1002
Langmuir 1990,6, 1002-1007
Dynamic Contact Angle Measurements on Glass Fibers: Influence of Fiber Diameter on Hysteresis and Contact Line Pinning Bryan B. Sauer*$+and Thomas E. Carney E . I. du Pont de Nemours and Company, Inc., Central Research and Development and Fibers Department, Experimental Station, Wilmington, Delaware 19898 Received October 5, 1989. In Final Form: December 26, 1989
The microstructure of glass fiber surfaces treated with silane and fluorocarbon monolayers has been investigated by use of dynamic contact angle analysis. Advancing and receding contact angles on single glass fibers as small as 10 pm in diameter were measured at low meniscus velocities by using the Wilhelmy method. A unique aspect of this work is that the dynamic wetting of fibers, in the limit of small diameters, takes the form of contact line pinning. This is induced by surface chemical inhomogeneities with domain sizes comparable to the fiber diameter, giving direct evidence regarding the surface heterogeneity of coated fibers. Several fiber surface treatments were investigated including a silane, a fluorochemical surfactant, and oxidative cleaning agents. Clean glass fibers of varying diameters were prepared from molten glass. From the fiber diameter dependence of contact line pinning, we determined that monolayers of octadecyltrichlorosilane (OTS) adsorb in patches with domains on the order of microns. To confirm our analysis, larger diameter fibers were studied, leading to a suppression of contact line pinning due to an averaging effect over the larger perimeter, although the classical contact angle hysteresis was still present. The results for OTS treated fibers are direct experimental evidence of the free energy barriers predicted to be the thermodynamic basis of contact angle hysteresis caused by surface chemical inhomogeneity. Analysis of the domain sizes and other surface features in terms of wettability should be possible for a wide variety of fiber coatings and surface treatments using this method.
Introduction Interfacial strength and stability of the solid/polymer interphase govern the properties and performance of fiberreinforced polymer composites.lY2 A wide variety of materials including silanes are used to improve interfacial properties by promoting fiber interaction with the matrix, either by chemical bonding, improved wettability, or resistance to water. The focus of this report is in part to develop methods to study the fiber surface directly. Surface energetic methods lead to both chemical and thermodynamic characterization of surfaces while elemental analysis techniques such as XPS only give chemical information. The former technique is directly relevant in practical situations in terms of wetting and adhesion. One method of evaluating fiber surface wettability with a high degree of precision is based on the Wilhelmy techn i q ~ e ,where ~ ? ~ the contact angle is determined directly from the wetting force for the stationary mode or advancing and receding modes. Contact angle hysteresis (defined by the difference between advancing and receding contact angles) occurs because of metastable states which are probed as the liquid front advances or recedes over surface roughness or surface chemical inhomogeneities with which the liquid makes different intrinsic contact angle^.^-^ If hysteresis Research and Development. (1)Wu, S. Polymer Interface and Adhesion; Marcel Dekker: New York, 1982. (2) Plueddemann, E. P. Silane Coupling Agents; Plenum Press: New York, 1982. (3) Wilhelmy, L. Ann. Phys. 1863, 119, 177. (4) Johnson, R. E.; Dettre, R. H. In Surface and Colloid Science; Matijevic, E., Ed.; Wiley Interscience: New York, 1969; Vol. 2, p 85. (5) Good, R. J. J . Am. Chem. SOC.1952, 74, 5041. (6) Johnson, R. E.; Dettre, R. H.; Brandreth, D. A. J. Colloid Interface Sci. 1977, 62, 205. + Central
0743-7463/90/2406-1002$02.50/0
is neglected, not only is valuable information discarded, but for the majority of the systems the “equilibrium” contact angle state is impossible to attain because of energy barriers between metastable state^.^^^ Thus static contact angles can be quite misleading since equilibrium is never reached. A common procedure invoked to estimate the surface free energy of the solid is to use the advancing contact angle in Young’s equation.’ Only values for the low-energy component of the surface are determined with this scheme since the advancing contact angle is only representative of the low-energy material. To fully characterize the material, the advancing and receding contact angles must be measured.6pB-lO Aside from the technical interest of developing methods to evaluate the fibers, the fiber surface is also readily available for a wide variety of surface chemical studies. Although contact angle hysteresis is drastically affected by small meniscus dimensions, fibers with diameters on the order of 10-100 pm are flat on the scale of molecular adsorption at the surface. Since the meniscus height for a fiber advancing vertically into a liquid is roughly proportional to the fiber diameter for small fibers,11J2 a perturbation of the meniscus height by only a few microns leads to a significant change in contact angle. For a moving contact line, perturbation of the meniscus height can take the form of contact line pinning, which occurs as the liquid front advances over surface roughness or sur(7) Penn, L. S.; Miller, B. J. J . Colloid Interface Sci. 1980, 78, 238. (8) Good, R. J.; Koo, M. N. J. Colloid Interface Sci. 1979, 71, 283. (9) Neumann, A. W.; Good, R. J. J. Colloid Interface Sci. 1972, 38, 341. (IO)Cain, J. B.; Francis, D. W.; Venter, R. D.; Neumann, A. W. J. Colloid Interface Sci. 1983,94, 123. (11)Derjaguin, B. C. R. (Dokl.)Acad. Sci. URSS 1946,51, 519. (12) Huh, C.; Scriven, L. E. J. Colloid Interface Sci. 1969, 30, 323.
