Stable superhydrophobic surface of hierarchical carbon nanotubes on Si micropillar arrays
© He et al.; licensee Springer. 2013
Received: 21 August 2013
Accepted: 18 September 2013
Published: 7 October 2013
It is of great importance to construct a stable superhydrophobic surface with low sliding angle (SA) for various applications. We used hydrophobic carbon nanotubes (CNTs) to construct the superhydrophobic hierarchical architecture of CNTs on silicon micropillar array (CNTs/Si-μp), which have a large contact angle of 153° to 155° and an ultralow SA of 3° to 5°. Small water droplets with a volume larger than 0.3 μL can slide on the CNTs/Si-μp with a tilted angle of approximately 5°. The CNTs growing on planar Si wafer lose their superhydrophobic properties after exposing to tiny water droplets. However, the CNTs/Si-μp still show superhydrophobic properties even after wetting using tiny water droplets. The CNTs/Si-μp still have a hierarchical structure after wetting, resulting in a stable superhydrophobic surface.
KeywordsCarbon nanotube Hierarchical architecture Superhydrophobic surface
Interfacial interaction between liquid and solid is of great importance for materials in various applications, such as absorption, adhesion, lubrication, and transference. Due to easy deformation of liquid, large droplets slide on a solid surface easier than the small ones. The mobility of droplets depends not only on the properties and size of liquid but also on the surface state of solid. Superhydrophobic surfaces which have a static contact angle (CA) larger than 150° are desired in collecting and delivering tiny water droplets in some cases[3, 4]. Various approaches have been established to construct superhydrophobic surfaces, such as coating with hydrophobic materials[5–7], increasing roughness[8, 9], and fabricating hierarchical micro/nanoarchitectures[10–12]. Interfacial interaction hinders the motion of stationary water droplets on a solid surface, resulting in CA hysteresis. The CA hysteresis on a superhydrophobic surface might result from high adhesive force and absorption[13, 14], which implies that it is not easy for tiny water droplets to move on such surface. Up to now, most of the research on superhydrophobic surface focused on measuring the CAs and sliding angles (SAs) of water droplets with a volume not smaller than 2 μL (approximately 1.6 mm in diameter). However, we often observe water droplets with a volume lower than 2 μL, such as fog droplets, existing or sliding on a solid surface in nature. There is a need to reveal the interfacial interaction between superhydrophobic surface and tiny water droplets.
Generally, pristine carbon nanotubes (CNTs) are hydrophobic materials, which have also been used to construct a superhydrophobic surface[15, 16]. By making micropatterns, the hydrophobicity of a CNT surface is further enhanced. The CA between water and CNT pattern is usually larger than 150°, but the SA is also large (usually larger than 30°)[17, 18]. However, the superhydrophobic CNT forest might also absorb water, resulting in collapsing into cellular foams when water evaporates from interstices of nanotubes. After wetting, the CNT forest might lose its superhydrophobic properties. It needs to construct a stable and durable superhydrophobic surface even wetted by vapor or tiny water droplets. Here, we fabricate the superhydrophobic hierarchical architecture of CNTs on Si micropillar array (CNTs/Si-μp) with large CA and ultralow SA. The CNTs/Si-μp show a durable superhydrophobic surface even after wetting using tiny water droplets.
The samples were characterized using a scanning electron microscope (SEM). The CA and SA were measured using a contact angle goniometer (Rame-hart 300, Rame-hart Instrument Co., Succasunna, NJ, USA). The CNT samples mounted on an inclined substrate with a slope of 5° were exposed to tiny water droplets (50 to 500 μm in diameter) generated from a nebulizer. The tiny water droplets on the CNT forest were observed using a stereomicroscope (Stemi 2000, Carl Zeiss, Inc., Oberkochen, Germany).
