Novel AutoCAD method for performing surface energetics analyses.
Numerous fields of science and engineering require detailed understanding of material surface characteristics in order to select or properly prepare surfaces. A common and familiar example is that paints are many times formulated for very specific applications. Spreading and adhesion of paint onto a target surface strongly influences the appropriateness of that paint for particular surfaces. For these reasons paint intended for coverage of wood surfaces are often unsuitable for application to plastic or metal surfaces. Additionally, considerations must also be made concerning the surface characteristics of dried paints: Dried paints formulated for applications such as metal surfaces exposed to outdoor conditions should have low surface energies in order to repel precipitation and thus protect the underlying materials from oxidation. In another entirely different and strongly contrasting utilization, surface wetting characteristics of plant and tree leaves have also been studied by environmental toxicologist to demonstrate exposures to automobile exhaust (Pal et al. 2002) and exposure to sulfur and deposition of heavy metals (Turunen et al. 1997).
Under certain circumstances, observing changes in wetting properties of solid surfaces may be the only rapid and reliable method for confirming the presence or absence of modifications made to those surfaces. The application of self assembled monolayers (SAMs) to gold-coated silica or glass is one such situation. A readily accessible and inexpensive technology capable of imaging these chemical monolayers is currently unavailable. Yet, by observing the contact angles of probe liquids of known polarities on these surfaces pre-and post-application of a SAM, researchers can compare changes in surface energies and determine the degree of success of the procedure. Hydrophobic/hydrophilic interactions between the solid surfaces and the applied probe liquids rapidly allow inferences to be made regarding any observed changes.
Since surface energetics analyses and tensiometry are essential to many fields and have been in use for decades, many methods of acquiring and analyzing contact angles have been developed, and much debate has gone into determining which methods produce the most accurate and precise results. In fact, So many methods exist that to list and describe each here would not give fair treatment to any, and would be beyond the scope of any single manuscript. A review of literature comparisons reveals the Wilhelmy Balance method to be the benchmark, followed by the tilted plate and the sessile drop methods (Lander et al. 1993; Chibowski et al. 2002; Krishnan et al. 2005).
While this need to testing the wetting properties of solid surfaces has widespread application, the associated costs may be prohibitive to small laboratories where research grants may not be prevalent: Low-end commercial manual goniometers can cost $5,000-$6,000 new, and frequently require additional proprietary software and dedicated computer equipment in order to be fully functional. This report represents the use of inexpensive apparatus adapted for goniometric contact angle [theta] analyses and an innovative use of AutoCAD [C] 2005 drafting software for determining [theta] values.
MATERIALS AND METHODS
Probe liquids and surface preparation.--Ninety-nine percent pure diiodomethane (Alfa Aesar, Pelham, NH, USA), 99.0% ethylene glycol (VWR International), and ultrapure water were utilized as probe liquids. Selection of this triad was based on the simplification of surface free energy ([[gamma][S.sup.TOT]) calculations via their utilization, and due to the preponderance of use in literature.
Droplets of probe liquids (1.0[micro]l) were applied to test surfaces via hand delivery with 10.0[micro]l glass syringes (Hewlett Packard pn5181-1267). Syringes were washed and autoclaved prior to use. Separate syringes were used for each probe liquid. The syringe for diiodomethane was wrapped in Al foil during the procedure to prevent photodegradation of its contents. Droplet images were acquired within 3-5sec of deposition in order to minimize evaporative loss.
Glass and silica wafers were immersed in chromic acid for 24 h, thoroughly rinsed with ultrapure water, and dried in a Precision Model 70D Laboratory Oven for 12h at 100[degrees]C. Gold layers applied to glass slides and silica wafers were tested within one week of the electrospray gold deposition and atomic force microscopy. Self-assembled monolayers were likewise tested immediately upon completion (l-2h) of their deposition and drying.
Self-assembled monolayer.--SPT-0014 ([C.sub.29][H.sub.50][O.sub.9][S.sub.2]; SensoPath Technologies, Bozeman, MT, USA) a dithiol-tethered SAM precursor molecule, was dissolved in an organic solvent and deposited onto gold coated glass slides from solution. Upon completion, films were rinsed with anhydrous ethanol (Fisher Scientific, Pittsburg, PA 15219, USA) and dried under nitrogen stream.
