pubs.acs.org/Langmuir © 2010 American Chemical Society
Formation of Nanodents by Deposition of Nanodroplets at the Polymer-Liquid Interface Xuehua Zhang,*,† Xiaoxuan Wei,† and William Ducker*,‡ †
Department of Chemical and Biomolecular Engineering and Particulate Fluid Processing Center, University of Melbourne, Melbourne 3010, Australia, and ‡Department of Chemical Engineering, 142A Randolph Hall, Virginia Tech, Blacksburg, Virginia 24061 Received September 18, 2009. Revised Manuscript Received January 7, 2010
Recent work has shown that it is easy to form nanoscale droplets of fluid phase at interfaces. Here we demonstrate the use of these small droplets as templates for facile modification of a polymer surface. Small dents with the depth of less than 20 nm were created at the interface between polystyrene and water by the deposition of toluene droplets. Polystyrene is much more soluble in the toluene than in the water, so modification of the polystyrene occurs within the droplet but not under the water. The modification is seen to be permanent because the surface pattern remains on the polystyrene after removal from the liquids. Two mechanisms for surface modification are discussed: plastic deformation of the polystyrene due to interfacial tension and transport of dissolved polystyrene to the three-phase line during dissolution of the droplet (the coffee stain effect).
Introduction The generation of microdents on a solid surface has drawn interest recently because microdents have potential application as microreactors or microlenses. Several techniques, such as solvent etching,1-3 contact printing,4,5 condensation of the vapor of reactive solvents,6-9 soft lithography,10 and chemical etching,11 have been used to generate microdents on polymer surfaces. Often a solvent droplet or array of droplets is deposited on a solid polymer surface, which then dissolves into the solvent. The microdents are formed after the evaporation of the solvent. Recent work has demonstrated a new way of forming droplets that can now be applied to the formation of a type of nanodent. That work shows that it is relatively easy to produce very small droplets of hydrocarbon at a solid-liquid interface.12,13 The droplets can be prepared in one of two ways. The first is by direct adsorption of emulsion drops at the interface,13 and the second is by generation of droplets at the interface.12 Generation at the interface is achieved by changing to a solvent with low solubility for the hydrocarbon. When the solubility is decreased, the solute *Corresponding authors. E-mail:
[email protected] (X.Z.); wducker@ vt.edu (W.D.). (1) Bonaccurso, E.; Butt, H. J.; Hankeln, B.; Niesenhaus, B.; Graf, K. Appl. Phys. Lett. 2005, 86, 124101. (2) Li, G. F.; Butt, H. J.; Graf, K. Langmuir 2006, 22, 11395–11399. (3) Li, G. F.; Graf, K.; Bonaccurso, E.; Golovko, D. S.; Best, A.; Butt, H. J. Macromol. Chem. Phys. 2007, 208, 2134–2144. (4) Khan, F.; Zhang, R.; Unciti-Broceta, A.; Diaz-Mochon, J. J.; Bradley, M. Adv. Mater. 2007, 19, 3524–3528. (5) Li, G. F.; Hohn, N.; Graf, K. Microtopologies in polymer surfaces by solvent drops in contact and noncontact mode. Appl. Phys. Lett. 2006, 89 (24), 241920. (6) Pericet-Camara, R.; Best, A.; Butt, H. J.; Bonaccurso, E. Langmuir 2008, 24, 10565–10568. (7) Pericet-Camara, R.; Bonaccurso, E.; Graf, K. ChemPhysChem 2008, 9, 1738–1746. (8) Bates, C. M.; Stevens, F.; Langford, S. C.; Dickinson, J. T. J. Mater. Res. 2007, 22, 3360–3370. (9) Haschke, T.; Wiechert, W.; Graf, K.; Bonaccurso, E.; Li, G.; Suttmeier, F. T. Nanoscale Microscale Thermophys. Eng. 2007, 11, 31–41. (10) Xia, Y. N.; Whitesides, G. M. Angew. Chem., Int. Ed. 1998, 37, 551–575. (11) Ma, Z. Y.; Hong, Y.; Ma, L. Y.; Ni, Y. L.; Zou, S. L.; Su, M. Langmuir 2009, 25, 643–647. (12) Zhang, X. H.; Ducker, W. Langmuir 2007, 23, 12478–12480. (13) Zhang, X. H.; Ducker, W. Langmuir 2008, 24, 110–115.
