superhydrophobic coating for stone. The densely packed particles of the surface resist fluid penetration via a layer of air molecules that form beneath the water droplets. According to the scientists, the coating can be produced inexpensively and applied in large quantities outdoors, making it suitable for building construction.
Other research breakthroughs demonstrate water control via surface shape and tunability. For example, mechanical engineers at MIT have recently determined that the assembly of a particular bowl-like form on hydrophobic surfaces can further enhance water runoff. By etching concave microstructures into the exterior faces of materials, the researchers can reduce surface tension—and water interaction—by an additional 40 percent. According to the engineers, the technique may be used to “limit heat loss under precipitation [and the] icing of surfaces, reduce salt deposition on a surface exposed to ocean spray, or inhibit the formation of a water film on wings or wind turbine blades.”
Harvard University scientists have created a smart, tunable surface with adaptable pores. Depending on if the hydrophobic material is relaxed or under tension, it interacts with water differently—either stopping it in place or shedding it rapidly. This bio-inspired, “switchable wettability” promises a new level of control over water–material surface interactions—suggesting that the future of superhydrophobic materials will look quite different from pre-Lotusan technologies.
