Fluid-Holding Time Parameter and Fluid Holding Capability of

The fluid holding capability of polymeric materials was semiquantitatively characterized with a new parameter called the fluid holding time (FHT). FHT...
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Langmuir 2000, 16, 5169-5177

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Fluid-Holding Time Parameter and Fluid Holding Capability of Polymeric Materials Christopher M. Weikart, Masayo Miyama, and Hirotsuga K. Yasuda* Center for Surface Science and Plasma Technology and Department of Chemical Engineering, University of MissourisColumbia, Columbia, Missouri 65211 Received August 26, 1999. In Final Form: February 28, 2000 The fluid holding capability of polymeric materials was semiquantitatively characterized with a new parameter called the fluid holding time (FHT). FHT measures the time it takes a continuous film to recede down the side of a Wilhelmy plate, which varies on different surfaces with the same degree of wettability. Both pure water and an artificial tear fluid (i.e., protein/salt solution) were used to systematically wet uncoated and plasma polymer coated polymeric plates. The FHT was calculated from Wilhelmy force loop plots in the event that a continuous water or aqueous film adhered to a polymeric surface. Such events are easily verified by consecutive wetting. Dynamic Wilhelmy force measurement is probably the most sensitive technique for monitoring wettability and surface configuration change, two important phenomenological factors that affect the FHT. The FHT measured by the Wilhelmy balance method can be effectively used to compare the liquid holding capabilities of different surfaces. The value of the FHT depends on experimental parameters and thus cannot be used in an absolute sense. In general, spontaneous wetting is the most favorable condition for producing continuous aqueous films on unperturbable surfaces. However, moderately hydrophilic and possibly even some hydrophobic surfaces perturbable by water were found to be capable of holding continuous films of water.

Introduction It is now generally accepted that water and aqueous solutions in close proximity to solid surfaces exhibit physical properties that are uniquely different from those of the bulk. Specifically, viscosity,1,2 dielectric constant,1-3 surface tension,1,4 hydrogen ion concentration,4 and, consequently, pH of this vicinal water layer have been found to be significantly different from the corresponding bulk liquid properties. The surface state moieties of the interfacing solid dictate the properties of this vicinal water and the extent of wetting. The water wettability in cellular structures and tissue interfaces is vitally important in biology. Living cells are thought to be composed of mostly vicinal water owing to the low volume-to-surface area ratio (∼0.01 µm) in such structures. The surface properties of the interfacing solid affect the thickness of the altered vicinal water layer, which may be as thick as 0.01-0.1 µm. The wetting characteristics of the interfacing solid in cellular structures directly affect the stability of the bounded aqueous solution. Wettability and stability of thin aqueous films is also critically important in the ocular environment.5-7 Placement of a contact lens in the eye introduces new interfaces, which require specific polymer/tear fluid interactions to * To whom all correspondence should be addressed. (1) Drost-Hansen, W. Structure of Water Near Solid Interfaces. Ind. Eng. Chem. 1969, 61, 10-47. (2) Davies, J. T.; Rideal, E. K. Interfacial Phenomena, 2nd ed.; Academic Press: London, 1963; p 369. (3) Hazelwood, C. F. Cell-Associated Water; Academic Press: New York, 1979; p 165. (4) Peters, R. A. Interfacial Tension and Hydrogen-Ion Concentration. Proc. R. Soc. London, Ser. A 1931, 33, 140-154. (5) Holly, F. J. Wettability and Wetting of Corneal Epithelium. Exp. Eye Res. 1971, 11, 239-250. (6) Holly, F. J. Formation and Rupture of the Tear Film. Exp. Eye Res. 1973, 15, 515-525. (7) Holly, F. J.; Lamberts, W.; Buesseler, J. A. The Human Lacrimal Apparatus: Anatomy Physiology, Pathology, and Surgical Aspects. Plas. Recon. Sur. 1984, 74 (3), 438-445.

