Coating is an esntial step in adjusting the surface
properties of materials. Superhydrophobic coatings with contact angles greater than 150° and roll-off angles below 10° for water have been developed, bad on low energy surfaces and roughness on the nano- and micrometer scales. However, the surfaces are still wetted by organic liquids such as surfactant-bad solutions, alcohols, or alkanes. Coatings that are simultaneously
superhydrophobic and superoleophobic are rare. We designed an easily fabricated, transparent, and oil-rebounding superamphiphobic coating. A porous deposit of candle soot was coated with a 25 nm thick silica shell. The black coating became transparent after calcination at 600°C. After silanization the coating is superamphiphobic and remained so even after its top layer was damaged by sand impingement.
A major goal in coating rearch is to design lf-cleaning surfaces (1–4). Many surfaces in nature are superhydrophobic – for example lotus leaves (5). Mimicking their surface
morphology led to the development of a number of artificial superhydrophobic surfaces (6, 7), opening many applications in industrial and biological process (8–13). Microscopic pockets of air are trapped beneath the water drops (14–17). This composite interface leads to an increa in the macroscopic co
ntact angle and a reduced contact angle
hysteresis, enabling water drops to roll off easily, taking dirt with them. However, the addition of an organic liquid such as alcohol or oil, decreas the interfacial tension sufficiently to induce homogeneous wetting of the surface. Drops, initially resting on air pockets (Cassie state), pass the transition to complete wetting (Wenzel state) (14). No naturally occurring surface is known to show a contact angle θ of greater than 150° and roll-off angles below 10° for water and organic liquids. The superhydrophobic and superoleophobic surfaces are called superamphiphobic (18). In contrast to superhydrophobicity, the term
“superamphiphobicity” is not uniquely defined in particular with respect to the liquid ud (19–22). According to Young’s equation, cos Θ = (γSV -γSL )/γLV , the lower the surface tension the higher the tendency of a liquid to spread on a solid surface (22, 23). Here, Θ is the macroscopic contact angle, γSV is the
surface tension of the solid, and γSL is the interfacial tension of the solid liquid interface. For organic liquids (30 ≤ γLV ≤ 18 mN/m) mainly van der Waals interaction act between the molecules. Therefore, γSV - γSL is positive and on planar surfaces Θ < 90°. Similarly, the contact angle on rough
surfaces depends on the surface tensions, becau roughness amplifies the wetting properties.
The key factors for superamphiphobicity are not clear yet. For water repellency, surface roughness and low surface energy are esntial (14). To fabricate superamphiphobic surfaces overhangs, re-entrant geometry, or convex curvature is also important (19–25). The complex interplay between surface roughness, low surface energy and topography has made it difficult and expensive to fabricate superamphiphobic surfaces. Tuteja et al. showed that careful design of the topography of a surface allows to construct surfaces with a contact angle for hexadecane clo to 160°, although the flat surface was oleophilic (19, 23). They explained their exceptional oil-repellency by overhang structures and re-entrant geometry.
Here we demonstrate a simple way to make robust, transparent, superamphiphobic coatings. The surface to be coated, in our ca a glass slide, is held above a flame of a paraffin candle (Fig. 1A). Deposition of a soot layer turns the glass black. Scanning electron microscopy reveals that the soot consists of carbon particles with a typical diameter of 30 - 40 nm, forming a loo, fractal-like network (Fig. 1, B and C) (26). A water drop gently deposited on the surface shows a contact angle above 160° and rolls off easily, demonstrating the surface’s superhydrophobicity (27). However, the
structure is fragile as the particle-particle interactions are only physical and weak. When water rolls off the surface, the drop carries soot particles with it until almost all of the soot deposit is removed and the drop undergoes a wetting transition (movie S1).
Inspired by the promising morphology of soot, we
developed a technique to coat the soot layer with a silica shell making u of chemical vapor deposition (CVD) of
tetraethoxysilane (TES) catalyzed by ammonia. The soot-coated substrates are placed in a desiccator together with two open glass vesls containing tetraethoxysilane (TES) and
Candle Soot as a Template for a Transparent Robust Superamphiphobic Coating
Xu Deng,1,2 Lena Mammen,1 Hans-Jürgen Butt,1 Doris Vollmer 1*
1
Max Planck Institute for Polymer Rearch, Ackermannweg 10, D-55128, Mainz, Germany. 2Center of Smart Interfaces, Technical University Darmstadt, 64287 Darmstadt, Germany.
*Towhomcorrespondenceshouldbeadddresd.E-mail:***********************.de
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jiajiammonia, respectively (fig. S1). Similar to a Stöber reaction, silica is formed by hydrolysis and condensation of TES. The shell thickness can be tuned by the duration of CVD. After 24 hours the particles are coated by a 20 ± 5 nm thick silica shell (Fig. 1, D and E). Calcinating the hybrid carbon/silica network at 600°C for 2 hours in air caus combustion of the carbon core (Fig. 1F) and a decrea in the shell thickness, while the layer keeps its roughness and network texture. Only isolated chains of particles, which are not linked in the network, broke during calcination (Fig. 1B). To reduce the surface energy, the hydrophilic silica shells were coated with a mi-fluorinated silane by CVD. Therefore, the substrates and an open beaker with the volatile silane were put in a desiccator for 3 h. After CVD a water drop placed on top of the coating formed a static contact angle of 165° ± 1° (Fig.