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“precipitates” out of solution and onto the solid. It is unclear at this point whether the precipitation occurs directly on the solid or near the solid. The interesting feature of these droplets is that they are very small. Typical dimensions of the droplet are 10-50 nm in height and 0.3-3 μm in diameter. The same method can also be used to produce interfacial bubbles of similar dimensions.14-19 The idea that we are presenting here is that the droplets can be used as templates for permanent modification of the solid, thereby providing as route to surface modification on a nanometer scale. After forming a set of hydrocarbon droplets or bubbles at the interface between the solid and water, one can exploit the different properties of the fluids to modify the solid, on the scale of the original droplet. For example, the solid could be selectively etched under the droplet or between the droplets, depending on the properties of the fluids. Because the droplet protects from, or facilitates, modification, the modification of the solid should be on the scale of the droplet. Note that in this work we use the term “drop” for liquid dispersed in bulk and “droplet” when the liquid is adsorbed to an interface. In this case we demonstrate the modification of the solid under the droplet by forming a droplet on the surface that is mutually soluble with the solid. Toluene droplets are formed on polystyrene in water: the water is a poor solvent for polystyrene, and the toluene is a good solvent. Following prior work,12,13 we report two methods to produce the nanodroplets on the surface of polystyrene by adsorption of toluene droplets. In the first method, very small toluene droplets are created by spontaneous emulsification and adsorb onto the polystyrene. In the second method, toluene droplets are created at the interface between water and polystyrene by replacing a solution of toluene in ethanol with a (14) Lou, S. T.; Ouyang, Z. Q.; Zhang, Y.; Li, X. J.; Hu, J.; Li, M. Q.; Yang, F. J. J. Vac. Sci. Technol. B 2000, 18, 2573–2575. (15) Zhang, X. H.; Zhang, X. D.; Lou, S. T.; Zhang, Z. X.; Sun, J. L.; Hu, J. Langmuir 2004, 20, 3813–3815. (16) Zhang, X. H.; Maeda, N.; Craig, V. S. J. Langmuir 2006, 22, 5025–5035. (17) Zhang, X. H.; Khan, A.; Ducker, W. A. Phys. Rev. Lett. 2007, 98, 136101. (18) Zhang, X. H.; Zhang, X.; Sun, J.; Zhang, Z.; Li, G.; Fang, H.; Xiao, X.; Zeng, X.; Hu, J. Langmuir 2007, 23(4), 1778–1783. (19) Zhang, X. H.; Quinn, A.; Ducker, W. A. Langmuir 2008, 24(9), 4756–4764.
Published on Web 01/25/2010
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Figure 1. Images of the dents in air that were formed by exposure of polystyrene to a spontaneous emulsion. (A) Optical image. (B) Grayscale AFM image. (C) Three-dimensional view. (D) Cross section through a diameter of the dent.
solution of toluene in water. The rapid decrease in solvency leads to supersaturation of the solution and formation of the droplets.
Experimental Section Preparation of Polystyrene Plate. Polystyrene (Mw =
350 kDa and Mn =170 kDa, Aldrich) was heated to 160 °C and compressed by a clean glass or silicon slide into a flat plate with thickness of several millimeters. The polystyrene plate was smooth (Figure 1) with rms less than 20 nm over 10 μm10 μm and had contact angles of water 97 °C (advancing) and 82 °C (receding). Prior to contact with the polystyrene, each glass slide was cleaned by piranha solution (H2SO4:H2O2 = 7:3 v/v). (Caution: this mixture reacts violently with organic materials and must be handled with great care.) A hydrophobic glass was used as an inert surface. To prepare the hydrophobic glass, we modified the bare glass surface by octadecyltrichlorosilane (OTS). First, a bare glass slice was cleaned using a freshly prepared hot piranha solution (7:3 (v/v) H2SO4 (96%) and H2O2 (30%)) for 30 min, then rinsed with MilliQ water, and dried in oven. The dry slice was immersed in ∼0.5 vol % OTS in dry toluene solution for 24 h, within a dry container. Upon removal, the slice was quickly rinsed with chloroform and sonicated for 15 min each in chloroform, toluene, and ethanol, respectively. Before use, the slice was cut and then sonicated in toluene, ethanol, and water for 5 min each and dried under a stream of nitrogen gas. The advancing and receding angles of the surface are 110° and 93°, respectively. Preparation of the Dents. All dents were created in a flow cell (Stovall Life Science, Greensboro, NC) that was also used for the deposition of droplets on the surface. This flow cell allowed Langmuir 2010, 26(7), 4776–4781