ensure the stability of the tear film over and under the lens. An aqueous film over a hydrophilic solid is generally the most stable; however, characterizing water film stability over solids of variable wettability is not so straightforward. Since hydrophobic surfaces cause water to bead up into a sessile droplet, the only way to study aqueous film stability on such surfaces is to use a solid trough filled with water or a solid island submerged in water. One study,6 which used both approaches, showed that the thickness of these bounded water films became thinner than the maximum limiting thickness of an unbounded sessile droplet of water, which can be quite high on hydrophobic solids. As the thickness of the films in troughs and over islands decreased owing to evaporation, a critical thickness was reached at which time the films ruptured. The critical thickness was comparable in magnitude in both cases, but an order of magnitude lower than the limiting thickness of a sessile droplet. Characterizing aqueous film stability on highly hydrophilic surfaces is more difficult with the troughs and islands method, since the film may be stable down to the thickness of a monolayer without rupturing. The current study employed the Wilhelmy balance method and a new parameter called the fluid holding time (FHT) to characterize aqueous film stability or the fluid holding capability on mostly hydrophilic plasma polymer coated polymeric materials. If the surface properties of a material are favorable for the formation of a continuous aqueous film of fluid after wetting, its presence and stability can be characterized by the corresponding Wilhelmy force loop. More importantly, many different surfaces are wetted by water forming a continuous film, but the film recedes down the side of a vertical Wilhelmy plate at different rates. The origin of this discrepancy is not known. The objective of this study is to provide the principles of FHT measurement, which is a necessary prerequisite step for understanding the factors that govern the fluid holding capability of polymers. Therefore, the effects of phenomeno-

10.1021/la991152e CCC: $19.00 © 2000 American Chemical Society Published on Web 04/14/2000

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Figure 2. Wetting stages of Wilhelmy force loops on two different polymeric plates; the gray (i.e., LDPE plate) and black (i.e., O2 plasma treated glass slide) lines depict the absence and presence of a continuous water film, respectively. The consecutive wetting stages of a Wilhelmy force loop are as follows: (A) first immersion (Adv.1), (B) first emersion (Rec.1), (C) second immersion (Adv.2), and (D) second emersion (Rec.2). Immersion/ emersion speed was fixed at 5 mm/min.

Figure 1. Pictures of a continuous film of water on a plasma polymer coated glass slide (A) just after wetting and (B) after 2 min of exposure to air. Arrows indicate the region of the continuous water film. Plasma polymer was deposited from trimethylsilane + O2 (1 + 4sccm) at 115 W, 50 mTorr, and 4 min exposure time.

logical and methodological parameters and two fluid mediums, water and artificial tear fluid, on FHT were investigated. This paper lays the methodological foundation for subsequent papers related to this subject. Experiment Materials. Substrate materials included the following: microscope cover glasses (22 × 22 × 0.153 mm) and glass slides (25 × 25 × 1.0 mm) supplied by Fisher Scientific; nylon-6, PMMA, and PTFE plates (20 × 25 × 1.0 mm) supplied by Goodfellow Corporation; and siloxane-based hydrogel plates (24 × 24 × 0.3 mm) called Lotrafilcon A, i.e., a proprietary material courtesy of Ciba Vision Corporation. Lotrafilcon A is a biphasic block

