pubs.acs.org/Langmuir © 2009 American Chemical Society
Wetting Behaviors of Individual Nanostructures Tak-Sing Wong, Adam Po-Hao Huang,† and Chih-Ming Ho* Mechanical and Aerospace Engineering Department, University of California, Los Angeles, California 90095. † Current address: Department of Mechanical Engineering, University of Arkansas, Fayetteville, Arkansas 72701. Received March 11, 2009. Revised Manuscript Received May 13, 2009 Pinning of a liquid contact line by micro/nanoscale defects is attributed as the physical origin of macroscopic contact angle hysteresis. However, direct experimental quantification of the pinning effect at the nanoscale has yet to be fully explored to establish this link. Here we present an experimental technique to systematically investigate the wetting behaviors of individual hydrophilic nanostructures with diameters from 2000 nm down to 75 nm. Our results show that the macroscopic pinning behavior is preserved for nanostructures with dimensions down to ∼200 nm. In addition, the estimated depinning liquid contact angle at the nanoscale is in agreement with the macroscopic receding contact angle, which indicates a physical link between nanoscopic pinning to the macroscopic liquid receding phenomenon.
Introduction For an ideal solid surface (e.g., flat and homogeneous), the equilibrium liquid contact angle can be uniquely defined.1 However, real surfaces are rarely ideal. Liquids sitting on these surfaces exhibit a variety of contact angles bound by two extreme values.2 The upper limit is known as the advancing contact angle, whereas the lower limit is referred to as the receding contact angle. The difference between these values is known as contact angle hysteresis, whose origin is attributed to physical roughness and chemical heterogeneity present on a solid surface.2-4 Understanding the quantitative nature of macroscopic contact angle hysteresis has tremendous technological implications, which recently was made evident in the development of super water-repellent surfaces.4-6 Advancements in micro- and nanofabrication technologies allow for the creation of highly refined surfaces with controlled physical and chemical properties, which greatly facilitates the quantitative studies of the macroscopic wetting phenomenon. For example, Gupta et al.7 coated surfaces with self-assembled monolayers of mixed alkanethiolates onto ultrasmooth surfaces, and showed that chemical heterogeneity at the molecular level does not contribute to macroscopic contact angle hysteresis. This observation highlights the importance of physical structures in influencing the macroscopic wetting phenomenon. Additionally, recent studies by Ramos et al.8,9 and Fadeev and McCarthy10 demonstrate that the physical topography of surfaces at the nano- and molecular scales has direct impact on macroscopic contact angle hysteresis. Pinning of liquid contact line (CL) is attributed as the physical mechanism of macroscopic contact angle hysteresis. When liquids spread on or recede from roughened surfaces, they become pinned *Corresponding author. E-mail:
[email protected]. (1) Young, T. Phil. Trans. R. Soc. London 1805, 95, 65–87. (2) Johnson, R. E.; Dettre, R. H. J. Phys. Chem. 1964, 68(7), 1744–1750. (3) de Gennes, P. G. Rev. Mod. Phys. 1985, 57(3), 827–863. (4) Quere, D. Annu. Rev. Mater. Res. 2008, 38, 71–99. (5) Quere, D. Rep. Prog. Phys. 2005, 68(11), 2495–2532. (6) Dorrer, C.; Ruhe, J. Soft Matter 2009, 5(1), 51–61. (7) Gupta, P.; Ulman, A.; Fanfan, S.; Korniakov, A.; Loos, K. J. Am. Chem. Soc. 2005, 127(1), 4–5. (8) Ramos, S. M. M.; Charlaix, E.; Benyagoub, A. Surf. Sci. 2003, 540(2-3), 355–362. (9) Ramos, S. M. M.; Charlaix, E.; Benyagoub, A.; Toulemonde, M. Phys. Rev. E 2003, 67(3), 031604. (10) Fadeev, A. Y.; McCarthy, T. J. Langmuir 1999, 15(11), 3759–3766.
