Hierarchical micro/nano structures for super-hydrophobic surfaces and super-lyophobic surface against liquid metal
© Yoon et al.; licensee Springer 2014
Received: 13 November 2013
Accepted: 27 May 2014
Published: 3 September 2014
Non-wetting super-hydrophobic or super-lyophobic surfaces are of great interest in a variety of applications. Natural water repelling surfaces show micro-/nano- combined hierarchical structure which shows extremely low wettability and self-cleaning characteristics. Inspired by such natural wonders, there have been tremendous efforts to create artificial non-wetting super-hydrophobic or super-lyophobic surfaces. In this paper, recent progress in artificial super-hydrophobic surfaces based on hierarchical micro-/nano- dual-scale structure is reviewed. In addition, application of hierarchical micro-/nano- dual-scale structure as super-lyophobic surfaces against gallium-based liquid metal alloy is also reviewed.
Natural water repelling surfaces, such as lotus leaves, butterfly wings, and shark scales exhibit high water contact angle in excess of approximately 150° –. Surfaces with water contact angles greater than 150° is called super-hydrophobic. It has been reported that such super-hydrophobicity stems from a combination of hierarchical micro-/nano-scale surface texturing and surface chemical composition . Combination of micro- and nano-scale structure is crucial as water droplet on these hierarchical structures may touch on the apex of the nano-scale structures without fully sitting on the surface because air pockets fill in the vicinity of nano-scale structures beneath the droplet resulting in low contact area of water droplet, high contact angle, low contact hysteresis, and low adhesive force .
Mimicking such natural wonders and creating artificial super-hydrophobic surfaces on various solid substrates has been of great interest over the past few decades and there have been considerable amount of reported literatures on the topic. Although a variety of materials and technologies were studied to create artificial hierarchical micro- /nano-scale surface textures for super-hydrophobicity, many of the studies are based on materials that are solid and optically opaque in visible wavelengths. Super-hydrophobic surfaces with high optical transmission in visible wavelengths have considerable potential to be used in a variety of self-cleaning surface applications such as optical component, lens, displays, electronic equipment, automobile glass, anti-fog consumer glass, solar panels, among others. In order to maintain high optical transparency, it is critical that structural roughness on the artificial lotus effect surface should be much smaller than the wavelength of transmitted light .
Along with super-hydrophobic surfaces, there have been great interests in creating anti-wetting surfaces against oils  as well as microfluidic platforms for manipulating gallium-based non-toxic liquid metals . With the phenomenal advance of artificial super-hydrophobic surfaces, hierarchical micro-/nano-scale surfaces may provide starting platforms for study on creating super-oleophobic surfaces and super-lyophobic surfaces against liquid metals.
This review discusses recent research progress on hierarchical micro-/nano-scale surfaces as artificial super-hydrophobic surfaces as well as super-lyophobic surfaces against gallium-based liquid metal alloys.
Definition of lyophobic/lyophilic
The definition of ‘lyophobic/lyophilic’ is the physical property of a molecule that is repelled from or attracted to a mass of liquid. Therefore, the ‘lyophobic or lyophilic’ should be used with indication of target liquid except hydrophobic/hydrophilic or oleophobic/oleophilic that implies target liquid is water or oil, respectively. The ‘lyophobic/lyophilic’ is the superordinate concept to hydrophobic/hydrophilic.
Generally, when the contact angle is smaller than 90°, the solid surface is considered to be ‘lyophilic’ which means wetting of liquid on the surface is favorable. When the contact angle is larger than 90°, the solid surface is considered ‘lyophobic’ which indicates the liquid will minimize its contact on the surface and form a compact liquid droplet. When the contact angle is greater than 150°, the solid surface is considered to be ‘super-lyophobic’. And when the contact angle is almost 0°, the surface is considered to be ‘super-lyophilic’ .
