ARTICLE pubs.acs.org/JPCC
Spontaneous Formation of Micrometer-Scaled Cell-like Patterns on nardMarangoni Alkoxide-Derived Silica Gels Induced by Be Convections Hiroaki Uchiyama,* Yuichiro Miki, Yuto Mantani, and Hiromitsu Kozuka Faculty of Chemistry, Materials and Bioengineering, Kansai University, 3-3-35 Yamate-cho, Suita 564-8680, Japan ABSTRACT: Cell-like patterns were spontaneously formed on the silica gels prepared by dropping acid-catalyzed tetramethyl orthosilicate (Si(OCH3)4) solutions containing poly(vinylpyrrolidone) (PVP) on a stationary substrate. The height of the cell-like patterns increased with increasing the contents of PVP in the precursor solutions and the substrate temperature. On the other hand, the width of the patterns decreased with increasing the substrate temperature, while did not change with the addition of PVP. Such pattern formation was caused by the Benard Marangoni convections occurring in the precursor solutions during the solvent evaporation. The cell-like patterns on the silica gels disappeared during the aging of the gels in the solvent vapor.
1. INTRODUCTION Solgel coating processes are widely used as the methods for preparing metal oxide films, where micrometer-scaled patterns such as unevenness in thickness or ridges are formed on the surface of the films under certain conditions. Such surface patterns are responsible for the scattering and reflection of light, while they can be utilized as gratings or microlenses when aligned regularly and periodically. Because of such reasons, the mechanism of the formation of such patterns has been investigated by several approaches.17 BenardMarangoni convection occurring in the coating layer during solvent evaporation is known as one of the important factors for the spontaneous pattern formation on solgel films.17 BenardMarangoni convection is caused by the surface tension inhomogeneity of fluid layers induced by the solvent evaporation-driven thermal or compositional gradient,3,4 and the pattern formation induced by BenardMarangoni convection has been studied using colloids815 and polymer solutions.1620 The measure of the convection occurrence tendency can be characterized by the Marangoni number, Ma:3,4,18,19 Ma ¼
ð∂γ=∂TÞH 2 ∇T μα
ð1Þ
Ma ¼
ð∂γ=∂CÞH 2 ∇C μD
ð2Þ
when the concentration gradient is the main driving force. In the case of solgel coating, such cell-like convections are connected via the flow of the coating solutions during the coating process, and consequently striations are often formed on the surface of films.37 For example, radiative striations appear on the spinning substrate during spin-coating process.36 On the other hand, we prepared solgel dip-coating films at high substrate withdrawal speeds of 70140 cm min1 and found that linear striations parallel to the substrate withdrawal direction are formed on the surface of films.7 The height and spacing were found to increase with increasing the film thickness both for the radiative and linear striations.57 Such spontaneous pattern formation on solgel films due to Benard Marangoni convection is regarded as a self-organization process, and the surface structure thus obtained holds great promise for the application to photonic devices such as diffraction gratings and microlens arrays. Cell-like BenardMarangoni convection occurring in the coating layer on the substrate significantly influences the size and shape of the surface patterns of the resultant solgel films. We previously observed a sol dropped on a stationary substrate and found the occurrence of cell-like convections in the sol and the formation of cell-like patterns on the surface of the resultant gel.1,2 However, the size and shape of cell-like convections occurring in a stationary sol have not been quantitatively discussed, and the essential factor in determining the surface patterns on the resultant gel remains unclear. The precise mechanism of the cell-like patterns would allow us to control the spontaneous pattern formation on solgel films during the solvent evaporation. Here, we examined the evolution of cell-like patterns on a silica gel prepared by dropping acid-catalyzed tetramethyl orthosilicate