0 1990 American Chemical Society
Langmuir, Vol. 6, No. 5, 1990 1003
Dynamic Contact Angle Measurements on Glass Fibers face chemical inhomogeneitie~.l~-~ For the surface roughness case, materials with well-defined periodic surface roughnesses have been prepared, and the results are readily explained by theory.13-15J9 To our knowledge, the analogues of these experiments in terms of reversible contact line pinning of fiber surfaces with intrinsic surface chemical inhomogeneities have not been reported. Instead of attempting to design surfaces with periodic chemical inhomogeneities, we found that reduction of the fiber diameter to the dimensions of the surface chemical inhomogeneities was more feasible. Since contact angle hysteresis on planar surfaces is attributed to surface chemical inhomogeneities, the wide variety of systems which exhibit contact angle hysteresis should give the desired behavior in terms of contact line pinning with small diameter fibers. Surface treatment with OTS is known to give a chemically adsorbed, highly oriented monolayel.2lIn which is stable under a variety of conditions. Applying OTS to fibers resulted in the desired contact line pinning, so this treatment was applied to a range of fiber diameters. Dynamic Wilhelmy contact angle measurements of silanes on glass plates have been but no contact line pinning was found because of the large perimeter of the contact line. In principle, both intrinsic surface inhomogeneities and surfactant adsorption could lead to contact line pinning. It is important to make the distinction between the two because the former is reversible to the extent that no desposition of surface-active material occurs on the surface. This is taken with the qualification that vapor adsorption of the test liquid is not considered as an impurity, for it is well-known that high-energy surfaces such as glass and metals are readily modified by water vapors and other organic vapors in terms of surface energ e t i c ~ .Contact ~~ line pinning of an irreversible nature occurs when surfactants or impurities adsorb a t the contact line.13Js-z0~z5Surfactant deposition a t the contact line leads to contact line pinning for substrates of any dimension, making this the prevalent case observed experimentally. If the solid surface is higher energy than the depositing surfactant, then as the contact line advances it will be pinned periodically when it comes to regions of lower surface energy. Once it advances past the deposited surfactant to a fresh surface region, the deposition cycle repeats itself. Thus the surface is modified, making the process irreversible. Only reversible contact line pinning is addressed in this report. Our first evidence that fibers were drastically different than larger substrates was obtained when studying commercial fibers. It was found that for the advancing (13) Bayramili, E.; van de Ven, T. G. M.; Mason, S. G. Colloids Surf. 1981. 3. 131. (14) Bayramili, E.; van de Ven, T. G. M.; Mason, S. G. Can. J. Chem. 1981,59,1954. (15) Bayramili, E.; Mason, S. G. Can. J . Chem. 1981,59, 1962. (16) Kamath, Y. K.; Dansizer, C. J.; Weigmann, H.-D. J.Appl. Polym. Sci. 1984, 29, 1011. (17) de Gennes, P.-G. Rev. Mod. Phys. 1985,57, 827. (18) Cohen Stuart, M. A.; Cazabat, A. M. h o g . Colloid Polym. Sci. 1987, 74, 64. (19) Cazabat, A. M.; Cohen Stuart, M. A. Prog. Colloid Polym. Sci. 1987, 74, 69. (20) Princen, H. M.; Cazabat, A. M.; Cohen Stuart, M. A.; Heslot, F.; Nicolet, S. J . Colloid Interface Sci. 1988, 126, 84. (21) Cohen, S. R.; Naaman, R.; Sagiv, J. J. Phys. Chem. 1986, 90, 3054. (22) Gun, J.; Sagiv, J. J. Colloid Interface Sci. 1986, 112, 457. (23) Park, J. M.; Andrade, J. D. In Polymer Surface Dynamics;Andrade, J. D., Ed.; Plenum: New York, 1988, p 67. (24) Zisman, W. A. In Adhesion Science and Technology; Lee, L.-H., Ed.; Plenum: New York, 1975; p 55. (25) Peng, J. B.; Dutta, P.; Ketterson, J. B. Thin Solid Films 1988, 159, 215.