copolymer composed of a highly-oxygen permeable siloxane-based polymeric phase and a hydrogel phase with 24 wt % water when fully hydrated. Microscope cover glasses and slides were cleaned ultrasonically in ethanol, thoroughly rinsed in distilled/deionized (DDI) water, and then dried in air. All conventional polymers were ultrasonically cleaned in soap water, rinsed thoroughly, and then dried in air. Lotrafilcon A plates were used as received in a dehydrated state without any further cleaning and stored in a refrigerator at 40 °C. The following gases were used to deposit plasma polymers: trimethylsilane (TMS) (97% minimum purity, PCR, Inc.), oxygen (99.99% minimum purity, Scott Specialty Gases, Inc.), methane (99.0% minimum) supplied by Matheson Gas Products, Inc.; air supplied by Midstates Airgas, Inc.; oxygen (99.9% minimum) supplied by Scott Specialty Gases, Inc.; and argon (99.9% minimum) supplied by Acetylene Gas Company. The probing fluids used in Wilhelmy balance measurements included DDI water and artificial tear fluid. DDI water was produced in-house: water was first distilled and then passed once through an ion exchange column. Ciba Vision Corporation provided the recipe for artificial tear fluid. The following ingredients for this fluid were purchased from Sigma Chemical Company: lysozyme (from chicken egg white 3× crystallized, dialyzed, and lyophilized, approximately 95%), albumin (bovine fraction V powder, 98% minimum), calcium chloride dihydrate (approximately 99%), sodium chloride (99.5% minimum), and phosphate buffer solution (8.3 mmol/L in DDI water). The artificial tear fluid recipe calls for dissolving 2.5 mg/mL lysozyme, 0.1 mg/mL albumin, 0.3 mmol/L calcium chloride, and 0.9% by vol. sodium chloride in phosphate buffer solution. Each component was slowly dissolved in a 200 mL volumetric flask; care was taken to avoid denaturing the lysozyme or albumin by stirring too rigorously. The surface tensions of DDI water and artificial tear fluid were determined by the DuNou¨y ring method employing the tensiometric Wilhelmy apparatus. The surface tensions for DDI water and artificial tear fluid were 71.9 and 54.5 mN/m, respectively, at 25 °C. The resistivity of the DDI water was 40 MΩ cm. Wilhelmy Balance Measurements. The Wilhelmy balance apparatus was composed of a Sigma 70 (KSV Instruments, Ltd., Finland) automatic tensiometer interfaced with a PC computer. The tensiometer measures the force exerted by water on a partially immersed thin plate with a measuring range of 25 mN

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Figure 3. Interpretation of fluid holding phenomena on vertical plates immersed in solution to a depth of 15 mm (indicated by the dotted line on each plate) and then raised to a depth of 5 mm. Corresponding force loop plots with fluid holding times of (A) 0, (B) 57, and (C) 115 s were calculated from a constant immersion/emersion speed of 5 mm/min. and a resolution of 1 µN. The total force exerted on a sample when it touches the surface of the water is given by the following force balance equation

Ftotal ) Mg - FgtHd + LγL cos θ

(1)

where Ftotal is the total force exerted on the sample, M is the mass of the plate, g is the gravitational acceleration, F is the liquid water density, t is the thickness of the plate, H is the width of the plate, d is the immersion/emersion depth, L is the plate perimeter (L ) 2(thickness + width)), γL is the liquid water surface tension, and θ is the contact angle at the liquid/solid/air contact line. Since the tensiometer is automatically zeroed when a sample plate touches the surface of the water, the gravitational force of the sample may be neglected. Therefore, the actual force measured by the tensiometer is given by the following equation:

F ) LγL cos θ - FgtHd

(2)

The measured force, F, in eq 2, is the difference between the interfacial force between the water and sample plate (wetting force) and the buoyant force from the immersed portion of the plate. The total force, F, is divided by the sample plate perimeter, L, to give force per unit length, F/L (expressed in mN/m), or the abscissa of force loop plots. Plasma Reactor System. Plasma polymer deposition and plasma modification of polymeric materials were performed in a bell-jar reaction chamber. A 15-kHz audio-frequency (AF) PE-

1000 generator (Advance Energy Industries, Inc.) powers a pair of titanium electrodes (18.1 × 18.1 × 0.16 cm) separated by a distance of 10 cm. A magnetron system3 is mounted on the back of each electrode. Each electrode is electrically insulated by a layer of glass slides. The magnetron system confines the glow discharge to the interelectrode space. Each sample spends about 20% of the total discharge time continuously rotating in and out of the glow discharge on a wheel. Thus, the total exposure time, not the total discharge time, is given in the caption of each figure. Continuous movement in and out of the glow discharge combined with magnetron enhancement improves the uniformity and reproducibility of resulting plasma polymer films. Further details of the reactor setup can be found elsewhere.8

Results and Discussion Surface Energy, Wettability, Surface Configuration Change, and Fluid Holding Capability. To clarify the discussions in the following sections, the intended meanings of several terms are presented below. The surface energy of a solid is generally referred to as the interfacial energy or interfacial tension of the solid with ambient air. In a strict sense, a “surface” is a hypothetical concept. The surface of a solid exists only as an interface. (8) Miyama, M.; Yasuda, H. K. Surface Dynamics of Plasma Polymers Studied by the Wilhelmy Force Measurement. Langmuir 1997, 14, 9960-964.

