Why a Lotus-like Superhydrophobic Surface Is Self-Cleaning? An

Oct 22, 2014 - ABSTRACT: The unique self-cleaning feature of the lotus- ... However, as far as the lotus-like SH surface and typical dust are concerne...
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Why a Lotus-like Superhydrophobic Surface Is Self-Cleaning? An Explanation from Surface Force Measurements and Analysis Miao Yu,† Sheng Chen,† Bo Zhang,† Dengli Qiu,‡ and Shuxun Cui*,† †

Key Laboratory of Advanced Technologies of Materials (Ministry of Education), Southwest Jiaotong University, Chengdu 610031, China ‡ Bruker (Beijing) Scientific Technology Co. Ltd., Beijing 100081, China S Supporting Information *

ABSTRACT: The unique self-cleaning feature of the lotuslike superhydrophobic (SH) surface attracted worldwide interest in recent years. However, the mechanism of the selfcleaning phenomena remains unclear. Here, we attempt to provide a comprehensive understanding of why self-cleaning of the particles with a broad range of size can be realized on the lotus-like SH surfaces. After measurements and analysis of the force involved at the interface, we conclude that there are four main preconditions for self-cleaning: (1) contact angle (CA) > 90°, (2) low enough sliding angle, (3) low enough adhesion force, and (4) proper particle size. However, as far as the lotus-like SH surface and typical dust are concerned, all the preconditions will be satisfied automatically. We also observe that the particles with a broad range of size (from submicron level to the millimeter level) and density (virtually no limit) can be driven by a water droplet on the lotus-like SH surface. This interesting finding may be helpful for the design of novel engineering system at the micron-millimeter scale in the future.

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INTRODUCTION In general, surfaces with a static contact angle (CA) higher than 150° are defined as superhydrophobic (SH) surfaces. According to the sliding angle and CA hysteresis, the SH surfaces can be categorized into five different states, i.e., the Wenzel state, the Cassie state, the “lotus” state, the transitional state between Wenzel and Cassie states, and the “gecko” state.1,2 Lotus leaf is a well-known example of a SH surface with very high CA and very low sliding angle, which result in the famous self-cleaning “lotus effect”. Jiang et al. proposed that the micro- and nanoscale hierarchical surface topographies are crucial for this effect.3 Currently, studies of SH surfaces mainly focus on the preparation of lotus-like surfaces4−7 because the self-cleaning properties of such surfaces are very useful in a broad range of applications,8−13 such as self-cleaning windows,14,15 stain-proof textiles,16 the separation of water and oil,17 and so forth. In the last decade, a number of methods have been developed to prepare SH surfaces.5,6,18−21 The self-cleaning is the final result of the interplay among the water/air interface, the particle, and the solid surface. Several forces are involved in the self-cleaning process, including the interface force between the water/air interface and the particle, the adhesion force between the solid surface and the particle, the friction force between the solid surface and the particle, together with the gravity and buoyancy of the particle.22 In these forces, the interface force and the buoyancy are positive for self-cleaning, while the adhesion force, the friction force, and the gravity are negative. For typical dust particles on surfaces, the densities are larger than that of water. Thus, the © XXXX American Chemical Society

buoyancy will only partially counteract the gravity of the particle. Therefore, the interface force will be the only one that leads to self-cleaning. Recently, the interactions between the particle and the water/air interface have been studied extensively.22,23 It has been reported that a number of factors can influence the interface force, such as contact angle,23 surface tension of liquid,24 size of the particle,25 and moving velocity of the liquid−gas interface.23,24 However, due to the complexity of the forces involved, the mechanism of self-cleaning on SH surfaces is not fully revealed yet. Here in this paper we aim at providing a general understanding of why self-cleaning of the particles with a broad range of size can be realized on the lotus-like SH surfaces. First, we measure the adhesion force between a glass colloid probe and three kinds of surfaces of different hydrophobicities by atomic force microscopy (AFM). Then we carry out force analysis on the involved forces for the hydrophilic and hydrophobic surfaces, respectively. On the basis of the results of force analysis and adhesion force measurements, we predict the critical sizes of the four kinds of particles that can be removed by the flow of the water droplet on the SH surface, which are verified by the experimental results. Received: October 18, 2014