contact angle for fibers such as aminopropylsilanetreated Pittsburgh Paint and Glass-3540 E-glass fibers (nominal diameter 10 pm) a t typical speeds (5-100 pm/s), a high-amplitude noise in the measured force was recorded which interferes with the measurement. I t was found that when the speed was lowered down to a fraction of a micron per second, the "noise" actually was quite periodic and had the form of contact line pinning reported previously.20 Since the PPG fibers are available in only one diameter and the specific surface treatment is not known, we proceeded to coat freshly made glass fibers in our lab with a well-characterized monolayer such as OTS.= OTS-treated fibers gave the desired contact line pinning similar to that found for the PPG fibers due to surface chemical heterogeneities on the small diameter fibers. The premise of this work was that by studying a range of diameters we could suppress the contact line pinning a t sufficiently large diameters as the effect of surface chemical heterogeneities was averaged out. It was evident from the start that we could obtain this goal because contact line pinning is absent for silane-treated glass plates6pZ3 due to the large perimeters involved, even in cases of large hysteresis.
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Experimental Section Materials. Fibers of different diameters were prepared by drawing molten glass at different rates after heating with an oxygen/methane torch. The glass used was E-glass (Si02, CaO, A1203, BzO3; 54%,22%, 16%, 7%,respectively,and other minor components less than l % ) Z 7 or borosilicate (Corning 2947 slides composed at 0, Si, C, B; 63%, 28%, 4.4%, and 2.3%, respectively, with other minor componentsless than 1%).23 For adsorption studies on glass plates, the slides were 2.52 em wide and 0.1 cm thick and were usually cut into pieces about 3 cm long. They were cleaned by using the method of Maoz and Sagivzs by sonication for 10 min in a hot 10% Micro detergent solution, 3 min in a hot 10% NaOH solution, 1min in 1%HC1, and 10 min in hot deionized water with additional rinses in deionized water between each step. Glass fibers pretreated with aminopropylsilanes were supplied by Pittsburgh Paint and Glass (PPG-3540,nominal 10-rmdiameter E-glass fibers). HPLC grade distilled water (Baker Chemical Co.) was used for contact angle measurements and gave the same results as double-distilled, deionized water. The surface tension was 72.0 0.05 dyn/cm (25 OC) measured by the Wilhelmy method with fibers of known diameter. All glass and Teflon containers were cleaned in Nochromix (Godax Labs, NY) solutions in sulfuric acid. Stock solutions of octadecyltrichlorosilane (OTS; CH3(CH2)17SiC13,95%, Aldrich) were made in CCl4 (EM Science, HPLC grade, glass distilled). The OTS solution was made by adding the CCl4/OTS solution to hexadecane (Aldrich, 99%) and CHC13 (EM Science, HPLC grade, glass distilled with 1% ethanol added) to give 80% hexadecane, 12% CCl4, and 8% CHC13.26 Sometimes dicyclohexyl (Aldrich, C&IC&~) was substituted for hexadecane. The surface tension of hexadecane was 27.0 f 0.1 dyn/cm (25 "C). The OTS solution was stored in a dry environment and discarded after 2 days. All reported fiber experiments with OTS coatings were done at relative humidity below 20-30% at 25 "C. At higher R h , the contact line pinning was not as reproducible,although contact angle hysteresis would still occur. Nonadecafluorodecanoic acid (CF~(CF~)SCOOH) was supplied by Aldrich. Methods. Advancing and receding contact angles were determined by the Wilhelmy method.4 Figure 1 is a schematic of the instrument. A single fiber was attached to a wire hook by using a cyanoacrylate adhesive and suspended on a Cahn 2000
*
(26) Sagiv, J. J. Am. Chem. SOC.1980, 102,92. (27) Rubio, J.; Oteo, J. L J. Mater. Sci. Lett. 1989,8, 119. (28) Maoz, R.; Sagiv, J. J. Colloid Interface Sci. 1984,100,465.
