Static and Dynamic Contact Angles of Water on Polymeric Surfaces

Scattering of data points at cos θS = 0.6 to 0.9 was caused by an inconsistency of the force loop shape, which distorted a parallelogram, due to inco...
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Static and Dynamic Contact Angles of Water on Polymeric Surfaces Masayo Miyama,† Yanxia Yang,† Takeshi Yasuda,‡ Tsumuko Okuno,‡ and Hirotsugu K. Yasuda*,† Center for Surface Science and Plasma Technology and Department of Chemical Engineering, University of MissourisColumbia, Columbia, Missouri 65211, Department of Home Economics, Mukogawa Women’s University, Nishinomiya 663, Japan Received September 5, 1996. In Final Form: July 14, 1997X Static contact angle and dynamic (advancing and receding) contact angles of water on polymeric surfaces were investigated using microscope cover glasses coated with various plasma polymers of trimethylsilane and oxygen. By variation of the mole fraction of the TMS/oxygen mixture, glass surfaces having varying degrees of wettability were prepared. The advancing contact angle of a sessile droplet, which is independent of the droplet volume, is considered as the static contact angle of water on a polymeric surface, θS, which is a parameter characteristic to a polymeric surface. The dynamic contact angle of water refers to the contact angle of which three-phase contact line is in motion with respect to the surface. The dynamic advancing (immersing) contact angle, θD,a, and receding (emerging) contact angle, θD,r, were measured by the Wilhelmy balance. The difference between θD,a and θD,r is mainly due to the direction of dynamic force acting on the three-phase contact line. The discrepancy between the immersion and the emersion buoyancy lines and the corresponding values of contact angles can be used to indicate the hysteresis due to the dynamic factor (the dynamic hysteresis). The dynamic hysteresis is largely determined by the critical immersion depth in which the three-phase contact line remains at the same place on the surface while the shape of meniscus changes when the motion of the sample is reversed. The dynamic hysteresis may contain the contribution of the change of static contact angle due to the surface-configuration change caused by the wetting of the surface (the intrinsic hysteresis). The dynamic hysteresis varies according to the value of cos θS, with the maximum at the threshold value around 0.6 and linearly decreases above this value, as the emersion line approaches the limiting buoyancy line determined by the surface tension of the liquid. The intrinsic hysteresis follows the same trend with the maximum at around 0.8. The three contact angles are related by cos θS ) (cos θD,a + cos θD,r)/2.

Introduction and Background Information The surface properties of a polymeric material are determined by the surface configuration, which is the spatial arrangement of ligands and atoms in the top surface region (surface-state) of the material. This is a rephrasing of the principle pointed out by Langmuir in 1938.1 The surface configuration of a polymer is not a sole function of molecular configuration of the polymer but is a function of factors involved in the phase which contacts with the surface. It is important to recognize that the surface of a material exists only as an interface with a contacting medium. This aspect is particularly important when dealing with surfaces of polymeric materials which have a highly perturbable surface configuration. While it is generally recognized that the properties of a material in an interface can be significantly different from what one might anticipate from the bulk material properties, the surface characteristics of a material are often expressed by the interfacial characteristics with air or vacuum. The interfacial characteristics of a material in real applications, many of which involve aqueous environments, are usually quite different from those determined in air or vacuum. This situation can be best understood by considering the top surface region of a material as the “surface state”, which is significantly different from the state of bulk material.2,3 The surface state of a polymeric material, or the surface configuration of the polymer in the interface, changes according to the nature of the contacting medium. Furthermore, the surface state of * To whom all correspondence should be addressed. † University of MissourisColumbia. ‡ Mukogawa Women’s University. X Abstract published in Advance ACS Abstracts, September 15, 1997. (1) Langmuir, I. Overturning and anchoring of monolayers. Science 1938, 87, 493.

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the contacting medium (such as vicinal water) also changes according to the nature of the solid material in order to establish an equilibrium between the surface states. Consequently the surface state of a solid in contact with liquid water cannot be fully explained or predicted from observations of the surface state of the same material in vacuum or in air. The configuration of a dry polymer may change quickly when a surface is immersed in liquid water.2-6 Such a change can be easily detected by the change of contact angle of water as a function of the contact time. Indeed the contact angle change is much more sensitive to the quick change of surface configuration than many other surface analytical means, although it does not provide details of molecular level configuration changes. A modern analytical tool such as X-ray photoelectron spectroscopy (XPS) has very limited use in dealing with the quick surface-configuration change which occurs in water at ambient temperature. While the simple method of contact angle measurement could provide information directly relevant to the surfaceconfiguration change due to the change of surrounding medium, the interpretation of data depends entirely on (2) Yasuda, H.; Charlson, E. J.; Charlson, E. M.; Yasuda, T.; Miyama, M.; Okuno, T. Dynamics of surface property change in response to changes in environmental conditions. Langmuir 1991, 7, 2394-2400. (3) Yasuda, T.; Miyama, M.; Yasuda, H. Dynamics of the surfaceconfiguration change of polymers in response to changes in environmental conditions. 2. Comparison of changes in air and in liquid water. Langmuir 1992, 8, 1425-1430. (4) Yasuda, T.; Okuno, T.; Yoshida, K.; Yasuda, H. A study of surface dynamics of polymers. II. Investigation by plasma surface implantation of fluorine-containing moieties. J. Polym. Sci., Part B: Polym. Phys. 1988, 26, 1781-1794. (5) Yasuda, T.; Miyama, M.; Yasuda, H. Effect of Water Immersion on Surface-configuration of an Ethylene-Vinyl Alcohol Copolymer. Langmuir 1994, 10, 583-585. (6) Yasuda, T.; Okuno, T.; Tsuji, K.; Yasuda, H. Surface-configuration change of CF4 plasma treated cellulose and cellulose acetate by interaction of water with surfaces. Langmuir 1996, 12, 1391-1394.