Apparatus.--An adjustable instrument base with bubble leveler (Clay Adams, Parsippany, NJ 07054, USA) provided the foundation of this apparatus (Figure 1). Aluminum optical mounting dovetails (40 by 50mm) with 1/4"-20 counter bores (Thorlabs, Inc., 435 Route 206 North Newton, NJ 07860) were mounted onto the metal base as attachment points for the digital microscope and stage. A Digital Blue[TM] QX5 [TM]Computer Microscope (e-bay, USA) was mounted by using two, 1/2" posts and 3" post holders (Thorlabs, Inc.) which were mounted to optical mounting dovetails. These mounting dovetails were joined to a single base-mounted dovetail via 40mm double dovetail clamps (Thorlabs, Inc.). This arrangement allowed horizontal and vertical adjustments of the microscope. To the second base-mounted dovetail, an in-house machined three-planar goniometer was similarly affixed. A 49.0 c[m.sup.2] aluminum plate was attached to the goniometer to serve as the stage. A 30.5cm section of 40mm optical dovetail rail was mounted to the metal base for attaching the diffuse light source.
[FIGURE 1 OMITTED]
Analysis.--Fifty kilobyte, 512 x 384 digital images (JPG) of l.0[micro]l droplets liquids resting on solid surfaces were acquired with a Digital Blue [TM] QX5 [TM] Computer Microscope with the focus of the image on the profile of the droplet at its greatest height and diameter (Figure 2). Images were then imported into the AutoCAD [C] 2005 design window where tools were employed to overlay simple geometric shapes onto the images. First, a circle was overlaid onto the image and manipulated to match the profile of the droplet surface. This was accomplished by altering the diameter of the circle and moving the location of the mathematical center. Once this circle was best fit onto the image, a single straight line (Line 1) was applied to match the edge of the solid surface onto which the droplet is resting. The combination of this circle and Line 1 become the basis for which [theta] calculations are performed.
[FIGURE 2 OMITTED]
Contact angle was determined by the application of two additional straight lines and a rectangle to the image. Line 2 was formed by connecting the center of the circle and the point of intersection of the circle and Line 1, and was facilitated by the AutoCAD [C] design software: Points of intersection and center points of circles are highlighted during modification of these files and are readily accessed by the user. The point where Line 2 intersects the circle forms one vertex of a rectangle that has Line 2 as the bottom side. A rectangle was then inserted into the design window where the program auto-positions this shape so that two sides will parallel Line 1. The rectangle was rotated to a position where one side is coincident with Line 2. A third line (Line 3) was then applied to the image using the side of the rectangle which lies closest to the circle and is perpendicular to Line 2. At this juncture, all elements are in position for determining [theta] for the droplet and solid surface (Figure 3).
[FIGURE 3 OMITTED]
<TOOLS> <INQUIRY> <DISTANCE> are chosen from the AutoCAD [C] toolbar at the top of the design window. Two points within the design window are selected in order to receive output from this function. Two points along Line 3 are selected to generate [theta]. Theta is reported in degrees by the AutoCAD [C] software in the Command dialog box at the bottom of the screen as "Angle in XY Plane = ...".
Surface free energy estimation.-Surface free energy estimations were calculated by inserting experimentally determined mean [theta] values into the van Oss-Chaudhury-Good (vOCG) equation
(1 + cos[theta])[gamma]L = 2([square root of ([gamma][S.sup.LW])] [square root of ([gamma][L.sup.LW])] + [square root of ([gamma][S.sup.+])] [square root of ([gamma][L.sup.-])] + [square root of ([gamma][S.sup.-])] [square root of ([gamma][L.sup.+])])
(Van Oss 2004). For the vOCG computational method, known reference values of the polar and dispersion components of a triad of pure liquids (Table 1) are utilized, along with experimentally determined [theta] values for each. Three equations with three unknowns are then solved to obtain the values of [gamma][S.sup.LW], [gamma][S.sup.+] and [gamma][S.sup.-]. The total surface free energy is then calculated as [gamma][S.sup.TOT] = [gamma][S.sup.LW] + 2([square root of ([gamma][S.sup.+])] [square root of ([gamma][S.sup.-])]). Microsoft (R) Excel (C) (XP, 2003) was used to generate surface free energy estimations.