2A), with a roll-off angle lower than 1°. Owing to the extremely low adhesion of the coating to water, it was difficult to deposit water drops, becau they immediately rolled off (movie S2). When drops of
organic liquid were deposited, the static contact angles ranged from 154° for tetradecane up to 162° for diiodomethane, (Fig. 2B, Table 1, and fig. S3). The maximal roll-off angle was 5°, even for tetradecane with a surface tension of 26 mN/m.
Hexadecane drops, radius of 1 mm, impinging with a velocity up to v = 1 m/s did not penetrate into the layer. The drop’s kinetic energy was transformed into vibrational energy allowing the drop to rebound twice, before it underwent damped oscillations and finally rested on the surface in the Cassie state (Fig. 2D, figs. S4 and S5, and table S1) (28–30). The coating retained its superamphiphobicity even after impinging of at least thousands of water drops of a radius of 1.3 mm with a velocity of 1.4 m/s (fig. S6), or flushing the coating with water for veral hours.
At velocities between 1 - 1.5 m/s the drop started to penetrate into the coating. As a result a satellite drop was left on the surface after rebound. Typically at the cond impact the satellite drop merged with the primary drop and rolled off (fig. S5).
Self-cleaning properties for water and alkane were verified by depositing drops of either liquid on a superamphiphobic layer and monitoring the taking up of contaminants (fig. S7).
sarcasticFor applications on glass surfaces such as goggles, touch screens, or difficult-to-access windows, th
e superamphiphobic coating needs to be thermally stable, transparent, and mechanically robust. To quantify the thermal stability, the coatings were annealed at temperatures up to 450°C for 1 hour. The static contact and roll-off angle remained constant up to 400°C (Fig. 3A). Annealing at even higher temperatures decompod the fluorosilane. The silica network remained almost unaltered until annealing up to 1000°C (fig. S8). Annealed coatings can recover their superamphiphobicity after repeating CVD of a fluorosilane. After calcination of the black carbon template, the silica
network has a shell thickness well below the wavelength of
light. Such thin shells are highly transparent, as verified by
UV-VIS (Fig. 3B). The transmittance of a 3 µm thick coating
is reduced by less than 10 % compared to pristine glass for a wavelength above 500 nm. This transparency is reflected in
the easy readability of the letters underneath the coated glass
plate and its superamphiphobicity is depicted in the high
contact angle for a wide variety of liquid drops (Fig. 3C).
In outdoor applications superamphiphobic surfaces need to survive harsh conditions. To investigate the mechanical resistance, water drop impact and sand abrasion tests were performed. Sand grains, 100 to 300 µm in diameter impinged
the surface from a height of 10 ~ 40 cm, corresponding to an impinging energy of 1-90 × 10–8 J per grain (Fig. 4A). The
silica shells were not sufficiently robust to completely sustain sand impact. A cave formed underneath the impacting area (Fig. 4C). However, zooming into the cave revealed an
almost unaltered sub-micrometer morphology (Fig. 4D).
藕的作用
Owing to the coating’s lf-similarity, the surface kept its superamphiphobicity until the layer was removed after
extended impact. The mechanical durability depends on the amount of sand impinging per unit of time and area, the size
of the grains, the height of fall, and the thickness of the silica shell. The mechanical stability increas with the thickness of
the silica shell, but at the expen of the coating’s transparency. The surface retains its superamphiphobicity for
5 min sand abrasion from a height of 25 cm (2 m/s). (movie
S3). While the coating can be eroded through wear and
abrasion it keeps its superamphiphobicity as along as its thickness remains above 1 µm (fig. S11).
The coating consists of a fractal-like asmbly of nano-spheres. With increasing duration of CVD of TES or
annealing above 1100°C the necks between particles fill with silica and more rod-like shapes evolve, which reduces the superamphiphobicity (figs. S8 and S10). This can be understood from Nosonovsky’s prediction that convex small-scale roughness can provide a sufficient energy barrier
against wetting (22, 31), thus rendering superamphiphobicity possible. A spherical shape should provide a higher energy
barrier against wetting than a rod-like shape (figs. S8 and
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S10).
Our easy to fabricate oil and water repellent coating is
made from soot encad by a silica shell. The coating is sufficiently oil repellent to cau rebounding of impacting
drops of hexadecane. Even low surface tension drops of tetradecane roll off easily when tilting the surface by 5°,
taking impurities along. The surface keeps its superamphiphobicity after annealing at 400°C. The coating is transparent and can be applied to a variety of heat resistant surfaces, such as aluminum, copper, or stainless steel.
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References and Notes
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(1944).
15. A. Lafuma, D. Quere, Nat. Mater.2, 457 (2003).
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Lammertink, F. Mugele, Phys. Rev. Lett.106, 014501
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19. A. Tuteja, W. J. Choi, G. H. McKinley, R. E. Cohen, M.