© 1997 American Chemical Society

Contact Angles of Water on Polymeric Surfaces

the basic concept of the interfacial phenomenon. The advancing and receding contact angles, or contact angle hysteresis, has been often dealt with a priori concept that such parameters represent “characteristics of the surface” under examination; i.e., the surface does not change during the measurement. In such an approach, the surface dynamic change recognized by the terms such as surfaceconfiguration change, surface reconstruction, surface rearrangement, etc. might belong to the old solved problems of contact angle hysteresis. In reality, however, it is relatively recently that researchers became aware of the significance of the surface dynamic change. The seemingly old problems evolved as important new topics of the interfacial phenomena. Dealing with highly perturbable polymer surfaces, particularly hydrophilic polymer surfaces, the measurement of contact angle by the sessile droplet method as well as by the Wilhelmy balance is often overwhelmed by the change of material itself due to swelling or imbibing of water into the bulk phase. The water-surface interaction should be viewed by the two distinctly different “interactions”.6 One is the interfacial interaction, which is caused by the interfacial free energy difference and can be represented by the term “γ-interaction”. The other is the chemical (molecular) interaction of water and polymer, which can be represented by the term “χ-interaction” using the thermodynamic interaction parameter χ. There seems to be a general trend that the term “interaction” is interpreted as the “γ-interaction” by surface scientists and as the “χ-interaction” by polymer scientists. For highly hydrophilic polymer, it is nearly impossible to isolate the change due to the “γ-interaction” due to an overwhelmingly large contribution of “χ-interaction”, unless one could create a hydrophilic surface state on a hydrophobic bulk. Such a unique situation can be created by the plasma polymerization technique discussed in this paper. A plasma polymer with thickness in the range of 10-50 nm deposited on a glass plate provides an unique opportunity to investigate some fundamental aspects of surface dynamic changes in the context of “γ-interaction” which occur when a hydrophilic surface is immersed in liquid water or a contact is made with water droplet. Since the plasma polymer is very thin and the surface of the glass plate (microscope cover glass) is smooth, the variation of factors, such as roughness, inhomogeneity, buoyancy change, etc., which might hamper the accurate measurement of force or contact angle in dealing with conventional polymers, can be minimized if not completely eliminated. The extreme thinness of a plasma polymer layer and the fact that an unperturbable substrate (glass) is used as the substrate make it easier to identify changes due to “γ-interaction”, rather than “χ-interaction” for various plasma polymers with varying surface energies. In this study, the surface force measurement by the Wilhelmy balance and the sessile droplet contact angle measurement were performed for microscope cover glasses coated by the plasma polymer (trimethylsilane + oxygen) of varying wettability. It is intended to reexamine the meaning of contact angles obtainable by these methods and to gain insight into the causes for the hysteresis observed in contact angle measurement and also in the surface force loop observed by the Wilhelmy balance method. Experimental Section Materials. Microscope cover glasses (22 × 22 × 0.153 mm) were used as substrates for the Wilhelmy force and the sessile droplet contact angle measurements. Prior to plasma polymerization, they were cleaned ultrasonically in ethanol, thoroughly rinsed, and then dried in air. For measurement of deposited

