Characterization of Oil Droplets under a Polymer Film by Laser

The films were then observed by laser scanning confocal fluorescence microscopy (LSCFM). On quartz, triolein breaks up into approximately spherical, ...
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Langmuir 2000, 16, 3391-3400

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Characterization of Oil Droplets under a Polymer Film by Laser Scanning Confocal Fluorescence Microscopy J. P. S. Farinha† and M. A. Winnik* Department of Chemistry, University of Toronto, 80 St. George Street, Toronto, Ontario, Canada M5S 3H6

K. G. Hahn ICI Paints Research Center, 16651 Sprague Road, Strongsville, Ohio 44136-1739 Received June 4, 1999. In Final Form: December 2, 1999 We studied the wetting behavior of thin oil films on quartz and poly(ethylene terephthalate) (PET) substrates before and after being coated with a polymer colloidal dispersion. The triolein oil was doped with a trace of a fluorescent dye, and the films were prepared by casting a dilute solution of dyed triolein in acetone onto the substrates. The films were then observed by laser scanning confocal fluorescence microscopy (LSCFM). On quartz, triolein breaks up into approximately spherical, micrometer-sized droplets as the acetone evaporates. The oil droplets were coated with a water dispersion of a film-forming latex dispersion of poly(butyl methacrylate) (PBMA). Most of the droplets adhered to the quartz substrate as the water evaporates. With LSCFM, we could image the triolein droplets under the transparent polymer latex film. These had larger volumes than those formed in air, and their micro-contact angle increased from 〈θ〉 ) (32 ( 5)° at the quartz-air interface to 〈θ〉 ) (71 ( 4)° at the quartz-PBMA interface. When deposited on PET, the triolein wets the substrate surface and forms an almost homogeneous film. Upon coating with a PBMA latex film, we observed some dewetting of the oil from the PET substrate, and the micro-contact angle of the oil increased from 〈θ〉 ) (8 ( 4)° at the PET-air interface to 〈θ〉 ) (18 ( 5)° at the PET-PBMA interface.

Introduction The behavior of oil-like films forced to spread onto nonwetting substrates has long been of interest.1-5 The wetting behavior of thin oil films is of great importance for many engineering applications such as lubrication, oil recovery and separation, detergency, printing, and stability of different kinds of coating films. Our aim in this paper is to understand the wetting behavior of a thin oil film deposited on a solid surface that is subsequently coated with a latex dispersion, which is allowed to dry to form a solid polymer film. From the point of view of the coatings industry, this system serves as a model that might allow one to develop paint capable of efficiently covering an oily surface without first having to remove the oil. From a surface science perspective, we are interested in characterizing the effect of the polymer dispersion, as it evolves toward a dry polymer film, on the wetting properties of a thin oil film previously forced to spread onto a solid substrate. This constitutes a rather complex system. It starts as a ternary liquid mixture (oil plus latex polymer particles dispersed in water) wetting a solid substrate. This ternary mixture gradually evolves toward a solid polymer film covering the liquid oil film. The wetting * To whom correspondence should be addressed. Email: [email protected]. † On leave from Centro de Quı´mica-Fı´sica Molecular, complexo I, Instituto Superior Te´cnico, Av. Rovisco Pais, 1096 Lisboa codex, Portugal. Email: [email protected]. (1) Wilkinson, M. C.; Zettlemoyer, A. C.; Aronson, M. P.; Vanderhoff, J. W. J. Colloid Interface Sci. 1979, 68, 508, 545, 560, 575. (2) Basu, S.; Sharma, M. M. J. Colloid Interface Sci. 1996, 181, 443. (3) Redon, C.; Brochard-Wyart, F.; Rondelez, F. Phys. Rev. Lett. 1991, 66, 715. (4) Sharma, A.; Ruckenstein, E. J. Colloid Interface Sci. 1989, 133, 358. (5) Brochard-Wyart, F.; Martin, P.; Redon, C. Langmuir 1993, 9, 3682.

properties of the oil on the initial mixture are expected to change as the composition of the mixture is modified by the evaporation of the water. Eventually, the configuration of the oil film is frozen due to the decrease in mobility imposed by the formation of the solid polymer film. The problem of the dewetting of a binary mixture was recently discussed by Fondecave and Brochard Wyart6 for the case in which the solvent wets the substrate but the solute does not. Their results apply to different systems in which dewetting and phase separation are coupled but do not consider the effect of solvent evaporation and formation of a solid film. The problem we address here is significantly more complex. One of the points we wish to examine is whether the final polymer-coated oil film is in an equilibrium configuration determined by the wetting properties of the polymer/oil binary mixture, or whether it reflects instead some intermediary state of the oil/ dispersion mixture. An intermediate state may be frozen in place because of the reduction in the oil film mobility caused by water evaporation. We are interested both in the nature of the thin oil film formed on the surface of a solid substrate and on how this film is affected when a dispersion of film-forming latex is spread over the substrate and allowed to dry. Two different substrates were investigated, poly(ethylene terephthalate) (PET) and quartz. While quartz is a high-energy, hydrophilic substrate and causes the deposited oil film to partially dewet into micrometer-sized spherical shape droplets, PET is a low-energy, hydrophobic substrate that can be covered by a nearly homogeneous oil film. As previously described,7 we employed triolein, a liquid triglyceride, as the oil; PET and quartz as substrates; and (6) Fondecave, R.; Brochard-Wyart, F. Macromolecules 1998, 31, 9305. (7) Farinha, J. P. S.; Winnik, M. A.; Hahn, K. G. Langmuir 1999, 15, 7088.