e
Sauer and Carney
1004 Langmuir, Vol. 6, No. 5, 1990
Lab Jack
Table 11. Contact Angle Hysteresis of OTS-Treated Fibers in Water 8,, velocity, d,rrma d,rma 8.aV,b.‘ , , e &, (Hs0) (hexadecane) deg deg deg deg pm/s
Cahn 2000 Mino-Stila~~
la
33.4
45.9 130
33.2 45.9
97
96
63 58
99 88
86 79
0.41 0.33 0.37
96 64 96 93 Borosilicate Glass 52.9 52.9 100 84 a Thicknesses were determined by using the Wilhelmy method assuming zero contact angle hefore OTS treatment. Only freshly prepared fibers were used. *The values of 8., were obtained after prewetting the fibers, which tended to suppress contact line pinning (see Figure 4). e Because of contact line pinning, the precision of contact angle measurements was reduced to roughly +Z0 for a given fiber.
Figure 1. Dynamic contact angle instrument. Table I. Comparison of Fiber Diameters (in wm) Determined by the Wilhelmy Method and Optical Microscopy oDtic.4 microacoDe Wilhelmv (hexadecane) Wilhelmv.~ (H20) 42i.2 41.3 0.2 40.3 0.2 90*4 91.4 i 0.4 90.4 i 0.4 electrobalance having a sensitivity of 0.1 pg. The coarse position of the balance head is controlled by a lab jack, and the fine position is controlled by raising and lowering the test liquid with a Inchworm piezoelectric positioner (Burleigh Instrumenta, Fishers, NY 14453) which is driven by a Model 6200 Burleigh controller interfaced with a personal computer (PC) by use of a Burleigh 6200 Inchworm interface card. The position is also monitored by the PC. A Lucite box resting on a rigid balance table surrounds the equipment to reduce air currents and provide environmental control. Sometimes a thin sheet of soft rubber was placed on the positioner platform in an attempt to minimize transmitted vibrations from the positioner. At the slow speeds used in this study, this did not have any noticeable effect, and the noise level was the same with and without the platform in motion. The Cahn balance output is also recorded on the PC with a 12-bitanalog-to-digital converter hoard. For convenience, both the position readout and Cahn output were sent to a chart recorder. To obtain the surface tension by use of the Wilhelmy method, the mass is recorded before the fiber contacts the liquid. The liquid is then raised by the piezoelectric positioner and the mass recorded as the fiber is immersed, giving a change in force A F related to the contact angle by AF = gAm = Py cos 8-pgah (1) where g is the gravitational constant, P the perimeter of the fiber, y the air/liquid surface tension, 8 the contact angle, p the density of the liquid, a the fiber cross sectional area, and h the depth of immersion. The second term on the right is the buoyancy correction and is negligible for most fibers studied here (Le., the buoyancy correction is only 0.2% for a 50-pm fiber immersed to h = 1mm). For surface-modified fibers with nonzero contact angles, advancing and receding contact angles at rates from 0.005 to 5000 pmls can be measured once the fiber perimeter is determined.