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Figure 4. Wilhelmy force loops of CH4 + air plasma treated cover glasses in (W) DDI water and (T) artificial tear fluid at varying second immersion velocities: (1) 2, (2) 5, (3) 10, and (4) 20 mm/min. Plasma discharge conditions were 1 sccm CH4, 2 sccm air, 38 W, 50 mTorr, 4 min. First and second emersion and first immersion velocities were fixed at 20 mm/min. The use of water and tear fluid yielded advancing contact angles, θD,a,1, means, and standard deviations of 47° ( 1 and 48° ( 3, respectively.

When the contacting medium is ambient air or vacuum, such an interface is generally termed a “surface.” Wettability is the ease with which an air/solid interface can be converted to a liquid/solid interface. The interfacial tension of a liquid/solid system is a direct measure of wettability. Surface configuration change due to wetting refers to the change in the configuration of chemical moieties in the surface state of a solid resulting from a change in the contacting medium from air to liquid (wetting). Probably the earliest known work confirming the necessary general condition for the formation of continuous liquids on high-energy solid surfaces, such as mica, quartz, and silica, was performed by Hardy et al.9 The subsequent (9) Hardy, W. B. The Tension of Composite Fluid Surfaces and the Mechanical Stability of Films of Fluid. Proc. R. Soc. London, Ser. A 1912, 86, 610-635.

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work of Derjaguin10 introduced the well-known empirical parameter called the disjoining pressure for the characterization of thin films in contact with high-energy solid surfaces. The disjoining pressure was thermodynamically related to regions of continuous liquid films up the side of a Wilhelmy plate. An extensive study11 of the regions of these liquid films showed that between 0 and 10 cm from the bottom of a Wilhelmy plate the corresponding film thickness varied from 0.04 to 0.5 µm, respectively. A continuous film of water on a hydrophilic (i.e., highenergy) plasma polymer coated glass slide moments after it was immersed to a depth of 3 cm in a beaker of DDI water is shown in Figure 1A. The water film remained continuous as it receded toward the bottom of the plate. After about 2 min the water film front receded to approximately 1 cm from the bottom of the plate, as shown in Figure 1B. The presence and stability of a continuous water film can be easily detected and quantified by the Wilhelmy method of systematic wetting. Most conventional polymers are moderately hydrophobic (i.e., they possess low surface energy); thus, spontaneous wetting by water does not occur on such polymers. When a liquid is placed on these moderately hydrophobic surfaces by the sessile droplet method, the liquid will not spread but will form a droplet with a measurable contact angle between the dry solid and sessile droplet surfaces at the three-phase contact line. Similarly, when a moderately hydrophobic plate is immersed in a liquid by the Wilhelmy balance method, a depressed meniscus is formed on the side of the plate with a definite contact angle that can be calculated by eq 2. Nonspontaneous wetting of polymeric solids occurs more often than not; however, improvement of the wettability of polymeric solids usually requires some type of surface modification. Spontaneous spreading of a liquid on a solid surface occurs when the interfacial tension is zero, in which case the adhesive attraction of the solid and liquid molecules is stronger than the cohesive attraction among the liquid molecules. Spontaneous wetting or a contact angle of zero is the postulated necessary condition for the formation of stable and continuous water films, but some moderately to poorly wettable surfaces are also capable of holding a continuous film of pure liquid water by virtue of surface configuration change. For example, the water droplet-rolling-off-angle (WDRA) is the angle at which a sessile droplet starts to roll down a sample surface that is being inclined. A sessile droplet of water placed on an extremely hydrophobic (i.e., with a static contact angle of ∼130°) and unperturbable (i.e., without surface configuration change) surface such as Teflon will bead up and exhibit a WDRA of approximately 40°. Thus, it is clearly not possible to form continuous water films on Teflon owing to the low adhesive tension at the solid/liquid boundary. One study15 showed that perturbable fluorinated plasma polymer coatings on nylon 6 fabrics exhibited contact angles and WDRA’s similar to Teflon. However, after repeated washing and drying, the WDRA increased to over 90° while the contact angle remained virtually the same. The change in the WDRA independent of the contact angle is a result of surface configuration change, which increased the adhesive ten(10) Derjaguin, B. The Repulsive Forces Between Charged Colloid Particles and on the Theory of Slow Coagulation and Stability of Lyophobe Sols. Trans. Faraday Soc. 1939, 36, 203-214. (11) Padday, J. F. Cohesive Properties of Thin Films of Liquids Adhering to a Solid Surface. Spec. Discuss. Faraday Soc. 1970, 1, 6474. (12) 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.

