Article pubs.acs.org/Langmuir
Underwater Drag-Reducing Effect of Superhydrophobic Submarine Model Songsong Zhang,†,‡ Xiao Ouyang,†,‡ Jie Li,‡ Shan Gao,‡ Shihui Han,†,‡ Lianhe Liu,*,†,‡ and Hao Wei*,†,‡ †
Key Laboratory of Superlight Materials and Surface Technology of the Ministry of Education and ‡Institute of Advanced Marine Materials, Harbin Engineering University, Harbin 150001, PR China S Supporting Information *
ABSTRACT: To address the debates on whether superhydrophobic coatings can reduce fluid drag for underwater motions, we have achieved an underwater drag-reducing effect of large superhydrophobic submarine models with a feature size of 3.5 cm × 3.7 cm × 33.0 cm through sailing experiments of submarine models, modified with and without superhydrophobic surface under similar power supply and experimental conditions. The drag reduction rate reached as high as 15%. The fabrication of superhydrophobic coatings on a large area of submarine model surfaces was realized by immobilizing hydrophobic copper particles onto a precross-linked polydimethylsiloxane (PDMS) surface. The pre-cross-linking time was optimized at 20 min to obtain good superhydrophobicity for the underwater drag reduction effect by investigating the effect of pre-cross-linking on surface wettability and water adhesive property. We do believe that superhydrophobic coatings may provide a promising application in the field of drag-reducing of vehicle motions on or under the water surface.
■
INTRODUCTION The phenomenon of superhydrophobicity originated from the research on lotus leaves, which refers to a surface with a water contact angle above 150° and a small roll-off angle.1−3 In the early stages, the research mainly focused on the fabrication strategy of superhydrophobic surfaces by combining a suitable surface roughness and a low-surface-energy coating to mimic the extreme surface wettability in nature, such as lithography,4 vapor deposition,5 template methods,6,7 layer-by-layer methods,8−10 sol−gel processing,11,12 electrochemical methods,13,14 electroless metal deposition,15−18 and so forth. Currently, the research on superhydrophobic surfaces confronts three major challenges: the improvement of the mechanical and chemical stability,19−21 the fabrication of transparent coatings with a large area of superhydrophobicity,22,23 and new functions of superhydrophobic surfaces for wide applications, including water−oil separation,24−26 self-cleaning coatings,27 drag reduction,28−30 and so on. Among them, the drag-reducing properties of superhydrophobic coatings are the most promising for practical use, which could improve the sailing velocities of vehicles on or under the water surface and also effectively reduce the fuel consumption. Most research31−36 on the drag-reducing effect of superhydrophobic surfaces was carried out by investigating flow behaviors on static superhydrophobic surfaces and demonstrated the effective drag-reducing property of superhydrophobic coatings under this condition with the exception of Steinberger’s report.37 Until now there have been few reports to evaluate the drag-reducing effect by measuring the velocity of moving objects with superhydrophobic coatings. Debates remain over whether the superhydrophobic coating can reduce drag in this situation. Zhang et al.38 have drawn the conclusion © 2014 American Chemical Society
that superhydrophobic surfaces are favorable for drag reduction and achieve a remarkable 70% velocity increase in millimeter gold thread modified with superhydrophobic coatings, compared to a normal hydrophobically modified one. Similarly, we have also obtained a drag-reducing rate of 49.1% on superhydrophobic model ships as long as 33 cm on the water surface.39 However, some reports argue that superhydrophobic surfaces are not always favorable to drag reduction and even cause dragincreasing results in the situation of evaluating the velocity of moving superhydrophobic objects. For example, Lu and coworkers40 showed a lower underwater moving velocity of a superhydrophobic glass ball than a normal glass ball, which indicated that the superhydrophobic coatings caused a dragincreasing effect under the water surface. But they did not compare the moving distances between the superhydrophobic ball and the highly hydrophilic one in a similar situation. On one hand, for the initial energy provided by releasing the balls into water, there is a large difference at the point where the balls penetrated the water from air. For the superhydrophobic ball, there are two upward forces (buoyancy force and curvature force) against the gravity force; for the highly hydrophilic ball, the curvature force provides a downward force,9 which will lead to an initial velocity of the hydrophilic ball that is larger than that of the superhydrophobic one. On the other hand, in the underwater vertical motions, the bubble layer with a thickness of 74 μm around the superhydrophobic ball40 will provide extra buoyancy against gravity. The size of balls is small (diameter Received: November 12, 2014 Revised: December 11, 2014 Published: December 13, 2014 587
