Superhydrophobicity
Literally meaning, “fear of water,” superhydrophic materials (SHM) are those which
exhibit a high resistance to water penetration and in turn create a water droplet to surface contact
angle greater than 150° (as seen below). This discovery means that any material, cotton to steel,
could be treated with SHMs to resist water penetration. This type of treatment, as will be
explained in more detail later, has immense applicability in everything from bridge construction
to fighting bacteria.
Do these materials really deserve any attention at all? The applications for SHMs may not
be obvious at first. But before anyone can understand how to apply something, it is necessary to
first know how that something works. For most, visual representations are the most effective
educational tools for total comprehension of the subject at hand. So before any mention of the
microscopic or atomic chemistry at work in these materials, a good understanding of what is
happening macroscopically is of the utmost importance. On the following page is just that, a
visual for one of the more elementary concepts behind the nature of SHMs.
Figure 1. The illustration above represents the contact angle (CA) of a water droplet on the surface of a
superhydrophobic material. To be considered superhydrophobic, the CA must be greater than 150° (Banschbach,
2013).
As can be observed from the illustration above; instead of breaking down and creating a
thin film of water on the surface, the water droplet retains its spherical geometry and sits gently
on the surface as would a basketball on concrete. But what is really happening here? To
understand this question, it is first necessary to understand what drives the variable contact
angles. According to research being done on “Wettability, non-wettability and contact angle
hysteresis,” the contact angle is generally attributed to surface roughness, but also surface
heterogeneity, solution impurities adsorbing on the surface, or swelling (MIT, 2005). With
regards to surface roughness, the more you ‘rough up’ a material at the (ideally) nano-level, the
smaller the contact angle between the SHM and the water droplet. Through this process, the
appearance of the nanostructure is being ‘reconfigured’ into a dense formation of submicrometer
sized needle-shaped crystals (less than 0.2 – 0.3 micrometer in diameter and 2.2 -2.5
micrometers long) which together act as a barrier against water based liquids. These smaller
crystals more effectively enhance the superhydrophobicity. With the help of water’s surface
tension and the large fraction of air between the pillars, the droplet has no other choice but to rest
on the surface or in the case of SHMs, fear away. This phenomenon means lower surface
contact. Lower surface contact means a higher contact angle. The larger the contact angle, the
more hydrophobic the material is (IPSE, 2013).
Figure 2 (left). Botanischer Garten München-Nymphenburg, 2009. The image on the left represents a macroscopic
view of a water droplet on a lotus leaf.
Figure 3 (right). A. Otten and S. Herminghaus, Göttingen, Germany, A.Marshall, Stanford University, A.Snyder,
Exploratorium, 2013. The image on the right represents the nanostructure of the lotus leaf.
SHMs do not exist solely in the lab, but in fact have been around for thousands of years.
The lotus plant, for example, (as seen above) is a great example of a SHM found in nature. The
“bed of really tiny pillars” mentioned earlier is better seen upon review of figure 3. The whitish
“pillars” are the drivers behind superhydrophobicity. Paying close attention to the “pillars” and
also realizing the scale of the image in comparison to the size of a single water droplet, add to the
picture a water droplet that is a 100,000 times larger and it becomes easier to see how the pillars
suspend the water over the underlying surface. Keeping these basic concepts in mind, the
applications for these materials is now much easier realized.
Staphylococcus aureus, streptococcus pheumoniae, klebiella pneumonia, escherichia coli,
and clostridium difficile are five types of bacteria that commonly plague people all around the
world every year. These bacteria also share another common bond; they all could be controlled
and/or prevented with the use of superhydrophobic coatings and/or materials. In fact, bacteria
spread leading to hospitalization and/or death, costs the health care industry $28 to $45 billion
dollars a year (Freschauf, 2012)! SHMs ability to resist water penetration means more than meets
the eye. Most bacteria growth occurs in wet environments. Given that SHMs don’t allow water
to settle on them, due to their non-polar nature, no bacteria would be able to grow. In the event
the product treated with SHMs was designed to retain water, an individual could simply tilt the
object and watch as the bacteria rolls away. In this application, SHMs could be used as a coating
on nearly everything including, but not limited to, hospitals, schools, and residential areas.
Another application for SHMs would be in energy production. According to scientists
with MIT who are conducting more research on SHMs, but this time with regards to fossil fuels
and the application of SHMs in cooling towers. Current cooling towers have a reduced efficiency
due to a built up of water. A simple application of SHMs and their efficiency increases (MIT,
2005).
SHM’s can be applied to any material that may be affected by corrosion. Experiments
were conducted on both ‘as delivered’ and superhydrophobic treated titanium and steel in a salt
water environment. Results of testing concluded successful. The treated specimen showed no
signs of corrosion or microbial buildup whereas the ‘as delivered’ were both excessively
corroded (though the titanium ‘as delivered’ did remain free of microbial buildup). The article
concluded by saying further research is being done in finding a feasible way to coat building
materials for practical uses (Mahalakshmi , 2011). To name a few final applications, SHMs
could also be used for military purposes ranging from clothing to weapons and everything in
between.
Superhydrophobic materials are recently seeing the light of day. Breakthroughs in
manufacturing have been made to create cheaper SHM products and it is likely more consumers
will see them on shelves directly and/or indirectly in the near future (if not already). Already
there have been breakthroughs in the field. Some positive research is being done on materials
that are able to completely resist all liquids including strong acids (i.e. hydrofluoric, nitric,
hydrochloric, and even fluorosulfuric). These materials are called superomniphobic materials,
and with further research guarantee a bright a future.
Citations
A. Otten and S. Herminghaus, Göttingen, Germany, A.Marshall, Stanford University,
A.Snyder,Exploratorium. Nasturtium Leaf Images. [Scanning Electron Microscope],
Retrieved: 4/17/2013 from: URL (http://www.nisenet.org/image-collection/tool%3A-
scanning-electron-microscope)
T. Banschbach. Droplet on the Surface. [Microsoft Paint], Created: 4/17/2013.
Botanischer Garten München-Nymphenburg. (2009). Water droplet on a lotus leaf.
[Photography], Retrieved: 4/17/2013 from: URL (http://www.deutsches-
museum.de/en/exhibitions/new-technologies/nanotechnology/nanocosmos/lotus-leaf/)
Freschauf, L. R., McLane, J., Sharma, H., Khine, M., & Bansal, V. (2012). Shrink-Induced
Superhydrophobic and Antibacterial Surfaces in Consumer Plastics. Plos ONE, 7(8), 1-7.
doi:10.1371/journal.pone.0040987
IPSE: Crone, W. C., Gimm, J. A., deProphetis, W., Zenner, G., and Derenne, T. (2013).
Education and Outreach: Nanotechnology Activity Guides. Retrieved: 4/17/2013 from:
http://web.mit.edu/nnf/education/wettability/wetting.html
Mahalakshmi, P. V., Vanithakumari, S. C., Gopal, J., Mudali, U., & Raj, B. (2011). Enhancing
corrosion and biofouling resistance through superhydrophobic surface modification.
Current Science (00113891), 101(10), 1328-1336.
Nanotechnology and Interdisciplinary Research Initiative (NIRT) and Agrawal, Abhinandan
(June 2005). Wettability, non-wettability and contact angle hysteresis. Retrieved:
4/17/2013 from: http://web.mit.edu/nnf/education/wettability/wetting.html