10.1021/la990710m CCC: $19.00 © 2000 American Chemical Society Published on Web 03/11/2000

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poly(n-butyl methacrylate) (PBMA) as the film-forming latex dispersion. Triolein is a component of olive oil and serves as a model for oils and greases that might appear on a wall to be painted. It is also a good model for lubricant traces on the surface of aluminum sheet that is to be coated. PBMA is an excellent model for a variety of acrylic dispersions that form films at or near room temperature.8,9 Typical latex films are 30- to 60-µm thick and are coated over the quartz or PET substrates, and both substrates contain a significantly thinner oil film. To quantify the effect of the latex polymer coat on the oil film, we wished to determine the change in contact angle between the oil and the substrate. The quartz substrate is the more appropriate for this effect since the oil films dewet into a distribution of micrometer-sized droplets for which the micro-contact angles can be determined more precisely than for the oil film formed on PET substrates. Contact angles θ define the tendency of a nonwetting fluid either to adhere to the surface (θ < 90°) or separate from it (θ > 90°) and can be measured by a variety of techniques.10-12 Traditional methods for determining contact angles cannot, however, be used to study a system consisting of micrometer-sized oil droplets on a quartz substrate coated with a solid polymer film. To characterize these droplets, we needed a technique with 3D imaging capability, submicrometer resolution, and the ability to detect the oil droplets through the transparent polymer film. Laser scanning confocal fluorescence microscopy (LSCFM)13-18 fulfils these requirements and allows us to characterize the shape and dimensions of oil droplets tagged with a fluorescent dye inside a polymer film that is transparent to the wavelengths used. The LSCFM technique is widely used in the biological sciences17,18 and has also been used to image domain shapes and sizes in a variety of solid polymer blend systems.19-27 LSCFM relies on the confocal effect to enhance both the lateral and in-depth resolution with respect to conventional optical microscopy.13-18 The laser beam is focused on the sample, and the light emitted is directed back (8) Farinha, J. P. S.; Martinho, J. M. G.; Kawaguchi, S.; Yekta, A.; Winnik, M. A. J. Phys. Chem. 1996, 100, 12552. (9) Winnik, M. A. In Emulsion Polymerization and Eumlsion Polymers; Lovell, P. A., El-Aasser, M. S., Eds.; Wiley: New York, 1997; pp 467-518. (10) Hiemenz, P. C. Principles of Colloid and Surface Chemistry; Marcel Dekker: New York, 1986. (11) Israelachvili, J. N. Intermolecular and Surface Forces; Academic: New York, 1992. (12) Bascom, W. B. Adv. Polym. Sci. 1988, 85, 89. (13) Confocal Microscopy; Wilson, T., Ed.; Academic: London, 1990. (14) Multidimentional Spectroscopy; Cheng, P. C., et., Eds.; Wiley: New York, 1994. (15) Corle, T. R.; Kino, G. S. Confocal Scanning Optical Microscopy and Related Imaging Systems; Academic: New York, 1996. (16) Gu, M. Principles of Three-Dimensional Imaging in Confocal Microscopes; World Scientific: London, 1996. (17) Three-Dimensional Confocal Microscopy: Volume Investigation of Biological Specimens; Stevens, J. K. et., Eds.; Academic: New York, 1994. (18) Handbook of Biological Confocal Microscopy; Pawley, J. B., Ed.; Plenum: New York: 1995. (19) Li, L.; Sosnowski, S.; Chaffey, C. E.; Balke, S. T.; Winnik, M. A. Langmuir 1994, 10, 2495. (20) Li, L.; Chen, L.; Bruin, P.; Winnik, M. A. J. Polym. Sci. B: Polym. Phys. 1996, 35, 979. (21) Kumacheva, E.; Li, L.; Winnik, M. A.; Shinozaki, D. M.; Cheng, P. C. Langmuir 1997, 13, 2483. (22) White, W. R.; Wiltzius, P. Phys. Rev. Lett. 1995, 75, 3012. (23) Jinnai, H.; Nishikawa, Y.; Koga, T.; Hashimoto, T. Macromolecules 1995, 28, 4784. (24) Ribbe, A. E.; Hashimoto, T.; Jinnai, H. J. Mater. Sci. 1996, 31, 5837. (25) Jinnai, H.; Koga, T.; Nishikawa, Y.; Hashimoto, T.; Hyde, S. T. Phys. Rev. Lett. 1997, 78, 2248. (26) Jinnai, H.; Hashimoto, T.; Lee, D.; Chen, S-H. Macromolecules 1997, 30, 130. (27) Ribbe, A.; Hashimoto, T. Macromolecules 1997, 30, 3999.