Table 1. Total surface tension and components (in mJ/[m.sup.2]) for water (W), diiodomethane (DIM), and ethylene glycol (EG) used for surface energy estimations. [gamma] [gamma] [gamma] [gamma] Liquid [gamma]L [L.sup.LW] [L.sup.AB] [L.sup.-] [L.sup.+] Water 72.8 21.8 51 25.5 25.5 DIM 50.8 50.8 0 0 0 EG 48 29 19 47 1.92
Single factor analyses of variances (ANOVAs) were performed on contact angles for each probe liquid between surface treatments (none, gold, SAM). Statistical comparisons were not made between data for glass and silica, nor were they made between the separate probe liquids.
Mean contact angles for droplets of probe liquids resting on foundations of silica wafers and glass slides and the surface treatments were consistent, with standard errors generally increasing as the [gamma][S.sup.TOT] values increased (Table 2, Figures 4 & 5). Standard deviations for contact angles of droplets of all three probe liquids resting on each surface as determined by this method ranged from 1.07 to 9.59 (Table 2, Figures 4 & 5), with the poorer performance occurring for water on gold (glass foundation).
[FIGURE 4 OMITTED]
[FIGURE 5 OMITTED]
Table 2. Sessile drop contact angles (in degrees) of probe liquids (Water, DilodoMethane, and Ethylene Glycol) on foundations of glass microscope slides and silica wafers with various surface treatments and total surface free energies (mJ/[m.sup.2]) calculated with the vOCG formula. Surface [gamma] Foundation treatment [theta] W [theta] DIM [theta] EG [S.sup.TOT] Glass None 32.4 50.9 19.0 46.00 [+ or -] [+ or -] [+ or -] 1.07 2.96 2.94 Glass 20A gold 78.7 45.5 55.2 41.20 [+ or -] [+ or -] [+ or -] 9.59 1.72 4.52 Glass SAM 37.9 22.5 21.4 62.84 [+ or -] [+ or -] [+ or -] 9.21 3.31 6.83 Silica None 62.0 54.7 50.5 39.62 [+ or -] [+ or -] [+ or -] 3.43 2.00 3.27 Silica 20A gold 60.1 37.6 42.3 50.91 [+ or -] [+ or -] [+ or -] 4.36 3.10 3.56 Silica SAM 53.1 31.1 30.0 55.54 [+ or -] [+ or -] [+ or -] 9.24 8.85 6.48
Surface free energy estimations for solid foundations and treatments, as expected, were highly variable depending on uppermost exposed layer contacting the probe liquids, and ranged from 31.81 mJ/[m.sup.2] (silica, no surface treatment) to 62.84mJ/[m.sup.2] (Au-coated glass with SAM) (Table 2). One way ANOVA p-values (n=10, [alpha]=0.05) within probe liquids between surface treatments indicated extremely significant differences (p[less than or equal to]1.04[E.sup.-9]) existed between all surface treatments for both glass slides and silica wafers.
The treated and untreated surfaces were expected to interact uniquely when brought into contact with the probe liquids. This was experimentally verified by comparing the statistical mean of the respective contact angles. Statistically significant and consistent probe liquid contact angle results between these surface treatments confirm the successful deposition of gold and the subsequent deposition of the SAMs.
The applicability of this method to research is demonstrated most fully by the contact angle results shown after the SAM deposition onto the gold surfaces. While the application of the gold was confirmed visually and through atomic force microscopy prior to these analyses, no analytical method is capable of confirming the successful deposition of a chemical monolayer as rapidly and as contact angle determination. Although the standard deviations from the means of the [THETA] values were elevated above desirable levels for probe liquids applied to SAMs on both foundations, the statistical analyses demonstrate that relatively inexpensive equipment and software can be coupled in a manner which is capable of producing reliable results.
