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Micro-patterned composite films with bowl-like SnO2 microparticles
Materials Letters 92 (2013) 284–286
Contents lists available at SciVerse ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/matlet
Micro-patterned composite films with bowl-like SnO2 microparticles Yong-Qiang Liu 1, Ge-Bo Pan n, Meng Zhang, Feng Li nn Suzhou Institute of Nano-tech and Nano-bionics, Chinese Academy of Sciences, Suzhou 215123, PR China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 30 July 2012 Accepted 27 October 2012 Available online 3 November 2012
Micro-patterned composite films consisting of bowl-like tin dioxide (SnO2) microparticles with smooth surfaces and well-defined shapes were prepared by the breath figures method using a homogeneous solution of tin (IV) chloride/polystyrene/chloroform (SnCl4/PS/CHCl3). The samples were characterized by scanning electron microscopy, energy dispersive X-ray spectroscopy, and X-ray diffraction. The shape and size of SnO2 microparticles could be adjusted by varying parameters of concentration and relative humidity. A possible mechanism was proposed for illustrating the formation of composite films with bowl-like SnO2 microparticles. The present strategy for the fabrication of bowl arrays offers new prospects for the application in biosensors, microreactor, and microfluidic devices. & 2012 Elsevier B.V. All rights reserved.
Keywords: Breath figures Composite film Tin dioxide Microstructure Phase transformation
1. Introduction Patterned organic or organic–inorganic films have potential applications in many fields, such as photonic crystals [1], miniaturized sensors [2], and functional membranes [3]. The fabrication of patterned films generally involves templating, including colloidal particles array, phase-separated block copolymers, and bacteria. Besides, lithographic techniques are widely used to fabricate micropatterns [4,5]. However, the above methods are either complicated or expensive. Breath figures (BFs) method, a dynamic templating method, is a promising strategy for fabricating ordered structures. On the other hand, asymmetrical particle arrays have attracted particular interest in photonic, biological and microfluidic devices. Organic asymmetrical particles can be obtained by seeded polymerization, microfluidics, and wet etching. However, inorganic asymmetrical particles can only be fabricated using only more complicated and rigorous methods [6,7]. To assemble asymmetrical particles remains a great challenge, in particular, for inorganic bowl-like particles. Despite some progress in fabricating bowl-like structures, most approaches are based on template strategy [8,9]. The resultant architectures generally have very rough surfaces and deformed shapes. In addition, no report described the formation of bowl-like SnO2 particles though various studies were carried out on SnO2 thin films [10,11]. Herein, we report for the first time the fabrication of patterned bowl-like SnO2 microparticles with smooth surfaces and welldefined shapes. The architectures were formed by the BFs method using a homogeneous solution of tin (IV) chloride/polystyrene/
n
Corresponding author. Fax: þ86 512 62872663. Corresponding author. E-mail addresses: [email protected] (G.-B. Pan), fl[email protected] (F. Li). 1 Also at Graduate University of Chinese Academy of Sciences, PR China. nn
0167-577X/$ - see front matter & 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.matlet.2012.10.106
chloroform (SnCl4/PS/CHCl3) and investigated by scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDX), and X-ray diffraction (XRD). The influence of humidity and concentration on the morphology of composite films was investigated, and a possible mechanism was proposed for the formation of patterned composite film with bowl-like SnO2 microparticles.
2. Experimental section Polystyrene (PS) was purchased from Aladdin Chemical Company, China. SnCl4 ( 499%) and CHCl3 ( 499%) were purchased from Sinopharm Chemical Reagent Co. Ltd. PS and SnCl4 were used as received. CHCl3 was purified with anhydrous magnesium sulfate (MgSO4, Sinopharm Chemical Reagent Co. Ltd.). PS was dissolved into CHCl3 and SnCl4 was added to PS/CHCl3. The concentration of PS in CHCl3 was 1 wt% and the volume ratios of V SnCl4 /V SnCl3 were 0.1%, 0.2%, and 0.4%. The homogeneous solution of SnCl4/PS/CHCl3 was dropped onto silicon and the solvent was allowed to evaporate in a moist air. The relative humidity (RH) was controlled in the range of 40–72%. The as-prepared composite films were calcined at 600 1C for 3 h. SEM images were obtained by using a scanning electron microscope (Hitachi- S4800). A 10 kV electron beam was used for the observation with a working distance of 8 mm. The EDX spectra were recorded on a Quanta 400 (ESEM with EDX from FEI Company) instrument. The XRD patterns were obtained on a D8-Advance Bruker-AXS diffractometer using Cu Ka irradiation.