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as a result of the CL encountering a sharp solid edge.11,12 The first quantitative relationship of the pinning phenomenon was proposed by Gibbs in 1870s,11 which broadly states that a solid edge can resist the motion of a CL until its local contact angle reaches a critical value. The validity of this relationship was later confirmed by studies on millimeter-sized cylindrical structures with micro/ nanoscale sharp edges by Mason and co-workers,12,13 and more recently on millimeter-sized spherical surfaces by Extrand and Moon.14 At the microscale, Lipowsky and co-workers have experimentally investigated wetting morphologies of microstructured surfaces,15 where they demonstrated that the CL pinning and different aspect ratios of the microstructures can induce a variety of liquid morphologies. In addition, Ondarc-uhu and Piednoir have experimentally investigated CL pinning on solid surfaces with microscale terraces of nanometric step heights,16 where they showed a critical step height is necessary for pinning to occur. While the pinning phenomenon has been studied in the macro- and microscale domains, direct experimental quantification of pinning on individual nanostructures (nanoscale dimensions in all three dimensions) has yet to be fully established because of various experimental constraints at the nanoscale.17-25 In order (11) Gibbs, J. W. The Scientific Papers of J. Willard Gibbs, New Dover ed.; Dover Publications: New York, 1961. (12) Oliver, J. F.; Huh, C.; Mason, S. G. J. Colloid Interface Sci. 1977, 59(3), 568–581. (13) Mori, Y. H.; Vandeven, T. G. M.; Mason, S. G. Colloids Surf. 1982, 4(1), 1–15. (14) Extrand, C. W.; Moon, S. I. Langmuir 2008, 24(17), 9470–9473. (15) Seemann, R.; Brinkmann, M.; Kramer, E. J.; Lange, F. F.; Lipowsky, R. Proc. Natl. Acad. Sci. U.S.A. 2005, 102(6), 1848–1852. (16) Ondarc-uhu, T.; Piednoir, A. Nano Lett. 2005, 5(9), 1744–1750. (17) Radigan, W.; Ghiradel, H; Frisch, H. L.; Schonhor, H; Kwei, T. K. J. Colloid Interface Sci. 1974, 49(2), 241–248. (18) Heslot, F.; Fraysse, N.; Cazabat, A. M. Nature 1989, 338(6217), 640–642. (19) Heslot, F.; Cazabat, A. M.; Levinson, P.; Fraysse, N. Phys. Rev. Lett. 1990, 65(5), 599–602. (20) Pompe, T.; Herminghaus, S. Phys. Rev. Lett. 2000, 85(9), 1930–1933. (21) Gao, Y. H.; Bando, Y. Nature 2002, 415(6872), 599–599. (22) Xu, H.; Shirvanyants, D.; Beers, K.; Matyjaszewski, K.; Rubinstein, M.; Sheiko, S. S. Phys. Rev. Lett. 2004, 93(20), 206103. (23) Gang, O.; Alvine, K. J.; Fukuto, M.; Pershan, P. S.; Black, C. T.; Ocko, B. M. Phys. Rev. Lett. 2005, 95(21), 217801. (24) Checco, A.; Cai, Y. G.; Gang, O.; Ocko, B. M. Ultramicroscopy 2006, 106(8-9), 703–708. (25) Checco, A.; Schollmeyer, H.; Daillant, J.; Guenoun, P.; Boukherroub, R. Langmuir 2006, 22(1), 116–126.
Published on Web 05/21/2009
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to provide a physical link between nanoscopic pinning to the macroscopic wetting phenomenon, a fundamental understanding on the wetting behaviors of individual nanostructures is necessary. Here, we present an experimental study to systematically quantify the wetting behaviors of individual hydrophilic structures with diameters down to ∼75 nm during the receding motion of macroscopic liquid droplets. Specifically, we aim to quantify the liquid wetting morphologies of individual hydrophilic nanostructures, and compare their local liquid contact angles with that of the macroscopic values.