where (∂γ/∂T) is the temperature derivative of the surface tension, rT the temperature gradient near the solution surface, (∂γ/∂C) the concentration derivative of the surface tension, rC the concentration gradient near the solution surface, H the thickness of the liquid layer, D the mass diffusivity of the component, and μ and α the viscosity and thermal diffusivity of the solution, respectively. Equation 1 applies for the case where the temperature gradient dominates the Marangoni effect, whereas eq 2 applies r 2011 American Chemical Society
Received: October 1, 2011 Revised: November 19, 2011 Published: December 14, 2011 939
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Table 1. Compositions and Viscosity of the Coating Solutions mole ratio of the coating solutions Si(OCH3)4
H2O
HNO3
CH3OH
A0
1
2
0.01
10
0
1.30
A0.05
1
2
0.01
10
0.05
3.60
A0.10
1
2
0.01
10
0.10
A0.15
1
2
0.01
10
0.15
19.9
A0.20
1
2
0.01
10
0.20
28.7
B0
1
2
0.01
5.13
0
B0.05
1
2
0.01
5.13
0.05
B0.10 B0.15
1 1
2 2
0.01 0.01
5.13 5.13
0.10 0.15
14.8 28.3
B0.20
1
2
0.01
5.13
0.20
55.3
solution
(Si(OCH3)4) solutions on a stationary substrate. For the investigation of the effect of evaporation rate on the size and shape of BenardMarangoni convections, methanol (CH3OH) and 2-methoxyethanol (CH3OCH2CH2OH) with different boiling points (CH3OH: 64.7 °C; CH3OCH2CH2OH: 125 °C) were used as the solvents of precursor solutions. The Ma value, the convection occurrence tendency, depends on the temperature gradient near the solution surface (rT) and the viscosity of the precursor solutions (μ), as seen in eq 1. Thus, we controlled rT and μ by heating the substrate and by adding an organic polymer, poly(vinylpyrrolidone) (PVP), respectively. The height and width of the surface patterns were quantitatively evaluated by optical microscopy and surface roughness measurement, and the mechanism of the pattern formation was discussed. Furthermore, we performed the aging of the silica gels in solvent vapor and examined the stability of the surface patterns on the gels.
CH3OCH2CH2OH
PVP
viscosity (mPa s)
8.56
2.88 8.83
Figure 1. Schematic illustration of the sample preparation (a) and of the aging of silica gels in solvent vapor (b).
Hereafter, the Si(OCH3)4 solutions containing CH3OH and CH3OCH2CH2OH as the solvents are denoted as solutions A and B, respectively. The solutions A00.20 and B00.20 indicate that the PVP/Si(OCH3)4 mole ratios, x and y, in solutions A and B, respectively, are 00.20. The silica gels prepared from the solutions A00.20 and B00.20 are denoted as silica gels A00.20 and B00.20, respectively. The precursor solutions were dropped on a stationary Si(100) substrate (20 mm 40 mm 0.85 mm), where the substrate was heated at 2580 °C using an electric hot plate (C-MAG HS4, IKA, Osaka, Japan) (Figure 1a). Silica gel samples were obtained by drying the precursor solutions on the stationary substrate heated on the electric hot plate for a few minutes. 2.3. Aging of Silica Gels. Silica gel samples were dried in air or aged in solvent vapor at room temperature. In the case of drying in air, the silica gels were exposed to the ambient atmosphere for 1 day. On the other hand, in the case of aging in the solvent vapor, silica gel samples were placed with 5 mL precursor solutions in a sealed glass vessel 120 mL in volume for 1 day, as shown in Figure 1b. 2.4. Characterizations. The viscosity of the precursor solutions was measured at 25 °C using an oscillating-type viscometer (VM-1G, Yamaichi Electronics, Tokyo, Japan). Microscopic observation was made using an optical microscope (VHX-2000, Keyence, Osaka, Japan). The width of the surface patterns on silica gels was estimated from the optical micrographs, where the mean value of width was obtained from ten cell-like patterns as shown in Figure 2a. Microscopic observation was also made using a scanning electron microscope (SEM) (Model JSM-6510, JEOL, Tokyo, Japan) and a laser microscope (LEXT OLS3500, Olympus, Tokyo, Japan).