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(see Figure 2). For a given particle density, the sizes of the particles were increased gradually in the drag test. In these tests, the maximum

EXPERIMENTAL SECTION

Materials and Sample Preparation. All chemicals were analytically pure and used without further treatment, unless specified otherwise in this study. Deionized (DI) water (>15 MΩ·cm) was used when water was involved. Preparation of Hydrophilic Surface. The quartz slides were cleaned by air plasma for 5 min followed by immersion in a hot piranha solution (98% H2SO4 and 35% H2O2, 7:3, v/v; caution: extreme care should be taken when preparing and using the piranha solution) for 30 min.26 After that, the slides were thoroughly rinsed with DI water and dried under vacuum. Preparation of Hydrophobic Surface. The above-prepared hydrophilic quartz slides were silanized by 3-triethoxysilylpropylamine (APTES).27−30 After that, the silanized slides were soaked in a stearic acid/n-hexane solution (10 mM) for 24 h. Finally, the slides were rinsed thoroughly by DI water and were dried under vacuum. Preparation of SH Surface. In this paper, the sol−gel method was used to prepare the SH surface.5,31 Briefly, aluminum isopropoxide was dissolved in toluene to a concentration of 0.433 M, then ethyl acetoacetate was added to the solution to a concentration of 0.866 M, and finally DI water was added until a white sol was obtained. The sol was cast on a clean hydrophilic quartz slide. After that, the slide was placed in an oven at 60 °C. When dried, the slide was soaked in a stearic acid solution in n-hexane (10 mM) for 24 h, washed, and dried by air flow. Contact Angle Measurements. The sessile drop method was used for contact angle measurements (5 μL water droplet, DSA-100, KRUSS Corp.). The contact angles were measured at five different points for each sample at ambient conditions. The contact angles (CA) for the hydrophilic surface, hydrophobic surface, and SH surface are 13 ± 3°, 97 ± 3°, and 161 ± 3°, respectively (Figure S1 in Supporting Information). The sliding angle of the SH surface is about 3°.5 Adhesion Force Measurements. The preparation of the colloid probe has been described elsewhere.32 In this paper, AFM cantilevers (model: SNL-A and FESP, Bruker Inc.) were used to prepare the colloid probe. A glass microsphere (R = 20 μm, R = 2.5 μm, Thermo Scientific Inc.) was glued to the V-shaped cantilever (see Figure 1a and

Figure 2. Water droplet is dragged by the dropper, which drives the particle to move on the surface of a lotus leaf. The arrow indicates the moving direction of the dropper. particle radius that can be driven by the flow of droplet was recorded as R1 (Video 1 in Supporting Information), while the minimum particle radius that could not be driven was recorded as R2 (Video 2 in Supporting Information). In order to approach the real critical particle size for the drag test, many drag tests of various particle sizes had been carried out. Finally, the average value of R1 (same for R2) is used (N = 3) to calculate the experimental critical particle size, which is defined as Rexp = (R1 + R2)/2. Four kinds of particles with different densities, i.e., sand grains, iron scrap, tin grains, and copper grains, were tested by the same procedure mentioned above. The densities of the particles were measured by a home-built instrument,36 which was built on the basis of an electronic balance (model JA2003, Heng Ping Co., China). The measured densities (with an estimated error of 5%) of these particles are 2.6, 3.4, 7.5, and 8.5 g/cm3.