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through the same optical path and passed through a small aperture (confocal pinhole) before reaching the photomultiplier. Only light emitted from the focal plane is detected because the light coming from other planes cannot pass through the pinhole. It is thus possible to obtain optical sections of the sample and reconstruct its threedimensional structure. In this way, we were able to determine the shape of the oil droplets, even when they are coated with a polymer film. To achieve good contrast between the polymer and oil phases, we used the confocal microscope in the fluorescence mode. The oil phase contained a trace amount of a water-insoluble fluorescent dye, 2-octadecyl-6-thio-2-azo-benzo-[def]chrysene-1,3-dione (Hostasol Yellow, HY), which can be excited at the wavelength (488 nm) of an argon laser and which emits above 500 nm, where the polymer is transparent. When the quartz substrates containing a thin triolein film are covered with surfactant-free latex dispersion, most of the oil adheres to the substrate. Some droplets appear at the edge of the latex film. A tiny fraction is displaced to the latex-air interface, but no droplets can be detected within the latex film itself. The droplets maintain their spherical shape but increase their average volume. The average micro-contact angle 〈θ〉 increases from (32 ( 5)° at the quartz-air interface to (71 ( 4)° at the quartzPBMA interface. Heating the latex films on quartz to a temperature (90 °C) well above the polymer glass transition temperature (30 °C) produces only a slight further increase in the oil droplets’ micro-contact angle to 〈θ〉 ) (80 ( 3)°, with no major change in the droplet diameter. This seems to indicate that upon coating of the oil film with the latex dispersion, the oil droplets have time to readjust to a near-equilibrium shape before the dispersion dries and the oil distribution is frozen under the solid polymer film. On PET substrates, the oil spreads and wets the surface. The resulting film is not perfectly homogeneous, probably because of the conjugation of surface irregularities on the PET substrate and the small average thickness of the oil film. We could observe the formation of thicker patches in the oil films, for which the approximate micro-contact angle was calculated as 〈θ〉 ) (8 ( 4)°. This value is close to the contact angle θ ) 5° determined for macroscopic triolein droplets in the same PET substrate but immersed in water.28 When the oil film on PET is coated with a PBMA dispersion, the dispersion dries to form a transparent polymer film. All the oil remains attached to the PET, but there is a small amount of dewetting, with the micro-contact angle increasing to 〈θ〉 ) (18 ( 5)°. This dewetting is probably due to some instability of the oil film, caused by the tendency of the system to squeeze the oil between the PET substrate and the PBMA film in order to maximize the contact between the two polymers.29 Experimental Section Materials and Sample Preparation. PET was uncoated Melinex film, obtained from ICI Films. Triolein (Sigma, practical grade: 65%), was used as received. We used practical grade triolein because it contains more hydrophobic material than the 98% pure triolein. Rharbi in our group measured the macroscopic contact angle of triolein on PET, immersed in water, and found θ ) 5° for the practical grade triolein and θ ) 22° for the 98% pure sample.28 The fluorescent dye 2-octadecyl-6-thio-2-azobenzo[def]chrysene-1,3-dione, [Hostasol Yellow (HY), Clariant], was a gift from Clariant Canada Inc. We used a concentration of 0.1 wt % of HY in triolein.7 To obtain thin films of the model (28) Rharbi, Y.; Winnik, M. A. Private communication. (29) Martin, P.; Silberzan, P.; Brochard-Wyart, F. Langmuir 1997, 13, 4910.

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very useful for fluorescence microscopy experiments. Also, HY is very soluble in triolein at room temperature but virtually insoluble in water. This allows us to cover HYdyed triolein with an aqueous polymer colloidal dispersion while maintaining the HY in the oil phase. All experiments described here involve triolein containing 0.1 wt % HY. This amount of dye gives the strongest fluorescent emission because at higher concentrations, self-absorption and self-quenching decrease the fluorescence emission.7

Figure 1. Normalized fluorescence excitation and emission spectra of a 0.1 wt % HY solution in triolein at room temperature. In the LSCFM experiments, the excitation wavelength was 488 nm, and emission from the sample was collected above 515 nm. oil confined to a limited area (ca. 4 mm) of the substrates, we used a solution of 1 wt % triolein (containing 0.1 wt % of HY) in acetone (Aldrich, spectrograde), which was deposited on the substrate with a microsyringe.7 The area-averaged thickness of the oil films on PET and quartz substrates calculated from mass balance was approximately 1 µm.7 The PBMA latex dispersion at 10 wt % solids (diameter 144 nm, Mn ) 1.4 × 105, Mw/Mn ) 2.7) was prepared by a standard emulsion polymerization technique30 using sodium dodecyl sulfate as the surfactant. The dispersion was cleaned by three successive exposures to a purified ion-exchange resin (Biorad AG-501-X8) to remove the surfactant.7 The quartz or PET substrates were completely covered by a few drops of the dispersion and were dried at 38 °C over a period of approximately 12 h to yield transparent and defect-free PBMA films, approximately 50-µm thick (mass balance average).7 Laser Scanning Confocal Fluorescence Microscopy. With the LSCFM technique, we obtained enhanced lateral and in-depth resolution with respect to conventional optical microscopy.13-18 This allowed us to determine the shape of the grease droplets and, from it, measure both their diameters at the substrate level, D, and their heights, h. As reported previously,7 the LSCFM measurements were performed with a Bio-Rad MRC600 confocal microscope, using the 488-nm line of an ArKr laser. All the images were obtained in the fluorescence mode, using a 488-nm interference filter for the excitation and a cutoff filter at 515 nm for emission. To image large regions of the oil film, we used a low magnification 10× dry objective (Nikon). From the images obtained in this configuration, we could calculate distributions of droplet diameters. In the text below, we refer to these experiments as low-resolution measurements. To image single particles and determine their heights from optical sectioning and depth profiling, we used a 100× magnification dry objective (Nikon) as well as a higher resolution configuration of the microscope, in which the pinhole was kept at its smallest diameter.7 The depth resolution for this configuration for visible light at λex ≈ 500 nm is estimated to be 0.3 µm. We refer to these measurements as high-resolution experiments.