F. Rubner, MRS Bull.33, 752 (2008).
20. A. Steele, I. Bayer, E. Loth, Nano Lett.9, 501 (2008).
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Chem. C114, 2936 (2010).
22. L. Joly, T. Biben, Soft Matter5, 2549 (2009).
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25. L. Cao, T. P. Price, M. Weiss, D. Gao, Langmuir24, 1640
(2008).
26. C. M. Megaridis, R. A. Dobbins, Combust. Sci. Technol
71, 95 (1990).
27. M. Callies, D. Quere, Soft Matter1, 55 (2005).会计证报考条件
28. D. Bartolo et al., Europhys. Lett.74, 299 (2006).
朋友用英语怎么写29. A. Tuteja, W. Choi, J. M. Mabry, G. H. McKinley, R. E.
Cohen, Proc. Natl. Acad. Sci. U. S. A.105, 18200 (2008).
30. D. Richard, C. Clanet, D. Quere, Nature417, 811 (2002).
31. M. Nosonovsky, Langmuir23, 3157 (2007). Acknowledgments: We are grateful to G. Glar, K.
Kirchhoff, G. Schäfer, S. Pinnells, J. Ally, and P.
Papadopoulos for technical support and stimulating
discussions. We acknowledge financial support from DFG
SPP 1273 (D.V.), SPP 1420 (H.J.B.), and SPP 1486
(L.M.).
Supporting Online Material
www.sciencemag/cgi/content/full/science.1207115/DC1 Materials and Methods
SOM Text
Figs. S1 to S11
Tables S1 and S2
References
Movies S1 to S3
8 April 2011; accepted 8 November 2011
Published online 1 December 2011; 10.1126/science.1207115 Fig. 1. Morphology of porous structure. (A)Photograph depicting sample preparation. A glass slide is held in the
flame of a candle until a few µm thick soot layer is deposited. (B) Scanning electron microscope (SEM) image of the soot deposit.(C)High resolution SEM image showing a single particle chain made up of almost spherical carbon beads of 40
± 10 nm diameter.(D) SEM image of the deposit after
coating with a silica shell (e fig. S2 for a cross ction of
the deposit). (E) High resolution SEM image of a cluster after removing the carbon core by heating for 2 hours at 600°C. (F) High resolution transmission electron microscopy image of a cluster after calcination, revealing the silica coating with
holes that were previously filled with carbon particles. The
silica shell is 20 ± 5 nm thick.
Fig. 2. Superamphiphobicity of the surface. A2 µl water drop (A) and 5 µl hexadecane drop (B) deposited on the surface posss a static contact angle of 165°±1° and 156° ± 1°, respectively. (C)Cartoon of a liquid drop deposited on the
fractal-like composite interface. (D) Time resolved bouncing
杭州计算机培训
of a 5 µl hexadecane drop on a super-amphiphobic surface.
Just before impinging the drop’s kinetic energy exceeds its interfacial energy by 2.4, i.e. the Weber number is 2.4 (28).
Fig. 3. Thermal stability and light transmittance of a superamphiphobic surface. (A)Static contact and roll-off
angle of hexadecane measured after annealing the samples for
1 hour at various temperatures. The surface los its super-amphiphobicity after annealing at temperatures above 400°C
due to thermal degradation of the fluorosilane (shadow area). (B)UV-Vis transmittance spectra of a 3 µm thick superamphiphobic surface compared to pristine glass. (C) Photograph of dyed water (γlv = 72.1 mN/m; blue), peanut oil
(γlv = 34.5 mN/m; white), olive oil (γlv = 32.0 mN/m; yellow),
and dyed hexadecane drop (γlv = 27.5 mN/m; red) deposited
on a superamphiphobic glass slide. The coated slide was
placed on labeled paper.
Fig. 4. Mechanical resistant quantified by sand abrasion.(A) Schematic drawing of a sand abrasion experiments. (B)
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Hexadecane drop deposited on the coating after 20 g sand abrasion from 40 cm height. The 100 to 300 µm sized grains have a velocity of 11 km/hour just before impingement. After impingement drops rolls off after tilting the substrate by 5°. (C ) SEM image of a spherical crater (orange circle) after sand abrasion. (D ) SEM image of the surface topography inside the cavity.
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Table 1. Comparison of the static contact angle (SCA) and roll-off angle of drops with different surface tension, deposited on a flat fluorinated glass substrate and on a superamphiphobic coating.
Liquid Surface tension (mN/m ) Flat surface SCA° Superamphiphobic Roll-off angle°
surface SCA°
Water 72.1 108 ± 1 165 ± 1 1 ± 1 Diiodomethane 50.9 91 ± 1 161 ± 1 2 ± 1 Ethylene glycol 47.3 89 ± 1 160 ± 1 2 ± 1 Peanut oil 34.5 70 ± 1 158 ± 1 4 ± 1 Olive oil 32.0 69 ± 1 157 ± 1 4 ± 1 Hexadecane 27.5 64 ± 1 156 ± 1 5 ± 1 Tetradecane 26.5 54 ± 1 154 ± 1 5 ± 1
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