Results and discussion
The Si-μp arrays used in the experiment have a square shape with spacing equal to the dimension. The area fraction of the Si-μp arrays is f = 0.25 (f = a2 / (a + b)2, where a is the dimension of micropillars and b is the spacing between the neighboring pillars). Figure 1a is a tilted-view SEM image of the Si-μp array with a dimension of 8 μm, showing well-defined pillars with a smooth surface. The height of the micropillar is about 15 μm.
Figure 1b is a SEM image of the CNT forest growing on Si-μp arrays, showing the hierarchical architecture of CNTs/Si-μp. The forest comprises a large amount of loose CNTs. Figure 1c is a SEM image of a single Si-μp with mutually orthogonal CNT forests. The forests growing on two neighbor micropillars already join together after 6-min CNT growth. For comparison, we prepared the CNT forest on planar Si wafers (CNTs/Si) using the same growing parameters. Some CNTs extruding from the forest are observed during SEM examination, forming a rough surface (see Figure 1d). The density of CNTs within the forest growing on the planar Si is similar to that growing on the Si-μp arrays. The height of the forest is approximately 10 μm after 6-min CNT growth.
CA and SA of water droplets (7 μL) on various CNT surfaces
5-μm CNTs/Si (deg)
8-μm CNTs/Si (deg)
10-μm CNTs/Si (deg)
15-μm CNTs/Si (deg)
CNTs/Si-μp, 16-μm Si pillar (deg)
CNTs/Si-μp, 8-μm Si pillar (deg)
where f x is the areal fraction of x and θ x is the contact angle of water with surface x. Because the Si micropillars are covered by CNTs, the CA of CNTs/Si-μp is larger than that of CNTs/Si. The CA increases slightly from 153° to 155° when the dimension of Si micropillars reduces from 16 to 8 μm (see Table 1).
The mobility of water droplets on a CNT forest surface was investigated by measuring the SA. Figure 2c shows an image of a water droplet which begins to slide on an inclined CNTs/Si surface with a slope of approximately 50°. It shows a significant CA hysteresis of approximately 77° with an advancing angle of Φa = 163° and a receding angle of Φr = 86°. The SA of CNTs/Si varies from 40° to 50° according to the height of the CNT forest (see Table 1). The large CA hysteresis implies that it is hard for water droplets to slide on the CNTs/Si surface. Figure 2d shows an optical image of a water droplet sliding on CNTs/Si-μp. The water droplet on hierarchical CNTs/Si-μp has no evident hysteresis with an ultralow SA of 3° to 5°. The ultralow SA implies that water droplets are easy to slide on the CNTs/Si-μp surface.
For comparison, we provide a microscopic image of CNTs/Si exposed under nebulizer fogs in Figure 3d. It is not easy for water droplets to slide on the CNTs/Si surface due to large SA. Some water droplets sprayed into CNTs/Si disperse into the cavities of the CNT forest, making the wetting surface of the CNTs and some tiny water droplets gather into large drops. The large water droplets on the CNTs/Si surface deform into irregular shapes due to wetting, which are quite different from those on the CNTs/Si-μp. The water droplets we observed on the CNTs/Si surface have a diameter above 5 mm (approximately 52 μL).
In summary, the hierarchical architecture of CNTs/Si-μp has a superhydrophobic surface with large CA and ultralow SA of only 3° to 5°. Tiny water droplets larger than 0.3 μL can slide on CNTs/Si-μp with a tilted angle of 5°, showing a high capacity of collecting water droplets. After wetting using tiny water droplets, the CNT forest growing on planar Si wafer loses its superhydrophobic properties, but the CNTs/Si-μp still have a superhydrophobic surface because they still have a hierarchical structure. The CNTs/Si-μp show stable superhydrophobic properties.
This work is financially supported by the National Natural Science Foundation of China (51172122, 11272176), Foundation for the Author of National Excellent Doctoral Dissertation (2007B37), Program for New Century Excellent Talents in University, and Tsinghua University Initiative Scientific Research Program (20111080939).
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