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RESULTS AND DISCUSSION Surface Adhesion Forces. The surface adhesion forces between the glass microsphere and each of the sample surfaces are measured by AFM under the same conditions. As shown in Table 1, the adhesion force measured on the hydrophobic Table 1. Adhesion Force Measured by AFM on Different Sample Surfaces (RH = 45%, T = 25°C) force/nN

Figure 1. (a) Micrograph of the colloid probe. (b) Scheme of the adhesion force measurements. Figure S2). All the adhesion force measurements were performed on a commercial AFM (MFP-3D, Asylum Research, Santa Barbara, CA) in air at room temperature (∼25 °C) and 45% relative humidity (RH). Prior to force measurements, the spring constant of the cantilever was calibrated by thermal excitation method,33−35 which was in the range of 0.25−2.5 nN/nm. Figure 1b shows the scheme of the adhesion force measurements. The adhesion force (Figure S3) was measured more than 100 times at random positions on each kind of surface. The average of the measured values was used to represent the adhesion force of a certain surface. Estimate of the Critical Particle Size by Drag Test. A simple but effective method described below was developed to estimate the critical size of the particle that can be driven by the flow of water droplet. A piece of fresh lotus leaf was fixed on a horizontal surface. A particle of a certain size was placed on the surface of a lotus leaf, and water was dropped on the particle so that it was immersed. Then, a dropper (note that the opening of the dropper should be larger than the particle diameter) was used to slowly drag (∼2 cm/s) the droplet

surface

hydrophilic

hydrophobic

SH

R = 20 μm R = 2.5 μm

71.32 ± 1.87 29.08 ± 1.73

18.53 ± 0.81 7.72 ± 0.98

4.36 ± 0.06 1.53 ± 0.42

surface is much less than that on the hydrophilic surface, and a reduction of ∼74% is observed. The result is reasonable since the hydrophobic surface was modified by stearic acid that has low surface energy. The adsorbed water layer on the hydrophobic surface is thinner than that on a hydrophilic surface.37 Both of the two points will lead to a lower adhesion force. What is more interesting is that the adhesion force of the SH surface is much less than that of the hydrophobic surface, though both hydrophobic and SH surfaces are modified by stearic acid. The main reason for this result is that the SH surface has a hierarchical micro−nano rough structure, which dramatically reduces the contact area between the microsphere and the SH surface. Consequently, a very low adhesion force is observed on the SH surface. B

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Force Analysis of the Interactions at the Sample Surfaces. The interface force, Fγ, would act on a particle if a particle is entering into or leaving a water droplet. Fγ can be calculated by Fγ = 2πRγ sin δ sin(α − δ)

(1)

where R is the radius of the particle, γ is the water/air interfacial tension (72 nN/μm at 25 °C), δ is the angle determining the position of the water/gas interface on the particle surface, and α is the contact angle of water and particle.38,39 Fγ acts along the normal direction of the water/air interface (40 see Figure 3).

Figure 4. Force analysis when a particle is leaving the water droplet on a hydrophilic (a) and hydrophobic (b) surface. Force arrows do not represent force magnitudes. The upper large arrow indicates the direction of the droplet flow. Fγ is the interface force, Fb is the buoyancy, G is the gravity of the particle, Fad is the adhesion force, and f is the friction between the particle and surface. The dashed line is the water/air interface.

In the above force analysis, the size of the particle is not considered as a factor. That is to say, self-cleaning will not be realized on the hydrophilic surfaces, no matter if the particle is big or small. Force Analysis on a Hydrophobic Surface. According to Figure 4b, the interface force (Fγ) on a hydrophobic surface points inclined upward. The vertical component of Fγ will partially or fully counteract the adhesion force and gravity of the particle, while the horizontal component of Fγ will drive the particles to move along with the water droplet. Both horizontal and vertical component of Fγ have contributed to self-cleaning. According to eq 4, it is clear that when θ > 90°, the bigger the CA, the greater the vertical component force of Fγ (Figure S5). However, whether self-cleaning can be realized is determined by the surface properties and the size and density of the particle. According to CA, hydrophobic surfaces can be classified into SH surface and common hydrophobic surface. It has been found that the majority of the SH surfaces are self-cleaning, while most common hydrophobic surfaces are not. Indeed, there is an exception that some SH surfaces are not selfcleaning. This will be discussed later. The following section will discuss the reason why the SH surfaces can realize self-cleaning but the common hydrophobic surfaces cannot. The adhesion force of a particle on a surface is affected by the particle size. So far, a consensus has not been reached on the relationship between the adhesion force and the particle size. The particle size dependence of the adhesion force has been proposed to be linear,41,42 nonlinear,43 and even irrelevant.42 In this paper, the colloid probe is glass and the surface is quartz, which is very close to that in Corn’s study.41 Therefore, the relationship between the particle size and the adhesion force is assumed to be linear.41 On the basis of the adhesion force of the colloid probe of R = 20 μm and R = 2.5 μm (Table 1), the function between the particle size and the adhesion force on a hydrophilic quartz surface can be obtained as