Contact angle hysteresis
Along with static contact angle, in order to quantify the ‘dynamic’ wetting property of liquid on a solid surface (when the droplet is in the transitional motion), ‘dynamic’ contact angle is an important parameter. When a droplet is placed on an inclined surface, the droplet can experience gravitational force so that the shape of the droplet becomes asymmetric; on the downhill side, the droplet advances but on the uphill side, the droplet recedes. Therefore, the contact angle of the droplet on the downhill side is an advancing contact angle and that of the droplet on the uphill side is a receding contact angle. The difference between the advancing angle and the receding angle is called contact angle hysteresis. Due to the difference of advancing and receding contact angles, the droplet can stick to the surface against gravitational force. Additionally, the dynamic contact angle can be measured by changing the volume of liquid. The volume of liquid is increased or decreased until the droplet has a maximum or minimum contact angle without changing the surface area between the liquid and the solid substrate. The maximum and minimum contact angles are called the advancing and receding contact angle, respectively, and the advancing and receding contact angles are obtained from a series of images from a recorded video just before the contact line changes. When the advancing and receding contact angles of liquid on a solid substrate are close each other, which means the lower contact angle hysteresis, it indicates the substrate is lyophobic to the liquid .
Super-hydrophobic surfaces in nature
Artificial super-hydrophobic surfaces
Understanding reasons for the super-hydrophobicity in various plant surfaces and insect wings among others in nature has allowed extensive researches in artificial super-hydrophobic surfaces using various materials. A surface with combined characteristics of high water contact angles (>150°) with low sliding angles (tilted angle which allows free rolling of water droplets on a surface) typically smaller than 10° is commonly known as self-cleaning surfaces. One of the motivations for the popularity in the research community on the super-hydrophobic surfaces is because of potential manufacturing of artificial self-cleaning surfaces. Inspired by natural super-hydrophobic surfaces, a wide variety of fabrication processes such as deep reactive ion etch (DRIE) process ,, electrodeposition –, self-assembly , plasma treatment ,, chemical vapor deposition ,,, and layer-by layer deposition  have been studied to fabricate artificial biomimetic super-hydrophobic hierarchical micro-/nano-scale structures.
Among various artificial super-hydrophobic surfaces, optically transparent super-hydrophobic surfaces have great potential to be used in a variety of self-cleaning surface applications such as the prevention of the adhesion of dust and snow to window glasses, traffic indicators, goggles, and solar panels, etc. Optically transparent coatings, self-cleaning surfaces and anti-reflective coatings are active research topics targeted for the coating industry, consumer glass, optical component/lens manufactures, displays/electronic equipment and aerospace. In order to make surfaces transparent to certain range of wavelengths of light, it is essential that the surface roughness of the artificial super-hydrophobic surfaces should be much smaller than the wavelength of light . This provides one of the key design criteria for the optically transparent self-cleaning surfaces which should have nano-scale roughness much smaller than wavelength of visible light (400 ~ 700 nm) while having micro-scale roughness for enhanced air-trapping.
Among various hierarchically textured super-hydrophobic surfaces, in order to introduce various materials and methods to fabricate them, we have studied four different super-hydrophobic surfaces: silica nanoparticles, carbon nanotube (CNT), PDMS, and SU-8 based super-hydrophobic surfaces.
Silica nanoparticles based super-hydrophobic thin film
Carbon nanotube based super-hydrophobic thin film
SU-8 based super-hydrophobic thin film
SU-8, one of the most popular materials used in the field of microelectromechanical systems (MEMS), shows a water contact angle of 73.1 ± 2.8° and surface energy of 45.5 ± 0.3 mJ/m2. Although the material itself does not show hydrophobicity, SU-8 is one of the great materials to create super-hydrophobic thin film because it is readily fabricated in high aspect ratio microstructures to have roughened surface which can be tuned its wetting property. In order to realize the property, it is essential to combine artificial nano-scale textures with the micro-patterned SU-8, forming hierarchical micro-/nano-scale structured surface. Recently, there have been a few efforts to create artificial super-hydrophobic thin film using SU-8 as surveyed below.
Hong et al. prepared a direct mixture of polytetrafluoroethylene (PTFE) nanoparticles into SU-8, spin-coated the PTFE-SU-8 nano-composite on transparent substrates such as glass and polymers, and photo defined micro-scale patterns with a minimum feature size of 50 μm . This film showed water contact angle of 150°, which is significant improvement in hydrophobicity from normally not so hydrophobic SU-8 flat surface. This is attributed to the effect of formation of hierarchical roughness on the surface as well as reduction of effective surface energy of the substrate. Although this result seemed good, optical transparency of this PTFE-SU-8 nano-composite film was only 31%, prohibitively low in practical optical transparency applications. They also reported an alternative method in which the PTFE nanoparticles were spray coated and thermally immobilized onto the exposed SU-8 matrix. During the SU-8 developing process, PTFE nanoparticles sprayed onto the unexposed SU-8 layer were also removed. The PTFE-SU-8 film prepared by this alternative method showed a water contact angle of 165° ~ 167° and an optical transparency up to 80%.