2. EXPERIMENTAL SECTION 2.1. Materials. The starting materials were Si(OCH3)4 (ShinEtsu Silicones, Tokyo, Japan), nitric acid (69 mass%, Wako Pure Chemical Industries), methanol (CH3OH) (Wako Pure Chemical Industries), 2-methoxyethanol (CH3OCH2CH2OH) (Wako Pure Chemical Industries), and poly(vinylpyrrolidone) (PVP) (K90, 6.3 105 in viscosity-average molecular weight, Tokyo Kasei Kogyo Co., Tokyo, Japan). 2.2. Preparation of Silica Gels. The compositions of the starting solutions are listed in Table 1. Starting solutions of molar compositions, Si(OCH3)4:H2O:HNO3:CH3OH:PVP = 1:2:0.01:10:x (x = 00.2) and Si(OCH3)4:H2O:HNO3:CH3OCH2CH2OH:PVP = 1:2:0.01:5.13:y (y = 00.2), were prepared by the following procedure, where the mole ratio for PVP was defined for the monomer (polymerizing unit) and the volume fractions of the solvents (CH3OH and CH3OCH2CH2OH) were equal to each other. First, PVP was dissolved in a half of the prescribed amount of solvents (CH3OH or CH3OCH2CH2OH), and then Si(OCH3)4 was added. The remaining amount of solvents was added to purified water, and then nitric acid was added. The solution containing CH3OH or CH3OCH2CH2OH, purified water, and nitric acid was added dropwise to the Si(OCH3)4 solution under stirring. The solutions were kept standing at room temperature in a sealed glass container for 30 min and served as precursor solutions. 940
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Surface roughness of the silica gels was measured using a contact probe surface profilometer (SE-3500, Kosaka Laboratory, Tokyo, Japan). The measurement was conducted on a straight test line 8 mm in length at the center of the gel. The height of the patterns was estimated using a surface roughness parameter, RZ (ten point height of irregularities), which was calculated from the transverse profile as shown in Figure 2b.
Figures 3 and 4 show the optical micrographs of silica gels A00.20 and B00.20, respectively. Smooth surface was observed in the silica gels A0 and B00.10 (Figures 3a and 4ac). Inhomogeneity in interference color was found on the gels A0.050.10, where cell-like patterns were slightly confirmed (Figure 3b,c). Cell-like patterns clearly appeared on the surface of the gels A0.150.20 and B0.150.20 (Figures 3d,e and 4d,e). Figure 5a shows the dependence of the width of surface patterns on the PVP contents. Surface patterns were not detected at PVP/ Si(OCH3)4 mole ratios below 0.10 (A00.10 and B00.10), where the width was not evaluated. The values of width were dispersed, not changing with PVP content for the gels A0.150.20 and B0.150.20. The width was smaller for A0.150.20 than for B0.150.20. Figure 5b shows the dependence of the roughness parameter, RZ, on the PVP contents for the silica gels A00.20 and B00.20. RZ was low and almost unchanged at the PVP/ Si(OCH3)4 mole ratios below 0.10 (A00.10 and B00.10), where cell-like patterns were not clearly observed in the optical micrographs. RZ increased with increasing the PVP/Si(OCH3)4 mole ratios over 0.10, which shows the increase in the height of the cell-like patterns. The height of the patterns was larger for A0.150.20 than for B0.150.20. 3.2. Effect of the Substrate Temperature on the Pattern Formation. Silica gels were prepared from the solutions A0.10 and B0.10 on the substrates heated at 2580 °C. Silica gels A0.10 were cracked when prepared on the substrates heated over 60 °C. On the other hand, cracks were not observed for the gels B0.10 irrespective of the substrate temperature. Figures 6 and 7 show the optical micrographs of the silica gels A0.10 and B0.10, respectively, prepared on the substrates heated at 2580 °C. Smooth surface was found in the silica gels prepared at 25 °C (Figures 6a and 7a). Cell-like patterns were formed on the surface of the gels prepared on the substrates heated over 40 °C (Figures 6b,c and 7bf). Figure 8a shows the dependence of the width of the surface patterns on the substrate temperature for the silica gels A0.10 and B0.10. The silica gels A0.10 and B0.10 had almost the same width when prepared at 40 and 50 °C. In both gels, the width of the celllike patterns slightly decreased with increasing the substrate temperature. Figure 8b shows the dependence of the roughness parameter, RZ, on the substrate temperature for the silica gels A0.10 and
3. RESULTS 3.1. Effect of the Addition of PVP on the Pattern Formation. Silica gels were prepared from the precursor solutions
A00.20 and B00.20 of various PVP/Si(OCH3)4 mole ratios on the substrate of 25 °C. The viscosity of the solutions increased with increasing the PVP/Si(OCH3)4 mole ratios as shown in Table 1.
