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Langmuir 2009, 25, 3240-3248
Self-Cleaning Efficiency of Artificial Superhydrophobic Surfaces Bharat Bhushan,*,† Yong Chae Jung,† and Kerstin Koch‡ Nanoprobe Laboratory for Bio- and Nanotechnology and Biomimetics (NLBB), The Ohio State UniVersity, 201 West 19th AVenue Columbus, Ohio 43210-1142, and Nees Institute for BiodiVersity of Plants, Rheinische Friedrich-Wilhelms UniVersity of Bonn, Meckenheimer Allee 170, 53115 Bonn, Germany ReceiVed NoVember 21, 2008. ReVised Manuscript ReceiVed December 23, 2008 The hierarchical structured surface of the lotus (Nelumbo nucifera, Gaertn.) leaf provides a model for the development of biomimetic self-cleaning surfaces. On these water-repellent surfaces, water droplets move easily at a low inclination of the leaf and collect dirt particles adhering to the leaf surface. Flat hydrophilic and hydrophobic, nanostructured, microstructured, and hierarchical structured superhydrophobic surfaces were fabricated, and a systematic study of wettability and adhesion properties was carried out. The influence of contact angle hysteresis on self-cleaning by water droplets was studied at different tilt angles (TA) of the specimen surfaces (3° for Lotus wax, 10° for n-hexatriacontane, as well as 45° for both types of surfaces). At 3° and 10° TA, no surfaces were cleaned by moving water applied onto the surfaces with nearly zero kinetic energy, but most particles were removed from hierarchical structured surfaces, and a certain amount of particles were captured between the asperities of the micro- and hierarchical structured surfaces. After an increase of the TA to 45° (larger than the tilt angles of all structured surfaces), as usually used for industrial self-cleaning tests, all nanostructured surfaces were cleaned by water droplets moving over the surfaces followed by hierarchical and microstructures. Droplets applied onto the surfaces with some pressure removed particles residues and led to self-cleaning by a combination of sliding and rolling droplets. Geometrical scale effects were responsible for superior performance of nanostructured surfaces.
1. Introduction Superhydrophobic and self-cleaning surfaces with a high static contact angle above 150° and low contact angle hysteresis (the difference between the advancing and receding contact angles) play an important role in technical applications ranging from self-cleaning window glasses, paints, and textiles and include low-friction surfaces for fluid flow and energy conservation.1 Lotus (Nelumbo nucifera) leaves have been the inspiration for the development of several commercially available self-cleaning products.2 Self-cleaning is not restricted to plants; indeed, it has been found in insects, in particular those insects with large wings, in which case legs cannot be used to clean the wings.3 The self-cleaning property of plant surfaces was demonstrated by Barthlott and Neinhuis.4 The surfaces of plants were artificially contaminated with various particles and subsequently subjected to artificial rinsing by a sprinkler or fog generator. In the case of waterrepellent leaves, particles were removed completely by water droplets that rolled off the surfaces. If water moves over a structured hydrophobic surface, contaminating particles are picked up by the water droplets or they adhere to the surface of the droplets and are then removed with the droplets. Their experimental data, carried out on smooth and rough water-repellent plants, showed that the interdependence between surface roughness, degree of particle adhesion, and water repellency is the key to the self-cleaning efficiency of many plant surfaces. The leaves of Lotus afford an impressive demonstration of this effect, which is therefore often called the “Lotus-Effect.” The superhydrophobicity and self-cleaning * To whom correspondence should be addressed. E-mail: bhushan.2@ osu.edu. † The Ohio State University. ‡ Rheinische Friedrich-Wilhelms University of Bonn. (1) Nosonovsky, M.; Bhushan, B. Multiscale DissipatiVe Mechanisms and Hierarchical Surfaces: Friction, Superhydrophobicity, and Biomimetics; SpringerVerlag: Heidelberg, Germany, 2008. (2) Koch, K.; Bhushan, B.; Barthlott, W. Soft Matter 2008, 4, 1943. (3) Wagner, T.; Neinhuis, C.; Barthlott, W. Acta Zool. 1996, 77, 213. (4) Barthlott, W.; Neinhuis, C. Planta 1997, 202, 1.