Marquez-Velasco et al. reported super-hydrophobic surfaces fabricated in SU-8 by having micro- and nano-scale topography . In this work, SU-8 was patterned to have low aspect ratio cylindrical and square pillars with thickness of 45 μm and 75 μm. Then, the SU-8 pillars were O2 plasma treated to roughen the SU-8 pillar surface to have 20 nm (30 sec. etch) or 2 μm (5 min. etch) sub-pillars formed on the SU-8. It was shown that 2 μm sub-pillars were too tall and readily clung together to lose its purpose (dual-scale topography). However, dual-scale topography SU-8 formed by 20 nm sub-pillars clearly showed super-hydrophobicity (contact angles > 157°, contact angle hysteresis as low as 5º). Although there was a potential, optical transparency of the dual-scale topography of this SU-8 film was not reported.
PDMS based super-hydrophobic thin film
Although SU-8-based artificial super-hydrophobic thin films overall showed excellent hydrophobicity, the optical transparency in visible wavelengths turned out to be less than 80%. For practical optically transparent self-cleaning thin film applications, it is highly desirable to have greater than 80% optical transparency. Polydimethylsiloxane (PDMS) was one of the popular materials studied by various groups for optically transparent artificial self-cleaning thin film applications. The popularity for the PDMS was due to many favorable inherent material properties. Some of the key material properties of the PDMS for the optically transparent super-hydrophobic thin film applications are its low surface energy of 19.8 mJ/cm2, high optical transparency throughout the ultraviolet and visible wavelengths , and extremely low Young’s modulus (<4 MPa)  which makes the PDMS ideal material for flexible self-cleaning super-hydrophobic thin film application to conform to arbitrary shaped non-planar surfaces. It is also well-known for reliably maintaining dimensions in the replicated inverse images of a master mold using soft lithography .
Super-lyophobic surfaces against liquid metals
Liquid metal is the metal in liquid phase at room temperature. As it has liquid property which is continuously deformable as well as metallic property such as high thermal and electrical conductivity, it can be applied to various applications such as radio frequency (RF) MEMS switch , electro-wetting on dielectrics (EWOD) ,, tunable and flexible antennas , frequency selective surfaces (FSS) , and heat transfer  among others. For those various applications, as the surface oxidation of liquid metal drastically increases the wettability, the super-lyophobic surface against liquid metal has been of interest.
Super-lyophobic surfaces against mercury
The most well-known liquid metal is mercury. The surface tension and contact angle of various substrates against mercury have been studied from 1920s. The reported surface tension of mercury is in the range of 400 ~ 516 mN/m  in air environment at room temperature. The typically acceptable value of surface tension is ~ 480 mN/m  which is much greater than that of water (72.9 mN/m). Therefore, the contact angle of mercury is much higher than that of water on the same solid substrate based on Young’s equation .
As the contact angle study was investigated without development of micromachining technology, most studied substrates were simply flat surface but treated with different chemicals such as low surface energy material like Teflon (surface energy ~ 18.5 mN/m). Yarnold reported that the advancing and receding contact angle of mercury on the steel which was washed in ether were 165° and 130°, respectively . Gray also reported dynamic contact angle of mercury on low-energy solid substrates such as polyethylene, paraffin wax, and PTFE. The highest advancing angle of 151.7° was obtained on the PTFE substrate and the lowest contact angle hysteresis of ~ 0.2° was achieved on the paraffin wax with pure mercury by acid treatment . Ellison et al. studied the dynamic contact angles of mercury on various substrates (tungsten, stainless steel, nickel, quartz, glass, and Teflon) under varying temperatures in the range of 25 ~ 150°C . The highest advancing contact angle of 157° was measured on the Teflon as the Teflon is ‘low surface energy’ substrate compared with other ‘high surface energy’ substrates. In addition, most substrates showed that contact angle hystereses were in the range of 0° ~ 3° with pure cleaned mercury which means that the mercury can be easily roll off from the surface. The temperature effect for surface tension of mercury was negligible. Awasthi et al. reported the static contact angle of mercury on graphite as 152.5 ± 2° by applying ‘spot technique’ which can be also used for high temperature measurement, as the contact angle of liquid metal at high temperature is useful for practical applications such as metal casting, welding, and brazing .