Results and Discussion Hostasol Yellow (HY, structure 1) has a strong absorption at the 488-nm argon laser line and very strong yellow fluorescence emission above 515 nm. In Figure 1, we present the normalized fluorescence excitation and emission spectra of a 0.1 wt % HY solution in triolein at room temperature. The excitation and emission wavelengths used in the LSCFM experiments are also shown. The spectral characteristics of HY, along with its resilience to photobleaching by the argon laser light, makes this dye (30) Feng, J. Ph.D. Thesis, University of Toronto, 1996.

Triolein is a modestly viscous fluid at room temperature. When small amounts of triolein were placed on PET or quartz substrates, the resulting films were always several tens of micrometers thick. However, for our model system, we were interested is much thinner oil films. By spin coating, we could obtain thinner films, but these coated the entire substrate, and this poses a problem for the subsequent polymer-coating step we wished to examine. Even though the triolein film dewets into droplets on quartz, if the triolein film covers the entire quartz substrate, the latex dispersion cannot wet the substrate sufficiently to form a continuous liquid film. To get a triolein film of about 1-µm thick with an area of approximately 4 mm2 by direct deposition of the oil, we would need to deposit about 4 × 10-3 µL of triolein on the subtract. An alternative strategy to dispense such a small amount was developed, in which a dilute 0.1 wt % solution of the dyed triolein in acetone was spread over a portion of the substrate using a glass capillary tube or a microsyringe. The solvent spreads the oil over the substrate and evaporates almost instantly, leaving thin and stable oil films at the surface of the substrate.7 Oil Films on Quartz Substrates. Oil films cast from an acetone solution onto a quartz substrate consist of micrometer-sized droplets confined to a small area of about 4 mm2 at the center of the substrate. The dewetting of the oil into micrometer-size droplets, gives us an indication that for quartz substrates, the oil films are below the critical thickness for dewetting of the oil in acetone solution. In this case, the solvent (acetone) wets the quartz substrate, but the solute (dyed triolein) does not. The theory of the dewetting process for this kind of mixture was described recently.16 After the deposition of a drop of acetone solution, wetting of the quartz occurs with accelerated spreading caused by the volatile solvent.31 The drop spreads too much and too fast, producing a Merangoni instability, and its profile becomes flat and surrounded by an unstable rim. The contact line recedes, producing undulation of the rim and digitation, and the drop breaks up into a distribution of micrometer-sized droplets.6 In Figure 2 (top), we show a low-magnification image of the micrometer-sized triolein droplets formed on quartz. For all images obtained with the confocal microscope, fluorescence from the triolein doped with HY is shown in dark. The droplets are approximately hemispherical in shape, with an average elongation (defined as the ratio of the major axis to the minor axis of the droplets) of 1.2 ( 0.3. The particles have an average maximum equivalent diameter of 20 µm. The diameter D is defined from the (31) Redon, C.; Brochard-Wyart, F.; Rondelez, F. J. Phys. II 1992, 2, 1671.

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Figure 3. Confocal depth-scan of a small region of a triolein film cast from acetone onto a quartz plate (scale bar 25 µm, pinhole aperture in closest position). LSCFM images 1 (at droplet top) to 3 (at quartz surface) correspond to different focal depths. From the droplet height h and diameter at the substrate surface D, we calculate the micro-contact angle θ using eq 1. The micro-contact angle 〈θ〉 ) (32 ( 5)° is approximately equal for all micrometer-sized droplets, since their height was found to be proportional to the diameter.

Figure 2. LSCFM image (top) of the surface of a quartz plate in which a triolein film was deposited (scale bar 250 µm) and distribution of the diameter at the surface of the substrate, D, for the oil droplets on the same image (bottom).

calculated area A of the particles at the substrate surface D ) 2 xA/2π. The histogram of droplet diameters obtained from the image in Figure 2 (top) is shown in Figure 2 (bottom). The distribution is broad and its detailed shape changes for different films, but the global features remain the same: one main maximum D1 and one or more secondary maxima (e.g., D2, D3) at higher diameters, corresponding to aggregates of two or more particles (D2 ) x2D1, D3 ) x3D1). When 1 µL of acetone solution was used, the films had an area-averaged thickness of 1 µm by weight balance. The actual height of the individual oil droplets could be determined by optical sectioning the droplets using LSCFM (Figure 3), and typical values ranged from 3 to 6 µm. In this way, we could also determine the diameter of the droplets at the substrate surface. Since the droplets have a spherical cap shape, we use geometrical arguments to calculate the micro-contact angle θ from the radius R at the substrate surface and the height h of the droplets:

tan

(2θ) ) Rh

(1)

From the scan of numerous droplets in each of over 20 triolein films, we calculated an average value of 〈θ〉 ) (32 ( 5)° for the micro-contact angle of uncoated triolein films on quartz. The large uncertainty in the micro-contact angle value is in part related to the uncertainty in droplet height determination. The in-depth resolution is limited, by the wavelength used (ca. 500 nm) and the number aperture of the lens, to a value of 0.3 µm. We define θ as a micro-contact angle in distinction from the normal contact angle measured for macroscopic liquid