of the lotus leaves was found to be a result of a hierarchical surface structure built by randomly oriented small hydrophobic wax tubules on the top of convex cell papillae. The surface waxes (also termed epicuticular waxes) are the hydrophobic component of the leaf surfaces.5 Scanning electron microscope (SEM) studies showed that the waxes occur in different morphologies, such as tubules, platelets, rodlets, granules, or films.6-8 Wax tubules, which contain high amounts of secondary alcohols and alkandiols, and other species have been reported for the leaves of lotus (N. nucifera) and nasturtium (Tropaeolum majus).7,9 These tubules are hollow structures, usually 0.3-1.1 µm in length and 0.1-0.2 µm in diameter.7,10,11 Recrystallization experiments with total waxes, as well as with isolated compounds, showed that the wax layer occurs by self-assembly of the dominating chemical compounds, in particular nonacosan-10-ol, as well as the corresponding nonacosandiols.10,12-14 (5) Holloway, P. J. Section I-Reviews. Plant cuticles: Physiochemical characteristics and biosynthesis. In Air Pollutants and the Leaf Cuticle; Percy, K. E., Cape, C. N., Jagels, R., Simpson, C. J., Eds.; Springer: Berlin, Heidelberg, Germany, 1994. (6) Barthlott, W.; Neinhuis, C.; Cutler, D.; Ditsch, F.; Meusel, I.; Theisen, I.; Wilhelmi, H Bot. J. Linn. Soc 1998, 126, 237. (7) Jeffree, C. E. The Fine Structure of the Plant Cuticle. In Biology of the Plant Cuticle; Riederer, RiedererM., Mu¨ller, C., Eds.; Blackwell: Oxford, 2006; pp 11-125. (8) Koch, K.; Bhushan, B.; Barthlott, W. Functional plant surfaces, smart materials. Handbook of Nanotechnology, 3rd ed.; Bhushan, B., Ed.;SpringerVerlag: Heidelberg, Germany, 2010. (9) Barthlott, W.; Theisen, I.; Borsch, T.; Neinhuis, C. Epicuticular waxes and vascular plant systematics: Integrating micromorphological and chemical data. Deep Morphology: Toward a Renaissance of Morphology in Plant Systematics; Stuessy T. F., Mayer V., Ho¨randl E., Eds.; Reg. Veg. Gantner Verlag Ruggel: Liechtenstein, 2003; pp 189-206. (10) Jetter, R.; Riederer, M. Bot. Acta 1995, 108, 111. (11) Koch, K.; Dommisse, A.; Barthlott, W. Crys. Grow. Design 2006, 6, 2571. (12) Jetter, R.; Riederer, M. Planta 1994, 195, 257. (13) Dommisse, A. Self-assembly and pattern formation of epicuticular waxes on plant surfaces, Dissertation, Rheinische Friedrich-Wilhelms Universita¨t Bonn, Germany, 2007. (14) Koch, K.; Dommisse, A.; Barthlott, W.; Wandelt, K.;Nonacosan-ol and alkandiol wax tubules: molecular architecture and self-assembly. Surf. Sci. 2009, in press.
10.1021/la803860d CCC: $40.75 2009 American Chemical Society Published on Web 02/10/2009
Artificial Superhydrophobic Surfaces
Superhydrophobic plant surfaces always consist of threedimensional surface (epicuticular) waxes.15,16 Self-cleaning ability is optimized by the hierarchical structures resulting from the micropatterned cell papillae and the wax.1,14,17-19 Water on such a surface forms a spherical droplet, and both the contact area and adhesion to the surface are dramatically reduced.20,21 Contact angle hysteresis is responsible for the sticking of liquids to a surface and is defined as the difference between the advancing and receding angles of a moving or evaporating water droplet. The degree of contact angle hysteresis is correlated with the wetting state of a liquid on a surface. A droplet may roll in addition to slide with little resistance if the contact angle hystersis is small.20-22 The tilt angle (TA) is defined as the tilting degree of a surface on which an applied drop of water starts to move. Surfaces with low contact angle hysteresis have a low tilt angle. Wetting states of superhydrophobic surfaces are described by the Wenzel- and Cassie-states. On rough surfaces, an applied water droplet in the Wenzel-state creates a wet-contact mode with a high contact angle hysteresis. In the Cassie-state, the droplet sits on top of the structure asperities, and a low contact angle hysteresis lets the liquid roll off, in addition to slide, at low tilt angles. In the past, several experimental and theoretical attempts have been made to characterize the influence of the surface structure on the wettability of biological and artificial surfaces, and a large number of publications describe the development and properties of superhydrophobic surfaces.1,20-27 Several authors have compared the surface properties of fabricated superhydrophilic surfaces to the self-cleaning properties of lotus leaves, but selfcleaning tests were not performed. Most existing papers about self-cleaning surfaces are based on an alternative self-cleaning approach. In this approach, photocatalytic and superhydrophilic TiO2 has achieved considerable self-cleaning success.28-30 Only two studies describe the self-cleaning of surfaces by water repellence. These are self-cleaning tests performed with plant surfaces by Barthlott and Neinhuis,4 and self-cleaning test performed with