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Figure 1. Schematic representation of the substrates assembly used: (a) side view; and (b) front view. film thickness, a piece of silicon wafer was used after ultrasonic cleaning in acetone. For plasma polymerization, trimethylsilane (TMS) (97% min., PCR Inc.) and oxygen (99.99% min., Scott Specialty Gases, Inc.) gases were used without further purification. Water was used as a liquid for measurement of the Wilhelmy force and the sessile droplet contact angle. It was purified by distillation and deionization (for measurement of the Wilhelmy force and advancing sessile droplet contact angle), or reverse osmosis and ion exchange (for measurements of advancing and receding sessile droplet contact angle with increasing or decreasing water droplet volume, respectively). The electrical resistivity of the water was not less than 11 MΩcm. Plasma Polymerization (Plasma Polymer of TMS/ Oxygen). The plasma polymerization on the glass plates was carried out in a glass bell jar reactor, PlasmaCarb MPR-500P, which has capacitively coupled parallel internal magnetic enhancement electrodes. Power was supplied to the electrodes by a 40 kHz power source PE-1000 generator (Advanced Energy Industries, Inc.) and controlled by wattage. The inside diameter of the glass bell jar is 43.5 cm, and the total volume of the reactor chamber is approximately 75 L. A cooling box with circulation of coolant at 20 °C was attached on the back side of each titanium electrode (18.1 × 18.1 × 0.16 cm) with electrical insulation to minimize heating up. Six pieces of permanent magnet bars were evenly arranged on the back side of each cooling box in a circular configuration with the south poles oriented toward the center. By using the cooling system, the strength of the magnetic field can be kept constant. The electrode separation was fixed at 10 cm. Substrates were clamped on a rotating disk frame (36.2 cm diameter) which was placed midway between the electrodes shown in Figure 1. Both sides of the substrates were exposed to the plasma glow and coated by the plasma polymer at the same time, because the substrates were located in the open space of the disk frame. Radial positions of the substrates were 13 cm from the center of the disk frame. For uniform deposition, the disk frame was rotated at a speed of 15 rpm during the process. Therefore, the substrates were located between the two electrodes and exposed to the intense glow for approximately 25% of the total plasma polymerization time. The bell jar chamber was evacuated by two pumps in series, one rotary pump and one mechanical booster pump. After the pressure in the reactor reached its minimum attainable value of less than 0.13 Pa (1 mTorr), gases were fed through a tube with its outlet located above the interelectrode space facing the bell jar wall. The pressure was measured with a MKS Baratron pressure meter. The volume flow rate of each gas was controlled with a MKS mass flow controller and determined by using the ideal gas equation in conjunction with ∆p/∆t measurement, taken with the pump valved-off. Oxygen gas was added to 1 sccm of TMS in mole percent of 0 to 80. The polymerization was performed under a W/FM7 from 700 to 820 MJ/kg (W/FM (7) Yasuda, H. Plasma Polymerization; Academic Press, Inc.: Orlando, FL, 1985; p 303.

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represents the energy input per unit mass of the monomer, where W is the discharge wattage, F is the volume flow rate of monomer, and M is the molecular weight of monomer) and a system pressure of 6.7 Pa (50 mTorr), which was controlled by a throttle valve coupled with the pressure meter. The thickness of the plasma polymers were measured by using an AutoEL-II Automatic Ellipsometer (Rudolph Research Corporation), which is a null-seeking type with 632.8 nm heliumneon laser light source. The plasma polymers with thickness of around 15 and 50 nm were prepared by adjusting plasma polymerization time. Sessile Droplet Contact Angle. Advancing and receding contact angles with increasing or decreasing water droplet volume were measured in a temperature and humidity controlled room (20 °C, 65%) using a Kyowa contact angle measuring device, CA-A (Kyowa Interface Science Co., Ltd., Japan). A water droplet of 2.7 µL was placed on the surface, and a picture of the droplet was taken by a camera within 5 s. A droplet was added every 20 s and the procedure was repeated 10 times (total 27 µL) for the measurement of advancing contact angles. Then 2.7 µL of water was withdrawn every 20 s for the measurement of receding contact angles. The contact angle of water was measured using an enlarged image of the developed film. Using this photographic approach, the time necessary to measure the contact angle can be eliminated in the process of increasing and decreasing the water droplet volume and an accurate measurement of the advancing and receding contact angles can be performed with a controlled advancing and receding rate. The diameter of contact area, which is the surface area under the sessile droplet, was also measured on the enlarged image of the developed film, and the actual contact area was calculated. For the comparative study of the sessile droplet method and the Wilhelmy balance method, the both measurements should be performed within a relatively short time interval using samples from the same batch. For this purpose, after plasma polymerization (within a day) a water droplet of 10 µL was placed on the surface of the plasma polymer and the contact angle of water (advancing) was measured within 5 s with a protractor using enlarged image on a monitor. The Wilhelmy Balance. A fully computer controlled and automatic tensiometer, Sigma 70 (KSV instruments, Ltd., Finland), was used for measurement of the surface force. The measured force acting on a thin plate which is partially immersed in a liquid can be represented by the force balance equation

F ) -FgtHd + LγL cos θ

(1)

where F is the measured force on the electrobalance, F is the liquid density, g is the gravitational acceleration, t is the thickness of the plate, H is the width of the plate, d is the immersion depth, L is the perimeter of the plate, i.e., 2(t + H), γL is the liquid surface tension, and θ is the contact angle at the liquid/solid/air three-phase contact line. The first term is the buoyancy force and the second term is the interfacial interaction force (wetting force of the liquid on the plate). Because the balance is reset to zero after the plate is installed each time, the measured force, F, does not include the term for the gravitational force, Mg. Measurements were performed within a day after plasma polymerization. The measuring range 25 mN giving a resolution of 1 µN was used in this study. Water was poured in a beaker and placed inside the closed test chamber. Immersion and emersion of the glass plate coated by the plasma polymer (both sides identically) were accomplished by raising and lowering the beaker stage at a constant speed of 5 mm/min. Data points, including depth and force of the measuring point, were collected and stored by a computer at the rate of 1 data set/s. The balance is automatically reset to zero when each test is started. The immersion depth is also reset to zero automatically when the bottom edge of the plate first contacts with water, and calculated from the time and speed of immersion or emersion. In cycle 1, the glass plate was immersed into the water to 10 mm depth (A-B-C in Figure 2), and emerged from the water to 5 mm depth (C-D-E). In cycle 2, the plate was immersed again, but much deeper until 15 mm (E-F-G-H-I), and emerged to 5 mm (I-J-K). In cycle 3, the same immersion and emersion as cycle 2 were repeated (K-L-M-N-O).