Figure 2. Definitions of the mean width of the cell-like patterns (a) and RZ (ten point height of irregularities) (b).
Figure 3. Optical micrographs of the silica gels A0 (a), A0.05 (b), A0.10 (c), A0.15 (d), and A0.20 (e) prepared on the substrate of 25 °C. 941
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Figure 4. Optical micrographs of the silica gels B0 (a), B0.05 (b), B0.10 (c), B0.15 (d), and B0.20 (e) prepared on the substrate of 25 °C.
agreed with the value of RZ evaluated using a contact probe surface profilometer (Figure 8b). 3.4. Effect of the Aging of Silica Gels on the Cell-like Pattern. Silica gels were prepared from the precursor solutions A0.20 and B0.20 on the substrate of 25 °C and then dried in air or aged in the solvent vapor for 1 day. The gels were cracked during drying in air but not cracked when aged in the solvent vapor. Figure 11 shows the optical micrographs of the silica gels A0.20 and B0.20 aged in the solvent vapor. Although cell-like patterns were found on the as-prepared gels A0.20 and B0.20 (Figures 3e and 4e), such patterns disappeared during the aging in the solvent vapor for 1 day. The aging resulted in inhomogeneity in interference color on the gels A0.20 (Figure 11a), while smooth surface with homogeneous color was observed in the gels B0.20 (Figure 11b). Figure 12 shows the transverse profiles of the silica gels A0.20 and B0.20 before and after the aging. It is seen that the surface roughness was reduced by the aging. Especially, the smoother surface was observed on the aged gel B0.20 than A0.20 (Figure 12b,d). Table 2 shows the roughness parameter, RZ, of the as-prepared and the aged silica gels A0.20 and B0.20. The aging in the solvent vapor led to the decrease in the RZ, which agreed with the optical micrographs.
Figure 5. Dependence of the width of the cell-like patterns (a) and RZ (b) for the silica gels A00.20 and B00.20 prepared at 25 °C on the PVP/Si(OCH3)4 mole ratio in the precursor solutions.