artificial surfaces by Fu¨rstner et al.31 In the latter case the influence of the kinetic energy on self-cleaning was quantified for smooth and two different microstructured surfaces, but not for nanostructured or hierarchical structured surfaces. In these studies, cleaning efficiency by water for surfaces contaminated by particles was studied. (15) Baker, E. A. Chemistry and morphology of plant epicuticular waxes. In The Plant Cuticle; Cutler, D. F., Alvin K. L., Price C. E., Eds.; Academic Press: London, 1982; pp 139-166. (16) Jeffree, C. E. The cuticle, epicuticular waxes and trichomes of plants, with reference to their structure, functions and evolution, In Insects and the Plant Surface; Juniper, B. E., Southwood S. R., Eds.; Edward Arnold: London, 1986; pp 23-63. (17) Neinhuis, C.; Barthlott, W. Ann. Bot. 1997, 79, 667. (18) Wagner, P.; Fu¨rstner, R.; Barthlott, W.; Neinhuis, C. J. Exp. Bot. 2003, 54, 1295. (19) Nosonovsky, M.; Bhushan, B. Mater. Sci. Eng. R 2007, 58, 162. (20) Bhushan, B.; Jung, Y. C. J. Phys.: Condens. Matter 2008, 20, 225010. (21) Nosonovsky, M.; Bhushan, B. J. Phys.: Condens. Matter 2008, 20, 225009. (22) Marmur, A. Langmuir 2003, 19, 8343. (23) Extrand, C. W. Langmuir 2002, 18, 7991. (24) Lafuma, A.; Que´re´, D. Nat. Mater. 2003, 2, 457. (25) Patankar, N. A. Langmuir 2003, 19, 1249. (26) Bormashenko, E.; Pogreb, R.; Whyman, G.; Erlich, M. Langmuir 2007, 23, 6501. (27) Roach, P.; Shirtcliffe, N. J.; Newton, M. I. Soft Matter 2008, 4, 224. (28) Nakajima, A.; Hashimoto, K.; Watanabe, T.; Takai, K.; Yamauchi, G.; Fujishima, A. Langmuir 2000, 16, 7044. (29) Mills, A; Lepre, A.; Elliott, N.; Bhopal, S.; Parkin, I. P.; O’Neil, S. A. J. Photochem. Photobiol. A. Chem 2003, 160, 213. (30) Zhang, X. T.; Sato, O.; Taguchi, M.; Einaga, Y.; Murakami, T.; Fujishima, A. Chem. Mater. 2005, 17, 696. (31) Fu¨rstner, R.; Barthlott, W.; Neinhuis, C.; Walzel, P. Langmuir 2005, 21, 956.
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In contrast to the few data about self-cleaning tests, several studies have been carried out to characterize the efficiency of cleaning procedure and cleaning detergents. Different techniques are available to characterize the efficiency of cleaning, such as techniques for soiling carpets and hard floors.32 Relevant techniques include colorimetric measurements, which are based on the color change induced by dirt compared to that of a clean surface. Another nondestructive technique is the radiochemical method, based on gamma-ray emission.33 In practical situations, the method selected is based on the cleanliness level required.34 For larger particles, foil sampling (transfer of particles on an adhesive tape) and optical detection are the most practical methods.35,36 In our recent studies, artificial hierarchical surfaces inspired by lotus leaves with higher static contact angles and lower contact angle hysteresis have been produced.37-39 Self-cleaning requires contact between the water droplets and the contaminant particles and movement of the water droplets to remove the contaminant from the surfaces. In the present study, self-cleaning efficiency studies on flat, nanostructured, microstructured, and hierarchical structured surfaces were systematically carried out. SiC particles in the size range of 1-10 and 10-15 µm were used, and particles were detected by light microscopy and were counted on the surfaces before and after self-cleaning tests. The impact of contact angle hysteresis and kinetic energy of artificial rain on the selfcleaning efficiency were studied.
2. Experimental Details 2.1. Samples. Microstructures were fabricated using lotus leaves, after removing the original wax structures,39 and a microstructured Si surface with pillars of 14 µm diameter and 30 µm height with 23 µm pitch by soft lithography.37,38 The replication is a two-step molding process, in which a negative replica of a template is generated using a polyvinylsiloxane dental wax and a positive replica is made with a liquid epoxy resin.40 Nanostructures were created by self-assembly of tubule forming plant wax, isolated from leaves of N. nucifera39 and alkane n-hexatriacontane (C36H74) for the development of platelet nanostructures.37,38 The tubule forming N. nucifera waxes are referred to as lotus wax. The chemical structures of the major components of the wax forming tubule and alkane n-hexatriacontane are shown in Table 1. The complete chemistry of the plant waxes used is presented in Koch et al.11 The lotus wax and n-hexatriacontane were deposited on the specimen surfaces by thermal evaporation. The amounts were 0.2 µg/mm2 for n-hexatriacontane and 0.8 µg/mm2 for lotus wax, respectively. After coating, the specimens with lotus wax were exposed to ethanol vapor for three days at 50 °C and then left in the oven at 50 °C in total for 7 days,39 and the specimens with n-hexatriacontane were placed in a desiccator at room temperature for 3 days for creating tubule and platelet structures, respectively. The n-hexatriacontane forms platelets at room temperature within 3 days, without any treatment by temperature or solvents.37,38 (32) Burrows, J. Laboratory techniques for soiling carpets and hard floors, In Proceedings of the 39th International Detergency Conference. wfk. September 6.