Article optical observation of the plates and exchange of solutions. The room temperature was 23 °C. The droplets were produced by two methods: Method 1: Using Droplets Prepared from Emulsion. An emulsion of very small toluene drops was prepared as follows. 4 mL of 3 vol % toluene in ethanol was put into the flow cell, and then 6 mL of water was added immediately. (Note: toluene is miscible with ethanol but has very low solubility in water.) When water was added into toluene-ethanol solution, the liquid became cloudy and the toluene emulsion formed spontaneously. This process is also known as the “Ouzo effect”.20-23 The size of toluene drops depended on several factors: the concentration of toluene in ethanol, the ratio of ethanol to water, and the temperature. The polystyrene was removed from the emulsion after 5 s and rinsed with ethanol, dried with nitrogen, and heated at 60 °C overnight to remove the residue of toluene. Method 2: Solvent Exchange. Two solutions were prepared by addition of toluene until saturation: (A) toluene-saturated aqueous-ethanol solution (80 vol % ethanol) and (B) toluenesaturated water. The interfacial droplets of toluene were formed by first passing solution A and then immediately solution B through the flow cell (solvent exchange) with the polystyrene plate on the bottom. The duration for the surface in first solution was less than 15 s because the solution could soften PS. The polystyrene plate was removed from the flow cell after 30 min, rinsed with ethanol, dried with nitrogen, and then heated at 60 °C overnight. Optical microscopic pictures (Supporting Information Figure 1) showed that the dents had been formed once the surface was removed from the liquid. Drying the surface further at 60 °C did not influence the formation of dents but could remove the excess toluene on surface and prevent the cracks on surface. Atomic Force Microscopy. The polystyrene plate was first observed by optical microscopy and then by atomic force microscopy. The dents were imaged in air after the preparation. All AFM images were captured using an MFD 3D (Asylum Research) with nominally 0.32 N/m cantilevers (NP probe, Veeco) that were treated by UV for 20 min before use. All images were recorded using contact mode in air. Observation of the Formation of Macrodents. A videobased contact angle meter (OCA surface tensiometer, DataPhysics Instruments GmbH, Germany) equipped with a fluid cell was used to record the morphology of a macroscopic toluene droplet (∼0.2 μL) on polystyrene surface in water and in ethanol aqueous solution with time. The liquids were saturated with toluene.
Results Method 1: Generation of Dents Using Droplets Adsorbed from an Emulsion. The polystyrene plate is exposed to the spontaneous emulsion for about 5 s, and then the emulsion was removed and the plate was rinsed with ethanol and dried. The polystyrene plate was imaged optically and by AFM, as shown in Figure 1. The optical images show a distribution of circular objects, and the AFM images show the three-dimensional shape of each dent with nanometer-scale height resolution. There is a variety of cross sections of dents: many are circular, and some are asymmetric (see Figure 2). The width of the dents varies in a large range from several micrometers to several tens of micrometers. The depth of the dents varies from 10 nm to several hundreds of nanometers. The height, depth, and the width are defined in Figure 1D. The aspect ratio of depth to width is around 10-3 (20) (21) (22) (23)
Ruschak, K. J.; Miller, C. A. Ind. Eng. Chem. Fundam. 1972, 11, 534–540. Ganachaud, F.; Katz, J. L. ChemPhysChem 2005, 6, 209–216. Vitale, S. A.; Katz, J. L. Langmuir 2003, 19, 4105–4110. Scholten, E.; van der Linden, E.; This, H. Langmuir 2008, 24, 1701–1706.
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Figure 2. Cross sections through the diameter of dents observed by AFM imaging. These dents were formed by spontaneous emulsion. Table 1. Morphology of the Dents Formed by Method 1 (Dents 1-7) and by Method 2a width height depth aspect ratio height/depth height/depth dent (μm) (nm) (nm) (depth/width) (measured) (calculated) 5 8.7 88 19 2.2 10-3 1 49 640 491 1.0 10-2 2 15.5 153 80 5.2 10-3 1 16.8 76 134 8.0 10-3 -3 1 8.9 27 47 5.3 10 -4 8 17.6 70 8.6 4.9 10 1 27 236 167 6.2 10-3 4 12.2 42 12 9.4 10-4 -4 13 7.2 8 3 2.2 10 -3 1 6.4 10.5 11 1.5 10 1 8 21 21 2.9 10-3 a The height/depth (calculated) is estimated from eq 1.