Experimental Section We consider a situation where an aqueous solution, with a known ionic concentration, recedes from a nonwetting substrate onto hydrophilic nanostructures (Figure 1). Liquid droplets are retained on the structures as a result of pinning, provided that the thicknesses of the structures are above a critical length scale.16 Subsequently, the solvent in the droplets will evaporate, allowing the ions to crystallize and form inorganic nanoparticles. Assuming the ions are arranged in a certain cubic crystal structure, the characteristic length of the particle can be defined as
L ¼
Vd ðθg , DÞCAg a3 Z
!1=3 ð1Þ
where L is the characteristic length of the particle, which is defined as the cube root of the measured particle volume; C is the concentration of the inorganic ions in the liquid, Ag is the Avogadro constant, a is the length of the unit cell of the respective cubic crystal structure, Z is the total number of ions present in the unit cell, and Vd(θg, D) is the volume of the liquid droplet, which is a function of the local contact angle, θg, and the wetting diameter, D, provided the droplet size is less than its capillary length.26 Specifically, Vd(θg, D) can be further expressed as27 πβ D Vd ¼ 24 sin θg
!3 ð2Þ
Figure 1. Schematic diagram showing the trapping of a liquid droplet with dissolved binary salt on a hydrophilic surface during macroscopic liquid receding process. Upon solvent evaporation, an inorganic nanoparticle is formed. Inset shows the process of pinning at the convex edge of a hydrophilic structure. We assume the local contact angle of the receding CL maintains an angle of θ1 (i.e., liquid contact angle of the nonwetting layer) before it reaches the edge, which is valid if the thickness of the structure is small. The substrate was tilted to allow the solution to recede from the surface, resulting in water droplets being retained on the Pt structures. The experiments were performed under a humidity-controlled environment at room temperature. The complete evaporation of the solvent resulted in the formation of inorganic nanoparticles on the structures. The humidity conditions were investigated to ensure single nanoparticle formation (Supporting Information). Since all the experiments were carried out under very high relative humidity conditions (i.e., > 80% at ∼24 °C), and the macroscopic liquid droplets were removed within a short amount of time (i.e., < 1 min), no solid deposits at the CL were observed on the substrates before and after the experiments. We used a scanning electron microscope (SEM) to further verify that no solid deposits were formed on the HMDS surface. Both atomic force microscopy (AFM) and SEM were used to obtain the threedimensional volumes of the resulting particles. These nanometrology tools are required to characterize the nanoparticles because AFM will produce overestimated volumetric measurements when the size of the particles approaches that of the AFM tip. Therefore, SEM was utilized to determine the two-dimensional base areas of the particles, as well as the diameters of the Pt structures, to compensate for the volumetric measurement errors induced by AFM (Figure 2).
where β ¼ 2 -3 cos θg þ cos θg 3
ð3Þ
Here the local contact angle is defined as the angle between the three-phase CL of the droplet and the protruded horizontal surface of a hydrophilic structure, as depicted in Figure 1. Equation 1 provides a theoretical estimation of a particle size, L, for a given D and θg. Conversely, if L is measured experimentally, then the volume of the liquid droplet retained on the hydrophilic nanostructures, Vd, can be determined by rearranging the terms in eq 1. A heterogeneous surface was prepared with hexamethyldisilazane (HMDS) on a silicon/silicon dioxide (Si/SiO2) substrate as a (relatively) nonwetting layer, and platinum (Pt) structures, which were patterned by electron beam lithography, as hydrophilic regions (Table 1). The diameters of the Pt structures ranged from ∼75 to 2000 nm and measured ∼18 nm in height. The height of the structures is greater than the critical step height to ensure effective pinning (i.e., typically on the order of the characteristic length of the liquid molecules).16 An aqueous solution with 4.5 M of binary salts, such as potassium chloride (KCl, face-centered-cubic crystal structure), was prepared and applied to the patterned surface. (26) de Gennes, P.-G.; Brochard-Wyart, F.; Quere, D. Capillarity and Wetting Phenomena: Drops, Bubbles, Pearls, Waves; Springer: New York, 2004. (27) McHale, G.; Rowan, S. M.; Newton, M. I.; Banerjee, M. K. J. Phys. Chem. B 1998, 102(11), 1964–1967.