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Figure 5. Effect of Wilhelmy plate second immersion velocity (Adv.2) on the fluid holding time of water and artificial tear fluid on CH4 + air plasma treated cover glasses. FHT’s were calculated from the corresponding force loops of Figure 4. Plasma discharge conditions were 1 sccm CH4, 2 sccm air, 38 W, 50 mTorr, 4 min.

sion between the coated surface and liquid. thus making possible the formation of an aqueous film of water. Wettability alone becomes an increasingly inadequate criterion of fluid holding capability the more easily the polymer surface is perturbed by water. This problem stems from that which contact angles actually reflect, the force balance at the three-phase (i.e. solid/liquid/air) contact line. Aqueous film stability is dependent on the adhesive force or interfacial tension at the two-phase (i.e., solid/ liquid) boundary. The force balance at the two-phase boundary may change independently from the three-phase force balance owing to surface configuration change of interfacing surface state moieties, which occurs in order to minimize interfacial tension with water.12-16 Thus, an equilibrium contact angle formed at a stationary threephase contact line will not change within the time scale of contact angle measurement as a result of surface configuration change in the two-phase region. To see this effect, the three-phase contact line must move over the prewetted surface in order to reflect any changes in surface configuration from contact angle measurement. This can be done using the Wilhelmy method, but it is difficult to achieve by the sessile droplet method. When surface configuration change occurs, the three-phase contact line does not recede as water volume is reduced; in other words, water volume decreases without changing the contact area. Wilhelmy Force Loops and Fluid Holding Time. Wilhelmy force measurement is a versatile tool for studying the wetting properties as well as the surface dynamics of polymeric materials.8,11,17-19 Numerous studies with untreated and plasma polymer modified conventional polymers have shown that there are two (13) Yasuda, H.; Miyama, M.; Yasuda, H. Dynamics of the Surface Configuration 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. (14) Yasuda, H. K.; Okuno, T.; Sawa, Y.; Yasuda, T. Surface Configuration Change Observed for Teflon-PFA on Immersion in Water. Langmuir 1995, 11, 3255-3260. (15) Iriyama, Y.; Yasuda, T.; Cho, D. L. Plasma Surface Treatment on Nylon Fabrics by Fluorocarbon Compounds. J. Appl. Polym. Sci. 1990, 39, 249-264. (16) 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. (17) Miyama, M.; Yang, Y.; Yasuda, T.; Okuno, T.; Yasuda, H. K. Static and Dynamic Contact Angles of Water on Polymeric Surfaces. Langmuir 1997, 55, 5494-5503. (18) Wang, J. H.; Claesson, P. M.; Parker, J. L.; Yasuda, H. K. Dynamic Contact Angle and Contact Angle Hysteresis of Plasma Polymers. Langmuir 1994, 10, 3887-3897.