3. Results and discussion Fig. 1a shows a typical SEM image of composite film, which is from evaporation of a homogeneous solution of SnCl4/PS/CHCl3
Y.-Q. Liu et al. / Materials Letters 92 (2013) 284–286
285
Fig. 1. SEM images of composite film obtained from SnCl4/PS/CHCl3 (a) before calcining, (b) after calcining for 3 h, and (c) after calcining for 6 h. The concentration of SnCl4 is 0.4% v/v and the RH is 40%. (d) and (e) EDX spectra of microparticles before and after calcining, respectively. (f) XRD spectra.
Fig. 2. Schematic diagram for the formation of composite films.
(PS, 1 wt%; SnCl4, 0.4% v/v) dropped onto silicon. A porous film with microparticles trapped in the holes can be observed. The diameters of particles are in range of 1–2 mm and the number density of particles is up to 1.5 107 features/cm2. The EDX spectrum of microparticles (Fig. 1d) shows the elements C, Sn, Cl, and O. This implies the preliminary hydrolysis of SnCl4 inside composite films. Similar spectrum has also been recorded in surrounding matrix, while the intensities of peaks are much lower than those of microparticles, indicating strong Sn uptake by microparticles. After calcining at 600 1C for 3 h, bowl-like particles with smooth inner surfaces are exposed (Fig. 1b). It can be seen that the composite film matrix maintains relative order before and after calcination. Increasing the calcination time to 6 h, the film matrix is destroyed and bowl-like microparticles were unfolded completely (Fig. 1c). The corresponding EDX spectrum (Fig. 1e) shows that the bowl-like microparticles mostly consist of tin and oxygen elements. This indicates that the intermediate hydrolysis product has been transformed to SnO2 upon calcinations. The XRD spectra (Fig. 1f) further confirm this assumption. Similar to the literatures [12,13], the particles are amorphous before calcination, while crystalline after calcination at 600 1C for 3 h. In addition, the indentified peaks are indexed to cassiterite SnO2 (JCPDS card no. 41-1445). The formation of micro-patterned composite films with bowllike microparticles is interpreted by BFs mechanism. When a surface of polymer solution is exposed to moist air, water droplets are condensed on solution surface via the evaporative cooling between volatile solvent and water, grow with time, and finally
form well-ordered patterns. Fig. 2 shows a schematic diagram of the formation of composite films. Firstly, a homogeneous solution of SnCl4/PS/CHCl3 is dropped onto silicon in moist air. As CHCl3 evaporates, water droplets condense continuously and nucleate onto the surface of composite film. This is a typical cooling effect caused by the thermocapillary convection between CHCl3 and water [14]. Meanwhile, the hydrolysis of SnCl4 occurs in the presence of water, and the immediate hydrolysis product, Sn(OH)nCl4 n, accordingly forms the inorganic sol droplets with the mixture of water. Because the sol droplets are denser than the solvent (CHCl3), they tend to sink into the composite films. With continuous evaporation of solvent and water, the sol droplets become more concentrated and self-organize into a preliminary micro-pattern due to the Marangoni force [15]. The resultant sol microparticles are on the surface of polymer matrix. This is different from the traditional BFs process, in which only pores are formed [4]. Subsequently, as water further evaporates upon calcining, the sol microparticles begin to shrink and the solutes precipitate gradually, leading to the formation of solid particles trapped inside composite films, similar to sessile water droplets evaporation on solid surface [16]. Further experiments reveal that the shape of microparticles inside micro-patterned composite films is dependent on the precursor concentration. Fig. 3 shows SEM images of calcined composite films with different concentrations of SnCl4. An array of crescent-like microparticles is obtained for 0.1% v/v SnCl4/ CHCl3, half-bowl-like microparticles for 0.2% v/v SnCl4/CHCl3, and bowl-like microparticles for 0.4% v/v SnCl4/CHCl3 (Fig. 1b).