-8 Luxemburg. 294-297, 1999. (33) Pesonen-Leinonen, E.; Redsven, I.; Neuvonen, P.; Hurme, K. R.; Pa¨a¨kko¨, M.; Koponen, H. K.; Pakkanen, T. T.; Uusi-Rauva, A.; Hautala, M.; Sjo¨berg, A. M. Appl. Rad. Isotopes 2006, 64, 163. (34) Chawla, M. K. How clean is clean? Measuring surface cleanliness and defining acceptable level of cleanliness. In Handbook for Critical Cleaning; Kanegsberg, B., Kanegsberg, E., Eds.; CRC Press: Boca Raton, FL, 2001. (35) Schneider, T.; Nilsen, S.; Dahl, I. Building EnViron. 1994, 3, 369. (36) Nilsen, S.; Dahl, I.; Jørgensen, O.; Schneider, T. Building EnViron. 2002, 37, 1373. (37) Bhushan, B.; Koch, K.; Jung, Y. C. Soft Matter 2008, 4, 1799. (38) Bhushan, B.; Koch, K.; Jung, Y. C. Appl. Phys. Lett. 2008, 93, 093101. (39) Koch, K.; Bhushan, B.; Jung, Y. C.; Barthlott, W. Fabrication of artificial Lotus leaves and significance of hierarchical structure for superhydrophobicity and low adhesion. Soft Matter 2009, DOI: 10.1039/b818940d. (40) Koch, K.; Schulte, A. J.; Fischer, A.; Gorb, S.; Barthlott, W. Bioinsp. Biomim. 2008, 3, 046002.
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Table 1. Chemical Structure of the Major Components of Lotus Wax and n-Hexatriacontanea
a
The major component is shown first.
Hierarchical structures were fabricated using a two-step fabrication process, including the production of microstructured surfaces by soft lithography and subsequent development of nanostructures on top by self-assembly of plant waxes and n-hexatriacontane, as described above. In order to produce specimens with unstructured coverage of lotus wax and n-hexatriacontane, flat epoxy resin and a micropatterned Si replica were covered with flat lotus wax and n-hexatriacontane, and a microstructure in lotus leaf replica were covered with flat lotus waxes. Flat wax layers were made by melting the deposited waxes (3 min at 120 °C) and subsequent rapid cooling of the specimen to 5 °C. Then the specimens were stored for 7 days at 21 °C in a desiccator. The fast cooling of the wax prevents the formation of nanostructure roughness. Figure 1a shows the SEM micrographs of flat surfaces with the tubules nanostructure. Microstructures shown in Figure 1b are the lotus leaf and micropatterned Si replica covered with a wax film. Hierarchical structures were fabricated with microstructured lotus leaf replicas and micropatterned Si replicas covered with a nanostructure of lotus wax tubules as shown in Figure 1c. SEM micrographs show an overview (left column), a detail in higher magnification (middle column), and a large magnification of the created flat wax layers and tubules nanostructures (right column). All surfaces show a homogeneous distribution of the wax mass on the specimen. The recrystallized lotus wax shows tubular hollow structures with random orientation on the surfaces, as shown in Figure 2. Their diameter varied between 100 and 150 nm, and their length varied between 1500 and 2000 nm. The nanostructure is formed by threedimensional platelets of n-hexatriacontane, as shown in detail in Figure 2. Platelets are flat crystals, grown perpendicular to the substrate surface. The platelet thickness varied between 50 and 100 nm, their length varied between 500 and 1000 nm. 2.2. Artificial Contamination and Self-Cleaning Test. For deposition of contamination on the artificial surfaces, various structures were placed in a contamination glass chamber with 0.6 m in height and 0.3 m in diameter, as shown in Figure 3a. The chamber was divided by a removable panel at about of 0.3 m above the ground covering the specimens. Silicon carbide (SiC) (Guilleaume, Germany) particles in two different sizes ranges of 1-10 and 10-15 µm, were used as the contaminants. The SiC particles have been chosen because of their hydrophilicity and similarity in shape and sizes to natural dirt contaminations. The 0.1 g of the contaminants was filled in a bowl placed near the top of the chamber. The contaminants were blown into the upper part of the contamination chamber by pressurized air (5 s at 300 kPa). After 30 s, when the largest particles and particle aggregates had settled onto the removable panel, it was removed and the airborne particles were deposited onto the specimens. The time of exposure to the sample was 30 min. After each contamination, the chamber was cleaned with water. The number of particles per area was determined by counting them from a 280 µm × 210 µm image taken by an optical microscope with a CCD camera (Nikon, Optihot-2) before and after water cleaning. SEM was used to characterize the particle sizes and their relative distribution deposited on the surfaces.