DOI: 10.1021/la504451k Langmuir 2015, 31, 587−593
Article
Langmuir 1.8−1.9 cm), and the extra buoyancy could seriously affect its motion. Even though the hydrophilic and superhydrophobic balls weighed the same, the difference between gravity and buoyancy exerted on the superhydrophobic ball is around 4.3%, less than that of the highly hydrophilic ball. Hence, to exclude the influence of initial energy and extra vertical buoyancy forces, herein we investigated drag reducing effects on horizontal movements of an electricity powered submarine model with a feature size of 33 cm. This research can clarify whether superhydrophobic coatings reduce drag for the following reasons: (1) the electrical power within the submarine model is basically the same to provide a similar initial velocity; (2) the feature size of the submarine model is large enough to ignore the extra displacement volume induced by the bubble layer (S1 in Supporting Information); (3) the vertical downward gravity or upward buoyancy has little influence on horizontal movements and thus excludes uncertain parameters. In this article, we have fabricated a homogeneous superhydrophobic coating on the submarine model and investigated the underwater drag-reducing property of superhydrophobic coatings by comparing the moving velocity of the superhydrophobic submarine model with a normal one. The results showed that in the range of electricial power from 0.3 to 0.7 W the superhydrophobic submarine model always presented a higher velocity than that of the normal one, and the drag-reducing effect could reach as high as 15%. This result demonstrated that superhydrophobic coatings could also exhibit a good underwater drag-reducing effect under the same initial power supply.
■
Figure 1. (a) Illustration of the fabrication of submarine model with superhydrophobic coatings. (b) Optical photographs of submarine models without any surface modification (left, gray one) and modified with a PDMS/copper superhydrophobic coating (right, rose bengal one). to immobilize the copper particles. After the cross-linking of PDMS, redundant copper particles were removed by water. The superhydrophobicity of the as-prepared superhydrophobic coating on the submarine model was displayed with a plastron effect in Figure S2. Measurement of the Water Adhesive Force. Following previous reports,41,42 the water adhesive force measurement was carried out with DCAT11, which was equipped with a sensitive balance in the upper part to sense force changes of hung water and a precise motor in the bottom part to drive the substrate at the desired speed. Before the test, a water droplet of around 5 μL was hung on a metal ring connected to the balance, and the as-prepared substrate was placed on the motorized stage. Once the measurement started, the substrate was driven upward by the motor to contact the hung water above at a constant speed of 0.1 mm/s. Upon contact, the instrument collected data because of the force change of the water droplet, during which the substrate was kept upward for another 0.1 mm at a slowed moving rate of 0.01 mm/s. Later on, the substrate moved downward back to its original position, which led to the detachment of the substrate and the water droplet. At the detaching point, the peak value recorded appeared and indicated the adhesive force between the water droplet and the substrate. Underwater Drag-Reducing Tests of the Submarine Models with and without Superhydrophobic Coatings. The sailing experiments were carried out in a water trough with dimensions of 6 m × 0.6 m × 0.3 m, which provided a sailing journey as long as 6 m. To guide the direction of the moving submarine, two copper wires of the same length were hung parallel to each other right above the water surface, and the submarine was connected to the copper wires with a nylon thread. For comparison, the masses of the normal submarine and the superhydrophobic one were adjusted to 305 and 309 g, respectively, whose gravity force is slightly larger than their corresponding buoyancy force measured by the Archimedes method. The resulting submarine models shared a similar density and were equipped with the same power supply. Before the start of sailing, they were both switched on to start the engine and released at the same starting line of the trough. The time to reach the end of the trough was recorded, and an average moving velocity was calculated correspondingly. For each condition of the velocity test, 10 measurements were carried out.