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Y.-Q. Liu et al. / Materials Letters 92 (2013) 284–286
rapid the hydrolysis of SnCl4 and more water droplets adsorb on composite films. As a result, larger pores are formed, which are similar to asymmetrical TiO2 particles [7]. In addition, micropatterned composite films are not formed when the precursor solution is drop-casted in a glove box, i.e., a non-humid condition. Therefore, it is concluded that the morphology of composite films strongly depends on the RH.
Fig. 3. SEM images of calcined composite film obtained from SnCl4/PS/CHCl3. The concentration of SnCl4 is (a) 0.1% v/v and (b) 0.2% v/v. The insets are magnified images of individual microparticles.
4. Conclusion Micro-patterned composite films consisting of bowl-like SnO2 microparticles with smooth surfaces and well-defined shapes were prepared by the breath figures method using a homogeneous solution of SnCl4/PS/CHCl3. The shape and size of SnO2 microparticles could be adjusted by varying parameters of concentration and humidity. Moreover, a possible mechanism was also proposed for illustrating the formation of composite films with bowl-like SnO2 microparticles. The present strategy for the fabrication of bowl arrays offers new prospects for the application in biosensors, microreactor, and microfluidic devices.
Acknowledgments
Fig. 4. SEM image of the composite film (SnCl4, 0.4% v/v) obtained at RH of 72%.
At higher or lower concentrations of SnCl4, only randomly distributed microparticles with irregular shape are observed. In addition, the size of asymmetrical microparticles increases accordingly with the concentration of SnCl4, as evidenced by the insets in Fig. 3a and b. The profound discrepancy in shape of SnO2 microparticles is mainly ascribed to the decrease of polymer concentration and rapid coagulation of water droplets [15,17]. Thus, the shape of microparticles can be tuned easily by adjusting the precursor concentration. Besides the precursor concentration, the formation of micropatterned films in the BFs process is affected by solvent, stabilization of water droplets, substrate type, and relative humidity (RH) [15,17]. Among these factors, RH is one of the crucial parameters for adjusting the morphology of polymer films. When the concentration of precursor solution is fixed at 1 wt% PS, and 0.4% SnCl4 (v/v), uniform pattern in composite films is obtained with bowl-like particles for RH of 40–50%, while it is only randomly distributed pores for larger RH of 72% (Fig. 4). This is possibly due to the conglomeration of rapidly condensed droplets and lower evaporation of solvent. Moreover, the larger the RH, the more
This work was supported by the National Basic Research Program of China (No. 2010CB934100) and the Chinese Academy of Sciences. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17]
Deutsch M, Vlasov YA, Norris DJ. Adv Mater 2000;12:1176–80. Geissler M, Xia YN. Adv Mater 2004;16:1249–69. Lewandowski K, Murer P, Svec F, Fre´chet JM. Anal Chem 1998;70:1629–38. Srinivasarao M, Collings D, Philips A, Patel S. Science 2001;292:79–83. Li L, Zhong YW, Ma CY, Li J, Chen CK, Zhang A, et al. Chem Mater 2009;21:4977–83. Li Y, Cai WP, Duan GT. Chem Mater 2008;20:615–24. Li XF, Zhang L, Wang YX, Yang XL, Zhao N, Zhang XL, et al. J Am Chem Soc 2011;133:3736–9. Xu MJ, Lu N, Xu HB, Qi DP, Wang YD, Chi LF. Langmuir 2009;25:11216–20. Wang Y, Chen X, Zhang J, Sun Z, Li Y, Zhang K, et al. Colloids and Surfaces A: Physicochem Eng Aspects 2008;329:184–9. Shirahata N, Hozumi A. Chem Mater 2005;17:20–7. MasudaY Ohji T, Kato KACS. Appl Mater Interfaces 2012;4:1666–74. Shirahata N, Masuda Y, Yonezawa T, Koumoto K. Langmuir 2002;18:10379–85. Shirahata N, Shin W, Murayama N, Hozumi A, Yokogawa Y, Kameyama T, et al. Adv Funct Mater 2004;14:580–8. Aversana PD, Banavar JR, Koplik J. Phys Fluids 1996;8:15–29. Bunz UHF. Adv Mater 2006;18:973–89. Birdi KS, Vu DT. J Adhes Sci Technol 1993;7:485–93. Ma HM, Hao JC. Chem Soc Rev 2011;40:5457–71.