1 2 3 4 5 6 7 8 9 10 11
4 6 5 5 4 5 5 5 4 4 4
(see Table 1), which is 1 order of magnitude smaller than that of dents formed by other methods.9 To elucidate the mechanism of formation, we would like to know whether there is a volume gain or loss on formation of the dent. We define a plane that is level with the plain surrounding the dent and examine the area of cross section above and below this plane. There is usually an area gain, indicating net swelling; that is, some toluene has remained in the dent area after drying. This is clearly shown in Figure 2C. The permanent intake of toluene into polystyrene was also observed in the work of Pericet-Camara 4778 DOI: 10.1021/la903530u
Figure 3. AFM images of nanodents formed by the droplets deposited from solvent exchange. (A) Grayscale AFM image. (B) Three-dimensional view of (A). (C, D) Cross sections through two dents in (A).
et al.,7 where the dents were formed on polystyrene surface masked by nonsolvent droplets. For majority of the dents, especially those with rather symmetric structure, the inside of the dent is smoother than the surrounding area (Figures 1D and Figure 2A,C). The increase in smoothness shows that there was restructuring within the region under the droplet, which is not surprising because toluene and polystyrene can diffuse mutually into each other. Method 2: Generation of Dents from Droplets Deposited by Solvent Exchange. To create droplets by solvent exchange, we used toluene-saturated water solution to replace toluenesaturated 80% ethanol aqueous solution in the presence of the polystyrene plate. From optical microscopy, we found that the dents could be formed, and they were generally smaller than those formed by method 1 (Figure 3). The aspect ratio of depth to width was around 10-4 (Table 1). As controls, we directly immersed polystyrene plates into toluene-saturated 80% ethanol solution or into toluene-saturated water solution. No dents were formed in either control, but toluene saturated ethanol solutions could soften the whole polystyrene plate. Also, no dent was observed if the polystyrene plate was taken out from the water soon (∼5 s) after ethanol solution was replaced with water. To form dents, the plate must be kept in toluene-saturated water for at least 30 min. At this stage, we can only roughly control the size of the droplets as in our previous study of decane droplets and nanobubbles.12,19 Our experiments showed that the formation of the dents was affected by the following parameters: (1) Concentration of Langmuir 2010, 26(7), 4776–4781
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Figure 4. Optical images of a macroscopic (∼0.2 μL) toluene droplet at the interface between a polystyrene solid and (A) water or (B) 40% ethanol aqueous solution. The magnification is 2. Both the water and ethanol solution were saturated with toluene. In water, the initial contact angle formed by the droplet was 45° and decreased to 25° after 2 min. The volume of the droplet decreased by ∼20% over 2 min. In 40% ethanol aqueous solution, the initial contact angle of the droplet was 40° and the contact angle decreased to 15° after 2 min. The volume of the droplet decreased by ∼40%.
ethanol: the density of dents increased with increasing concentration of ethanol (Supporting Information Figure 2). This is explicable in terms of the increased concentration of toluene that could be dissolved in the first solution and therefore the increased supersaturation of toluene and the greater deposition of toluene from the second solution. (2) Temperature: fewer dents are formed at high temperature (Supporting Information Figure 2). This is also consistent with the mechanism that solubility difference is essential for the formation of the droplets by exchange method. The solubilities of toluene in water and in 80% ethanol solution both increase with the increase of temperature, but the gap between them decreases with the temperature.24 (3) Exposure time: a minimum of 30 min was required to form any dents, whereas only a few seconds was required to form dents by method 1. (4) The type of liquid phase. If 10% ethanol aqueous solution instead of water (both saturated with toluene) was used for exchange, the dents were formed in much shorter time (less than 10 min). After the exchange was completed and the toluene droplets were formed, if water (toluene-free) was used to immediately exchange the toluene-saturated water again, the formation of dents took much shorter time in toluene-free water as shown in Supporting Information Figure 3. Macroscopic Toluene Droplets on Polystyrene Surface. A macroscopic toluene droplet was deposited on polystyrene surface immersed in toluene-saturated water, and the shape of the droplet was recorded with time (Figure 4). The initial contact angle formed by the droplet was 45°, and immediately after the deposition the droplet started spreading along the surface. The contact angle had decreased to 25° after 2 min, and the volume of the droplet decreased by ∼20%. Then the droplet volume decreased slowly all the time until the end of experiment. We also formed macroscopic dents on polystyrene with toluene droplets in 40% aqueous ethanol solution saturated with toluene. The initial contact angle of the droplet on the polystyrene surface was 40°, and the contact angle decreased to 15° after 2 min. At the same time the volume of the droplet decreased by ∼40%. We attribute the initial contact angle decrease to the fast mixing of toluene and polystyrene at the interface and the consequent decrease of the interfacial tension between toluene and polystyrene. (24) http://srdata.nist.gov/solubility/. The ratio of water to toluene in the coexisting phase of water-ethanol-toluene is 0.924:0.0528 at 25 °C and 0.924:0.055 at 50 °C in the water-rich phase. The ratio is 0.155:0.163 at 20 °C and 0.188:0.216 at 25 °C in the hydrocarbon-rich phase.