EXPERIMENTAL SECTION
Materials and Instruments. The following chemicals were used and purchased: copper powder was purchased from Sinopharm Chemical Reagent Beijing Co., Ltd. (Beijing, China). PDMS prepolymer and its curing agent (Sylgard 184) were obtained from Dow Corning. The motor-driven submarine models were purchased from ZT Model Co., Ltd. (Hangzhou, China). Scanning electron microscope (SEM) images and energy-dispersive X-ray spectroscopy (EDX) patterns were obtained with an Su1510 scanning electron microscope at 15.0 kV (Hitachi High-Technologies Corporation, Japan). The contact angle and roll-off angle were measured on an OCA15EC (Data Physics Instruments GmbH, Filderstadt, Germany). Adhesive force curves were tested through a dynamic contact angle measuring tensionmeter (DCAT11) from Data Physics. The volume of water droplets for the measurement of the contact angle and roll-off angle was 4.0 μL. For each sample, the contact angle, roll-off angle, and adhesive force were measured at 5 to 10 different positions on the substrates, which were averaged. Optical photographs were taken by a Nikon camera (D5000). Fabrication of Superhydrophobic Coatings on the Submarine Model. The fabrication of the superhydrophobic submarine model is illustrated in Figure 1a. First, a submarine model with a feature size of 3.5 cm × 3.7 cm × 33.0 cm was cleaned by ethanol; the PDMS prepolymer was mixed with its corresponding cross-linking agent in a weight ratio of 10:1, followed by stirring for 15 min and removing air bubbles in vacuum for 30 min. Second, the viscous fluid of the PDMS mixture was painted onto all exposed surfaces of the submarine model, which was kept in air for 1 h, and then the submarine model with a PDMS coating was held at 60 °C for 20 min for the partially thermal pre-cross-linking of PDMS, resulting in a sticky surface. The pre-cross-linking time of 20 min was optimized through checking the surface morphology and wettability at the following pre-cross-linking time intervals: 0, 15, 20, 30, and 40 min. Third, a layer of commercially available copper particles was adhered to the submarine model because of the partially cross-linked sticky PDMS surface, followed by full cross-linking of PDMS at 60 °C for 4 h
■
RESULTS AND DISCUSSION Until now, there have been few reports on drag-reducing effects of a superhydrophobic surface through directly measuring the 588
DOI: 10.1021/la504451k Langmuir 2015, 31, 587−593
Article
Langmuir
Figure 2. SEM images of the PDMS/copper surface at pre-cross-linking times of (a) 0, (d) 10, (g) 20, (j) 30, and (m) 40 min. (b, e, h, k, and n) and (c, f, i, l, and o) Corresponding local magnified SEM images and EDX patterns, respectively. The yellow color indicates the Cu element, and the blue color refers to the Si element.
aggregates. Moreover, a corresponding EDX pattern was used to analyze the surface composition in Figure 2c: the appearance of the Cu element (yellow color) and Si element (blue color) indicated the presence of copper particles and the PDMS coating, respectively; the copper particle aggregates scattered on the PDMS surface like islands, leading to a relatively small surface coverage. The reason for this phenomenon might be attributed to the total wrapping of copper particles within the un-cross-linked PDMS, which was a layer of viscous, flowable liquid and supplied little supporting force to the copper particles. When we increased the pre-cross-linking time to 15 min, the PDMS/copper composite film showed irregular clusters instead of stacked layers (Figure 2d), and the protruding and concave structures led to increased surface roughness (Figure 2e) and surface coverage (Figure 2f). In this situation, the copper particles were not highly wrapped within the PDMS because the partially cross-linked PDMS coating obtained a little rigidity to support the copper particles, which resulted in higher surface roughness than for the PDMS coating only. Following this principle, in further increasing the pre-cross-linking time to 20
moving velocity of vehicles on or under water surfaces with a large area of curved surfaces; this is because the fabrication of superhydrophobic coatings on this kind of surface is not easy work.38,39,41 Herein, we fabricated superhydrophobic coatings on a submarine model with a feature size of about ϕ = 4 cm × 33 cm by sticking hydrophobic copper particles onto the partially cross-linked PDMS coating as demonstrated in the Experimental Section. To control the surface morphology of the as-prepared coatings, we investigated the condition of copper particles immobilized on the PDMS layer by changing the pre-cross-linking time of PDMS before the adherence of copper particles. The surface morphology of the PDMS/copper hydrophobic substrates with a different pre-cross-linking time interval was checked with SEM images after the adhesion of copper particles and full cross-linking: 0, 15, 20, 30, and 40 min as summarized in Figure 2. Without pre-cross-linking, the surface presented a large area of stacked layers with a relatively homogeneous morphology (Figure 2a); from its local magnified image in Figure 2b, small particles aggregated to form bundles of clusters, indicating that the large stacked layers in Figure 2a were composed of closely packed particle 589