4. DISCUSSION
B0.10. The height of the cell-like patterns (RZ) increased with increasing the substrate temperature. The height of the patterns on the gels A0.10 prepared at 2550 °C was almost similar to that of the gels B0.10. 3.3. SEM and Laser Microscope Image of Cell-like Pattern. Figure 9a,b shows the SEM image of the surface of a silica gel prepared from the solution B0.10 on the substrate heated at 80 °C. Cell-like patterns of 50300 μm in width were observed on the surface of the gel (Figure 9a). Moreover, elevated parts were formed at the edge of cell-like patterns (Figure 9a,b). Figure 10a shows the laser microscope image of the silica gel B0.10 prepared on the substrate heated at 80 °C. Cell-like patterns of ca. 100 μm in width were observed on the surface of the gel. Figure 10b shows the surface profiles evaluated along the red line shown in Figure 10a. As indicated by arrows in Figure 10, the center and edge of cell-like patterns were depressed and elevated, respectively, which agreed with the SEM images (Figure 9). The height of the edge was 0.51.0 μm, which almost
4.1. Mechanism of the Evolution of Cell-like Patterns. Celllike patterns were formed on the surface of silica gels by dropping the precursor solutions on a stationary substrate (Figures 3, 4, 6, and 7). We previously performed in situ observation on silica and titania sols dropped on a stationary substrate, and cell-like convections in the sol layer were observed during the solvent evaporation.1,2 The cell-like convection induced by the solvent evaporation, which is known as BenardMarangoni convection, is thought to be the cause of the formation of cell-like patterns on the surface of resultant gels.14 Figure 13 shows the schematic illustration of the cell-like BenardMarangoni convections. In the case of evaporating liquid layer, the evaporation rate is locally increased due to disturbances, which results in a local decrease in the surface temperature, causing a local increase in the surface tension (Figure 13a).21 The liquids near the surface are dragged toward the high surface tension region, leading to the occurrence of convections. The convections result in the formation of elevated parts at the high surface tension regions, which are 942
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Figure 6. Optical micrographs of the silica gels A0.10 prepared on the substrate heated at 25 (a), 40 (b), and 50 °C (c). Silica gels were cracked on the substrates heated over 60 °C.
Figure 7. Optical micrographs of the silica gels B0.10 prepared on the substrates heated at 25 (a), 40 (b), 50 (c), 60 (d), 70 (e), and 80 °C (f).
Figure 9. SEM images observed from above (a) and an angle of 20° (b) of the silica gels prepared from solution B0.10 on the substrate heated at 80 °C.
Figure 8. Dependence of the width of the cell-like patterns (a) and RZ (b) for the silica gels A0.10 and B0.10 on the substrate temperature. The silica gels A0.10 were cracked on the substrates heated over 60 °C.
due to the convections. Finally, the gelation of the solution layer fixed the surface patterns. 4.2. Effect of the Addition of PVP on the Cell-like Patterns. When silica gels were prepared on the substrate of 25 °C, cell-like patterns appeared at PVP/Si(OCH3)4 mole ratios over 0.10
regarded as the edge of the cells, as shown in Figure 13. The concavo-convex shape of the cell-like patterns observed in our experiments (Figures 9 and 10) agrees with such surface patterns 943
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Table 2. Roughness Parameter, RZ, of the As-Prepared and Aged Silica Gels RZ/nm gel
as-prepared
aged
A0.20
670
180
B0.20
510
130
Figure 10. Laser microscope image (a) and surface profile (b) of the silica gel B0.10 prepared on the substrate heated at 80 °C.
Figure 11. Optical micrographs of the silica gels A0.20 (a) and B0.20 (b) prepared on the substrate of 25 °C and aged in the solvent vapor.
Figure 13. Schematic illustration of BenardMarangoni convection: cross section (a) and surface (b) of liquid layer.
of precursor solutions on the substrate, and the area of the droplets was almost the same irrespective of the solutions and of the substrate temperature. From these results, the thickness of the solution layer (H) before the solvent evaporation was assumed to be almost the same under all conditions. Thus, the Ma values were thought to depend mainly on rT and μ. In the present case, rT induced by the solvent evaporation is expected to be decreased with increasing the PVP contents because the vapor pressure of the volatile components (CH3OH, CH3OCH2CH2OH, and H2O) is decreased by the addition of nonvolatile components (PVP), as described by Raoult’s law. Moreover, the high viscosity due to addition of PVP also leads to the decrease in the Ma. However, cell-like patterns were formed at large PVP contents in spite of smaller Ma. As mentioned in section 4.1, the surface patterns induced by convections are fixed by gelation. Although the rate of solvent evaporation is slightly decreased by the addition of PVP, the high viscosities at large PVP contents provide fast gelation. In fact, we experimentally confirmed that the precursor solutions with larger PVP contents showed faster gelation. (For example, 5 mL of the solutions B0 and B0.20 was gelled in 3 and 2 h, respectively, where the solutions were kept standing at 80 °C in uncovered containers.)
