J. Phys. Chem. C 2009, 113, 13683–13688
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Contact Angle Hysteresis and Work of Adhesion of Oil Droplets on Nanosphere Stacking Layers Chien-Te Hsieh,* Fang-Lin Wu, and Wei-Yu Chen Department of Chemical Engineering and Materials Science, Yuan Ze Fuel Cell Center, Yuan Ze UniVersity, Taoyuan 320, Taiwan ReceiVed: April 22, 2009; ReVised Manuscript ReceiVed: June 11, 2009
Surface repellency of liquid droplets with different surface tensions (23.4-73.2 mN/m) on nanostructured surfaces built of one- and two-tier silica sphere stacking surfaces were investigated, and contact angle hysteresis (CAH) and work of adhesion were analyzed. It was shown that the binary nano- and sub-micrometer-scaled roughened surfaces exhibited better repellency toward oil drops with surface tension of 30 mN/m, and the CAH behavior strongly depended on both liquid types and density of three-phase contact line. This improvement of hysteresis from the two-tier texture is attributed to the facts that (i) the arrangement of submicrometer spheres generates a primary roughness that allows air cushion in the texture and (ii) the deposition of nanospheres creates a large number of point contact fashion to repel the oil penetration, reducing the length of the liquid-solid contact line (i.e., Cassie state). The existing air layer tends to impart the fraction of vapor-solid contact, which decreases the kinetic barrier of drop movement. Incorporated with the YoungDupre` equation, it has shown a linear relationship between the difference between cosines of advancing and receding angles and the work of adhesion. Accordingly, it reveals that the adhesion of solid-oil contact interface is reduced due to the design of surface topography. 1. Introduction Super water repellency (i.e., water contact angle (CA) > 150° and sliding angle 160°) with low sliding angle (160°, while their oil repellencies are quite different. An order for the oil repellency is shown as follows: 2T-SL (147.2°) > T-L (138.4°) > T-S (127.1°). This result implies that (i) super water repellency can be achieved based on sufficient roughness and surface fluorination treatment and (ii) surface repellency toward oil drops is also significantly affected by surface topography. In other words, the arrangement of silica sphere stacking plays a major role in affecting the wetted fraction by oil droplets. For in-depth investigation of the wetting behavior, Figure 3a depicts the variation of CA with surface tension of liquid droplets. These relationships deliver some messages: (i) the repellency of silica surfaces is generally decayed by surface tension of oil droplets and (ii) the oil repellency is strongly governed by the sphere arrangement.In comparison, 2T-SL surface exhibits the best oil repellency, capable of repelling oils with low surface tension 90°, Cassie’s model is thus considered for further investigation of oil repellency. Theoretically, the Cassie-Baxter model assumes that the droplet is partially in contact with the air trapped in the pores of the surface when it sits on the peaks of the surface. This relation between the apparent CA θ* observed on a rough surface and the equilibrium CA θ obtained on a smooth surface is based on the same chemical composition17-19
cos θ* ) -1 + f(cos θ + 1)
(1)
where f corresponds to the fraction of the surface of each pillar in contact with the liquid to the projected planar area. Equation 1 postulates that the porous surface is heterogeneous, which
Figure 3. (a) CA as a function of surface tension on various silica sufaces. In this plot, soild and empty symbols represent advancing and receding angles, respectively. (b) CAH as a function of surface tension on different silica sufaces.