Figure 2. A typical force loop of the Wilhelmy balance for a plasma polymer coated glass plate. The (average) dynamic advancing and receding contact angles were calculated from the extrapolated values of the corresponding F/L lines (for the immersion and for the emersion, respectively) to the zero immersion depth. When the line was not straight, a linear portion toward the end of immersion or emersion was used. The water was discarded and changed after each measurement to prevent any possible contamination, and the surface tension of water was measured by the Du Nouy ring method (72-74 mN/m at 20-25 °C) for each measurement. X-ray Photoelectron Spectroscopy (XPS). An X-ray photoelectron analyzer, ESCA 850, and a data processing system, ESPAC 1000 (Shimadzu, Japan), were used. A sample was placed on the sample holder by means of double-faced adhesive tape. C1s, O1s, and Si2p peaks were scanned by filtered (by 2 µm thick aluminum foil) Mg KR X-rays (6 kV, 30 mA) at the scanning rate of 0.5 eV/s, and atomic compositions of these atoms were calculated from peak areas which were corrected by sensitivity of each element. A takeoff angle of 90° was used.

Results and Discussion 1. Plasma Polymers of TMS/Oxygen Mixtures. One very unique feature of plasma polymerization is that diatomic gases such as H2, O2, N2, etc. can be incorporated into the structure of a plasma polymer in a very reproducible manner by simply mixing the gas with a main polymer-forming plasma gas (monomer). It has been shown that TMS/oxygen plasma polymer (prepared by the cathodic polymerization using a cold rolled steel plate as the cathode) changes its chemical structure from silane type to siloxane type, and further to SiO2 type structure according to the mole ratio of oxygen in the monomer mixture. The contact angle of water on a plasma polymer surface changes gradually according to the mole ratio of oxygen.8 TMS/oxygen plasma polymers deposited on a glass plate by a magnetron discharge with 40 kHz electrical power source used for this study also show an increase of contact angle with increasing oxygen mole content in the monomer mixture as shown in Figure 3. The static contact angle, θS, is an advancing sessile droplet contact angle of water at a water droplet volume of 10 µL. The range of contact angles of the plasma polymers covers 6-95°. (The details of θS will be discussed in the next section.) Table 1 shows surface atomic compositions, which were determined by XPS, of plasma polymers with 50 nm thickness. With increasing the oxygen mole content in the TMS/oxygen monomer, the amount of atomic oxygen in the plasma polymer increased. A linear relationship (8) Wang, T. F.; Yasuda, H. K. Modification of wettability of a stainless-steel plate by cathodic plasma polymerization of trimethylsilane-oxygen mixtures. J. Appl. Polym. Sci. 1995, 55, 903-909.

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Figure 3. Effect of oxygen mole content in the TMS and oxygen gas mixture on cos θS. θS: static contact angle of water determined by the sessile droplet method (advancing, 10 µl). Thicknesses of plasma polymer were 50 and 15 nm. Table 1. Surface Characteristics of Plasma Polymers oxygen mole content in TMS/oxygen monomer (%)

cos θSa of plasma polymer

0.0 50.0 63.4 67.4 71.2 73.7 73.7 74.5 75.0 79.9

0.02 0.24 0.38 0.49 0.52 0.68 0.81 0.68 0.89 0.95

atomic composition of plasma polymer determined by XPS (%) C1s

O1s

Si2p

57.9 53.9 47.8 46.8 42.4 30.7 15.8 20.8 15.3 11.1

21.2 29.6 35.2 38.3 42.5 49.0 57.1 54.5 55.4 61.0

20.9 16.5 16.9 14.9 15.1 20.3 27.2 24.7 29.3 27.9

a θ , static contact angle of water determined by the sessile droplet S method, which is a droplet volume-independent advancing contact angle.

Figure 4. Correlation between cos θS and amount of atomic oxygen in the plasma polymer determined by XPS. Thickness of plasma polymer was 50 nm.

was found between cos θS and amount of atomic oxygen from XPS as shown in Figure 4. 2. Contact Angles by Sessile Droplet. The contact angle which is measured in a static condition is referred to as “static contact angle”, and the contact angle which is measured under the condition that the three-phase contact line is moving with respect to the surface is referred to as “dynamic contact angle”. When the volume of a sessile droplet is increased by adding more water to a droplet placed on a surface, the three-phase contact line advances to the dry surface. Because of this situation, the contact angle measured on the volume increasing process is customarily referred to as the “advancing contact angle”. When water is withdrawn from a droplet, the droplet volume decreases and the three-phase contact line recedes, though not always.