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Results and Discussion From the volumetric measurements of the particles, we observed two distinct size regimes with transition at D ∼220 nm (Figure 3a). When D > ∼220 nm, the measured particle size fits very well with θg = 21.5° (R2 = 0.989, m = 215, where m represents the number of individual nanoparticles measured) using eq 1, with a and Z given by recently published X-ray crystallography data.28 The nanoscopic local contact angle, θg, is in good agreement with the macroscopic θg obtained by performing identical wetting experiments using millimeter-sized Pt structures (D ∼ 2 mm, θg = 18.7° ( 5.0° for n = 5, where n represents the number of independent measurements). When D < ∼220 nm, the sizes of the particles begin to deviate from the theoretical estimates. In particular, for D < ∼120 nm, the deviation is twice as large as the theoretical expectation. By converting the experimentally measured L into Vd using eq 1, and numerically solving for θg using eqs 2 and 3, the deviation is evident for D < ∼120 nm, in which θg changes from ∼20° to 30° for D > ∼220 nm and suddenly increases to >100° for D < ∼120 nm (Figure 3b). Based on these experimental observations, we hypothesize that this particle size deviation could be due to the drastic change of the droplet profile retained on the nanostructures when (28) Trzesowska, A.; Kruszynski, R. J. Mol. Struct. (THEOCHEM) 2005, 714(2-3), 175–178.
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Letter Table 1. Contact Angle Measurements for the Macroscopic Surfaces HMDS
platinum
liquid
θadv
θstatic
θrec
θadv
θstatic
θrec
water KCl (4.5 M)
78.4° ( 3.0° 84.1° ( 2.7°
66.7° ( 1.2° 73.8° ( 1.3°
63.9° ( 5.2° 70.7° ( 2.0°
47.0° ( 0.4° 57.4° ( 2.6°
22.8° ( 1.4° 38.0° ( 0.8°
11.3° ( 2.9° 11.9° ( 2.5°
Figure 2. Formation of inorganic nanoparticles on hydrophilic Pt nanostructures at different wetting diameters. Representative SEM and AFM images showing Pt nanostructures of different wetting diameters: (a) 98 ( 1 nm (b) 124 ( 1 nm (c) 630 ( 2 nm (d) 1230 ( 4 nm (e) 1830 ( 5 nm.
D < ∼120 nm, compared to when D > ∼120 nm. Another explanation is the locally nonuniform distribution of ions within the liquid medium. If the latter hypothesis is valid, then the particle size discrepancy could still occur when the droplet maintains the same profile for all D. To resolve this matter, additional geometric information of the particles is required. On the basis of the AFM measurements, we observed that, when D < ∼120 nm, the heights of the nanoparticles were at least twice as tall as the theoretical droplet height at θg = 20° (Figure 3c). This directly shows that the actual droplet heights for D < ∼120 nm should be larger than predicted. In addition, we measured the aspect ratio of the particles, which is defined as the ratio between the particle heights to the square root values of the measured particles base area. We found that the aspect ratio was ∼0.3-0.4 when D > ∼220 nm while suddenly increasing to ∼0.6 when D ∼75 nm (Figure 3d). This further confirms that there is extra height in the droplet (in the vertical direction) for the particles to grow when D < ∼120 nm. Examining the experimental data on the size, height, and the aspect ratio of the particles, it is evident that the droplet profiles on the nanostructures for D < ∼120 nm are significantly distinct from those when D > ∼220 nm. To account for these experimental observations, we studied the liquid wetting morphologies at the macroscale structures to gain further physical insight. For example, when the liquid solution receded from the HMDS surface onto a low aspect ratio Pt macroscale structure (i.e., aspect ratio , 0.1), the liquid was pinned at the upper convex edge, forming a thin spherical cap droplet on the top surface (configuration C1, Figure 4a). The measured macroscopic local contact angles agree very well with that of the nanoscopic values for D > 220 nm (Figure 3b), which suggests that the macroscopic description of liquid wetting Langmuir 2009, 25(12), 6599–6603