Figure 3. Schematic diagram of the interface force, Fγ, on a particle adhering to a hydrophobic surface.

According to the literature,22 Fγ will reach its maximum (Fγ,max) when δ = (π + α)/2. Thus ⎛π + α⎞ ⎟ Fγ ,max = 2πRγ sin 2⎜ ⎝ 2 ⎠

(2)

From eq 2, we can obtain the horizontal and vertical component of Fγ,max, i.e. Fγx,max = Fγ ,max sin θ

(3)

Fγz,max = Fγ ,max cos θ

(4)

Besides the interface force, the water flow can generate a drag force, which can be calculated by Stokes’ law. However, this Stokes’ force will not be a dominant factor in typical situations (see details in the Supporting Information). This is in good agreement with the fact that on a common surface the dust cannot be removed effectively by rain or tap water. In the next section, we will carry out the force analysis of a particle that interacts with water on a hydrophilic and a hydrophobic surface. Force Analysis on a Hydrophilic Surface. On a hydrophilic surface, the forces that exerted on a particle when it is leaving the water/air interface are shown in Figure 4a. The interface force points incline downward, and the vertical component force of Fγ points downward. Thus, both of the apparent adhesion force and friction force of the particle will be enhanced, which in turn hinders the self-cleaning. According to eq 4, the smaller contact angle, the larger the vertical component of the interface force (Figure S4 in Supporting Information). On the other hand, the horizontal component of Fγ may drive the particle on the surface. This is the reason that the particle may slide/rotate slightly when the droplet flows through the hydrophilic surface. However, water tends to adhere and spread on the hydrophilic surface. Therefore, the particle will not be removed from the hydrophilic surface.

Fad = 2.413R + 23.05

(5)

where R is particle radius in the unit of μm and Fad is the adhesion force in the unit of nN. Similarly, one can obtain the function between the particle size and the adhesion force on the other two surfaces from Table 1. For a hydrophobic surface the function is

Fad = 0.618R + 6.176 C

(6)

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Figure 5. Theoretical value of Fzγ,max/(Fad + G′) as a function of particle radius on a common hydrophobic surface with θ = 100° and α = 60° (a) and on a superhydrophobic (SH) surface with θ = 160° and α = 60° (b).

Figure 6. Theoretical curves of Fad + G′ (solid line) and Fzγ,max (dotted line) as a function of particle size (R) for sand and iron scrap (a) and tin and copper (b). For both figures, θ = 160°. For the results of α = 90° and 120°, see Figure S7.