Results Hydrophilic Glass Fibers. Diameters of freshly prepared glass fibers were determined by the Wilhelmy method using zero contact angle liquids such as water and hexadecane (HD) and are recorded in Table I along with optical microscope measurements. Several other
126
thickness determinations using the Wilhelmy method in water and HD are reported in Table 11. The agreement with microscope determinations and between the two test liquids indicates that the Wilhelmy method can be used for rapid perimeter determinations and is accurate to a fraction of a percent. There was no variation in force upon advancing or receding along the length of the fiber in water or HD within 0.1% for freshly prepared fibers a t relative humidities Rh 2&40%. Consistent with this observation, freshly prepared fibers with diameters less than 100 pm are smooth to better than 0.1 pm in the scanning electron microscope. After -8 h of exposure of the glass fibers to air, noise was found in the advancing force due to contamination of the fiber surface. After treating this same fiber in a low-power air plasma for 2 min, the advancing and receding force was constant, the same as that for freshly prepared fibers. The absence of contact angle hysteresis for freshly prepared glass fibers led us to investigate possible effects of glass cleaning treatments to determine whether zero contact angle is obtained after applying chemical treatments. Fibers were treated with strong oxidizers such as Nochromix-surfuric acid and chromosulfuric acid mixtures. It was found that these agents generally introdhced some contact angle hysteresis and non-zero contact angles as had been reported p r e v i o ~ s l ydue ~ ~ to a change in the water structure or some other surface mcdification effects. Lelah and MarmurZ9 also found that heating close to the melting point was an important factor contributing to zero contact angle which explains why the fibers prepared from molten glass in this study resulted in zero contact angle. We pursued the effect of these cleaning agents in order to differentiate between hysteresis due to impurities and those due to inherent surface structure. Recent work has shown that hysteresis, including stick-slip phenomena or “Haines” jumps, are caused by surfactants or impurities adsorbing ahead of the advancing contact line. This causes the contact line to be periodically pinned,13.14Jg20 resulting in a variation in contact angle (or force) with immersion distance. The stick-slip can take two forms:2o (1)a periodic sawtooth behavior, which will he discussed in the next section in the context of silane treatments, or (2) more complicated traces. Princen et aLm have suggested that the cleaning procedure may contribute to the
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(29) b l a h , M. D.; Marmur, A. Am. Ceram. Sm.Bull. 1979,.?8,1121.
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Langmuir, Vol. 6, No. 5, 1990 1005
Dynamic Contact Angle Measurements on Glass Fibers A
1.0
c
jj 0.8 0.6
I
B
adv. 2.1 pm/s
I
C
adv. 0.4 wm/s
adv. 1.2 pm/s
D rev.
-
+
02
I
I
5
10
t pin)
Figure 2. Contact line pinning as a chromosulfuric acid treated glass fiber is advanced into hexadecane at different meniscus velocities. See text for discussion. more complicated type of stick-slip behavior and found that upon advancing on a glass plate in HD, stick-slip with an "overshoot" occurred. It was found that this stickslip behavior in HD could be reproduced after treatment in chromosulfuric acid for 1-2 min and rinsing in deionized water. Figure 2 is a representative trace of cos 6adv vs time. The meniscus velocity was changed to the indicated values a t points A, B, and C in the figure. A t point D, the direction is reversed giving Or,, = 0' i 5' where the errors reported here are an estimate of the precision for a single fiber. For the velocities shown here, the amplitude of the stick-slip in terms of the change in contact angle is O1 = 13' f 5' to 62 = 37' f 3'. The decrease in cos Oadv in the traces in Figure 2 is due to partial sticking of the meniscus as the fiber advances. A small bump occurs at a distance about halfway through the cycle due to the "footprint" where the meniscus first overshot and left a partial film of adsorbed molecules.20 As the fiber continues to advance, cos 6adv remains relatively constant until the contact line slips again as indicated by the sharp increase in cos and the cycle starts over. All these results indicate that the stick-slip is in part due to the chromosulfuric treatment causing a structural change in the glass surface or some other contamination effect. Because more experiments are needed to understand these phenomena fully, all experiments reported in the next sections are for freshly prepared glass fibers avoiding the problems associated with cleaning. Kinetics of OTS Adsorption. Because the force is monitored directly, continuous real-time contact angle determinations are easily made. Under proper conditions, measurement of the rate of OTS adsorption is possible as the surface becomes autophobic and repels the OTS solution. The increasing autophobicity leads to an increase in contact angle as the meniscus recedes even though the fiber is stationary. This effect was not detected on glass plates because the contact line is not sufficiently homogeneous over the entire length of the plate, and the meniscus also has to travel a much larger distance (meniscus height on a plate in water is 3.8 mm30) as 0 changes. The meniscus heights on glass fibers studied here are extremely small and are comparable to the fiber circumference.12 Figure 3 is a representative trace for a fiber which is advanced rapidly into 0.028 M OTS in HD/CCl4/CHCla to a clean area of a glass fiber and then stopped (represented by point A). cos 0 is equal to 1.0 corresponding ' within experimental error a t point A, before to 0 = 0 exponentially decreasing to 0.87 (0 = 29") with time as the silane adsorbs, making the surface a u t ~ p h o b i c .This ~~ later value is identical with Orec measured in HD independently. From point B to C in Figure 3, the fiber is receding a t -10 ym/s, and no change is seen in 0 as (30)Adamson, A. W.Physical Chemistry of Surfaces, 4th ed.;Wiley: New York, 1982;p 27. (31)Bigelow, W. C.;Pickett, D. L.; Zisman, W. A. J. Colloid Sci. 1946, I, 513.