The contact angle measured on the droplet-volume withdrawing process is referred to as the “receding contact angle” with an intuitive assumption that the three-phase contact line indeed recedes. In both cases, the contact angle of a stationary sessile droplet is measured. In other words, the three-phase contact line is not moving during the measurement of contact angle. Thus, the advancing contact angle in the sessile droplet contact angle measurement represents the static contact angle at the three-phase contact line for water/air/dry surface, and the receding contact angle represents the static contact angle at the three-phase contact line for water/air/wetted surface. It is important to recognize that “advancing” or “receding” merely refers to which process the contact angle is measured. The contact line in the receding process often does not recede from the contact line developed on the last step of the advancing process. The hysteresis (discrepancy) of contact angles in the advancing and the receding processes is a very common phenomenon. There are many factors which can be attributed to the hysteresis effect, such as surface roughness, surface chemical heterogeneity, surface deformation, adsorption and desorption, swelling and penetration, surface-configuration change during the process of contact angle measurement, etc.9-11 Figure 5 shows the effects of water droplet volume on advancing and receding cos θ for the sessile droplet method. From (a) to (f) the wettability of the plasma polymer increases as a result of increasing the oxygen content in the plasma polymer. The withdrawal of water does not follow the advancing process in a reversible manner for all samples, and the receding contact angle is strongly dependent on the droplet volume and/or the contact time of water and surface except for a higher wettability surface. No single value can be assigned for “receding contact angle”, in other words, a “receding contact angle” from the sessile droplet method has very little meaning by itself. The advancing contact angle, on the other hand, is less dependent on the droplet volume. It either becomes nearly independent of the droplet volume as the droplet volume increases beyond a certain value or is nearly independent within the experimental range beyond the threshold value. A characteristic advancing contact angle could be found for a surface in most cases, and this value is designated as the “static contact angle” of a surface, θS, in this paper. Because of the droplet volume dependent receding contact angle, the contact angle hysteresis cannot be simply described by a single value as the discrepancy between advancing and receding contact angles. The decreasing contact angle on the receding process can be attributed to the flattening of a droplet caused by the decrease of volume without decreasing the contact area of a droplet (surface area under the droplet). The contact area was plotted as a function of water droplet volume in Figure 6. The reluctance of the contact area to decrease has been explained by the creation of an attractive force between water and polymer molecules, which results from the change of the surface configuration to establish a new equilibrium between liquid water and the surface state of the contacting polymer solid.12 (9) Johnson, R. E., Jr.; Dettre, R. H. Wettability and contact angles. In Surface and Colloid Science; Matijevic, E., Eirich F. R., Eds.; Wiley-Interscience: New York, 1969, Vol. 2, pp 85-153. (10) Andrade, J. D.; Smith, L. M.; Gregonis, D. E. The contact angle and interface energetics. In Surface and Interfacial Aspects of Biomedical Polymers; Andrade, J. D., Ed.; Plenum Press: New York, 1985; Vol. 1, Chapter 7, pp 249-292. (11) Adamson, A. W. Physical Chemistry of Surfaces, 5th ed.; John Wiley and Sons: New York, 1990.

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Figure 5. Cos θ from the sessile droplet method as a function of water droplet volume for TMS with oxygen plasma polymer coated glass plates with varying degrees of wettability. Thickness of plasma polymer was 50 nm. Oxygen mole contents in the TMS and oxygen gas mixture were (a) 0.0%, (b) 67.4%, (c) 71.2%, (d) 74.5%, (e) 75.0%, and (f) 79.9%.

3. Contact Angles by the Wilhelmy Balance. a. Hysteresis in a Force Loop. The Wilhelmy force loops for plasma polymers with varying degree of wettability are shown in Figure 7. From (a) to (j) the wettability of the plasma polymer increases. The location of the loop within the plot as well as the shape of the loop change with the wettability of the surface. For hydrophobic surfaces, the loop is located at the lower force region and the width of the loop (separation of the immersion and the emersion lines) is small. As the wettability increases the position of the loop becomes higher and the width becomes larger. This trend continues until the emersion line approaches the limiting value which is determined by the surface tension of the liquid and the buoyancy at a given immersion depth. As the wettability increases further, the immersion line continues to shift to the higher position while the emersion line remains at the limiting value which yields the loop with a narrower width. There are at least two conspicuous factors in the Wilhelmy force loop which can be used to describe the extent of hysteresis. One is the separation width between (12) Yasuda, T.; Okuno, T.; Yasuda, H.; Contact angle of water on polymer surfaces. Langmuir 1994, 10, 2435-2439.