Figure 6. Wilhelmy force loops of CH4 + air plasma treated cover glasses in (W) DDI water and (T) artificial tear fluid at varying first emersion velocities: (1) 2, (2) 5, (3) 10, and (4) 20 mm/min. First and second immersion velocities were fixed at 20 and 10 mm/min, respectively, and second emersion was fixed at 20 mm/min. The use of water and tear fluid yielded advancing contact angles, θD,a,1, means, and standard deviations of 39° ( 2 and 48° ( 2, respectively. Plasma discharge conditions were 1 sccm CH4, 2 sccm air, 3 W, 50 mTorr, 4 min.

distinguishable hysteretic differences in Wilhelmy force loops: (1) the dynamic hysteresis (i.e., the difference between immersion and emersion F/L lines), which is related to the change of the meniscus shape at the beginning of each immersion and emersion step, and (2) the intrinsic hysteresis (i.e., the difference between the first and second immersion lines), which is related to the extent of surface configuration change resulting from wetting. Intrinsic hysteresis also varies according to surface wettability17 and Wilhelmy measurement parameters8 (i.e., immersion/emersion velocity and halting the plate motion after immersion or emersion). All of these factors can have a profound effect on the overall shape of a force loop. (19) Weikart, C. M.; Masayo, M.; Yasuda H. K. J. Surface Modification of Conventional Polymers by Depositing Plasma Polymers of Trimethylsilane + O2, II. Dynamic Wetting Properties. Colloid Interface Sci. 1999, 211, 28-38.

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Figure 7. Effect of Wilhelmy plate first emersion velocity (Rec.1) on the fluid holding time of water and artificial tear fluid on CH4 + air plasma treated cover glasses. FHT’s were calculated from the corresponding force loops of Figure 6. Plasma discharge conditions were 1 sccm CH4, 2 sccm air, 38 W, 50 mTorr, 4 min.

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Figure 9. Effect of halting the motion of Wilhelmy plates of CH4 + air plasma treated cover glasses in water and artificial tear fluid. FHT’s were calculated from the corresponding force loops of Figure 8. Plasma discharge conditions were 1 sccm CH4, 2 sccm air, 38 W, 50 mTorr, 4 min.

parallelogram-shaped force loops, since the intrinsic hysteresis is usually relatively small or even zero. This typical case is depicted in Figure 2. The deviation between the first and second immersion lines is a result of surface configuration change during the time scale of Wilhelmy force measurement. Several studies8,16,17 have shown that improving the wettability of moderately hydrophobic conventional polymers according to the ratio of trimethylsilane to oxygen gas mixture during plasma polymer deposition causes significant deviations from the ideal parallelogram-shaped force loops. Furthermore, the second immersion line starts to approach the first emersion line as the wettability increases. At some limiting wettability, a portion of the second immersion line retraces the first emersion line, as shown in Figure 2C. This retracing is a special case that has now been determined to be due to the presence of a continuous water film adhering to the polymer plate much like the one depicted in Figure 1. An extreme case of fluid holding capability occurs when the film is present over the entire wetted length of a Wilhelmy plate. This case is depicted in Figure 2 as the hook-shaped force loop formed during two wetting cycles. The continuous water film did not recede at all during the time scale of wetting, since the second immersion line retraced the first emersion line from a depth of 5-15 mm. The two contrasting force loops in Figure 2 are extreme cases of water film stability or the capability of polymeric surfaces to hold water. These and any intermediate cases in which the film recedes during the time scale of wetting may be quantitatively characterized by a new parameter called the fluid holding time (FHT) Figure 8. Wilhelmy force loops of CH4 + air plasma treated cover glasses in (W) DDI water and (T) artificial tear fluid with (1) no halting, (2) halting after first immersion for 20 min while immersed in liquid after first immersion to a depth of 10 mm, and (3) halting after first immersion for 20 min while immersed in liquid at a depth of 10 mm followed by halting after first emersion for 10 min in air at a depth of 5 mm. The use of water and tear fluid yielded advancing contact angles, θD,a,1, means, and standard deviations of 47° ( 3 and 48° ( 3, respectively. Plasma discharge conditions were 1 sccm CH4, 2 sccm air, 38 W, 50 mTorr, 4 min.