Contents lists available at SciVerse ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/matlet
Micro-patterned composite films with bowl-like SnO2 microparticles Yong-Qiang Liu 1, Ge-Bo Pan n, Meng Zhang, Feng Li nn Suzhou Institute of Nano-tech and Nano-bionics, Chinese Academy of Sciences, Suzhou 215123, PR China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 30 July 2012 Accepted 27 October 2012 Available online 3 November 2012
Micro-patterned composite films consisting of bowl-like tin dioxide (SnO2) microparticles with smooth surfaces and well-defined shapes were prepared by the breath figures method using a homogeneous solution of tin (IV) chloride/polystyrene/chloroform (SnCl4/PS/CHCl3). The samples were characterized by scanning electron microscopy, energy dispersive X-ray spectroscopy, and X-ray diffraction. The shape and size of SnO2 microparticles could be adjusted by varying parameters of concentration and relative humidity. A possible mechanism was proposed for illustrating the formation of composite films with bowl-like SnO2 microparticles. The present strategy for the fabrication of bowl arrays offers new prospects for the application in biosensors, microreactor, and microfluidic devices. & 2012 Elsevier B.V. All rights reserved.
Keywords: Breath figures Composite film Tin dioxide Microstructure Phase transformation
1. Introduction Patterned organic or organic–inorganic films have potential applications in many fields, such as photonic crystals [1], miniaturized sensors [2], and functional membranes [3]. The fabrication of patterned films generally involves templating, including colloidal particles array, phase-separated block copolymers, and bacteria. Besides, lithographic techniques are widely used to fabricate micropatterns [4,5]. However, the above methods are either complicated or expensive. Breath figures (BFs) method, a dynamic templating method, is a promising strategy for fabricating ordered structures. On the other hand, asymmetrical particle arrays have attracted particular interest in photonic, biological and microfluidic devices. Organic asymmetrical particles can be obtained by seeded polymerization, microfluidics, and wet etching. However, inorganic asymmetrical particles can only be fabricated using only more complicated and rigorous methods [6,7]. To assemble asymmetrical particles remains a great challenge, in particular, for inorganic bowl-like particles. Despite some progress in fabricating bowl-like structures, most approaches are based on template strategy [8,9]. The resultant architectures generally have very rough surfaces and deformed shapes. In addition, no report described the formation of bowl-like SnO2 particles though various studies were carried out on SnO2 thin films [10,11]. Herein, we report for the first time the fabrication of patterned bowl-like SnO2 microparticles with smooth surfaces and welldefined shapes. The architectures were formed by the BFs method using a homogeneous solution of tin (IV) chloride/polystyrene/
n
Corresponding author. Fax: þ86 512 62872663. Corresponding author. E-mail addresses: [email protected] (G.-B. Pan), fl[email protected] (F. Li). 1 Also at Graduate University of Chinese Academy of Sciences, PR China. nn
0167-577X/$ - see front matter & 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.matlet.2012.10.106
chloroform (SnCl4/PS/CHCl3) and investigated by scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDX), and X-ray diffraction (XRD). The influence of humidity and concentration on the morphology of composite films was investigated, and a possible mechanism was proposed for the formation of patterned composite film with bowl-like SnO2 microparticles.
2. Experimental section Polystyrene (PS) was purchased from Aladdin Chemical Company, China. SnCl4 ( 499%) and CHCl3 ( 499%) were purchased from Sinopharm Chemical Reagent Co. Ltd. PS and SnCl4 were used as received. CHCl3 was purified with anhydrous magnesium sulfate (MgSO4, Sinopharm Chemical Reagent Co. Ltd.). PS was dissolved into CHCl3 and SnCl4 was added to PS/CHCl3. The concentration of PS in CHCl3 was 1 wt% and the volume ratios of V SnCl4 /V SnCl3 were 0.1%, 0.2%, and 0.4%. The homogeneous solution of SnCl4/PS/CHCl3 was dropped onto silicon and the solvent was allowed to evaporate in a moist air. The relative humidity (RH) was controlled in the range of 40–72%. The as-prepared composite films were calcined at 600 1C for 3 h. SEM images were obtained by using a scanning electron microscope (Hitachi- S4800). A 10 kV electron beam was used for the observation with a working distance of 8 mm. The EDX spectra were recorded on a Quanta 400 (ESEM with EDX from FEI Company) instrument. The XRD patterns were obtained on a D8-Advance Bruker-AXS diffractometer using Cu Ka irradiation.