For the cleaning test, the specimens with the contaminants were subjected to water droplets using two microsyringes, as shown in Figure 3b. The water reservoir has two microsyringes with an orifice of 0.5 mm, which release droplets with a diameter of about 2 mm. The specimens were fixed to a tilted stage with various tilt angles. In order to obtain relative measure of self-cleaning ability of hierarchical structures which exhibit the lowest contact angle hysteresis and tilt angle as compared to other structures (flat, nanostructures, and microstructures), tilt angles chosen for cleaning tests were slightly above the respective tilt angles for the hierarchichal structures. Thus, experiments were performed with 10° for surfaces covered with n-hexatriacontane and 3° for surfaces with lotus wax.
Figure 1. SEM micrographs taken at 45° tilt angle, show three magnifications of (a) the tubule nanostructure on flat replica, (b) microstructures in lotus replica and micropatterned Si replica, and (c) hierarchical structure using lotus and micropatterned Si replicas. Nanoand hierarchical structures were fabricated with mass 0.8 µg/mm2 of lotus wax after storage for 7 days at 50 °C with ethanol vapor.39
Artificial Superhydrophobic Surfaces
Langmuir, Vol. 25, No. 5, 2009 3243 tilted to 45°, as used in many industrial weathering tests (e.g., International Standardization Organization (ISO) reference number ISO 1514:2004(E)). The tilt angle of 45° is much larger than tilt angle of all structured surfaces. The purpose of the low tilt angle experiments was to study the effect of various structures, whereas the purpose of large tilt angle experiments was to study the geometrical scale effects. Specimens were fixed on the stage by using a double-sided adhesive tape. The water cleaning test was carried out for 2 min (water quantity, 10 mL) with nearly zero kinetic energy of droplets. In order to apply different kinetic energies of the droplets, the distance (D) between the needles and the specimen was varied as 0, 0.02, 0.05, and 0.10 m, and resulting velocities were 0.63, 0.99, and 1.4 m/s and pressures were 0, 200, 500 and 1000 Pa, respectively. For watering with nearly zero kinetic energy, the distance between the microsyringes and surface was set to 0.005 m. These values represent a low value compared to a natural rain shower, where a water droplet of 2 mm radius can reach a fall velocity of 6 m/s (measured under controlled conditions).41 After being rinsed with water, wet specimens were put into an oven (at 50 °C) to remove small droplets remaining on the surface. The number of particles per area was counted using an optical microscope image.
3. Results and Discussion Figure 2. SEM micrographs taken at 45° tilt angle of three-dimensional tubules (top) and platelets (bottom) forming nanostructures on the surface fabricated with 0.8 µg/mm2 mass of lotus wax after storage for 7 days at 50 °C with ethanol vapor and 0.2 µg/mm2 n-hexatriacontane after storage for three days at room temperature, respectively.37,39
Figure 3. Schematics of (a) contamination chamber and (b) artificial water cleaning stage.
As it will be shown later, these tilt angles do not provide a high degree of self-cleaning and a larger tilt angle would be useful to better compare various surfaces. In a second study all surfaces were
3.1. Wettability and Adhesion Forces. To study the effect of tubule nanostructures on self-cleaning, the static contact angle, contact angle hysteresis, and tilt angle were measured on flat, microstructured lotus replica, micropatterned Si replica, and hierarchical surfaces shown in Figure 4a.39 The values measured for various surfaces are summarized in Table 2. For static contact angle, contact angle hysteresis, and tilt angle, droplets of about 5 µL in volume (with the diameter of a spherical droplet about 2.1 mm) were gently deposited on the surface using a microsyringe. For contact angle hysteresis, the advancing and receding contact angles were measured at the front and back of the droplet moving along the tilted surface, respectively. The tilt angle was measured by using a simple tilting stage. Figure 4a shows that for the hierarchical structured Si replica, the highest static contact angles of 173°, lowest contact angle hysteresis of 1°, and tilt angle varying between 1° and 2° were found. The hierarchical structured lotus leaf replica showed static contact angle of 171°, a contact angle hysteresis of 2°, and tilt angles of 1-2°. The recrystallized wax tubules are very similar to those of the original lotus leaf, but are 0.5-1 µm longer, the static contact angle is higher, and the contact angle hysteresis is lower than reported for the original lotus leaf (static contact angle of 164° and contact angle hysteresis of 3°).39 A static contact angle of 80° was found for flat epoxy resin, but the droplet still adhered at a tilt angle of 90°. The microstructured surfaces of the lotus leaf replica and the microstructured Si replica (covered with a wax film) are superhydrophobic (158° and 160°) but show much higher contact angle hysteresis (29 and 27°) and tilt angles (24 and 20°) than found in hierarchical structures. Superhydrophobicity, with a static contact angle of 167°, a contact angle hysteresis of 6° and tilt angle of 3-4°, was also found in nanostructured surfaces. Melting of the wax led to a flat surface with a flat wax film and a much lower static contact angle (139°), higher contact angle hysteresis (49°), and higher tilt angle of 43°. The data of a flat lotus wax film on a flat replica show that the lotus wax by itself is hydrophobic. To study the effect of platelet nanostructures on self-cleaning, the static contact angle, contact angle hysteresis, and tilt angle of four structures were measured, as shown in Figure 4b.38 The (41) van Dijk, A. I. J. M; Bruijnzeel, L. A.; Rosewell, C. J. J. Hydrology 2002, 261, 1.