DOI: 10.1021/la504451k Langmuir 2015, 31, 587−593
Article
Langmuir min in Figure 2g, we can observe that the surface roughness grew as the magnified image in Figure 2h displayed, which presented hierarchical structures accumulated by the particles for a high surface roughness. From its EDX pattern, the copper particles were firmly aggregated to result in a maximum surface coverage among other conditions, leaving few bare PDMS surfaces. However, as the pre-cross-linking time was prolonged to 30 min or even 40 min, the surface coverage gradually decreased and the copper particles became loosely distributed, as shown in Figure 2j−l,m−o, respectively. These results indicated that the pre-cross-linking degree of the PDMS layer strongly influenced the surface morphology of the as-prepared coating after the adhesion of copper particles (Figure S3). At a low pre-cross-linking degree, the copper particles were easily and entirely wrapped within the PDMS coating, and thus the surface roughness was not sufficient. At a high pre-cross-linking degree, the PDMS surface became rigid, and it was hard to adhere and trap the copper particles for the formation of stable PDMS/copper composites, which also lost surface roughness and coverage. Therefore, by comparing the surface morphology, we determined the optimized pre-cross-linking time to be 20 min, under which the PDMS/copper film provided sufficient surface roughness and coverage for superhydrophobicity. Because both the PDMS (22 mN/m) and the copper powder (30 mN/m)41 are low-surface-energy species, the surface wettability of the PDMS/copper composite coating with different pre-cross-linking times should be determined by the surface morphology. To investigate the relationship between the surface wetting property and the pre-cross-linking time, we measured the static contact angle and roll-off angle of the asprepared substrates versus the pre-cross-linking time. As shown in Figure 3a, without pre-cross-linking, the PDMS/copper surface had a contact angle about 140°, which was a highly hydrophobic surface but still not superhydrophobic. As the precross-linking time increased, the contact angle grew to 145° at 10 min, leveled off at around 153° in the range of 15 to 30 min, and dropped gradually to 140° at 50 min. The results matched well with the trend in the SEM images, i.e., the surface roughness and coverage showed a parabola-like trend with the growing pre-cross-linking time, and the optimized condition was indicated in the middle peak value. Correspondingly, the curve of the roll-off angle versus the pre-cross-linking time demonstrated an opposite trend to that of the contact angle. The substrates were highly water-adhesive at a low pre-crosslinking degree and had a large roll-off angle of up to 90°; after the contact angle was above 150°, the corresponding roll-off angle dropped to below 20°, under which the water droplet could easily roll off the substrate as shown in the inset of Figure 3a. With further increasing pre-cross-linking time, the roll-off angle grew back to 90° and presented a strongly water-adhesive surface again. The correlation of both the contact angle and roll-off angle with the pre-cross-linking time of the PDMS coating corresponded well with the situation for the surface morphology of SEM images. Therefore, the optimized precross-linking time at 20 min not only provided a suitable surface roughness and surface coverage but also favored the required superhydrophobicity. Although the surface wettability (contact angle and roll-off angle) was generally regarded as an important parameter in determining the drag-reducing properties of superhydrophobic surfaces, the superhydrophobic surface with a high roll-off angle displayed the opposite results. According to the report from Shi et al.,41 they proposed a metric to determine the drag-reducing
Figure 3. (a) Correlations among the static contact angle (left, red y axis), roll-off angle (right, blue y axis), and pre-cross-linking time, correspondingly. (b) Water adhesive force values versus the pre-crosslinking time.
effects of as-prepared superhydrophobic coatings through a water adhesive force with water capture status (Wenzel or Cassie) to precisely judge the drag-reducing (no water captured, Cassie mode)43 or drag-increasing (water captured, Wenzel mode)44 properties of superhydrophobic coatings, which well matched their experimental sailing results. In the current work, we applied this metric to evaluate the dragreducing properties of the as-prepared PDMS/copper composite coatings following the measurement of water adhesive force curves in Shi’s work. The adhesive force values were plotted versus the pre-cross-linking time of the PDMS layer, as displayed in Figure 3b. The force values declined from the original 110 μN without pre-cross-linking gradually to the minimum at around 20 μN at the pre-cross-linking time of 25 min; afterward, the adhesive force increased to a high level of 200 μN when the PDMS was pre-cross-linked for 50 min; in the pre-cross-linking time interval from 20 to 30 min, the average adhesive force values changed in a small range of 20 to 30 μN. The force trend in this result corresponded well to those in the surface morphology and surface wettability (contact angle and roll-off angle). Moreover, from one typical water adhesive force curve of the above PDMS/copper coatings in Figure S4, we could observe that there was no water captured within the above coatings, indicating that all of the as-prepared coatings would lead to drag-reducing results according to the reported metric by Shi et al.41 To save the pre-cross-linking time and in the meantime to obtain a good drag-reducing effect, we finally determined the pre-cross-linking time to be 20 min to obtain a superhydrophobic surface with low roll-off angle as the optimized condition for the further drag-reducing measurements. 590