Figure 4. Linear relation between the height h and the radius at the substrate surface R for different triolein droplets on quartz, before (9) and after (b) being coated with a PBMA film cast from the dispersion.

drops on a solid surface. Although there is a fairly broad distribution of droplet diameters in these films (Figure 2b), the droplet heights are proportional to the diameters, as expected for hemispherical shape droplets (Figure 4), and the micro-contact angle remains approximately constant for all the droplets measured. Contact angles for micrometer-sized droplets can be smaller than those for macroscopic droplets on the same substrate. The effect of droplet size on the contact angle was previously described using the concept of a line tension and a modified Young equation.32-34 More recently, Joanny and de Gennes35 showed, however, that the line tension itself derives from the effects of long-range forces between substrate and fluid. They also predicted that the effect of size on the drop shape should be significant only for micrometer-sized droplets. An example of this effect was presented by Rieutord and Salmeron36 for micrometersized droplets of sulfuric acid on mica examined by scanning polarization force microscopy. (32) Pethica, B. A. J. Colloid Interface Sci. 1977, 62, 567. (33) Good, R. J.; Koo, M. N. J. Colloid Interface Sci. 1979, 71, 283. (34) Gaydos, J.; Neumann, A. W. J. Colloid Interface Sci. 1987, 120, 76. (35) Joanny, J. F.; de Gennes, P. G. J. Colloid Interface Sci. 1986, 111, 94. (36) Rieutord, F.; Salmeron, M. J. Phys. Chem. B 1998, 102, 3941.

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Figure 5. LSCFM images of a triolein film on quartz while covering it in situ with water (top) and after water is allowed to evaporate slowly at 38 °C (bottom). Scale bar is 250 µm. Upon water addition, we observed a rapid coalescence of several oil droplets into larger irregular domains (top). After the water evaporated, the aggregates broke up and formed oil droplets with a similar size distribution and contact angle as before (bottom).

When the quartz plate containing the oil droplets in the center was covered with a film of water, the droplets began to move and to coalesce. A confocal image of the system under water is shown in Figure 5A. By eye, the larger aggregates seen in Figure 5A appear much less mobile than the droplets from which they were formed. We did not observe the system while water evaporated from the film, but we found that following slow water evaporation (at 38 °C and high humidity), the system had evolved to form a different but very similar distribution of droplets on the dry quartz surface (Figure 5B). Thus, upon drying, the large oil globules that form under water break up into smaller droplets. A cartoon illustrating our observations is shown in Figure 6. There is a feature of this phenomenon that we find striking. In the initial sample, the oil droplets are confined to a few square millimeters of the quartz plate. While the water film covers the entire substrate, the oil droplets, both under water and after evaporation of the water, remain confined to this area. We take this to mean that the oil under water remains in contact with the quartz surface. If the oil droplets were to become detached from the surface, they would float to the surface of the water and, after water evaporation, would move to other regions of the substrate. The aggregation of the oil droplets at the quartz surface is probably linked to an increase in the triolein contact angle driven by the surface tension between water and quartz. The driving force for globule break up would be the decrease in contact angle produced by water evaporation. The decrease in contact angle gives rise to thinner globules that are unstable and tend to dewet into smaller droplets (Figure 6). If an oil film freshly deposited on quartz is heated at 90 °C (well below the boiling point of triolein, Tb ) 235

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°C) for 1 h, the droplets become noticeably rounder and more regular (Figure 7). This effect shows up in a slight decrease of the average elongation of the oil droplets from 1.2 ( 0.3 to 1.1 ( 0.1 µm upon heating. We could not otherwise observe any major change in the droplet diameters or contact angle, even after the films were heated for several hours and then cooled back to room temperature. The behavior of the triolein films on quartz when perturbed either by the addition of water or by heating at 90 °C indicate that, although the triolein films on quartz are prepared by casting from a dilute acetone solution, the rapid spreading and evaporation of the solvent apparently does not freeze the oil film in a nonequilibrium configuration. During film formation, the oil droplets are able to achieve their equilibrium distribution of shapes and sizes. After the system is perturbed, either by the presence of water or by heating, the oil droplets recover the equilibrium shape and size distribution. Oil under Latex Films. After imaging the triolein films on quartz, they were covered with a few drops of the PBMA latex dispersion. The dispersion wets the free surface of the substrate but is turbid. As a consequence, it was not possible to image the triolein in the presence of the latex until the dispersion dried to form a transparent solid film. We imagine that in the presence of the dispersion, which contains 10 wt % latex solids, the triolein droplets undergo a coalescence process similar to that observed in the presence of water alone (Figures 5 and 6), and that these aggregates break up into droplets as the dispersion dries. After drying the dispersion at 38 °C and high humidity for about 12 h, we obtained transparent PBMA films with a mean thickness of about 50 µm, as calculated from the mass balance. Using LSCFM, we could determine the actual thickness of the polymer film at different locations across its surface. In this experiment, we used a “magic marker” to place dots of a red-emitting hydrophilic fluorescent dye on the quartz plate before coating it with the PBMA dispersion. After the latex film dried, we placed more dots on top of the latex film surface. Using high magnification and the smallest pinhole aperture, we focused on each of the bottom dots and, using the focusing calibrated step-motor of the microscope, focused on the corresponding dots at the latex surface, recovering the distance between these markings. The PBMA solid film in the region directly above the grease films was about 30-µm thick. Having determined the positions of both the quartz surface and the latex surface using the spots of the redemitting dye, we scanned the yellow-emitting triolein droplets containing HY. From these scans, in which we used the low magnification 10× objective, we found that in the dry film, approximately all of the triolein remains attached to the quartz surface. We observed that a small portion of the triolein had moved to the border of the latex film. A tiny fraction of droplets had floated to the top of the latex film, but no oil could be detected within the latex film. In Figure 8a, we show an image of the quartz-latex interface region for the same oil film as in Figure 2. In the center region of the quartz plate, the droplets rearrange to recover a diameter distribution similar to that of the oil film at the air/quartz interface. The corresponding histogram of droplet diameters (Figure 8b) shows certain features common to all of the films we examined. As in the case of triolein in air on quartz, the droplet shape distribution is not identical for all films, but the maximum in the size distribution is always shifted to higher diameter values than that for the uncoated grease film. We always observe the presence of secondary