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Figure 4. Bar chart showing the measured static contact angle, contact angle hysteresis and tilt angle on various structures fabricated with (a) 0.8 µg/mm2 of lotus wax after storage for 7 days at 50 °C with ethanol vapor and (b) 0.2 µg/mm2 of n-hexatriacontane after storage for 3 days at room temperature. The bar chart also shows adhesive forces for various structures, measured using a 15 µm radius borosilicate tip. The error bar represents (1 standard deviation.38,39
values measured for various surfaces are summarized in Table 2. The static contact angle of the flat surface coated with a film of n-hexatriacontane was 91° and increased to 158° when n-hexatriacontane formed a nanostructure of platelets on it. For the specimen with a microstructure the static contact angle was 154° but increased to 169° for the hierarchical surface structure. Contact angle hysteresis and tilt angle for flat, micro-, and nanostructured surfaces show similar trends. The flat surface showed a contact angle hysteresis of 87°, and the droplet still adhered at a tilt angle of 90°. The superhydrophobic micro- and nanostructured surfaces show a reduction of contact angle hysteresis and tilt angle, but a water droplet still needs a tilt angle of 51° and 26° and contact angle hysteresis of 36° and 23°, respectively, before sliding. Only the hierarchical surface structure with static contact angle of 169° and low contact angle hysteresis of 2° exceeds the properties of well-known superhydrophobic and self-cleaning lotus leaves. Bhushan et al.37,38 showed that
air pocket formation in the micro- and nanostructure decreased the solid-liquid contact. Adhesive force measured using a 15 µm radius borosilicate tip in an AFM also shows a similar trend as the wetting properties for the artificial surfaces (Figure 4). Adhesion force of the hierarchical surface structure was lower than that of micro-, nanostructured, and flat surfaces because the contact between the tip and surface was lower as a result of the contact area being reduced in both levels of structuring. 3.2. Self-Cleaning Test. Figure 5 shows SEM micrographs of 1-10 (top) and 10-15 µm (bottom) SiC particles on hierarchical structure using lotus replica. The particles are randomly distributed on the surfaces, and their shapes and sizes show some variation. Some particles contact only a few tubule nanostructures. SiC particles (1-10 µm) are found on top of the pillars and papillose cells and also within the cavities. However,
Artificial Superhydrophobic Surfaces
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Table 2. Summary of the Static Contact Angles, Contact Angle Hysteresis, and Remaining Particles Measured on Various Surfacesa remaining particles (%)
static contact angle (deg)
contact angle hysteresis (deg)
3° (Lotus wax) or 10° (n-hexatriacontane) tilt angle without impact pressure 1-10 µm 10-15 µm particles particles
45° tilt angle with 1000 Pa impact pressure 1-10 µm 10-15 µm particles particles
80 ( 2.8
82 ( 4.1
75 ( 3.7
39 ( 1.7
flat nanostructure microstructure (lotus replica) microstructure (micropatterned Si replica) hierarchical structure (lotus replica) hierarchical structure (micropatterned Si replica)
119 ( 2.4 167 ( 0.7 158 ( 2.7
71 ( 3.2 (162b, 91c) 6 ( 1.1 (170b, 164c) 29 ( 2 (165b, 136c)
Lotus Wax 65 ( 5.4 48 ( 3.4 56 ( 4.3
42 ( 7.9 24 ( 4.1 57 ( 5.3
17 ( 1.3 0 2.3 ( 0.3
0 0 0
160 ( 1.8
27 ( 2.1 (169b, 142c)
52 ( 6.8
59 ( 3.3
2.4 ( 0.4
0
171 ( 0.8
2 ( 0.7 (172b, 170c)
23 ( 2.9
30 ( 5.6
0.4 ( 0.1
0
173 ( 0.8
1 ( 0.6 (174b, 173c)
27 ( 2.7
28 ( 3.8
0.8 ( 0.3
0
flat nanostructure microstructure (micropatterned Si replica) hierarchical structure (micropatterned Si replica)
91 ( 2.0 158 ( 2.4 154 ( 1.9
87 ( 1.9 (141b, 54c) 23 ( 1.3 (165b, 142c) 36 ( 1.6(167b, 131c)
55 ( 4.7 26 ( 3.2 57 ( 3.2
15 ( 2.4 0 3.2 ( 0.8
0 0 0
169 ( 1.3
2 ( 0.8 (170b, 168c)
epoxy resin
a
The variation represents (1 standard deviation.
b
n-Hexatriacontane 74 ( 4.0 45 ( 6.2 70 ( 5.8 27 ( 2.6
30 ( 5.1
0
24 ( 2.4
0
Advancing contact angle. c Receding contact angle.