In particular, the contact angle of mercury on the dielectric substrate was an important parameter for EWOD applications ,. Shen et al. reported that the contact angle of line patterns with various contact ratio (line width per pitch) for mercury using photolithography . As the contact ratio decreases from 1 to 0.3, the contact angle increases from 142° to 158°. Latorre et al. reported that the contact angle of mercury on oxidized silicon wafer was 137° with a standard deviation of 8° .
Super-lyophobic surfaces against gallium-based liquid metal alloys
Non-toxic gallium-based liquid metal have been of interest lately as it has various favorable properties such as higher boiling point, higher thermal and electrical conductivity against mercury . The gallium-based binary (eutectic GaIn alloy ) and ternary alloy (e.g., Galinstan® ) have been studied for various applications. However, it has a challenging problem which is the fact that the surface of gallium-based liquid metal alloy is instantly oxidized in air and wets almost any surfaces . It is a critical issue to have non-wetting super-lyophobic surface against gallium-based liquid metal but there was very limited report on super-lyophobic surface for gallium-based liquid metal alloys, probably due to the strong wettability of the gallium-based alloys.
In order to prevent of wetting, Liu et al. measured, in the below 1 ppm oxygen environment, the advancing and receding contact angle of pure Galinstan® on the various substrates such as tungsten, silicon nitride, glass, parylene, Teflon®, phlogopite, and muscovite. It was found that Galinstan® was non-wettable on all surfaces under the condition. Among them, the muscovite showed the highest advancing of 163.6° and receding contact angle of 148.1°, resulting in the smallest contact angle hysteresis of 15.5°, as the substrate has higher surface roughness compared to others .
Oxide-free gallium-based liquid metal alloys are crucial for certain applications like micro switches. However, in other applications such as micro-cooling and FSS, maintaining true liquid phase of gallium-based liquid metal alloy is not necessary. Recently, we measured static contact angle of oxidized Galinstan® on a glass, Cytop, and Teflon, in air environment . Among them, Teflon substrate shows highest contact angle of 140.3°. However, the contact angle of the Galinstan® droplet was increased to be 152.5° by removing oxide layer using hydrochloric acid vapor.
Recent progress on the hierarchical micro-/nano-scale structures for the artificial super-hydrophobic surfaces and super-lyophobic surfaces against gallium-based liquid metal alloys are reviewed. As one of the key contributing factors for the natural super-hydrophobic surfaces is dual micro-/nano-scale surfaces, a wide variety of materials and numerous fabrication approaches were studied. Of those, optically transparent artificial super-hydrophobic surfaces are of great interest in many practical self-cleaning surface applications. Both SU-8 and PDMS-based super-hydrophobic self-cleaning surfaces have been demonstrated with promising characteristics. As demonstrated by a few groups, it is in its utmost importance to realize super-hydrophobic and super-oleophobic surfaces together to be used in real applications.
Gallium-based liquid metal alloys are one of the very interesting materials and have great potential to be utilized in a wide variety of novel applications due to their unique conductive characteristic along with constantly deformable liquid characteristic. Due to its oxidation problem, it is essential to have super-lyophobic surface against the oxidized gallium-based liquid metal. Recently hierarchical micro-/nano-scale structures have been applied to super-lyophobicity surfaces against gallium-based liquid metals. It is expected that, in the next few years, numerous unexplored potential of the liquid metal-based devices will be studied based on recent progress of the super-lyophobic surfaces.
The authors would like to thank Dr. Dong-Weon Lee of Chonnam National University and Dr. Wonjae Choi of The University of Texas at Dallas for invaluable technical/scientific discussions and UTD clean room staff for their support on device fabrication work. The authors also would like to thank Republic of Korea (ROK) Army for financial support.
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