the immersion and the emersion lines which can be visualized as the vertical separation width of the force loop. The other is the discrepancy between the first immersion line and the second immersion line, which is nearly the same as the step observed when the immersion depth is increased beyond that used in the first cycle. The first hysteresis is mainly attributable, as will be shown later, to the relative motion of the surface with respect to the water line and can be termed as “the dynamic hysteresis”. The second is due to the intrinsic change of surface configuration or surface state caused by the wetting of the surface with water and can be termed as “the intrinsic hysteresis”. Although the width of the force loop results from both the dynamic and the intrinsic hysteresis, a contribution of the intrinsic hysteresis is generally much smaller, particularly for hydrophobic polymers, than that of the dynamic hysteresis. The intrinsic hysteresis causes deviation from the ideal case represented by eq 1, which is under an assumption that the contact angle does not change during the measurement. If the contribution of the intrinsic hysteresis (within the time scale of the

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Figure 6. Contact area (surface area under the sessile droplet) as a function of water droplet volume for TMS with oxygen plasma polymer coated glass plates with varying degrees of wettability. Thickness of plasma polymer was 50 nm. Oxygen mole contents in the TMS and oxygen gas mixture were (a) 0.0%, (b) 67.4%, (c) 71.2%, (d) 74.5%, (e) 75.0%, and (f) 79.9%. These correspond to the samples used in Figure 5.

measurement) increases, the deviation from the ideal case becomes greater, and the shape of the force loop changes from a parallelogram to more distorted shapes. In order to express the both types of hysteresis in a quantitative manner, the F/L values at an immersion depth of 7.5 mm were used in this paper. According to this method, the difference of F/L at an immersion depth of 7.5 mm for the first immersion and for the first emersion is used as an indication of the dynamic hysteresis. Similarly, the difference of F/L at an immersion depth of 7.5 mm between the first immersion and the second immersion is used for the intrinsic hysteresis. In Figure 8, the values of F/L at an immersion depth of 7.5 mm for the immersion and the emersion in cycle 1 were plotted against cos θS. The values of F/L for the immersion and the emersion (advancing F/L and receding F/L, respectively) increase with increasing cos θS and the receding F/L reaches the limiting value shown by a dotted line. This limiting value of F/L corresponds to a calculated F/L (67.4 mN/m) using eq 1 at θ ) 0° and d ) 7.5 mm. The extrapolated value of F/L to zero immersion depth should be the surface tension of water. The measured force cannot exceed this limiting buoyancy line.

Due to the presence of the ceiling buoyancy line, the separation of immersion and emersion lines (dynamic hysteresis) becomes smaller as the wettability of surface increases. On the hydrophobic surfaces, the separation tends to increase as the wettability increases. Figure 9 clearly shows that the dynamic hysteresis can be shown by two lines with inflection point around cos θS ) 0.6. It is important to notice that the location of the ceiling buoyancy line is dependent on the surface tension of the liquid. If a liquid with a lower surface tension is used, a smaller separation, i.e., smaller dynamic hysteresis, is expected from these results. This is roughly the case according to reports seen in literature.13-16 (13) Yaminsky, V. V.; Claesson, P. M.; Eriksson, J. C. Wetting hysteresis and instability of hydrophobized glass and mica surfaces. J. Colloid Interface Sci. 1993, 161, 91-100. (14) Johnson, R. E., Jr.; Dettre, R. H.; Brandreth D. A. Dynamic contact angles and contact angle hysteresis. J. Colloid Interface Sci. 1977, 62, 205-212. (15) Penn, L. S.; Miller, B. A study of the primary cause of contact angle hysteresis on some polymeric solids. J. Colloid Interface Sci. 1980, 78, 238-241. (16) Richter, L. Wetting behavior of liquids towards Teflon. Tenside, Surfactants, Deterg. 1994, 3, 189-191.

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Figure 7. Wilhelmy force loops for TMS with oxygen plasma polymer coated glass plates with varying degrees of wettability. Thickness of plasma polymer was 50 nm. Oxygen mole contents in the TMS and oxygen gas mixture and cos θS were as follows: (a) 0.0%, -0.05; (b) 43.0%, 0.09; (c) 48.2%, 0.26; (d) 62.8%, 0.49; (e) 67.4%, 0.62; (f) 68.7%, 0.64; (g) 70.6%, 0.76; (h) 73.7%, 0.87; (i) 74.1%, 0.93; (j) 79.6%, 1.0.

The intrinsic hysteresis, which is represented by the difference of advancing F/L values at an immersion depth of 7.5 mm between cycles 1 and 2, is shown in Figure 10. The same wettability dependency as the dynamic hysteresis is seen. It increased with increasing cos θS followed by a decrease with inflection point around cos θS ) 0.8. Scattering of data points at cos θS ) 0.6 to 0.9 was caused

by an inconsistency of the force loop shape, which distorted a parallelogram, due to inconsistent behavior of the advancing F/L in cycle 2. b. Meniscus on a Moving Surface. When a sample plate is lowered, with respect to water surface, to make a contact with liquid water and continued to move at a constant speed, the three-phase contact line does not move

Contact Angles of Water on Polymeric Surfaces

Figure 8. Effect of wettability, cos θS, on the advancing and receding F/L at an immersion depth of 7.5 mm in cycle 1. Thicknesses of plasma polymer were 50 and 15 nm.