The standard deviations were consistently elevated for contact angles of probe liquids on SAM's and were randomly elevated for several other surfaces (Table 2). While no surface energetics analyses studies for a SAM identical or similar to the one used herein could be located, Lu et al. (1998) reported [theta] values for the same triad of liquids on a hexadecyltrichlorosilane (HTS) SAM, similarly formed on both glass and silicon. Due to the molecular differences between these SAM's, the contact angle means are not comparable, but the standard deviations for those measurements are useful and ranged from 0.8 to 5.0, whereas standard deviations in this study ranged from 3.31 to 9.24. Incomplete deposition of SAM's onto the gold substrates (e.g., islands of exposed gold) is a likely explanation for elevating the standard deviations of observed means. Additionally, 1.0[micro]l droplets of probe liquids were approaching the lower limit of volume that can be accurately measured and hand delivered. This phenomenon was addressed when Lander et al. (1993) determined that the sessile drop method gave the lowest contact angles and was the least reproducible when compared to the Wilhelmy balance and tilted plate methods. In spite of these limitations, the sessile drop method continues to be a valuable tool while receiving ongoing scientific validation (Tadmor & Yadav 2007). Under ideal circumstances, probe liquid droplets should be mechanically measured and delivered, but the results show that hand delivery of droplets was a suitably reproducible.
Of all methods of contact angle determination currently in use, the nearest equivalent to the [theta] values generated herein is the advancing contact angle ([[theta].sub.a]), regardless of which method is used, although it has been concluded that needle presence in droplets affects the three phase boundary line (Lander et al. 1993). Since sessile droplets are at static hydraulic pressure, while advancing and receding contact angles ([[theta].sub.r]) are at increasing and decreasing hydraulic pressures, respectively, the sessile [theta] for any given liquid will reside between [[theta].sub.a] and [[theta].sub.r], but is more closely associated with [[theta].sub.a]. Chibowski et al. (2002) reported that the [[theta].sub.a] for water, DIM, and EG on glass microscope slides using the tilted plate method were 33.07 ([+ or -]2.06), 46.12 ([+ or -]0.74), and 24.95 ([+ or -]2.36), which were in good agreement with corresponding experimentally determined [theta] values of 32.4 ([+ or -]1.07), 50.9 ([+ or -]2.96), and 19.0 ([+ or -]2.94) (Table 2). Chibowski et al. (2002) also found that [[theta].sub.a] values for DIM and water on glass microscope slides generated with the syringe method were 47.75 ([+ or -] 1.24) and 29.91 ([+ or -] 1.69), respectively, which also strongly agree with corresponding experimental [theta] values of 50.9 ([+ or -] 2.96) and 32.4 ([+ or -] 1.07) (Table 2). Radelczuk et al. (2002) reported similar findings for water and EG on glass slides with [[theta].sub.a] values of 33.6 ([+ or -] 1.1) and 48.8 ([+ or -] 0.8), respectively, which compared well with corresponding [theta] values of 32.4 ([+ or -] 1.07) and 50.9 ([+ or -] 2.96) (Table 2).
Contact angles determined on cleaned glass microscope slides and the subsequently calculated surface energies have become the predominant standard literature values for method and calculation comparisons. Observed [gamma][S.sup.TOT] estimate for cleaned glass slides (46.0mJ/[m.sup.2]) was in good agreement with literature values using the [[theta].sub.a] values of the same triad of probe liquids (Radelczuk et al. 2002; [[gamma s].sup.TOT] = 41.5mJ/[m.sup.2], Chibowski et al. 2002; [[gamma][S.sup.TOT] = 43.5[+ or -] 0.7mJ/[m.sup.2]). Surface free energy estimates for cleaned glass slides were also in strong agreement with the mean of [gamma][S.sup.TOT] calculations determined with [theta] hysteresis for seven probe liquids (Radelczuk et al. 2002; [gamma][S.sup.TOT] = 50.6 [+ or -] 9.7mJ/[m.sup.2]) and through the [[theta].sub.a] and [[theta].sub.r] measurements of eight combinations of six probe liquid triads by the tilted plate method (Chibowski et al. 2002; [gamma][s.sup.TOT]= 50.6 [+ or -]8.8mJ/[m.sup.2]). Since the [gamma][s.sup.TOT] estimates for cleaned glass slides were calculated with a commonly used triad of probe liquids and method, the agreement seen between three findings and literature values demonstrate the suitability of this method of calculating [theta].