Figure 12. Transverse profiles of the silica gels prepared on the substrate of 25 °C before and after aging in the solvent vapor: (a) asprepared gel A0.20, (b) aged gel A0.20, (c) as-prepared gel B0.20, and (d) aged gel B0.20.
(Figures 3 and 4), and the height of cell-like patterns (RZ) increased with increasing the PVP contents (Figure 5b). Generally, BenardMarangoni convections occur at large Marangoni numbers (Ma), which increase with increasing thermal gradient near the solution surface (rT) and the thickness of solution layer (H) and with decreasing viscosity (μ) of the precursor solutions, as shown in eq 1. Here, we dropped a fixed and constant volume 944
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The Journal of Physical Chemistry C The surface patterns on the solution layer could be quickly fixed at large PVP contents and consequently led to the formation of cell-like patterns on the silica gels. On the other hand, the surface patterns occurring at low PVP contents could easily disappear via solution flow before the fixing due to the gelation. Thus, the high viscosity could be required for the formation of cell-like patterns. 4.3. Effect of the Substrate Temperature on the Cell-like Patterns. The heating of substrates led to the increase in the height of cell-like patterns on the silica gels (Figure 8b). The value of Ma depends on the thermal gradient near the solution surface (rT) induced by the evaporation3,4 and thus increases with increasing the substrate temperature. Mizev et al. reported that the velocity of BenardMarangoni convections increases with increasing Ma.22 Thus, the heating of substrates activates the convections in the solution layer, which promote the local surface elevation via solution flow, consequently leading to the formation of cell-like patterns of larger heights. On the other hand, the width of the patterns decreased with increasing the substrate temperature (Figure 8a). The increase in the substrate temperature led to the increase in rT and Ma, resulting in the increase in the number of the convections per unit volume. 4.4. Effect of the Solvent on the Cell-like Patterns. In this work, CH3OH and CH3OCH2CH2OH were used as the solvents for the precursor solutions A00.20 and B00.20, respectively. The boiling points of CH3OH and CH3OCH2CH2OH are 64.7 and 125 °C, respectively. Since BenardMarangoni convections are evolved by the solvent evaporation, the more active convections would occur in A00.20 than in B00.20. In section 3.1, the height of cell-like patterns on the silica gels A0.150.20 prepared at 25 °C was larger than that of the gels B0.150.20 (Figure 5b), which could be caused by the small gelation rates for B0.150.20. As mentioned in section 4.1, the surface patterns induced by convections are fixed by gelation. The gelation of B0.150.20 slowly progresses because CH3OCH2CH2OH as the solvent hardly evaporates at 25 °C. Thus, the height of cell-like patterns could be reduced via solution flow during the slow gelation. On the other hand, the width of cell-like patterns on silica gels A0.150.20 was smaller than that on B0.150.20 (Figure 5a). As mentioned in section 4.2, the increase in Ma could lead to the decrease in the width of the patterns. The high evaporation rate of the solutions A0.150.20 led to the increase in the value of Ma, where the width of cell-like patterns could decrease. As described in section 3.2, when silica gels were prepared on substrates over 60 °C, cracking occurred for A0.10 while no cracks were observed for B0.10. The cracking of A0.10 could be caused by the rapid solvent evaporation due to the low boiling point of CH3OH (64.7 °C). When prepared on the substrates of 4050 °C, the silica gels A0.10 and B0.10 had cell-like patterns of almost the same height and width (Figure 8). More active convections are expected in A0.10 due to the lower boiling point of CH3OH. Faster gelation is also expected, on the other hand, in A0.10 due to the higher solvent volatility, which could suppress the convections to occur. These conflicting effects allow the convections to be similar in size between A0.10 and B0.10, leading to the cell-like patterns of same heights and widths. 