Figure 9. Effect of wettability, cos θS, on the dynamic hysteresis represented by the F/L difference at an immersion depth of 7.5 mm between immersion and emersion in cycle 1. Thicknesses of plasma polymer were 50 and 15 nm.

Figure 10. Effect of wettability, cos θS, on the intrinsic hysteresis represented by the difference of the advancing F/L at an immersion depth of 7.5 mm between cycle 1 and cycle 2. Thicknesses of plasma polymer were 50 and 15 nm.

immediately at the same speed. The steady movement of the contact line is reached after a transient stage in which the meniscus shape changes until a dynamic steady state meniscus, which represents the dynamic advancing contact angle, will be developed.13,17 When the motion of the plate is reversed (lifting upward), the contact line does not move up immediately by maintaining the dynamic meniscus developed in the previous motion. Instead, the contact line on the plate remains at the same position (17) Wang, J. -H.; Claesson, P. M.; Parker, J. L.; Yasuda, H. Dynamic contact angles and contact angle hysteresis of plasma polymers. Langmuir 1994, 10, 3887-3897.

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Figure 11. Depth for meniscus change in the transition stage C-D as a function of wettability, cos θS. C-D is transition stage of emersion in cycle 1 due to changing process from immersion to emersion. Thicknesses of plasma polymer were 50 and 15 nm.

while the plate itself is moving upward with respect to the water level and the shape of meniscus is changing. After the change of the meniscus shape is completed and the dynamic steady state for the upward meniscus, which represents the dynamic receding contact angle, is established, the contact line starts to move on the plate at the lifting speed. The characteristic immersion depth necessary to complete the meniscus change can be described by the immersion depth for meniscus change, dm, which can be obtained from the force loop. Region A-B (in Figure 2) results from the bottom edge of the sample first contacting water.17 For the most hydrophobic sample it showed very clearly (Figure 7 a), and at the level of cos θS ) 0.4 to 0.7 this transient stage is hardly detectable (Figure 7 d-f). When θS ) 90° no meniscus (rise or depression of liquid water line) develops in a static situation; however, the dynamic contact angles for advancing and receding menisci are different and dm is still identifiable. In regions C-D, E-F, I-J, K-L, and M-N (in Figure 2), the meniscus shape changes due to the changing direction of the three-phase contact line movement.13,17 The immersion depth for meniscus change, dm, for C-D was plotted as a function of cos θS in Figure 11. The trend found in Figure 11 is identical to that found in Figure 9, which confirms that the dynamic hysteresis is mainly caused by the meniscus change from immersion to emersion. The meniscus height at a partially immersed infinitely wide plate is given by eq 2,11,18

sin θ ) 1 - Fgh2/2γL

(2)

where θ is the contact angle at the liquid/solid/air threephase contact line, F is the liquid density, g is the gravitational acceleration, h is the meniscus height, and γL is the liquid surface tension. The meniscus height differences between C and D were calculated from eq 2, and compared with dm. Those showed very good agreement as shown in Figure 12. It indicates that dm directly corresponds to the meniscus height difference between immersion and emersion. Therefore, the force balance equation can be also represented by the meniscus height h as (18) Neumann, A. W.; Good, R. J. Techniques of measuring contact angles. In Surface and Colloid Science; Good, R. J., Stromberg, R. R., Eds.; Plenum Press: New York, 1979; Vol. 11, Chapter 2, pp 31-91.

5502 Langmuir, Vol. 13, No. 20, 1997

Miyama et al.

Figure 12. Depth for meniscus change in the transition stage C-D versus meniscus height difference calculated from eq 2. Thicknesses of plasma polymer were 50 and 15 nm.

F ) -FgtHd + LγL

x (

1- 1-

)

Fgh2 2γL

2

(3)

where F is the measured force on the electrobalance, F is the liquid density, g is the gravitational acceleration, t is the thickness of the plate, H is the width of the plate, d is the immersion depth, L is the perimeter of the plate, γL is the liquid surface tension, and h is the meniscus height. The force loop for an ideal case of a perfect parallelogram consists of an immersion line and an emersion line. The both lines can be divided into the stage-I (steep) and the stage-II (flat) lines. Equation 3 represents the stage-I immersion and the stage-I emersion lines. For the stageI, the first term in eq 3 is nearly a constant compared to the variation of the second term. The main variable for the stage-I is the second term. For the stage-II immersion and the stage-II emersion lines, the second term in eq 1, also the second term in eq 3, becomes nearly a constant. The main variable for the stage-II, therefore, is the first term. Because of this situation, the stage-II lines are customarily termed “buoyancy lines”. The hysteresis recognized as the separation of the stageII immersion line and the stage-II emersion line (the dynamic hysteresis) is determined by the magnitude of the second term in eq 3. In many cases, the contact angle in a stage-II may not be a constant, and consequently the slope of the stage-II buoyancy line may include a contribution of the change of contact angle. 4. Correlation between the Static Contact Angle and the Dynamic Contact Angles. The dynamic advancing and receding contact angles are calculated from the immersion and emersion lines in the Wilhelmy force loop according to the procedure described in the Experimental Section. It is important to notice that a single value of dynamic receding contact angle and its difference from the dynamic advancing contact angle can be obtained from the Wilhelmy force measurement, which is a strong contrast to the case of sessile droplet contact angle measurement. When the movement is stopped, the advancing F/L becomes larger while the receding F/L becomes smaller.13,19 This indicates that the dynamic advancing and receding contact angles approach the static contact angle when the movement is stopped. Hence, the dynamic advancing contact angle is anticipated to be greater than the static contact angle and the dynamic receding contact angle is (19) Hayes, R. A.; Ralston, J. The dynamics of wetting processes. Colloids Surf., A 1994, 93, 15-23.