The extent of intrinsic hysteresis is easily determined by exposing a Wilhelmy plate to two consecutive wetting cycles, each consisting of one immersion step followed by an emersion step, and observing the extent of deviation between the first and second immersion lines. Most lowenergy polymeric surfaces exhibit the more common ideal

FHT ) d/v

(3)

where d is the length of the continuous film up the side of the Wilhelmy plate and v is the velocity of the plate during the second immersion. The immersion/emersion velocities may be arbitrarily fixed, but the length of the continuous film is determined from the length of the region where the second immersion line retraces the first emersion line on the force loop. It should be noted that placing error bars on the FHT parameter is difficult since it depends on wetting parameters, i.e., immersion/emersion velocity, and environmentally induced surface dynamics. However, within a batch of glass slides coated with the same hydrophilic plasma polymer exposed to the same ambient conditions for the

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Figure 10. Wilhelmy force loops of (A) nylon-6, (B) PMMA, and (C) PTFE plates in artificial tear fluid. At the end of the first emersion, the plates were held out of the solution for (1) 0, (2) 5, and (3) 40 min at a depth of 5 mm. The use of tear fluid on nylon-6, PMMA, and PTFE yielded advancing contact angles, θD,a,1, means, and standard deviations of 68° ( 3, 91° ( 3, and 130° ( 1, respectively.

same amount of time, the FHT parameter was quite reproducible within a 5% error. Three different cases of fluid holding capability indicated by FHT are depicted in Figure 3. Corresponding force loops and diagrams of the continuous water film position up the side of each Wilhelmy plate after immersion to a depth of 15 mm are presented. Parts A and C of Figure 3 depict two limiting cases of FHT, and Figure 3B is just one of many possible intermediate cases. The FHT in all three cases was determined with a fixed second immersion velocity of 5 mm/min. Wilhelmy Balance Wetting Parameters. Wilhelmy plate wetting parameters, such as plate velocity during immersion or emersion and halting the motion of the plate after immersion or emersion, have been shown to affect the intrinsic hysteresis,8 which consequently affects the overall shape of the force loop. These wetting parameters were found to affect FHT differently depending on whether pure water or an aqueous artificial tear solution was employed as the wetting medium. The tear solution, which is essentially a protein/salt solution, was compared with water to determine which medium would be more suitable for use in the study of fluid holding capability.

Like many other substances with hydrophilic and hydrophobic groups, proteins tend to migrate toward and spread across the interface between air and water forming a thin film. This results in a notably lower surface tension and consequently a lower force loop position than are obtainable with pure water, as shown in Figure 4. Withdrawing a Wilhelmy plate from protein solution deposits a continuous aqueous protein film onto the surface by physical adsorption similar to Langmuir-Blodgett film deposition. In the present study, the adsorbed film masked and altered the original surface properties of the plasma polymer modified glass slides. This is indicated in Figure 4 by the large deviation between the first and second immersion lines; however, unlike the case of wetting with water, the second immersion line does not retrace the first emersion line. This suggests that a condensed tear film does not form within the time scale of wetting, but more than likely a hydrated protein film forms on the wetted portion of the plate. The effect of the second immersion velocity on the force loop shapes of water and artificial tear fluid on plasma polymer coated glass slides and depicted in Figure 4. Increasing the second immersion velocity had virtually

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Figure 11. Wilhelmy force loops of (A) untreated, (B) CH4 + air plasma treated, and (C) CH4 + air then O2 plasma treated siloxane-based hydrogel plates using artificial tear fluid. The emersion/immersion velocities were all fixed at 5 mm/min. Discharge conditions for CH4 + air plasmas were 2 + 1 sccm, 33 W, 50 mTorr, and 10 min; those for O2 plasma were 1 sccm, 30 W, 50 mTorr, and 1 min. The dynamic, advancing, firstcycle contact angles, θD,a,1, are indicated on the graphs.