According to the above discussion, we can draw a conclusion that a small sliding angle is one of the preconditions to realize self-cleaning. This can also explain why some special SH surfaces cannot realize self-cleaning: the sliding angle of those special SH surfaces is virtually infinite. A water droplet will not slide on such surfaces for any tilt angle.1 The theoretical curve of Fzγ,max/(Fad + G′) vs the particle size on a SH surface is shown in Figure 5b. In a large range of particle size, Fzγ,max is much larger than Fad + G′. This is an important reason that SH surfaces can realize self-cleaning. The ratio reaches the maximum at ∼20 μm, beyond which the ratio is smaller and smaller. It is clear that the ratio will reach to 1 as the particle size increases to a certain value. This size should be the upper limit for the particle to be removed on the SH surface, which is called the critical particle radius. This will be discussed in the next section. From Figure 5b we can find that the ratio of a SH surface is much greater than that of a common hydrophobic surface in a large range of sizes. Given that the SH surface has a very small sliding angle, the result can explain why the SH surfaces are much easier to realize self-cleaning than the common hydrophobic surfaces. According to above the force analysis, it is clear that there are four main key factors to realize self-cleaning on a surface, i.e., high enough contact angle, low enough sliding angle, low enough adhesion force, and the proper particle size. Theoretical Prediction and Verification of SelfCleaning Critical Radius. The lotus-like SH surface features with high enough contact angle, low enough sliding angle, and low enough adhesion force. As discussed above, whether selfcleaning can be realized on a SH surface is also determined by the size of the particle. When the gravity of the particle is too high, the interface force will be insufficient to drive the particle. Therefore, there is a critical particle size beyond which the selfcleaning cannot be realized. In this section, we will predict the

and for a SH surface the function is

Fad = 0.162R + 1.126

(7)

The forces on the vertical direction that act on the particle are Fzγ,max, Fad, and G′ (apparent gravity, G′ = G − Fb). (Fb is calculated according to the volume immersed in water, which is assumed to be a segment of a sphere; see Supporting Information for details.) On hydrophobic surfaces, Fzγ,max points upward, while Fad and G′ point downward. For the case that Fzγ,max is larger than (Fad + G′), the particle would be lifted by the interface force and tend to be removed by the water flow. Therefore, the ratio between Fzγ,max and (Fad + G′) can be used as an indicator: self-cleaning is possible when the ratio is larger than 1. It is also clear that the bigger the ratio, the easier to realize self-cleaning on the surface. The curve of the ratio (i.e., Fzγ,max/(Fad + G′)) as a function of the particle size on a common hydrophobic surface is shown in Figure 5a. In a certain range of radius, the ratio is larger than 1. The ratio reaches its maximum value at ∼30 μm. Beyond this size, the larger the particle, the smaller the ratio. According to the result in Figure 5a, common hydrophobic surfaces should be self-cleaning, since the ratio is larger than 1 for a considerable range of particle size. However, the opposite results are observed for most cases. The main reason for this paradox is that on the common hydrophobic surfaces the sliding angle is rather high. The water droplet tends to adhere on the surface and cannot slide easily. Thus, the particles tend to stay on the common hydrophobic surface. However, it is expected that if the hydrophobic surface is tilted beyond the sliding angle, the water droplet would slide and the particle would be removed from the surface. This has been verified by a recent study44 which shows that a hydrophobic surface with a CA of 104° can realize selfcleaning, when the surface is tilted at a certain angle. D

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preconditions for self-cleaning: (1) CA > 90°, so that the interface force points upward; (2) low enough sliding angle, which will facilitate the rolling of the water droplet on surfaces; (3) low enough adhesion force, which will make the motion of the particle easier; and (4) proper particle size. As far as the lotus-like SH surface is used, the preconditions 1−3 will be satisfied automatically. According to Figure 5b and Figure S7, the theoretical value of Fzγ,max/(Fad + G′) will be larger than 1 in a broad range of particle size and wettability (α), which safely covers the size range of common dust on surfaces from ∼2 to ∼100 μm. We find that even if α = 120° (corresponds to a hydrophobic particle), the critical particle size can be calculated to be >0.5 mm (Figures S7 and S9), which is much larger than that of a common dust. It is worth noting that a contact angle of 120° is the upper limit for a common hydrophobic object. For the case of α > 120°, the particle itself is actually superhydrophobic and can be easily removed by water flow from surfaces. Thus, any size of the typical dust can be removed on the lotus-like surface by water droplet. That is to say, all the four main preconditions for self-cleaning can be summarized into one precondition: use lotus-like SH surface.