DOI: 10.1021/la504451k Langmuir 2015, 31, 587−593
Article
Langmuir
those of the normal ones for the same power supply. The maximum drag-reducing effect achieved was as high as 15% at a power supply of 0.3 W. The mechanism of underwater drag-reducing effects caused by superhydrophobic coatings was interpreted as follows. For normal submarine models, the liquid contacting mode in the water was of a liquid−solid contacting type. In this situation, the fluid layer nearest the submarine surface produced a fluidic drag force (f) onto the submarine through a liquid−solid friction; meanwhile, for the nearest fluid layer, it was driven by a force (F) caused by the movement of the submarine. These two forces (F and f) acted as an impetus force and drag force for the nearest fluid layer and the submarine, respectively; they had equal magnitude and opposite direction according to the third law of Newton’s laws of motion. Therefore, the moving velocity of the nearest fluid layer (V1) should be close to that of the submarine (V). Because of Newton’s law of viscosity, the movement of this nearest fluid layer was slowed down by the second-nearest fluid layer because of fluid shear stress (if we considered the fluid to be an extremely thin-layered model). Similarly, the second-nearest fluid layer would be affected by the slower third-nearest fluid layer and so on. (Note that the farthest fluid layer has a velocity close to zero.) In this way, the fluid near the submarine formed a velocity gradient, which could be linearly correlated to the value of the shear stress if the viscosity of the fluid was a constant. In other words, the shear stress (fluid drag) increased with the growth of the velocity gradient. For the superhydrophobic submarine model, there is a bubble layer between the superhydrophobic solid surface and the nearest fluid layer due to the plastron effect of superhydrophobic surfaces.45 Under this condition, the nearest liquid−solid contacting mode was replaced with a liquid−gas− solid contacting mode. Correspondingly, the friction changed from the original solid−liquid friction ( f) to the combination of solid−gas friction ( f ′) and gas−liquid friction (f ″). Considering that f ′ and f ″ were both much lower than f,46 the driving force for the bubble and for the fluid layer nearest the bubble layer, F′ and F″, should also be much lower than F for a normal submarine. Hence, the resulting moving velocity of the fluid layer nearest the bubble layer (V1′) will be much smaller than that under the condition of a normal submarine (V1). Correspondingly, the velocity gradient was reduced and led to a lower shear stress of the fluid near the superhydrophobic submarine, contributing to a reduced drag force. This condition was quite similar to a slippery boundary layer,47,48 which was generally considered to be the drag-reducing essence of the superhydrophobic coatings in most reports investigating flow behaviors on static superhydrophobic surfaces.31−33 In summary, we have demonstrated that superhydrophobic coatings did show an underwater drag-reducing property through sailing experiments of submarine models modified with and without a superhydrophobic surface under a similar power supply and experimental conditions. The superhydrophobic coatings were fabricated on a submarine model with a large area of curved surfaces by immobilizing hydrophobic copper particles onto a pre-cross-linked PDMS surface. In the fabrication process, the pre-cross-linking time played a dominant role in the surface wettability and wateradhesive property of the as-prepared PDMS/copper composite coatings. We have investigated the effects of pre-cross-linking time on the surface morphology, surface wettability, and water adhesion independently, which corresponded well with each
After we obtained a superhydrophobic coating on a submarine model, which theoretically exhibited a drag-reducing effect,41 we wondered whether it could really achieve an underwater drag-reducing effect. We carried out the sailing experiments in a water trough with a 6 m journey as described in the Experimental Section. To keep other conditions similar, we switched on the engine of both superhydrophobic and normal submarine models and held them at the starting line before sailing, as shown in Figure 4a. Then the two submarine
Figure 4. Optical photographs of sailing experiments at a power supply of 0.3 W (a) before starting movement and (b) when the superhydrophobic submarine model reached the finish line. (c) Average moving velocity of normal and superhydrophobic submarine models versus the power supply.