3. Results and discussion Fig. 1a shows a typical SEM image of composite film, which is from evaporation of a homogeneous solution of SnCl4/PS/CHCl3
Y.-Q. Liu et al. / Materials Letters 92 (2013) 284–286
285
Fig. 1. SEM images of composite film obtained from SnCl4/PS/CHCl3 (a) before calcining, (b) after calcining for 3 h, and (c) after calcining for 6 h. The concentration of SnCl4 is 0.4% v/v and the RH is 40%. (d) and (e) EDX spectra of microparticles before and after calcining, respectively. (f) XRD spectra.
Fig. 2. Schematic diagram for the formation of composite films.
(PS, 1 wt%; SnCl4, 0.4% v/v) dropped onto silicon. A porous film with microparticles trapped in the holes can be observed. The diameters of particles are in range of 1–2 mm and the number density of particles is up to 1.5 107 features/cm2. The EDX spectrum of microparticles (Fig. 1d) shows the elements C, Sn, Cl, and O. This implies the preliminary hydrolysis of SnCl4 inside composite films. Similar spectrum has also been recorded in surrounding matrix, while the intensities of peaks are much lower than those of microparticles, indicating strong Sn uptake by microparticles. After calcining at 600 1C for 3 h, bowl-like particles with smooth inner surfaces are exposed (Fig. 1b). It can be seen that the composite film matrix maintains relative order before and after calcination. Increasing the calcination time to 6 h, the film matrix is destroyed and bowl-like microparticles were unfolded completely (Fig. 1c). The corresponding EDX spectrum (Fig. 1e) shows that the bowl-like microparticles mostly consist of tin and oxygen elements. This indicates that the intermediate hydrolysis product has been transformed to SnO2 upon calcinations. The XRD spectra (Fig. 1f) further confirm this assumption. Similar to the literatures [12,13], the particles are amorphous before calcination, while crystalline after calcination at 600 1C for 3 h. In addition, the indentified peaks are indexed to cassiterite SnO2 (JCPDS card no. 41-1445). The formation of micro-patterned composite films with bowllike microparticles is interpreted by BFs mechanism. When a surface of polymer solution is exposed to moist air, water droplets are condensed on solution surface via the evaporative cooling between volatile solvent and water, grow with time, and finally
form well-ordered patterns. Fig. 2 shows a schematic diagram of the formation of composite films. Firstly, a homogeneous solution of SnCl4/PS/CHCl3 is dropped onto silicon in moist air. As CHCl3 evaporates, water droplets condense continuously and nucleate onto the surface of composite film. This is a typical cooling effect caused by the thermocapillary convection between CHCl3 and water [14]. Meanwhile, the hydrolysis of SnCl4 occurs in the presence of water, and the immediate hydrolysis product, Sn(OH)nCl4 n, accordingly forms the inorganic sol droplets with the mixture of water. Because the sol droplets are denser than the solvent (CHCl3), they tend to sink into the composite films. With continuous evaporation of solvent and water, the sol droplets become more concentrated and self-organize into a preliminary micro-pattern due to the Marangoni force [15]. The resultant sol microparticles are on the surface of polymer matrix. This is different from the traditional BFs process, in which only pores are formed [4]. Subsequently, as water further evaporates upon calcining, the sol microparticles begin to shrink and the solutes precipitate gradually, leading to the formation of solid particles trapped inside composite films, similar to sessile water droplets evaporation on solid surface [16]. Further experiments reveal that the shape of microparticles inside micro-patterned composite films is dependent on the precursor concentration. Fig. 3 shows SEM images of calcined composite films with different concentrations of SnCl4. An array of crescent-like microparticles is obtained for 0.1% v/v SnCl4/ CHCl3, half-bowl-like microparticles for 0.2% v/v SnCl4/CHCl3, and bowl-like microparticles for 0.4% v/v SnCl4/CHCl3 (Fig. 1b).