0
1
2
3
4
5
t(min)
Figure 3. Change in cos 8 as OTS adsorbs on a glass fiber as of function of time of immersion. See text for discussion. Table 111. Kinetics of OTS Adsorption on Freshly Prepared Glass Fibers d, Gm 23.4 33.4 47.2 94.0 borosilicate glass 52.9 fiber borosilicate glass slide
E-glass fibers
T, s range of 0,a dea 25 f 5 0-03 f 1 30f5 0-29 25f 5 0-31 f 2 45f 15 0-32 30 f 10 0-30
29 f
eadv,b
deg
41 42 40* 1
0 The contact angle varies from 8 = Oo to 8 =, ,8 with time as the silanation proceeds and the contact angle recedes. The errors are standard deviations for different fibers. b These values are the advancing contact angles in hexadecane after OTS surface modification. Only OreC is given, because no reproducible change in contact angle was observed on the slides during OTS adsorption.
expected since the meniscus had already receded due to the autophobicity effect. At C, the direction is reversed, and the contact line advances over surface which has already been silanated, giving 6adv = 42' f 2'. Finally, a t point D Ore= is measured again, giving the same results as the values between B and C. The values of 6adv and Or= measured in this OTS/HD/CCL/CHClS mixture after OTS treatment are the same as those for the silanated fiber in pure HD, indicating that the surface tension of the silane mixture is the same as that of pure HD. The exponential time constants were roughly calculated as the time for cos 0 to drop to l / e (where e = 2.718) of the initial value giving 7 = 25 s with a standard deviation of f 5 for the different samples. The values were independent of fiber diameter and are summarized in Table 111. Contact angles for silanated glass slides in HD (Table 111) were found to be the same as those determined for the fibers. Values of 6adv approaching 46" have been reported for OTS monolayers,22 but recent results have shown that even monolayers which exhibit these high contact angles are i n ~ o m p l e t e . ~ ~ ~ ~ ~ Fluorinated Glass Fibers. Contact angles for fluorodecanoic acid treated fibers were measured to test the assumption that adsorbed monolayers on fibers have an equivalent microstructure to those on planar surfaces. Freshly prepared E-glass fibers were dipped into saturated solutions of fluorodecanoic acid in HD for 1-2 min. The contact angles measured in HD for several fibers with diameters of approximately 50 ym were 6adv = 84.5' (32)Garoff, S.;Hall, R. B.; Deckman, H. W.; Alvarez, M. S. R o c . Electrochem. SOC.1985,85,112. (33)Sabatani, E.; Rubinstein, I.; Maoz, R.; Sagiv, J. J.Electroanal. Chem. 1987,219,365.