morphology, C1, remains valid for D > 220 nm. In addition, it is known that the convex edge of surface structures can resist the motion of a receding CL until the local contact angle reaches the equilibrium contact angle of the surface.11 Further reduction of this local contact angle will depin the CL from the edge. On the basis of our nanoscale measurements, the depinning contact angle on these Pt nanostructures should be less than ∼20°, which agrees with the measured macroscopic receding contact angle of KCl (4.5 M) on Pt surfaces (i.e., 11.9° ( 2.5°, n = 3, Table 1). This result indicates that pinning at the nanoscale is closely related to the macroscopic liquid receding phenomenon. In another scenario, when the liquid solution receded from the HMDS surface onto a high aspect ratio Pt macroscopic structure (i.e., aspect ratio >0.1), a portion of the liquid was trapped onto the sidewall surface, leaving a thin-spherical cap droplet on the top surface (configuration C2, Figure 4b). This is caused by the high aspect ratio of the structures, where the surface area contribution from the sidewall is comparable to that of the top surface. The enhanced sidewall contribution provides additional surface area to trap the liquid during the liquid receding process. Similar liquid wetting morphologies at the microscale were observed in the study by Lipowsky and co-workers.15 In this situation, the local contact angle of the droplet is not unique and can vary from the depinning liquid contact angle of the Pt surface to the liquid receding contact angle on the HMDS surface (i.e., ∼12° < θg < ∼71°). This range of local contact angles agrees with that of the nanoscopic measurements for D ∼ 220 nm (i.e., ∼30° < θg < ∼60°, Figure 3b), therefore the liquid wetting morphology, C2, can be used to explain nanostructures of D ∼ 220 nm (Figure 4b). Since the measured local contact angles for nanostructures of D < 100 nm is greater than 100°, the macroscopic description, C2, DOI: 10.1021/la900874f
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Figure 3. Quantitative size measurements of inorganic nanoparticles on the hydrophilic structures based on AFM and SEM. (a) Characteristic lengths of the nanoparticles formed at different wetting diameters of Pt nanostructures, m = 322. The experimental data for D > 220 nm are fitted using eq 1 with θg = 21.5° (R2 = 0.989, m = 215). Inset shows an enlarged section for D = 0 to 250 nm, m = 107. Similar experiments have been performed using NaCl to verify the measurements (see Supporting Information). (b) Parametric representation of the liquid volume retained on the solids at different wetting diameters, m = 322. Macroscopic θg is highlighted within the dotted region for comparison. Inset shows an enlarged section from D = 0 to 250 nm, m = 107. (c) Comparison between computed droplet height and experimentally determined nanoparticle height, m = 322. Inset shows enlarged section from D = 0 to 250 nm, m = 107. Theoretical predictions for droplet heights at θ = 20°, 30°, and 105° are represented as dotted lines in the figure. (d) Aspect ratios of the nanoparticles (experimental) and the droplets (theoretical) change with the wetting diameters of the hydrophilic structures, m = 322. All the error bars represent standard deviations.