Figure 5. SEM micrographs of 1-10 (top) and 10-15 µm (bottom) SiC particles on hierarchical structure using lotus replica and lotus wax. The particles are randomly distributed on the surface and their shapes and sizes show some variation. Some particles contact only few tubule nanostructures.
a larger number of 10-15 µm SiC particles are sitting on the top of several pillarsand papillose cells. 3.2.1. Low Tilt Angle Studies. Self-cleaning by water rinsing with nearly zero kinetic energy of the applied water was studied with 1-10 and 10-15 µm particles, and tilt angles of the specimen surface were 3° for lotus wax and 10° for surfaces covered with n-hexatriacontane (reasons for selection of tilt angle given earlier). The values measured for various surfaces are summarized in Table 2. Figure 6 shows that none of the investigated surfaces was fully cleaned by water rinsing. The
data represent the average of five different investigated areas for each experiment. For lotus wax, which forms tubule nanostructures, and n-hexatriacontane, which forms platelet nanostructures, the same tendency of particle removal was found. With the exception of hierarchical structure on all surfaces, larger particles were removed more than small ones. Most particles (70-80%) remained on smooth surfaces, and 50-70% of particles were found on microstructured surfaces. Most particles were removed from the hierarchical structured surfaces, but ∼30% of particles remained. A clear difference of particle removal, independent of particle sizes, was only found in flat and nanostructured surfaces where larger particles were removed with higher efficiency. Observations of the droplet behavior during the movement on the surfaces showed that droplets were rolling only on the hierarchical structured surfaces. On flat, micro-, and nanostructured surfaces, the droplets first applied were not moving, but the continuous application of water droplets increased the droplet volumes and led to a sliding of these large droplets. During this, some of the particles had been removed from the surfaces. However, the rolling droplets on hierarchical structures did not collect the dirt particles trapped in the cavities of the microstructures. The data clearly show that hierarchical structures have superior cleaning efficiency. 3.2.2. High Tilt Angle Studies. In order to study the geometrical scale effects, a self-cleaning test with a standard tilt angle of 45° was performed. Additionally, the influence of water application with three different kinetic energies on particle removal was studied. After deposition of contamination on the surfaces, the surfaces with the contaminants were subjected to water droplets at a standard tilt angle of 45°. The values measured for various surfaces are summarized in Table 2. Figure 7(top) shows the optical microscope images of 1-10 mm SiC particles on hierarchical structured Lotus replicas before
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Figure 6. Bar charts showing the remaining particles after applying droplets with nearly zero kinetic energy on various structures fabricated using lotus and micropatterned Si replica lotus wax and n-hexatriacontane using 1-10 and 10-15 µm SiC particles. The experiments on the surfaces with lotus wax and n-hexatriacontane were carried out on tilted stages with 3° and 10°, respectively. The error bars represent (1 standard deviation.
Figure 7. Optical microscope images of 1-10 mm SiC particles sitting on hierarchical structure using lotus replica with lotus wax before and after artificial water cleaning test, with nearly zero kinetic energy. After droplets roll off the surface, some particles, which were trapped between pillars remained on the surface. After applying an impact pressure of 1000 Pa, such particles were removed.
and after artificial water rinsing with nearly zero kinetic energy. The images show large fractions of the particles were removed when the droplets rolled off the surface, but some particles which
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were trapped in the cavities of the cell papilla remained on the surface. In order to remove the particles trapped between the microstructures, different kinetic energies of the droplets hitting the surface were applied (Figure 7, bottom). Figure 8 summarizes data for self-cleaning experiments performed with the various surfaces at 45° tilt angle and with 1-10 and 10-15 µm SiC particles. The data represent the average of five different investigated areas for each experiment. An increase in impact pressure improves the cleaning efficiency. The schematic as shown in Figure 9 demonstrates that a kinetic energy (impact pressure) of the droplets removes the particles trapped between the microstructures. If the impact pressure of the droplet is zero or low, the air pockets in hierarchical structures still remain between the pillars or papillose cells, and the droplet cannot pick up the particles which are sitting at the bottom of the cavities between the pillars. However, if the impact pressure of the droplet is high, the air pockets do not exist below the droplet as a result of droplet impalement by the pillars. Therefore, all particles can be removed by the droplet after applying impact pressure of 1000 Pa. On flat epoxy resin and on the flat surfaces with a lotus wax or n-hexatriacontane film, the number of remaining particles decreased gradually with increasing of the pressure. The cleaning was more efficient on flat surfaces covered with lotus wax or n-hexatriacontane than on flat epoxy resin, where ∼40% of the particles remained. The cleaning experiment showed that the hydrophilic SiC particles show higher affinity to the hydrophilic surface of flat epoxy resin than to the wax coated surfaces. Fu¨rstner et al.31 performed selfcleaning tests with microstructured lotus leaf replica (without the wax nanostructure). The contaminant was a luminescent and hydrophobic powder, and self-cleaning tests were performed with artificial fog (droplets in the diameter range of 8-20 µm, without