1006 Langmuir, Vol. 6, No. 5, 1990
Sauer and Carney
Figure 4. Advancing and receding contact angles for OTStreated glass fiber in water. See text for discussion. f 2 O and 8rec = 78.5' f 2 O with no stick-slip. The error is the standard deviation between different fibers. These are in agreement with previous resultse for the same material on glass slides. Unlike the silanation process described above, the kinetics were too fast to be monitored for these saturated fluorodecanoic acid solutions. Also, the surface tension of the saturated solution is not the same as that of pure HD due to adsorption of fluorodecanoic acid a t the air/HD interface. Prerinsing the fibers in water before fluorodecanoic acid treatment did not change the results. The monolayers dissolved upon contact with water, so no contact angles in water are reported. OTS-TreatedGlass Slides. As discussed above, contact angles of OTS-treated fibers in HD indicated inhomogeneous coverage of the surface by OTS. This was pursued further by comparing the Wilhelmy and sessile drop techniques on OTS-treated glass slides. The slides were cleaned by using the method of Maoz and Sagiv28 and silanated in 0.028M OTS in HD/CC14/CHCls for 5 min while stirring. Under these conditions, the slides were dry when removed from OTS solution, and the same results were obtained if bicyclohexyl was substituted for HD. Contact angles determined by the sessile drop technique were eadv = 113' f 2' indicative of a surface of close packed methylene groups. When 8,,, was measured, it could be seen that the drop receded in jerks but settled to 112' f 4' the majority of the time. By use of the Wilhelmy method for velocities in the range 0.5-20 pm/s, 8adv = 113' f 1' and drec = 107' f 2' where the standard deviation is for differents plates. These results are consistent with those of Garoff et al.,s2 who reported approximately 6' hysteresis at high coverage. The hysteresis is attributed to an inhomogeneous monolayer, and the difficulties in obtaining an "equilibrium" state with the sessile drop method are not encountered with the Wilhelmy method. OTS-Treated Glass Fibers. Fibers were made with diameters ranging from 10 to 130 pm. Before OTS treatment, the diameter of the fiber was measured under zero contact angle conditions in water and HD by using the Wilhelmy method. Due to the high degree of curvature of the fibers, no drops of water or HD remained on the fibers as they were removed slowly from the liquids even though they were completely wetted by these liquids. This is because the energetics drive the system to minimize the exposed liquid surface area, and if viscous draining occurs rapidly enough, all the liquid will drain from the fiber surface into the bulk. As discussed earlier, the fibers were treated by immersing in 0.028 M OTS in HD/CCL/ CHCl3 for approximately 5 min. All fiber experiments using OTS were done a t relative humidity below 30% on freshly prepared glass surfaces. A t higher R h , the stickslip was less reproducible. A typical experiment is summarized in Figure 4 for a freshly prepared, OTS-treated 21.7-pm-diameter E-glass fiber. Upon advancing into water over unwetted fiber surface (represented by points A to B), the stick-slip behavior corresponding to variation in contact angle of 82
-
0
100
1000
Diameteripn
Figure 5. Amplitude of sticklip plotted as contact angle variation A0 = 6 2 - 01 versus fiber diameter.
-
91' to 81 72' for V = 0.35 pm/s was recorded. At point B, the velocity is increased to 0.7 pm/s, illustrating the inverse proportionality of the period with velocity. Slower velocities down to -0.05 pm/s gave essentially the same amplitude, but for velocities greater than 1pm/s the amplitude became more irregular as the vibrational energy of the liquid meniscus increased enough to inhibit meniscus pinning. At point C in Figure 4, the direction was reversed, and Orec = 55O was measured a t 4 pm/s. At point D, the fiber was advanced again but over the region which was already wetted. For the prewetted fiber surface, the stick-slip is suppressed, giving the values of Badv listed in Table I1 which are generally greater than or equal to 82. Without changing speed, the contact line reached the unwetted region a t D' and the stickslip behavior returned. At E and F, eadv (for the previously wetted surface) and ereCwere measured, respectively, and were consistent with the earlier values. For the prewetted regions, the stick-slip was suppressed for several hours but was found to return after allowing the fibers to dehydrate for 24 h, indicating it is not due to incomplete reaction of the monolayer but to surface chemical inhomogeneities. The fiber diameter results are summarized in Figure 5, where the amplitude of the stick-slip defined by A8 = d2 - 81 is plotted vs diameter. These values vary from A8 20" f 5' for 10-30-pm fibers to A8 3" for 130-pm fibers while the classical contact angle hysteresis 8adv erecdoes not change significantly with diameter (Table 11). This is strong evidence supporting our claim that the advancing contact line is pinned by regions or boundaries of lower surface energy when they are comparable in dimension to the fiber diameter (i.e., microns). With the larger diameter fibers, the pinning is suppressed because it is averaged out over the contact line, and only the common contact angle hysteresis is detected. This contact angle hysteresis (8,dV - 8, from Table 11)is approximately 37O f 6 O independent of diameter once the fiber has been prewetted. It has been shown previously that when the surface coverage of OTS is not complete the material organizes in patches of highly ordered materialF1 and these patches are essentially surface chemical inhomogeneities which would be expected to contribute to the large hysteresis. The values of 8& and 8r.c are significantly lower than those obtained on the glass plates. In terms of meniscus dimensions, a similar trend was seen for sessile drop measurements using small drop sizes.8 I t is known that the meniscus heights on fibers change dramatically with diameter due to the high degree of curvature.12 Due to the precise measurement of the depth of immersion, our experiment gives direct information on the meniscus dimensions for small diameter fibers.