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The rapid decrease in the toluene droplet volume immediately after the deposition of the droplet may be due to the fast diffusion of toluene into polystyrene. After the initial fast stage, the volume of toluene droplets in both water and 40% ethanol solution decreased gradually with time, which could be due to the slow diffusion of toluene into the liquid media. Formation and Stability of Toluene Droplets on a Nonsoluble Surface. We performed the same exchange procedure to form toluene droplets at the interface of water and a hydrophobic glass which is not reactive to toluene. From optical microscopy (shown in Supporting Information Figure 4) we could observe many interfacial droplets immediately after toluene saturated 80% ethanol solution was exchanged with toluene saturated water. It took less than 1 min after the exchange when we could obtain the first optic image. So the time for forming these droplets was very short (less than 1 min). This is consistent with the formation of interfacial nanobubbles by solvent exchange.25 We took images of some droplets with time and found small droplets gradually disappearing. So even if the medium is toluene-saturated water, there is still diffusion of toluene from these small droplets with time. Many toluene droplets could also be formed by using 10% ethanol solution to replace 80% ethanol solution (both saturated with toluene). We observed the droplets disappeared in 12 min, which is shorter than in toluene-saturated water. These results are consistent with earlier experiments using decane droplets, which shows that hydrocarbon droplets disappeared more quickly in the liquid phase with higher solubility for the hydrocarbon.13
Discussion Mechanism for Dents Formation. The deposition of toluene droplets from solution to the surface was the essential step for forming the dents. In method 1, the toluene drops were created through spontaneous emulsion in solution by adding water into a toluene in ethanol solution. This spontaneous emulsion is know as the “Ozuo effect”.20-23 The toluene drops could collide and deposit to the surface. Because the surface was present when the emulsion was forming, toluene drops with different diameters could deposit on the surface. This may be why the size distribution of the dents created by this method is very wide. In method 2, the mechanism for the deposition of toluene droplets is the same as the formation of nanobubbles and (25) Zhang, X. H. Phys. Chem. Chem. Phys. 2008, 10, 6842–6848.
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hydrocarbon nanodroplets described previously.12,15-19 The results on nonsoluble surface of hydrophobic glass can show that the same solvent pair can produce toluene droplets. The principle of this method is that, when a solvent is replaced by another solvent which has lower solubility for the solute, the solute becomes supersaturated and the solute can “precipitate” on the surface in the form of nanobubbles or nanodroplets. Factors that influence the supersaturation, e.g., concentration and temperature, also influence the droplet size and distribution. After the deposition of the toluene droplets on polystyrene surface, there are two possible mechanisms for the formation of the dents. One is plastic deformation of the solid and the other is due to transport of the polystyrene through the liquid phase. The temperature of our experiments (∼23 °C) is below the glass transition temperature of pure polystyrene (∼100 °C for PS with Mn ∼110-180).26 However, PS is soluble in toluene, so after the droplets have been formed, toluene and PS can diffuse into each other under the toluene droplet. The diffusion rate of a small molecule, such as toluene, into polystyrene is influenced by the ratio of toluene and polystyrene (i.e., crowding effect),27 which is not constant during our experiments. From the literature,7 for a similar molecular mass, the toluene penetrates to about 28 μm in 3 min, which is a large distance compared to the depth of the dents (∼0.1 μm). This is easily seen by exposing a PS surface to toluenesaturated ethanol solution; the surface becomes sticky within several seconds. Diffusion of the toluene into the PS decreases the glass transition temperature of PS, which falls to 25 °C at a PS volume fraction of 0.8.26 Therefore, underneath and near the toluene droplets where the toluene diffuses into the PS, the ambient temperature can be above the glass transition temperature and thus the PS can flow under an applied load. The well-known Young equation is derived from balancing the surface tension parallel to the plane of the solid. In the vertical direction, the surface tension is opposed by forces from the solid. In this case, the fluid-fluid interfacial tension is high (γ(toluene-water) ∼ 0.05 J m-2) and the contact angle is 45° initially, so the fluid-fluid interface exerts a large tension out of the plane of the plate. When the solid is soft, the surface tension can plastically deform the polystyrene surface at the three-phase line leading to the observed rim around the dent. Also, the Laplace pressure caused by the curved liquid-liquid interface will push down the polystyrene in the center of the droplet, causing the observed depression in the middle of the dent. Such effects have been observed previously,6,28 and the behavior for an elastic solid was described by White29and Shanahan.30,31 Although our solid is plastic, others have found some agreement with White’s theory for toluene-softened PS.6 When the influence of disjoining pressure is neglected, the following equation holds:7 "