Figure 4. Liquid wetting morphologies of micro- and nanostructures. (a) Macroscopic liquid wetting morphology for low aspect ratio structures (, 0.1): liquid is pinned at the convex corner of the structure, forming a thin spherical cap droplet on the top surface. The local contact angle in this situation is close to the receding contact angle of the surface. (b) Macroscopic liquid wetting morphology for higher aspect ratio structures (>0.1): a portion of the liquid is trapped at the sidewall surface, leaving a thin spherical cap droplet on the top surface. The local contact angle in this situation is not unique. (c) Computed droplet profile for sub-100 nm structures, where the CL is pinned at the sidewall/convex edge. These models were used to describe the liquid wetting morphologies of micro- and nanostructures of constant thickness (t ∼ 18 nm) for (a) D > 200 nm, (b) D ∼ 200 nm, and (c) D < ∼ 100 nm (see the corresponding AFM cross sections, where the aspect ratio of the x and y axes is 1:1).
cannot be extrapolated to account for the wetting behaviors of sub-100 nm structures. Instead, the measurements suggest that the original droplet profile could be a near-spherical droplet that 6602 DOI: 10.1021/la900874f
covers the entire Pt structure, with the three-phase CL pinned at either the convex edge or trapped at the sidewall (configuration C3, Figure 4c). To further identify whether the observed result is Langmuir 2009, 25(12), 6599–6603
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due to the aspect ratio effect, we fabricated a microscale version of high aspect ratio structures (e.g., D ∼ 1167.6 nm and t ∼ 236.6 nm, equivalent to an aspect ratio of ∼0.2) and performed similar wetting experiments (Supporting Information). The measured local contact angle on these microstructures were ∼40°, which can be well-described by the liquid wetting morphology C2. This result directly indicates that the observed high local contact angles were not caused by the high aspect ratio of the sub-100 nm structures. It is important to note that when the three-phase CL interacts with the sub-100 nm structures, the local curvature of the CL will be on the order of 10 nm. This is the length scale where the CL tension becomes significant compared to the surface tension.29 The line tension may affect the local receding CL of the liquid, thereby influencing the liquid wetting morphologies of the sub-100 nm structures.24 While the present study mainly focuses on the wetting behaviors of individual nanostructures, our experimental platform provides a unique opportunity to study the mechanisms associated with droplet evaporation27,30-32 and CL deposits33-35 at the nanoscale. According to macroscopic droplet evaporation models,27,30-32 a liquid droplet of initial contact angle greater than 90° will predominately follow a constant contact angle mode of evaporation in the majority of its evaporation history, whereas a droplet with initial contact angle smaller than 90° will follow a constant contact radius mode of evaporation. In particular, if a thin spherical cap droplet with dissolved substances evaporates under a constant contact radius mode of evaporation, solid deposits will be formed at the pinned CL as a result of the mass transport of the solids by the induced capillary flow.33-35 However, we did not observe any ring-shaped solid deposits on the nanostructures (Figure 2). Instead, all the ions in the droplets were collapsed into single nanoparticles (Supporting Information). These observations indicate two possibilities: (1) the thin spherical cap droplets retained on the nanostructures do not follow a constant contact radius mode of evaporation; or (2) the macroscopic CL deposit model does not apply at the nanoscale (i.e., induced capillary flow model). To understand whether the macroscopic models of evaporation can be applicable at the nanoscale, we compared the length scales between the mean free path of the vapor molecules, λ, and the characteristic length of the liquid droplets, D. The ratio between these two length scales is defined as the Knudsen number (Kn = λ/D), which characterizes the transition between the continuum regime (Kn < 0.1, where macroscopic evaporation models apply) and the free molecular flow regime (Kn g 1).36,37 The mean free path, λ, in air under ambient conditions (e.g., 25 °C, atmospheric pressure, and 0-100% relative humidity) is ∼70 nm,38 which suggests that the droplet evaporation in our (29) Amirfazli, A.; Neumann, A. W. Adv. Colloid Interface Sci. 2004, 110(3), 121–141. (30) Picknett, R. G.; Bexon, R. J. Colloid Interface Sci. 1977, 61(2), 336–350. (31) Birdi, K. S.; Vu, D. T.; Winter, A. J. Phys. Chem. 1989, 93(9), 3702–3703. (32) Rowan, S. M.; Newton, M. I.; Mchale, G. J. Phys. Chem. 1995, 99(35), 13268–13271. (33) Deegan, R. D.; Bakajin, O.; Dupont, T. F.; Huber, G.; Nagel, S. R.; Witten, T. A. Nature 1997, 389(6653), 827–829. (34) Deegan, R. D.; Bakajin, O.; Dupont, T. F.; Huber, G.; Nagel, S. R.; Witten, T. A. Phys. Rev. E 2000, 62(1), 756–765. (35) Sommer, A. P.; Rozlosnik, N. Cryst. Growth Des. 2005, 5(2), 551–557. (36) Davies, C. N. Faraday Symp. Chem. Soc. 1973, 7, 34–41. (37) Ray, A. K.; Lee, J.; Tilley, H. L. Langmuir 1988, 4(3), 631–637. (38) Jennings, S. G. J. Aerosol Sci. 1988, 19(2), 159–166.