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Langmuir, Vol. 6, NO.5, 1990 1007
Dynamic Contact Angle Measurements on Glass Fibers Table IV. Analysis of Stick-Slip on OTS-Treated E-Glass Fibers in Terms of Meniscus Height d , rm
21.7 21.7 23.4 33.4 45.9 47.2 130 130
Z
. rm ~ 112 116 89 154 167 116 800 740
O
~
~
rm
~~ O ~. d o b
72 72 78 104 135 139 295 295
large fibers, some slippage is expected because the energy barriers are lower due to an averaging effect over the larger perimeter.
velocity, rm/s 0.35 0.7 0.26 0.41 0.33 0.28 0.37 1.8
The experimentally determined extrapolated meniscus heights zo,exp were calculated from the stick-slip tracings such as those in Figure 4. The extrapolated meniscus heights zo,cdc at zero contact angle were obtained from published tables.lZ
Furthermore, by comparing the meniscus heights with theoretical predictions, one can address the question of the degree of contact line pinning. The measured change in contact angle A cos 6 corresponding to a given change in depth of immersion Ah is related by A h / A cos 6 = (P-y/g)Ahf Am (2) The slopes from tracings of individual sawteeth such as those in Figure 4 are directly related to A cos Blah if the time axis is converted to Ah since the rate of travel is constant. Assuming that the contact line is pinned, as varies from 0 ' to 90' the meniscus height would vary from the extrapolated full meniscus height ( ~ 0 ,to~zero, ~ ~ respectively. Thus, the slope is related to the full meniscus height by
Equations 2 and 3 are based on the assumption of complete contact line pinning and a uniform contact line. In reality, the contact line is not uniform and probably takes a more tortuous path due to the domains of different surface energy.34 The meniscus heights calculated from the experimental data are collected in Table IV and com~ I ~ pared with tabulated meniscus heights Z O , ~ published previously.12 There is qualitative agreement for this limited set of data for d e 50 pm, indicating that our assumption of contact line pinning is valid. The experimentally determined meniscus heights for the 130-pm fiber are almost 3 times larger than the theoretical ones. For the (34) Boruvka, L.; Neumann, A. W. J. Colloid Interface Sci. 1978,65, 315.
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Conclusions Fibers drawn from molten glass provided clean surfaces and were used both as surface tension probes in the Wilhelmy method and also for model surfaces for monolayer adsorption. Results for fluorodecanoic acid treated fibers indicated that the adsorbed state is similar to that on planar surfaces. This is because these surfaces are essentially flat with regard to adsorption on the molecular level. A method to quantify the kinetics of surface chemical reactions was developed. The silanation of clean fiber surfaces with octadecyltrichlorosilane (OTS) was monitored in situ by measuring the change in contact angle with time due to an autophobic effect as the surface repelled the OTS solution. Exponential time constants of 25 f 5 s independent of fiber diameter for d < 130 pm were obtained for the adsorption process. OTS-treated fiber surfaces were used in a study of contact line pinning and contact angle hysteresis caused by surface chemical inhomogeneities due to incomplete monolayer coverage. The advancing contact line on small-diameter fibers (10 pm e d e 130 pm) was intermittently pinned by boundaries or regions of lower surface energy when the fiber diameter was comparable in dimension with surface chemical inhomogeneities. As the fiber diameter was increased, the effect of surface chemical heterogeneities was averaged out because of the larger contact line, leading to a suppression of contact line pinning. Classical contact angle hysteresis similar to that found for planar substrates was always present. This indicates that contact line pinning on small diameter fibers is a specific but more quantitative example of the common phenomena of hysteresis due to free energy barriers. Dynamic contact angle analysis as a function of fiber diameter is a general method which should facilitate characterization of a wide variety of coatings in terms of surface chemical homogeneity and wetting. Acknowledgment. We thank Drs. L. L. Berger and S. Mazur for use of their microscopes and N. DiPaolo for help with the contact angle experiments. Discussions with Drs. C.-K. Shih and R. G. Kander were most beneficial. Registry No. OTS, 112-04-09;(CF3(CFz)sCOzH), 335-76-2.