theoretical critical radius (Rthe) of particles with different densities for self-cleaning and carry out verification test on a fresh lotus leaf surface. For simplicity, we only consider the vertical component of all forces. Four kinds of particles, i.e., sand grain, iron scrap, tin grain, and copper grain, are used in the prediction. The volume of the particle is denoted as V1; the particle volume that is immersed in water when Fγ reaches its maximum is denoted as V2. To calculate V2, the value of α should be provided (Figure 3 and Figure S6). However, this value has not been reported yet since it is very difficult to measure for small particles. Here, we consider four values of α (i.e., 30°, 60°, 90°, and 120°), which will cover all the typical situations. The theoretical curves of Fzγ,maxand Fad + G′ of different particles as a function of particle size are shown in Figure 6 and Figure S7. The dotted lines are the theoretical curve of Fzγ,max vs particle radius. The solid curves are theoretical curves between the Fad + G′ and particle radius. From the intersection of the dotted line and solid curves, the theoretical critical radius (Rthe) of the particles for self-cleaning can be obtained. Figure 6 indicates that the bigger the particle density, the smaller Rthe. A larger α will also result in a smaller Rthe. Drag tests (see Experimental Section and Figure S8 and videos in the Supporting Information) on a lotus leaf surface have been carried out to verify the predicted values of Rthe. From Figure 7 one can find that for each kind of particle the

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CONCLUSIONS In this paper, the mechanism of the self-cleaning phenomena on SH surfaces has been revealed. According to the force measurements and analysis, we can draw a conclusion that there are four main preconditions for self-cleaning: (1) CA > 90°, (2) low enough sliding angle, (3) low enough adhesion force, and (4) proper particle size. However, as far as the lotuslike SH surface and typical dust are concerned, all the preconditions will be satisfied automatically. If other type SH surfaces have similar surface property with lotus leaf, a similar model can be used to analyze the forces in the system. Another observation is that the particles with a broad range of size (from submicron level to the millimeter level) and density (virtually no limit) can be driven by water droplet on the lotus-like SH surface. This interesting finding may be helpful for the design of novel engineering system at the micron−millimeter scale in the future.

Figure 7. Experimental critical radius (Rexp) and theoretical critical radius (Rthe) of particles of different densities. In the prediction, α is assumed to be 30° and 60°. P1, P2, P3, and P4, in the order of increasing densities, represent sand grain, iron scrap, tin grain, and copper grain, respectively. For the results of α = 90° and 120°, see Figure S9.

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ASSOCIATED CONTENT

S Supporting Information *

experiment critical radii (Rexp) are very close to the theoretical values (Rthe) when α = 30° or 60°. The values for α are reasonable since the four particles used in the study are all hydrophilic. Because of the limitation of experimental condition and theoretical hypothesis, the results may have errors. Herein, the error mainly comes from the following aspects. First, the lotus leaf is not a complete horizontal surface. It is expected that the slightly tilted surface would lead to slightly larger results. However, when the tilt angle is less than 5°, the changes of the critical size can be ignored actually (Table S1). Second, the shape of the particle in the tests is irregular, not ideal sphere. Third, torque is not taken into account in the model, though torque is the actual factor that rolls the larger particles on surfaces. According to Figure 7, however, these factors only lead to limited effects. These results indicate that our simple model (both assumptions and force analysis) is reasonable. Self-Cleaning on Lotus-like SH Surface for Typical Dust. According to the above force analysis and experiments, we can draw a conclusion that there are four main

Contact angle measurements of the sample surfaces, additional force analysis, linear fitting of the adhesion force on a hydrophilic surface, and videos of the particle drag test. This material is available free of charge via the Internet at http:// pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (S.C.). Author Contributions

M.Y. and S.C. contributed equally to this work. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the Natural Science Foundation of China (21222401), the program for New Century Excellent Talents in University (NCET-11-0708), and the Fundamental Research Funds for the Central Universities (SWJTU11ZT05, E

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SWJTU12CX001). We thank Prof. Dr. Lidong Qin for providing the glass microspheres.

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dx.doi.org/10.1021/la5041272 | Langmuir XXXX, XXX, XXX−XXX