# h0 sin θ þ O R d " ¼π # h 4h0 sin θ 4ðln 2 -1Þ -ln w
ð1Þ
θ is the contact angle. We use the initial contact angle of 40° for method 1 and 45° for method 2. γlv is the interfacial tension (26) Bercea, M.; Wolf, B. A. J. Chem. Phys. 2006, 124, 174902. (27) Cherdhirankorn, T.; Best, A.; Koynov, K.; Peneva, K.; Muellen, K.; Fytas, G. J. Phys. Chem. B 2009, 113, 3355–3359. (28) Carre, A.; Gastel, J. C.; Shanahan, M. E. R. Nature 1996, 379, 432–434. (29) White, L. R. J. Colloid Interface Sci. 2003, 258, 82–96. (30) Shanahan, M. E. R.; Carre, A. Langmuir 1994, 10, 1647–1649. (31) Shanahan, M. E. R. J. Phys. D: Appl. Phys. 1988, 21, 981–985.
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(50 mN/m), h0 is the vertical range of the disjoining pressure (0.1 μm), and w is the width of the dents (see Figure 1D). We measured the depth and the width the dents (Table 1) and determined h/d: it is clear that h/d is not consistent as expected from White’s theory. For example, for dents 1 and 4 in Table 1, h/d decreases as the width increases. So the principal mechanism of drop formation is not elastic deformation. This is not totally unexpected as the solid is plastic when exposed to toluene. The extent of strain from plastic flow can be estimated from the product of the applied pressure and the creep compliance. The creep compliance of PS depends on Tg, but at 20 °C above Tg the value is ∼10-6 N-1,32 which yields a strain of about 3 10-3. For a 28 μm film under a Laplace pressure of about 3 103 Pa this represents a distance of about 80 nm. This is a very rough calculation but suggests that the Laplace may possibly play a role in contributing to the dent shape. The polystyrene will also dissolve into the toluene and diffuse throughout the droplet. Therefore, another possible mechanism for dent formation is direct diffusion of polystyrene from the center of the droplet to the three-phase line at the perimeter. We would expect deposition in the region of tensile stress created by the toluene-water interface at the three-phase line. In addition, if the toluene droplet decreases in volume, then we should consider this effect on the deposition of the polystyrene via the “coffeestain” effect. The coffee-stain effect occurs when a solute from a droplet is deposited near the three-phase line when a liquid droplet loses volume through evaporation into the vapor with a pinned three-phase line.33 Evaporation would reduce the height of the droplet at every point, but if the droplet perimeter is pinned, liquid must flow to the perimeter from the interior. The flow will carry any dispersed materials to the rim as the solvent evaporates.33 The coffee-stain effect has been shown for droplets that evaporate in air. We propose that it should also apply for a droplet that dissolves into a liquid. The loss of droplet volume at pinned perimeter is critical for the coffee-stain effect, not the mechanism of the loss of volume. For toluene nanodroplets on polystyrene, we have two components: the toluene and the polystyrene. Instead of evaporation of the toluene solvent, we can have dissolution of toluene from the droplet causing the reduction in droplet volume and therefore the coffee-stain effect. We know from our previous work that decane nanodroplets created by spontaneous emulsion lose volume with time.13 It is difficult to image the toluene droplets in real time by AFM on a softened surface, but we do see that macroscopic toluene droplets on PS in saturated solutions dissolve with time, as shown in Figure 4. Finally, we observe that small droplets on a nonsoluble surface of hydrophobic glass disappeared with time. The mechanism for this dissolution is that the toluene in the droplet is under a Laplace pressure because of the droplet curvature and so is transported to the edge of the container where flat toluene phase exists. We would expect that microscopic droplets, with greater curvature, would diminish even more quickly. In order for the coffee-stain effect to occur, the PS must diffuse into the toluene on a faster time scale than our measurements. A typical diffusion length is (6DΔt)1/2, where D is the diffusion coefficient of polystyrene in toluene and can be estimated according to the work from Li et al.2 It is about 2.3 10-12 m2/s for our sample (Mw = 350 kDa). Within 1 s, polystyrene can diffuse (32) Bernazzani, P.; Simon, S. L.; Plazek, D. J.; Ngai, K. L. Eur. Phys. J. E 2002, 8, 201–207. (33) Deegan, R. D.; Bakajin, O.; Dupont, T. F.; Huber, G.; Nagel, S. R.; Witten, T. A. Nature 1997, 389, 827–829.