Figure 13. Correlation between cos θD and cos θS. θD is the dynamic advancing or receding contact angle determined by the Wilhelmy balance method (cycle 1). θS is the static contact angle determined by the sessile droplet method (advancing, 10 µl). Thicknesses of plasma polymer were 50 and 15 nm.

anticipated to be smaller than the static contact angle. The deviation from the static contact angle is due to the frictional force created by the relative motion. The dynamic contact angles, therefore, are anticipated to be dependent on the speed of three-phase contact line movement. However, effects of the speed are beyond the scope of this study and will be discussed in future publications. The cosine of the dynamic advancing contact angles (cycle 1) and of the dynamic receding contact angles (cycle 1) are shown as a function of cos θS in Figure 13. cos θD,a is smaller, and cos θD,r is greater than cos θS. There are contradicting reports dealing with the correlation between contact angles calculated from the Wilhelmy force data and those measured by the sessile droplet method, i.e., the agreement of advancing contact angles between the Wilhelmy and the sessile droplet methods,14,20,21 and the agreement of the mean value of cos θ for immersion and emersion (the values obtained in cycle 2 were used) in the Wilhelmy method with advancing cos θ from the sessile droplet method.22 As mentioned above, however, the static contact angle should not be identical to the dynamic contact angle involved in the Wilhelmy force loop unless an extremely slow speed is employed. If the speed of the movement is low enough, the contact angle calculated from the Wilhelmy force loop is close enough to the static contact angle.10 Figure 13 clearly shows that there exists a general relationship that cos θD,a < cos θS < cos θD,r. The mean value of cos θD,a and cos θD,r is close to cos θS as shown in Figure 14. Namely, θS ) (cos θD,a + θD,r)/2. Conclusions Sessile Droplet Contact Angles. Although the terms “advancing” and “receding” are used, the sessile droplet method deals with a static contact angle, which varied depending on the droplet size and whether it is measured on “advancing” or “receding” process. (20) Morra, M.; Occhiello, E.; Garbassi, F. On the wettability of poly (2-hydroxyethylmethacrylate). J. Colloid Interface Sci. 1992, 149, 8491. (21) Parsons, G. E.; Buckton, G.; Chatham, S. M. Comparison of measured wetting behavior of materials with identical surface energies, presented as particles and plates. J. Adhesion Sci. Technol. 1993, 7, 95-104. (22) Uyama, Y.; Inoue, H.; Ito, K.; Kishida, A.; Ikada, Y. Comparison of different methods for contact angle measurement. J. Colloid Interface Sci. 1991, 141, 275-279.

Contact Angles of Water on Polymeric Surfaces

Figure 14. Correlation between mean cos θD and cos θS. Mean cos θD ) (cos θD,a + cos θD,r)/2, where θD,a ) dynamic advancing contact angle, and θD,r ) dynamic receding contact angle. Thicknesses of plasma polymer were 50 and 15 nm.

The contact angle changes with the size of droplet; however, the static advancing contact angle is nearly independent of droplet size as the droplet volume increases beyond a threshold value. The droplet volume independent static advancing contact angle can be dealt as the static contact angle of a surface, θS, which is a surface

Langmuir, Vol. 13, No. 20, 1997 5503

characteristic parameter describing the surface-state of the sample just before the measurement (wetting). On the other hand, the static receding contact angle per se cannot be dealt as a surface characteristic parameter. The Wilhelmy Force Loop. The discrepancy between the immersion and emersion buoyancy lines is caused by the fact that the three-phase contact line does not move on the sample surface in spite of the fact that the sample is moving with respect to the water level when the motion is reversed (the dynamic hysteresis). The immersion depth necessary for the meniscus change was found to be directly correlated to the meniscus height difference between the two processes and, hence, to the extent of the dynamic hysteresis observed in a force loop. The intrinsic hysteresis which is due to the change of surface properties during the process of force measurement also contributes to the overall hysteresis observed in the Wilhelmy force loop. When the extent of the intrinsic hysteresis increases, the force loop deviates from the ideal case and the shape of the force loop changes from a parallelogram to a distorted parallelogram and further to a completely different shape. In general, θD,a (dynamic advancing contact angle) > θS > θD,r (dynamic receding contact angle). The three contact angles are related by cos θS ) (cos θD,a + cos θD,r)/2. LA960870N