no effect on the overall shape of the force loops when tear fluid was used but increased the retracing region of the force loops when water was used. This means the adsorbed film from the artificial tear solution did not recede; however, the continuous water films receded down the side of the coated slides. The FHT’s of the tear fluid were calculated on the basis of the fact that the film did not recede providing a limiting case to compare with that of water as shown in Figure 5. Thus, the tear fluid FHT decreased with increasing second immersion velocity according to eq 3. The FHT for water is lower than the limiting value at 2 mm/min, since the water film recedes; however, it approaches the limiting FHT as the second immersion velocity is increased, since the water film does not have sufficient time to recede. The effect of first emersion velocity on force loop shape is shown in Figure 6. The force loops for tear fluid behave independently from the first emersion velocity, since the second immersion velocity is fixed in this case. Therefore, the tear fluid FHT is constant, as depicted in Figure 7. As evinced by Figure 6, the force loop shape for water is clearly dependent on the first emersion velocity, which controls the amount of time the plate and liquid are in

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contact before the next wetting cycle. The longer the plate and water contact each other (i.e., when there is low emersion velocity), the shorter the length of the retracing region. Apparently, the extended contact causes the surface to become more hydrophobic as indicated by the drop of the second immersion line past the first in Figure 6W1. This is most likely attributable to the washing away of residual hydrophilic plasma polymer oligomers from the surface. Furthermore, this may explain the peculiar spike in all the force loops for both water and tear fluid at a depth of 10 mm; however, examination of this phenomenon is beyond the scope of this study. Nevertheless, the hydrophobic transition disappears with increasing first immersion velocity, and the FHT approaches the limiting case, as shown in Figure 7. As depicted in Figures 8 and 9, respectively, halting the motion of the coated plates during immersion in the two liquids for 20 min before proceeding with the next wetting cycle had no significant effect on force loop shape or FHT. Halting the motion of the plates exposed to air after the first emersion was employed to force the continuous films to recede. The water film obviously receded while the hydrated protein film did not. However, the protein film dried somewhat, which lowered the position of the second immersion line slightly in Figure 8T3. Most conventional polymers wetted with water exhibit typical parallelogram-shaped force loops19 with no indication of the presence of a continuous aqueous film. However, when wetted with artificial tear fluid, a few conventional polymers of varying wettability displayed some second immersion line retracing of first immersion lines, as shown in Figure 10. This indicates that a short continuous aqueous film of artificial tear fluid was present on the polymersseven on highly hydrophobic PTFE. Halting the motion of polymer plates after the first emersion and exposing the wetted surface to air for extended periods caused the film to dry-leaving behind adsorbed proteins. This rendered it impossible to characterize fluid holding capability, since the original surface properties were masked regardless of surface wettability. Thus, as is evident in Figure 11, characterizing fluid holding capability of untreated and plasma-modified contact lens materials using artificial tear fluid is impossible. Conclusions The fluid holding time (FHT) measured by the Wilhelmy balance method was found to distinguish the liquid holding capabilities of different surfaces of variable wettability. More importantly, spontaneous wetting of hydrophilic surfaces was semiquantitatively characterized by the FHT, which is essentially the time it takes a continuous film of fluid to recede down the side of a Wilhelmy plate. The FHT cannot be used in an absolute sense because it depends on experimental parameters and because it depends on time-dependent surface configuration change, which is different for every surface and occurs during the time scale of wetting. This paper laid the methodological foundation for understanding the interfacial phenomena that governs the fluid holding capabilities of polymeric materials. In general, spontaneous wetting is the most favorable condition for producing continuous aqueous films on unperturbable surfaces. However, many different hydrophilic surfaces of similar wettability can hold continuous films but recede at different rates. This discrepancy depends on interfacial factors that are not known. Furthermore, moderately hydrophilic and possibly even some hydrophobic surfaces that are perturbable by

Fluid Holding Capability of Polymeric Materials

water were found to be capable of holding continuous films of water. Multicomponent fluid, such as a dilute solution of protein used as a simulated tear fluid, may yield misleading liquid holding characteristics of surfaces owing to preferential adsorption of components on a surface. However, the presence of adsorbed proteins on the surface of four different polymers of varying wettability was

Langmuir, Vol. 16, No. 11, 2000 5177

evident from the corresponding hook-shaped Wilhelmy force loops. Acknowledgment. The authors are grateful to Mr. Yasuo Matsuzawa of Ciba Vision Corp. for preparation of the contact lens material plates and to Ciba Vision Corp for project funding. LA991152E