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3.7 μm, which is much larger than the depth of the dents, and so there is sufficient time for diffusion. In order to discriminate between the surface tension and coffeestain mechanisms of dent formation, we can examine the effect of changing the rate of diffusion of toluene away from the droplets: an increase in rate of diffusion should accelerate the coffee-stain effect and not greatly affect the plastic flow. We reported in the Results section that the minimum time to form dents is greatly influenced by the type of liquid phase. It was 30 min in toluene-saturated water and less than 17 min in toluenefree water; i.e., the rate of dent formation was nearly doubled. The explanation is: dissolution of the droplet is accelerated by exposure to an unsaturated solution, and this accelerates the coffee-stain effect. It is difficult to imagine how the removal of the toluene could significantly either accelerate softening of the PS or increase the surface tension. Thus, while elastic deformation probably occurs, the rate of dent formation appears to be controlled by the coffee-stain effect. Indirect support for the dominance of the coffee-stain effect also comes from comparison of the time for dent formation in methods 1 and 2. The nanodents formed in seconds from the spontaneous emulsion (in 80% ethanol solution). For the exchange method, the dents formed in minutes in 10% ethanol solution from exchange but took half an hour in water. That is, the formation is accelerated in a better solvent for toluene and reduced for a greater surface tension force. Comparison to Earlier Work. Previous methods for forming dents1-4 differ from the work described here. The driving force for the reduction in drop volume in prior work was evaporation rather than diffusion though a mild chemical potential gradient supplied by the Laplace pressure in the droplet. One advantage of the method is that the dents can be very shallow (several nanometers), and therefore the volume is very small. The aspect ratio (depth/width) of the dents is around 10-3, which is much smaller than that of dents formed by other methods. The dents
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can be created over a large area in short time by using little amount of volatile reactive solvent. The dents are not arranged in regular arrays but are more or less randomly distributed over the solid. Such an array may find uses for changing lubrication or wetting properties of the polymer surface.
Conclusion Nanodents can be created on polystyrene by the deposition of toluene droplets at the solid-liquid interface. The toluene droplets can be deposited by exposing the surface to toluene emulsion (method 1) or by solvent exchange (method 2). The depth of the dents can be as small as several nanometers. The mechanism of dents formation was explored. The results are consistent with a mechanism in which dissolution of the toluene from the drop causes a flux of solvent toward the perimeter of the droplet. The toluene then deposits polystyrene at the perimeter to form the dents. Acknowledgment. We gratefully acknowledge Dr. Ziyang Lu for help in sample preparation and Mr. Nathan Nicholas for help in AFM imaging. We also gratefully acknowledge the anonymous reviewer for the insightful comments. This research was supported under Australian Research Council’s Discovery Project funding scheme (DP0664051, DP0880152). X. Zhang is the recipient of Australian Postdoctoral Fellowship (DP0880152). Supporting Information Available: Optical image of dents formed on polystyrene surface by solvent exchange (Figure 1); influence of ethanol concentration and temperature on the formation of dents (Figure 2); influence of toluene saturation on the minimum time required for the formation of dents (Figure 3); toluene droplets formed by solvent exchange on a hydrophobic glass surface (Figure 4). This material is available free of charge via the Internet at http://pubs.acs.org.
DOI: 10.1021/la903530u
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