may provide a more realistic description of hydrophobicity of the CNFs, compared to the Wenzel model. After calculations, the wetted fractions ( f ) can be estimated to be 5.2-82.2% (T-S), 7.4-49.6% (T-L), and 8.0-11.9% (2T-SL) within the range of surface tension (i.e., 23.4-73.2 mN/m). The results reveal that the 2T-SL surface shows the best capability to resist oil contamination, according to the lower wetted fraction even at low surface tension 0.990) passes through the
origin, obeying eq 6. The conformation implies that the present model incorporated with the Young-Dupre` equation enables characterization of the interfacial behavior. The slope of the linearity equation is estimated to be 0.092, which is considered as a proportional factor. The characteristic value may relate to the CA measurements in a specific environment, e.g., droplet size and surface chemistry. This means that on the basis of their similar chemical composition and droplet size the characteristic value (πr/w) can be used to predict the hysteresis behavior or the adhesion level. Figure 6 illustrates the relationship between (cos θr cos θa) and work of adhesion, also showing a linearly increasing trend. The plot clearly reflects that surface work of adhesion between the solid surface and the liquid droplet plays an important role in affecting the hysteresis behavior. The relation displays a significant benefit to predict the hysteresis of rough surfaces. Since the work of adhesion is a function of surface solid CA and Young’s CA, according to eq 3, it reveals that adhesion of solid-oil contact interface is reduced due to a lower surface fraction (i.e., shorter length of liquid-solid contact line), as illustrated in Figure 4. To confirm the feasibility, we deposited the silica sphere stacking layers on transparent glass and then used eight types of commonly used liquids, including sunflower oil, red wine, dilute juice water, soya sauce, coke, milk, coffee, and water, to examine the repellency toward these liquids. Figure 7 shows these droplets sitting on F-coated silica glass, confirming the anticontamination ability. The satisfactory results clearly reflect that an efficient approach contributes toward great benefit to
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Hsieh et al. repellency to oil contamination. This enhancement of oil repellency is presumably due to the fact that nanosized spheres provide a number of point contact fashion, preventing the oil droplet from entering the surface of the sub-micrometer spheres. It is suggested that the dual-size silica surface enables the creation of a tortuous three-phase contact line, inducing a large surface fraction to air (i.e., Cassie state). The CAH of different silica surfaces strongly depends on both surface tension of oil drops and sphere stacking. The hysteresis, as defined by a difference in cosines of advancing and receding angles, appears to be proportional to the surface work of adhesion, with a characteristic value (or called proportion factor) of 0.092. The linear relation shows that the surface work of adhesion between the solid surface and the liquid droplet plays an important role in affecting the hysteresis behavior. The relation shows a major benefit in simulating the hysteresis of rough surfaces.
Figure 8. Water and diethylene glycol CAs on 2T-SL surface as a function of water- and oil-immersion period. Four combinations of durability test were carried out as follows: water immersion water repellency (WW), ethylene glycol immersion water repellency (EW), water immersion ethylene glycol (WE), and ethylene glycol immersion ethylene glycol repellency (EE), respectively. The inset of this figure shows cross-sectional view of water and diethylene glycol droplets sat on 2T-SL surface, showing an excellent durability.
antifouling coatings on various substrates such as polymeric and flexible plates, wooden sheets, fabrics, cement plates, wafers, and so on. The durability test was carried out to examine both water and oil repellencies on 2T-SL surface for a long period. To examine its durability, a cycling test for the resulting surfaces was performed by immersing the surfaces into water and diethylene glycol solutions for 30 days. The CA measurement of water and diethylene glycol droplets was carried everyday after the wetted layer was dried at 105 °C for 1 h. The durability test delivers two crucial messages: (i) confirmation of adhesion between silica coating and glass substrate and (ii) evaluation of water and oil repellency for a long period, which can be an indicator for accelerating aging test. Figure 8 shows the variation of CAs of water and diethylene glycol on 2T-SL surface with immersion period. Four combinations of the durability test were carried out as follows: water immersion water repellency (WW), diethylene glycol immersion water repellency (EW), water immersion diethylene glycol repellency (WE), and diethylene glycol immersion diethylene glycol repellency (EE), respectively. A slight decline of CAs during the period confirms that the F-coated silica surface possesses the long-period durability, since both CAs of water and diethylene glycol exceed 150° after 30 days. Again, this satisfied result confirms (i) good adhesion between the silica layer and the glass substrate and (ii) super repellency against water and oil contamination. 4. Conclusions This study examined the repellency toward liquids with surface tension of 23.4-73.2 mN/m on three types of silica sphere stacking layers with one- and two-tier roughness. Among these silica surfaces, the binary-scaled texture, formed by nano and sub-micrometer sphere stacking layers, displays the best
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