4.5. Effect of the Aging on the Cell-like Patterns. The silica gels A0.20 and B0.20 were cracked when dried in air for 1 day. Generally, solgel films over submicrometer in thickness are easily cracked because of the stress evolution due to the film densification during the solvent evaporation.23 In this work, silica
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gels were prepared by dropping the precursor solutions on the substrate, where the thickness of gels became a micrometer scale (510 μm in thickness). The thick gel layer could be cracked during drying in air. On the other hand, the aging in the solvent vapor prevented cracking. In the solvent vapor, the densification of silica gels due to the solvent evaporation slowly progresses, which could result in the suppression of the cracking. The cell-like patterns on silica gels A0.20 and B0.20 disappeared during the aging in the solvent vapor (Figure 10). The slow evaporation rate in the solvent vapor also suppresses the gelation of solgel films, and thus the surface patterns on the films disappear via solution flow before the completion of the gelation.2,24 In the case of the gel B0.20 containing CH3OCH2CH2OH, the cell-like patterns significantly disappeared after the aging (Figures 10b and 11d), which could be caused by the slower evaporation due to the high boiling point of CH3OCH2CH2OH.
5. CONCLUSION Micrometer-scale cell-like patterns were spontaneously formed on the silica gels prepared by dropping tetramethyl orthosilicate (Si(OCH3)4) solutions on a stationary substrate. The height of the patterns increased with increasing the PVP contents and the substrate temperature. On the other hand, the width of the patterns decreased with increasing the substrate temperature. Such variation in the size of cell-like patterns was influenced by the solvent evaporation and gelation. The spontaneous pattern formation on silica gels was caused by the cell-like BenardMarangoni convections occurring in the precursor solutions during solvent evaporation. The cell-like patterns on the gels disappeared by the aging of the silica gels in the solvent vapor. Especially, in the case using CH3OCH2CH2OH with a high boiling point as the solvent, celllike patterns completely disappeared during the aging. ’ AUTHOR INFORMATION Corresponding Author
*E-mail:
[email protected]. Phone: +81-6-6368-1121 ext 5638. Fax: +81-6-6388-8797.
’ ACKNOWLEDGMENT The authors thank Prof. Masahide Takahashi, Department of Materials Science, Graduate School of Engineering, Osaka Prefecture University, for laser microscopy. ’ REFERENCES (1) Kozuka, H.; Takenaka, S.; Kimura, S.; Haruki, T.; Ishikawa, Y. Glass Technol. 2002, 43C, 265. (2) Kozuka, H.; Ishikawa, Y.; Ashibe, N. J. Sol-Gel Sci. Technol. 2004, 31, 245. (3) Birnie, D. P., III J. Mater. Res. 2001, 16, 1145. (4) Hass, D. E.; Birnie, D. P., III J. Mater. Sci. 2002, 37, 2109. (5) Kozuka, H. J. Ceram. Soc. Jpn. 2003, 111, 624. (6) Kozuka, H.; Hirano, M. J. Sol-Gel Sci. Technol. 2000, 19, 501. (7) Uchiyama, H.; Namba, W.; Kozuka, H. Langmuir 2010, 26, 11479. (8) Deegan, R. D.; Bakajin, O.; Dupont, T. F.; Huber, G.; Nagel, S. R.; Witten, T. A. Nature 1997, 389, 827. (9) Kim, D.; Jeong, S.; Park, B. K.; Moon, J. Appl. Phys. Lett. 2006, 89, 264101. (10) Hu, H.; Larson, R. G. J. Phys. Chem. B 2006, 110, 7090. (11) Ristenpart, W. D.; Kimm, P. G.; Domingues, C.; Wan, J.; Stone, H. A. Phys. Rev. Lett. 2007, 99, 234502. 945
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