models were released simultaneously, and their directions were guided by the parallel copper wires above the water surface. The two submarine models were totally immersed in water because the gravity forces exerted on them were slightly greater than the corresponding buoyancy forces. In the sailing process, the superhydrophobic submarine model soon ran ahead of the normal submarine, which continued until they reached the end of the trough (Figure 4b); the average moving velocities of the superhydrophobic and normal submarine model at a similar power supply of 0.3 W were 0.40 and 0.35 m/s, respectively. Considering the possible effect of moving velocity on the dragreducing property of superhydrophobic coatings, we have compared a series of moving velocities between the superhydrophobic and normal submarine models equipped with a different power supply through adjusting the resistance value in the power circuit. The sailing experiments were carried out pair by pair for superhydrophobic and normal submarine models, and the results are summarized in Figure 4c. We could observe that with the reduced power supply of the submarine model, the average velocity decreased. With the electrical power used ranging from 0.3 to 0.7 W, the average sailing velocities of superhydrophobic submarine models were always higher than 591
DOI: 10.1021/la504451k Langmuir 2015, 31, 587−593
Article
Langmuir
(6) Feng, L.; Li, S. H.; Li, H. J.; Zhai, J.; Song, Y. L.; Jiang, L.; Zhu, D. B. Super-Hydrophobic Surface of Aligned Polyacrylonitrile Nanofibers. Angew. Chem., Int. Ed. 2002, 41, 1221−1223. (7) Deng, X.; Mammen, L.; Butt, H.-J.; Vollmer, D. Candle Soot as a Template for a Transparent Robust Superamphiphobic Coating. Science 2012, 335, 67−70. (8) Zhai, L.; Cebeci, F. C.; Cohen, R. E.; Rubner, M. F. Stable Superhydrophobic Coatings from Polyelectrolyte Multilayers. Nano Lett. 2004, 4, 1349−1353. (9) Shi, F.; Wang, Z.Q.; Zhang, X. Combining a Layer-by-layer Assembling Technique with Electrochemical Deposition of Gold Aggregates to Mimic the Legs of Water Striders. Adv. Mater. 2005, 17, 1005−1009. (10) Li, Y.; Chen, X. Y.; Li, Q. Q.; Song, K.; Wang, S. H.; Chen, X. Y.; Zhang, K.; Fu, Y.; Jiao, Y.-H.; Sun, T.; Liu, F.-C.; Han, E.-H. Layerby-Layer Strippable Ag Multilayer Films Fabricated by Modular Assembly. Langmuir 2014, 30, 548−553. (11) Tadanaga, K.; Morinaga, J.; Matsuda, A.; Minami, T. Superhydrophobic-Superhydrophilic Micropatterning on Flowerlike Alumina Coating Film by the Sol-Gel Method. Chem. Mater. 2000, 12, 590−592. (12) Han, J. T.; Lee, D. H.; Ryu, C. Y.; Cho, K. Fabrication of Superhydrophobic Surface from a Supramolecular Organosilane with Quadruple Hydrogen Bonding. J. Am. Chem. Soc. 2004, 126, 4796− 4797. (13) Zhang, X.; Shi, F.; Yu, X.; Liu, H.; Fu, Y.; Wang, Z. Q.; Jiang, L.; Li, X. Y. Polyelectrolyte Multilayer as Matrix for Electrochemical Deposition of Gold Clusters: Toward Super-Hydrophobic Surface. J. Am. Chem. Soc. 2004, 126, 3064−3065. (14) Wang, S. T.; Feng, L.; Liu, H.; Sun, T. L.; Zhang, X.; Jiang, L.; Zhu, D. B. Manipulation of Surface Wettability between Superhydrophobicity and Superhydrophilicity on Copper Films. ChemPhysChem 2005, 6, 1475−1478. (15) Gao, Y. F.; Cheng, M. J.; Wang, B. L.; Feng, Z. G.; Shi, F. Diving-Surfacing Cycle Within a Stimulus-Responsive Smart Device Towards Developing Functionally Cooperating Systems. Adv. Mater. 2010, 22, 5125−5128. (16) Ju, G. N.; Cheng, M. J.; Xiao, M.; Xu, J. M.; Pan, K.; Zhang, Y. J.; Shi, F. Smart Transportation between Three Phases through a Stimulus-Responsive Functional Cooperating Device. Adv. Mater. 