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Figure 6. When a quartz plate containing oil droplets in the center is covered with water, the droplets begin to move and to coalesce into larger aggregates. As the water evaporates, the system evolves to form a different but very similar distribution of droplets on the dry quartz surface. Droplet aggregation at the quartz surface when water is added is probably related to an increase of the contact angle of the oil from θ1 to θ2. After water evaporation, the contact angle decreases to θ1, producing a decrease in the thickness of each globule. The now thinner globule is unstable and tends to dewet into smaller droplets similar to the original ones.

Figure 7. LSCFM images of a fresh triolein film on quartz, before (top) and after (bottom) heating at 90 °C (well below the boiling point of triolein, Tb ) 235 °C) for 1 h (scale bar 250 µm). After heating, the droplets become noticeably rounder and more regular, with their average elongation decreasing from 1.2 ( 0.3 to 1.1 ( 0.1 µm.

maxima, both at lower and higher diameters than the main maximum, the latter corresponding to aggregates of droplets. It is curious that there is no major change in

the average diameter value before and after coating the triolein film with the latex. To determine the dimensions, particularly the height, of individual oil droplets, we carried out measurements with the high resolution 100× objective. These highmagnification depth scans of individual oil droplets under the latex film were carried out in the same way as described above for uncoated triolein films. The measurements reveal that the heights of the droplets increased substantially upon coating the oil with the latex polymer film. Since the average diameter of the individual droplets does not change substantially, we learned that the oil droplets increase their average volume, and consequently, the grease film exhibits a lower droplet number density (compare Figures 2 and 8). To obtain better statistics on the droplet volume, and to quantify the total amount of oil remaining in the 4-mm2 area in the center of the substrate, we reanalyzed images taken with the low-magnification objective. Here, we used the image analysis software associated with the microscope to measure the distribution of droplet areas in the images. At low resolution, we could not measure directly the heights of individual droplets. For this analysis, we assumed that all the droplets in each of the lowmagnification images have the same micro-contact angle. Then, the height of each droplet, h, can be calculated as h ) R tan (θ/2), where the droplet radius R at the substrate surface is calculated from the area as described above. From geometrical arguments, the volume of each droplet can be calculated as V ) h2(3r - h)/3, where r is the sphere radius given by r ) (R2 + h2)/(2h). Within these approximations, we found that the sum of all droplet volumes in each image is approximately equal to (1.5 ( 0.4) × 105 µm3. Therefore, although the droplet number density in images of the latex-film-coated oil films is smaller by about 50% than the number density in images of oil droplets on

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Figure 9. LSCFM images of the edge of triolein films at the quartz/air (top) and at the quartz/PBMA (bottom) interfaces (scale bar 250 µm). We observe the formation of bigger droplets at the edge of the oil films, but this accumulation at the edges represent less than about 1% of the total amount of triolein. Figure 8. LSCFM image (top) of a triolein film deposited on a quartz substrate surface (same sample shown in Figure 2) coated with a PBMA latex film (scale bar 250 µm). In the dry film, approximately all of the triolein remains attached to the quartz surface. A small portion of the triolein moved to the border of the latex film, and a tiny fraction of droplets floated to the top of the latex film, but no oil can be detected within the latex film. In the center region of the quartz plate, the droplets rearrange to recover a diameter distribution similar to that of the oil film at the air/quartz interface. The histogram of droplet diameters D for the same image (bottom) show certain features common to all the coated films examined: the maximum in the size distribution is shifted to higher diameter values than that for the uncoated grease film, and there are secondary maxima.

quartz substrates exposed to air, the amount of triolein is very similar in the two types of images. Since the area covered by the images in Figures 2 and 8 is 2 × 105 µm2 and the calculated total volume of triolein is (1.5 ( 0.4) × 105 µm3, the average thickness of the oil film is approximately (0.8 ( 0.2) µm for both images. This value is only slightly smaller than the 1-µm thickness calculated from the mass balance. The small difference can be interpreted as a consequence of the accumulation of bigger droplets at the edge of the oil films, shown in Figure 9, both for PBMA coated and uncoated triolein films on quartz. From confocal depth sectioning of the coated oil film, we determined that the droplets maintain their approximate spherical shape upon coating with the polymer. This is also apparent in the linear relation obtained between the droplet height h and its radius R at the substrate (Figure 4). Martin et al.29 examined the case of liquid droplets squeezed between a solid substrate and a slab of rubber and observed that the droplets are deformed into a flat ellipse shape, for which R ∝ h2. This distortion of the droplet profile is described as being caused by the competition between the surface energy gain of establishing rubber/substrate contacts and the elastic energy loss for the deformation of the rubber to accommodate the