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Y.-Q. Liu et al. / Materials Letters 92 (2013) 284–286
rapid the hydrolysis of SnCl4 and more water droplets adsorb on composite films. As a result, larger pores are formed, which are similar to asymmetrical TiO2 particles [7]. In addition, micropatterned composite films are not formed when the precursor solution is drop-casted in a glove box, i.e., a non-humid condition. Therefore, it is concluded that the morphology of composite films strongly depends on the RH.
Fig. 3. SEM images of calcined composite film obtained from SnCl4/PS/CHCl3. The concentration of SnCl4 is (a) 0.1% v/v and (b) 0.2% v/v. The insets are magnified images of individual microparticles.
4. Conclusion Micro-patterned composite films consisting of bowl-like SnO2 microparticles with smooth surfaces and well-defined shapes were prepared by the breath figures method using a homogeneous solution of SnCl4/PS/CHCl3. The shape and size of SnO2 microparticles could be adjusted by varying parameters of concentration and humidity. Moreover, a possible mechanism was also proposed for illustrating the formation of composite films with bowl-like SnO2 microparticles. The present strategy for the fabrication of bowl arrays offers new prospects for the application in biosensors, microreactor, and microfluidic devices.
Acknowledgments
Fig. 4. SEM image of the composite film (SnCl4, 0.4% v/v) obtained at RH of 72%.
At higher or lower concentrations of SnCl4, only randomly distributed microparticles with irregular shape are observed. In addition, the size of asymmetrical microparticles increases accordingly with the concentration of SnCl4, as evidenced by the insets in Fig. 3a and b. The profound discrepancy in shape of SnO2 microparticles is mainly ascribed to the decrease of polymer concentration and rapid coagulation of water droplets [15,17]. Thus, the shape of microparticles can be tuned easily by adjusting the precursor concentration. Besides the precursor concentration, the formation of micropatterned films in the BFs process is affected by solvent, stabilization of water droplets, substrate type, and relative humidity (RH) [15,17]. Among these factors, RH is one of the crucial parameters for adjusting the morphology of polymer films. When the concentration of precursor solution is fixed at 1 wt% PS, and 0.4% SnCl4 (v/v), uniform pattern in composite films is obtained with bowl-like particles for RH of 40–50%, while it is only randomly distributed pores for larger RH of 72% (Fig. 4). This is possibly due to the conglomeration of rapidly condensed droplets and lower evaporation of solvent. Moreover, the larger the RH, the more
This work was supported by the National Basic Research Program of China (No. 2010CB934100) and the Chinese Academy of Sciences. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17]
Deutsch M, Vlasov YA, Norris DJ. Adv Mater 2000;12:1176–80. Geissler M, Xia YN. Adv Mater 2004;16:1249–69. Lewandowski K, Murer P, Svec F, Fre´chet JM. Anal Chem 1998;70:1629–38. Srinivasarao M, Collings D, Philips A, Patel S. Science 2001;292:79–83. Li L, Zhong YW, Ma CY, Li J, Chen CK, Zhang A, et al. Chem Mater 2009;21:4977–83. Li Y, Cai WP, Duan GT. Chem Mater 2008;20:615–24. Li XF, Zhang L, Wang YX, Yang XL, Zhao N, Zhang XL, et al. J Am Chem Soc 2011;133:3736–9. Xu MJ, Lu N, Xu HB, Qi DP, Wang YD, Chi LF. Langmuir 2009;25:11216–20. Wang Y, Chen X, Zhang J, Sun Z, Li Y, Zhang K, et al. Colloids and Surfaces A: Physicochem Eng Aspects 2008;329:184–9. Shirahata N, Hozumi A. Chem Mater 2005;17:20–7. MasudaY Ohji T, Kato KACS. Appl Mater Interfaces 2012;4:1666–74. Shirahata N, Masuda Y, Yonezawa T, Koumoto K. Langmuir 2002;18:10379–85. Shirahata N, Shin W, Murayama N, Hozumi A, Yokogawa Y, Kameyama T, et al. Adv Funct Mater 2004;14:580–8. Aversana PD, Banavar JR, Koplik J. Phys Fluids 1996;8:15–29. Bunz UHF. Adv Mater 2006;18:973–89. Birdi KS, Vu DT. J Adhes Sci Technol 1993;7:485–93. Ma HM, Hao JC. Chem Soc Rev 2011;40:5457–71.