Patterned Superhydrophobic Surfaces: Toward a Synthetic Mimic of the Namib Desert Beetle
By Sung Hoon Kang
Title: Patterned Superhydrophobic Surfaces: Toward a Synthetic Mimic of the Namib Desert Beetle
Reference: L. Zhai, M. C. Berg, F. C. Cebeci, Y. Kim, J. M. Milwid, M. F. Rubner, and R. E. Cohen, Nano Lett. 6, 1213-1217 (2006).
Soft matter keywords
superhydrophobic surface, hydrophilic pattern, contact angle, Namib desert beetle
Abstract from the original paper
The present study demonstrates a surface structure that mimics the water harvesting wing surface of the Namib Desert beetle. Hydrophilic patterns on superhydrophobic surfaces were created with water/2-propanol solutions of a polyelectrolyte to produce surfaces with extreme hydrophobic contrast. Selective deposition of multilayer films onto the hydrophilic patterns introduces different properties to the area including superhydrophilicity. Potential applications of such surfaces include water harvesting surfaces, controlled drug release coatings, open-air microchannel devices, and lab-on-chip devices.
Soft matter example
In the Namib Desert where there is limited water source for plants and animals, there are very interesting creatures that have unique ways to collect water from the atmosphere. For example, the Stenocara beetle utilizes its wings with hydrophilic/superhydrophobic patterns to acquire water from fog-laden wind. At dawn, the Stenocara beetle leans its body toward the wind to collect small water droplets in the fog. Then, these small water droplets coalesce and become bigger droplets. When the droplets reach certain sizes, they roll down into the beetle's mouth. Parker and his colleagues reported that the structure of the beetle's back provided this unique ability to collect water . According to their study, the beetle's back has an array of hydrophilic bumps with ~ 100 um in diameter on a superhydrophobic background. As a result, small water droplets in a fog are collected on the hydrophilic area and coalesce. When the weight of the droplet is big enough to overcome the adhesion forces of the hydrophilic region, it is detached from the surface and rolls down the superhydrophobic surface to the mouth of the beetle. In addition to this example of water collection, patterned surfaces with different wetting properties can be useful for many applications including microfluidic channels and rapid evaluation of complex bioactivities [2-4].
In this paper, the authors reported a method to make hydrophilic or superhydrophilic patterns on a superhydrophobic surface inspired by the Stenocara beetle's back. The details of experimental methods are described in the paper. Using their methods, they made an array of hydrophilic spots with size of 750 um onto a superhydrophobic surface by selected delivery of polyelectrolytes to the surface in a mixed water/2-propanol solvent. Then, the samples were characterized by various methods. From contact angle measurements, the advancing contact angle of water in the patterned region was 144' and the receding contact angle was 12'. They also tested their samples by spraying a mist of water onto the surface. As shown in Fig. 1, small water droplets (~250 um) did not wet the superhydrophobic surface and formed almost perfect spheres and most of the droplets did not stay on the superhydrophobic areas and formed large water droplets in the patterned hydrophilic region by coalescence of small drops.
As another example of a surface with patterns of different wetting properties, they also fabricated a surface with superhydrophilic canals on a superhydrophobic background. To make superhydrophilic canals, they selectively deposited multilayers of superhydrophilic materials onto hydrophilic stripes previously pattterned on a superhydrophobic surface. The wetting behavior of microcanals by water was studied using a contact angle instrument for samples with different numbers of PAH/SiO2 bilayers. As shown in Fig. 2A, it took more than 10 s for a water droplet to spread 1 cm along the 750 um wide canal made of 4 bilayers of PAH/SiO2. A small bulge at t=0.21 s in Fig. 2A indicates that the surface does not sprad the water fast enough to make the canal superhydrophilic due to the capillary force. However, if there is a sufficient number of PAH/SiO2 bilayers as the case of the microcanal with 14 bilayers of PAH/SiO2 shown in Fig. 2B, water spreads along a 6 cm long microcanal in 2 s.
Patterns with extreme wetting characteristics can be utilized for generating densely packed small reaction sites for fast evalulation of biomolecular interactions [5-7]. In this application, the patterned areas should have a uniform morphology and have no interactions between neighboring sites. The high contrast of wetting properties between the patterned area and the background as well as the cytophobicity and protein adsorption resistance of a superhydrophobic background can make the patterned superhydrophobic surfaces of this paper useful for such applications.
Fig 3 shows an array of circular hydrophilic spots with diameter of ~750 um on a superhydrophobic surface. Solution of 2 uL of UV-excitable fluorescent dyes were deposited onto the individual spots. By using the same technique, it would be possible to deliver a specific functionalizing reagent to a spot which can give us a simple way of studying cell viability, adhesion and response to different reagents.
This paper was interesting to me because they demonstrated synthetic approaches to mimic water collecting abilities of the Namib Desert beetle and showed the potential of their work that can be useful for making patterns with desirable chemical functionalities by selective delivery of chemicals or multilayer films.
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