2013, 25, 2915−2919. (17) Cheng, M. J.; Liu, Q.; Ju, G. N.; Zhang, Y. J.; Jiang, L.; Shi, F. Bell-Shaped Superhydrophilic-Superhydrophobic-Superhydrophilic Double Transformation on a pH-Responsive Smart Surface. Adv. Mater. 2014, 26, 306−310. (18) Song, M. M.; Cheng, M. J.; Ju, G. N.; Zhang, Y. J.; Shi, F. Converting Chemical Energy into Electricity through a Functionally Cooperating Device with Diving-Surfacing Cycles. Adv. Mater. 2014, 26, 7059−7063. (19) Li, Y.; Li, L.; Sun, J.Q. Bioinspired Self-Healing Superhydrophobic Coatings. Angew. Chem., Int. Ed. 2010, 49, 6129−6133. (20) Lee, C.; Kim, C.-J. Underwater Restoration and Retention of Gases on Superhydrophobic Surfaces for Drag Reduction. Phys. Rev. Lett. 2011, 106, 014502. (21) Li, Y.; Chen, S. S.; Wu, M. C.; Sun, J. Q. All Spraying Processes for the Fabrication of Robust, Self-Healing, Superhydrophobic Coatings. Adv. Mater. 2014, 26, 3344−3348. (22) Li, Y.; Liu, F.; Sun, J. Q. A Facile Layer-by-Layer Deposition Process for the Fabrication of Highly Transparent Superhydrophobic Coatings. Chem. Commun. 2009, 2730−2732. (23) Deng, X.; Paven, M.; Papadopoulos, P.; Ye, M.; Wu, S.; Schuster, T.; Klapper, M.; Vollmer, D.; Butt, H.-J. Solvent-Free Synthesis of Microparticles on Superamphiphobic Surfaces. Angew. Chem., Int. Ed. 2013, 52, 11286−11289. (24) Cheng, M. J.; Gao, Y. F.; Guo, X. P.; Shi, Z. Y.; Chen, J.-F.; Shi, F. A Functionally Integrated Device for Effective and Facile Oil Spill Cleanup. Langmuir 2011, 27, 7371−7375. (25) Cheng, M. J.; Ju, G. N.; Jiang, C.; Zhang, Y. J.; Shi, F. Magnetically Directed Clean-up of Underwater Oil Spills through a
Figure 5. Drag-reducing mechanism of the superhydrophobic coatings. Flow conditions of the boundary layer for (a) normal and (b) superhydrophobic submarine models.
other. On the basis of these results, we obtained an optimized pre-cross-linking time of 20 min to achieve good superhydrophobicity for the underwater drag-reducing property. We do believe that superhydrophobic coatings may provide a promising application in the field of drag reduction for vehicle motions on or under the water surface.
■
ASSOCIATED CONTENT
S Supporting Information *
Thickness of the bubble layer surrounding the superhydrophobic coatings, optical photograph of the plastron effect of the superhydrophobic coating, illustration of the surface morphology influenced by the pre-cross-linking degree of the PDMS layer, and adhesive force curve of water on the PDMS/ copper coatings. This material is available free of charge via the Internet at http://pubs.acs.org.
■
AUTHOR INFORMATION
Corresponding Authors
*E-mail:
[email protected]. *E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
■
ACKNOWLEDGMENTS This work was supported by the Special Program for International S&T Cooperation Projects of China (2011DFR50770).
■
REFERENCES
(1) Barthlott, W.; Neinhuis, C. Purity of the Sacred Lotus, or Escape from Contamination in Biological Surfaces. Planta 1997, 202, 1−8. (2) Sun, T. L.; Feng, L.; Gao, X. F.; Jiang, L. Bioinspired Surfaces with Special Wettability. Acc. Chem. Res. 2005, 38, 644−652. (3) Zhang, X.; Shi, F.; Niu, J.; Jiang, Y. G.; Wang, Z. Q. Superhydrophobic Surfaces: from Structural Control to Functional Application. J. Mater. Chem. 2008, 18, 621−633. (4) Shiu, J.-Y.; Kuo, C.-W.; Chen, P.; Mou, C.-Y. Fabrication of Tunable Superhydrophobic Surfaces by Nanosphere Lithography. Chem. Mater. 2004, 16, 561−564. (5) Kenneth, K. S. L.; Jose, B.; Kenneth, B. K. T.; Manish, C.; Gehan, A. J. A.; William, I. M.; Gareth, H. M.; Karen, K. G. Superhydrophobic Carbon Nanotube Forests. Nano Lett. 2003, 3, 1701−1705. 592
DOI: 10.1021/la504451k Langmuir 2015, 31, 587−593
Article
Langmuir
(48) Rothstein, J. P. Slip on Superhydrophobic Surfaces. Annu. Rev. Fluid Mech. 2010, 42, 89−109.