kinetic energy), and rain was simulated by a sprinkler (droplet sizes were 0.9 mm). Specimens were attached to a tilted stage at 45° tilt angle. Only the artificial rain (5 min rinsing with 1500 mL/m2) led to self-cleaning when a pressure of 500 Pa was used. In our study the microstructured lotus leaf replica showed similar self-cleaning behavior, but particles could be removed with less pressure and in much shorter time of water rinsing. This difference might be caused by the differences in water affinity of the particles used in both tests. Particles used by Fu¨rstner et al.31 are lipophilic, whereas SiC particles are hydrophilic and show a stronger affinity to the water droplet. The self-cleaning tests of plant surfaces described by Barthlott and Neinhuis4 were made by exposure of contaminated leaves to natural rain, artificial rain (droplets of 0.5-3 mm diameter), and fogging (droplets of 1- 20 µm diameter). They showed that on hierarchical structured leaves (lotus and Colocasia esculenta), a certain amount of very small particles (1-6 µm) remained, when surfaces were wetted without kinetic energy (fogging). They also reported that the particles were easily removed from the surfaces when subjected to heavy rain, or by rinsing with water for 5 min, and 50 mm distance to the leaves. The results for artificial hierarchical structured surfaces demonstrate that a much shorter time of surface rinsing, with a comparable kinetic energy, can remove the particles from the cavities of hierarchical structures. At a pressure of 500-1000 Pa, an elastic deformation of the water droplets allows them to penetrate between papillae and remove particles within the troughs. This low kinetic energy is reached at a droplet fall velocity of about 1 m/s and represents a very low value compared to a natural rain shower, where a water droplet of 2 mm radius can reach a fall velocity of 6 m/s, whereas the velocity of 4 mm droplets is already above 8 m/s (measured under controlled conditions).41 In natural rain, wind can dramatically increase the droplet speed, and the sizes of
Artificial Superhydrophobic Surfaces
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Figure 8. Bar chart showing the remaining particles as a function of impact pressure of droplets on various structures fabricated with lotus wax and n-hexatriacontane using (a) 1-10 and (b) 10-15 µm SiC particles. The error bars represent (1 standard deviation.
used are in general larger than the surface nanostructures, and the particles rest only on the very tips of the wax structures. Therefore, geometrical scale effects were responsible for superior performance of nanostructured surfaces. A comparison of particle sizes and self-cleaning efficiency shows that larger particles are more efficiently removed from all flat surfaces and, after increase of the impact pressure, also from the microstructured and hierarchical structured surfaces. The sizes of the larger particles are in the dimension of the cell papilla and Si-micropillar heights, so that some of them can still come in contact with a rolling water droplet. In contrast to that, smaller particles in the cavities do not come in contact with the droplet. The introduction of kinetic energy causes an elastic deformation of the water droplet when coming in contact with the specimen surface and provides the required contact between the trapped particles and the water surfaces.
4. Conclusions Figure 9. Schematic of wetting and self-cleaning of microstructure at two impact pressures of droplets hitting the surface.
droplets range in a wide distribution. The 0.2 mm droplets used here are in the lower size range, indicating that even a soft rain shower will be able to remove particles from hierarchical structures surfaces. It has been shown that nanostructures and more efficient hierarchical structures reduce the adhesive forces between the surfaces and a spherical borosilicate tip (Figure 4a,b). On the basis of this, it was assumed that the higher contact area between the larger particles and the surfaces increases their adhesion and leads to less efficient self-cleaning. The particles
Flat hydrophilic and hydrophobic, nanostructured, microstructured, and hierarchical structured superhydrophobic surfaces were fabricated, and their wettability, adhesion properties, and self-cleaning by water were studied. The hierarchical structured surfaces, created with wax tubules on the microstructure of a lotus leaf and micropatterned Si showed the largest static contact angle of 171° and 173° and the lowest contact angle hysteresis between 1-2°. The hierarchical structured surfaces, created with n-hexatriacontane on micropatterned Si, showed a static contact angle of 169° and contact angle hysteresis of 2°. On nanostructured surfaces static contact angles were 158° for platelet and 167° for tubules, and contact angle hysteresis was 23° and 4°, respectively. The influence of contact angle hysteresis on self-cleaning by water
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droplets was studied at tilt angles (TA) of the specimen surfaces (3° for tubules, 10° for platelets) which are comparable to that of contact angle hysteresis of hierarchical structure. No surfaces were cleaned by water applied onto the flat, microstructured, and nanostructured surfaces with nearly zero kinetic energy, but most particles were removed from hierarchical structured surfaces, and a certain number of particles were captured between the asperities of the micro- and hierarchical structured surfaces. After an increase of the TA to 45°, which is higher
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than contact angle hysteresis of all structured surfaces, all nanostructured surfaces were cleaned by water droplets moving over the surfaces. Droplets applied onto the surfaces with some pressure removed these particles and led to self-cleaning by rolling droplets. Geometrical scale effects minimizing the adhesion and hysteresis were responsible for superior performance of nanostructured surfaces. LA803860D