Table 1. Micro-contact Angle 〈θ〉 of Triolein Droplets on Quartz and PET before and after Coating with a PBMA Latex Film micro-contact angle 〈θ〉 oil on quartz

fresh film coated with latex film coated and annealed 1 hour at 90 °C

(32 ( 5)° (71 ( 4)° (80 ( 3)°

oil on PET

fresh film coated with latex film

(8 ( 4)° (18 ( 5)°

droplets. In our case, the polymer film is formed in situ around the droplets and, therefore, there is no driving force for droplet deformation. Using eq 1, we calculate that the average micro-contact angle of triolein increases from 〈θ〉 ) (32 ( 5)° at the quartz/ air interface to 〈θ〉 ) (71 ( 4)° at the quartz/PBMA interface (Table 1). In Figure 10, we show a cartoon illustrating the evolution of the system when the oil droplets are coated with a latex dispersion. The increase in the oil contact angle can be interpreted in terms of a higher affinity of the triolein for the latex polymer compared to the quartz, due to the hydrophobic character of the PBMA. The contact angle of the oil at the quartz/PBMA interface is not large enough for all the droplets to detach from the quartz surface. Complete detachment would occur only for θ > 90°. If such a large contact angle were achieved while the latex dispersion were still fluid, the oil droplets would float to the surface of the film due to their lower density. However, a small amount of droplets indeed float to the PBMA film surface. In Figure 11, we show two images taken in exactly the same position, one at the quartz/ PBMA interface and the other 30 µm above, at the PBMA/ air interface. High magnification confocal depth scans of the droplets on top of the PBMA film show that, once at the surface, these droplets do not wet the PBMA but instead maintain their spherical-cap shape, presenting a micro-contact angle of 〈θ〉 ) (31 ( 3)°. Upon heating of the

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Figure 10. When oil droplets in the center of a quartz plate are covered with a latex dispersion and the water is allowed to evaporate, a transparent polymer film forms around the droplets. Light scattering impedes the imaging of the oil droplets while the dispersion is still wet. After the dispersion dries, confocal depth scans through the polymer film reveal that the volume of the droplets increases substantially. Using eq 1, we calculate that the average micro-contact angle increases from 〈θ〉 ) (32 ( 5)° at the quartz/air interface to 〈θ〉 ) (71 ( 4)° at the quartz/PBMA interface.

Figure 11. LSCFM images of a triolein film on quartz, one at the quartz/PBMA interface (top) and the other 30 µm above, at the PBMA/air interface (bottom). The images were taken in exactly the same position (scale bar 250 µm). From high magnification confocal depth scans of the oil droplets on top of the PBMA film, we calculate a micro-contact angle of 〈θ〉 ) (31 ( 3)°. Upon heating of the sample at 90 °C, these droplets seem to spread over the polymer film to form thinner oil patches.

sample at 90 °C, these droplets seem to wet the PBMA and spread over the polymer film to form thinner oil patches. An interesting question is whether the 〈θ〉 ) (71 ( 4)° micro-contact angle measured for the triolein droplets under the PBMA latex film represents an equilibrium shape or a shape determined at some intermediate point

in the drying of the latex film. If the drying process affects this contact angle, then we could expect a change in contact angle if the dried films were heated above the polymer glass transition temperature. Under these conditions, the polymer in the coating becomes mobile, and the oil droplets would be free to relax the constraints imposed during the drying process and evolve toward an equilibrium configuration. We heated the polymer-coated films to a temperature of 90 °C, well below the boiling point of triolein, Tb ) 235 °C, but above the glass transition temperature of the PBMA (ca. 30 °C for the sample used). The polymer was maintained in this liquidlike state for 1 h and then rapidly cooled back to room temperature in order to freeze the oil droplet shape and size distributions. In Figure 12, we show the same PBMA-coated oil film before and after annealing at 90 °C for 1 h. Although the images do not show exactly the same location in the film, both images are representative of the whole oil film. First, we can observe that the number of smaller droplets that could be seen on the nonannealed film decreased after annealing. This effect was also observed to some extent for the uncoated oil films (Figure 7). It can be interpreted in terms of a ripening-type process, leading to the formation of bigger and more stable droplets. The histogram of the droplet diameters shows almost no change after the sample was annealed. From high magnification confocal depth scanning, we determined that upon annealing there is only a slight increase in the droplet microcontact angle, from 〈θ〉 ) (71 ( 4)° to 〈θ〉 ) (80 ( 3)° (Table 1). The minor changes observed after annealing of the latexcoated oil films give us some indication that the droplet shape and size distribution is not extensively constrained during the drying process. If that is the case, the relaxing of such constraints during annealing of the polymer film would lead to major changes in the droplet shape and size distribution. Therefore, upon coating of the oil film with the latex dispersion, the oil droplets have time to readjust to a near-equilibrium shape before the dispersion dries and the droplets are frozen under the solid polymer film. Oil Films on PET. Triolein films doped with HY and cast from a diluted acetone solution onto PET substrates