Functionally Integrated Device. J. Mater. Chem. A 2013, 1, 13411− 13416. (26) Ju, G. N.; Cheng, M. J.; Shi, F. A pH-Responsive Smart Surface for the Continuous Separation of Oil/Water/Oil Ternary Mixtures. NPG Asia Mater. 2014, 6, e111. (27) Yu, M.; Chen, S.; Zhang, B.; Qiu, D. L.; Cui, S. X. Why a Lotuslike Superhydrophobic Surface is Self-cleaning? An Explanation from Surface Force Measurements and Analysis. Langmuir 2014, DOI: 10.1021/la5041272. (28) Xiao, M.; Cheng, M. J.; Zhang, Y. J.; Shi, F. Combining the Marangoni Effect and the pH-Responsive SuperhydrophobicitySuperhydrophilicity Transition to Biomimic the Locomotion Process of the Beetles of Genus Stenus. Small 2013, 9, 2509−2514. (29) Xiao, M.; Guo, X. P.; Cheng, M. J.; Ju, G. N.; Zhang, Y. J.; Shi, F. pH-Responsive On-Off Motion of a Superhydrophobic Boat: Towards the Design of a Minirobot. Small 2014, 10, 859−865. (30) Xiao, M.; Jiang, C.; Shi, F. Design of a UV-responsive Microactuator on a Smart Device for Light-induced ON-OFF-ON Motion. NPG Asia Mater. 2014, 6, e128. (31) Conttin-Bizonne, C.; Barrat, J.-L.; Bocquet, L.; Charlaix, E. Lowfriction Flows of Liquid at Nanopatterned Interfaces. Nat. Mater. 2003, 2, 237−240. (32) Choi, C. H.; Westin, K. J.; Breuer, K. S. Apparent Slip Flows in Hydrophilic and Hydrophobic Microchannels. Phys. Fluids 2003, 15, 2897−2902. (33) Ou, J.; Perot, B.; Rothstein, J. P. Laminar Drag Reduction in Microchannels using Ultrahydrophobic Surfaces. Phys. Fluids 2004, 16, 4635−4643. (34) Shirtcliffe, N. J.; McHale, G.; Newton, M. I.; Zhang, Y. Superhydrophobic Copper Tubes with Possible Flow Enhancement and Drag Reduction. ACS Appl. Mater. Interfaces 2009, 1, 1316−1323. (35) Zhou, M.; Li, J.; Wu, C. X.; Zhou, X. K.; Cai, L. Fluid Drag Reduction on Superhydrophobic Surfaces Coated with Carbon Nanotube Forests (CNTs). Soft Matter 2011, 7, 4391−4396. (36) Bixler, G. D.; Bhushan, B. Bioinspired Rice Leaf and Butterfly Wing Surface Structures Combining Shark Skin and Lotus Effects. Soft Matter 2012, 8, 11271−11284. (37) Steinberger, A.; Cottin-Bizonne, C. C.; Kleimann, P.; Charlaix, E. High Friction on a Bubble Mattress. Nat. Mater. 2007, 6, 665−668. (38) Shi, F.; Niu, J.; Liu, J. L.; Liu, F.; Wang, Z. Q.; Feng, X.-Q.; Zhang, X. Towards Understanding Why a Superhydrophobic Coating Is Needed by Water Striders. Adv. Mater. 2007, 19, 2257−2261. (39) Dong, H. Y.; Cheng, M. J.; Zhang, Y. J.; Wei, H.; Shi, F. Extraordinary Drag-Reducing Effect of a Superhydrophobic Coating on a Macroscopic Model Ship at High Speed. J. Mater. Chem. A 2013, 1, 5886−5891. (40) Su, B.; Li, M.; Lu, Q. H. Toward Understanding Whether Superhydrophobic Surfaces Can Really Decrease Fluidic Friction Drag. Langmuir 2010, 26, 6048−6052. (41) Cheng, M. J.; Song, M. M.; Dong, H. Y.; Shi, F. Surface Adhesive Force: a Metric Describing the Drag-Reducing Effects of Superhydrophobic Coatings. Small 2014, DOI: 10.1002/ smll.201402618. (42) Wu, Y.; Liu, Z. l.; Liang, Y. M.; Pei, X. W.; Zhou, F.; Xue, Q. J. Switching Fluid Slippage on pH-Responsive Superhydrophobic Surfaces. Langmuir 2014, 30, 6463−6468. (43) Cassie, A. B. D.; Baxter, S. Wettability of Porous Surface. Trans. Faraday Soc. 1944, 40, 546−551. (44) Wenzel, R. N. Resistance of Solid Surfaces to Wetting by Water. Ind. Eng. Chem. 1936, 28, 988−994. (45) McHale, G.; Newton, M. I.; Shirtcliffe, N. J. Immersed Superhydrophobic Surfaces: Gas Exchange, Slip and Drag Reduction Properties. Soft Matter 2010, 6, 714−719. (46) Choi, C.-H.; Kim, C.-J. Large Slip of Aqueous Liquid Flow over a Nanoengineered Superhydrophobic Surface. Phys. Rev. Lett. 2006, 96, 066001. (47) Lauga, E.; Stone, H. A. Effective Slip in Pressure-Driven Stokes Flow. J. Fluid Mech. 2003, 489, 55−77. 593
DOI: 10.1021/la504451k Langmuir 2015, 31, 587−593