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experiments underwent transitions from the continuum regime (D > 1000 nm) to the free molecular flow regime (D < 100 nm). An interesting observation from our measurements is that the aspect ratios of the nanoparticles remain fairly constant in the range of ∼200 nm < D < ∼2000 nm, which indicates that the droplets underwent similar evaporation histories (Figure 3d). We further verified the measurements using different KCl concentrations: 1.0 M, 2.5 M, and 4.5 M, where similar experimental trends were observed (Supporting Information). These results show that the droplet evaporation histories are insensitive to the specified KCl concentration ranges. A recent study by Ondarc-uhu and co-workers experimentally demonstrated that the macroscopic evaporation model remains valid for droplets of D ∼ 1000 nm.39 If one assumes that the constant contact radius evaporation model applies for droplets of D > 1000 nm, then our measurements suggest that this model should remain valid down to D ∼ 200 nm (i.e., approaching the free molecular flow regime). A similar evaporation mode was observed in a recent molecular dynamics simulation study on a nanoscale droplet of dimension on the order of 10 nm.40 If the constant contact radius mode of evaporation applies, then it raises an interesting question of whether the macroscopic model of CL deposit33-35 would be applicable at the nanoscale.
Conclusions In summary, we systematically quantified the liquid wetting morphologies of individual nanostructures down to the sub-100 nm scale through the study of evaporation-induced nanoparticles. We have showed that the macroscopic pinning behavior is preserved for nanostructures with dimensions down to ∼200 nm. In addition, the estimated depinning liquid contact angle at the nanoscale is in agreement with the macroscopic receding contact angle, which indicates a physical link between nanoscopic pinning to the macroscopic liquid receding phenomenon. Furthermore, our experimental technique provides a unique platform to study nanoscale phenomena, such as the mechanisms associated with droplet evaporation and CL deposits. Acknowledgment. This work was financially supported by Center for Scalable and Integrated Nanomanufacturing (SINAM) under the National Science Foundation (Award Number CMMI-0751621). T.S.W. acknowledges a Ph.D. fellowship from Intel Foundation. The following colleagues are gratefully acknowledged for discussions: Prof. H.P. Kavehpour, Dr. A. Benahmed, Dr. B. Brough, T.H. Chen, Dr. N. Li, P. Lillehoj, Dr. Y.B. Ma, H. Tsutsui, and Dr. F. Wei of UCLA. The authors thank Prof. R.L. Garrell of UCLA for the use of optical contact angle goniometer. Supporting Information Available: Experimental details of nanoparticle formation and measurements, dependence of single nanoparticle formation on humidity, control experiments with different inorganic salts, influence of ionic concentrations to the aspect ratios of nanoparticles, liquid wetting morphologies of microscale and macroscale structures at different aspect ratios. This information is available free of charge via the Internet at http://pubs.acs.org/. (39) Arcamone, J.; Dujardin, E.; Rius, G.; Perez-Murano, F.; Ondarc-uhu, T. J. Phys. Chem. B 2007, 111(45), 13020–13027. (40) Koplik, J.; Pal, S.; Banavar, J. R. Phys. Rev. E 2002, 65(2), 021504.
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