Oil Droplets Under A Polymer Film

Figure 12. LSCFM images of a triolein film on quartz, before (top) and after (bottom) annealing at 90 °C for 1 h (scale bar 250 µm). Smaller droplets can be observed on the film before annealing. The number of these droplets decreased after annealing, but there is otherwise no major change on the droplet diameters after the sample was annealed.

spread rapidly with solvent evaporation. Unlike the films cast onto quartz, triolein wets the PET substrate, leaving an almost homogeneous thin film. Although these films contain some thicker patches (Figure 13a), these are sparse and probably more due to irregularities in the substrate than to dewetting of the oil. Measurements of the microcontact angle of the triolein in PET by LSCFM were only possible for the thicker patches. The main difficulty in this determination was the irregular shape of these patches compared to the oil droplets formed on quartz. We chose the rounder patches and, based on the assumption of spherical-cap shaped droplets, we calculated an approximate value of 〈θ〉 ) (8 ( 4)° for the micro-contact angle (Table 1). This gives an indication of the contact angle for triolein in PET, and indeed this value is close to the macroscopic contact angle value θ ) 5° measured for millimeter-sized droplets of triolein immersed in water on the same PET substrate.28 When a few drops of the PBMA dispersion were placed over the oil film on PET, the dispersion dried to form a solid and transparent polymer film. After the film dried, all the oil remained attached to the PET substrate (Figure 13b). There was, however, some coarsening of the oil film pattern that seems to indicate that a small amount of dewetting occurs upon coating the oil film with PBMA. High-magnification depth-scan profiling of the coated oil film shows that the pattern consists of approximately round shaped droplets. Again, assuming a spherical cap shape, the approximate micro-contact angle for these objects is 〈θ〉 ) (18 ( 5)° (Table 1). The increase in contact angle shows that some dewetting indeed occurs after

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Figure 13. LSCFM images of a triolein film on PET, before (top) and after (bottom) coating with a PBMA latex film (scale bar 250 µm). Triolein wets the PET substrate, leaving an almost homogeneous thin film containing some thicker patches (darker spots on top image). After coating with the latex, all the oil remains attached to the PET substrate (bottom), but there is a small amount of dewetting of the triolein from the PET.

coating. This dewetting is probably due to the hydrophobic character of the PBMA film, which increases as the dispersion dries and becomes more enriched in polymer. The system probably gains energy by creating contacts between the PET and the PBMA, squeezing the intercalated oil film. The films become unstable and dewet by nucleation and growth of dry patches.29 When the oil droplet environment is changed from the PET/air interface to the PET/PBMA interface formed upon drying of the dispersion, the increase in the micro-contact angle is not as large as that for coating oil films on quartz. Conclusions The possibility of imaging micrometer-sized droplets under a solid but transparent polymer film by LSCFM opens many opportunities for the study of wetting and dewetting phenomena related to coatings. The LSCFM technique has particularly advantageous features, since it allows one to image a fluorescently dyed material under a film transparent to the wavelengths used. In the present work, we applied this technique to study the wetting behavior of thin oil films on quartz and PET substrates. The oil films were then coated with a polymer colloid dispersion. The dispersion was dried to form a solid polymer film, and then the oil was imaged again. We used triolein as the oil, doped with a trace of a fluorescent dye (HY), and prepared the films by casting a dilute solution of the doped triolein in acetone onto quartz

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and PET substrates. The films were then imaged by laser scanning confocal fluorescence microscopy (LSCFM). On quartz, triolein breaks up into approximately spherical, micrometer-sized droplets as the acetone evaporates. After being exposed to water or heated at 90 °C, the ensemble of oil droplets recovers its initial shape and size distribution. When the oil droplets were coated with a dispersion of a film-forming latex [poly(butyl methacrylate) (PBMA)], most of the droplets adhere to the quartz substrate as the water evaporates. With LSCFM, we can image the triolein droplets under the solid polymer latex film. These droplets maintain their spherical shape but have larger volumes than those formed in air. Their micro-contact angle increases from 〈θ〉 ) (32 ( 5)° at the quartz-air interface to 〈θ〉 ) (71 ( 4)° at the quartz-PBMA interface. The oil droplets are not flattened by the polymer film because this film is formed in situ around the droplets. As a consequence, there is no loss of elastic energy to deform

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the polymer film in order to accommodate the droplets. Annealing of the coated films at 90 °C does not produce major changes in the oil film. On the other hand, when a triolein film is spread onto a PET substrate from a diluted acetone solution, the oil wets the PET substrate and forms an almost homogeneous film. Upon coating with the PBMA latex, we observed some dewetting of the oil from the PET substrate. The approximate micro-contact angle of the triolein oil increases from 〈θ〉 ) (8 ( 4)° at the PET-air interface to 〈θ〉 ) (18 ( 5)° at the PET-PBMA interface. Acknowledgment. The authors thank ICI, ICI Canada, and NSERC Canada for the support of this research and Clariant Canada Inc. for graciously supplying the dye Hostasol Yellow. J. P. S. Farinha acknowledges the support of FCT-PRAXIS XXI. LA990710M