Volpe et al. (2003) stated that there is no "handbook-level" collection of contact angles of common liquids on common solids, nor measured or calculated surface free energies of common solids, and that 21 st century scientists are not in agreement, even on the values of surface free energies of common solids. As such, it becomes apparent that variability between results remains inherent to the study of surface energetics, and that even large differences between data are to be expected. However, for the data presented herein, no such accommodations are required: Observed experimentally determined contact angles easily fall within range of frequently encountered data, while the calculated value for surface free energy of glass agrees strongly with values seen in current literature.
Several steps of this procedure rely heavily on the patience and consistency of the analyst. The first and most important step is acquiring an analysis-appropriate image of the droplet on a solid surface. As mentioned earlier, it is essential that the focal point of the image be the profile of the droplet at its greatest height (true midpoint, Figure 3). This is to ensure that the points of contact between the droplet and solid surface are also clearly in focus. Macro photographs can be focused along a continuum of the droplet from leading edge to beyond. If the contact points at maximum droplet height are not properly in focus, application of the circle and Line 1 in AutoCAD [C] cannot be accurately achieved. It is essential to the success of the analysis that the droplet be in sharp focus thereby blurring the edge of the solid surface: Figures 2 and 3 show the blurring of the solid surface while the droplet and its reflection at point of contact remain clearly visible. Gaclawski & Urbaniak-Domagala (2007) emphasize the significance of capturing the reflection point clearly for the sessile drop method. For the analysis described, images should be of the highest quality resolution so pixilization is avoided: As images can be enlarged in the AutoCAD [C] design window during [theta] determination to better fit the components, increased resolution ensures accurately fitting the components necessary for analysis.
An additional point is that the profile of a droplet resting on a surface is rarely the perfect arc of a circle bisected by a plane. This is especially true for droplets of low energy liquids laid onto low energy surfaces (eg., EG on glass, Figure 2b). While excessive droplet spreading may prevent the user from accurately applying a circle to such an image, the persistence of the user in fitting the circle to the best capabilities of the software increases the reliability and reproducibility of the results. Under instances where application of an ellipsoid to droplet profile is more appropriate, spheres in AutoCAD [C] can be elongated along the horizontal axis accordingly without negatively effecting results.
Surveys of existing literature for universal agreement on method of contact angle determination, acceptable probe liquids, and standard free energy values for common surfaces demonstrates that variability between these elemental components remains an open problem. Currently, investigators seem to have narrowed these problems and indications are that surface free energies are constant only under ideal conditions and, in reality, are best taken as the result of averaged values (Volpe et al. 2003, Chibowski et al. 2002).
In spite of the limitations to the method described herein, developing an in-house apparatus and method for contact angle analysis is beneficial in many aspects. As demonstrated, the usefulness of such a method in confirming successful surface treatments can be readily accomplished by novice operators. If these analyses are performed by using commonly accepted probe liquids, contact angle data can be inserted into surface free energy equations which result in estimations which concur strongly with literature values. In laboratories where surface energy studies play a small but significant role, such economical and reliable equipment and methods have great potential for applicability.
Primary debt is to the United States Army's Research, Development and Engineering Command (RDECOM) for providing the funding that made this project possible. Great appreciation also goes to Dr. Timothy Dallas, Ph.D. of Electrical Engineering at Texas Tech University for inspiring the development of this method and for his invaluable input and support.
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LM at: firstname.lastname@example.org
Les N. McDaniel, Nicholas A. Romero, George P. Cobb and Gopal Coimbatore
Department of Environmental Toxicology
The Institute of Environmental and Human Health
Texas Tech University, Box 41163, Lubbock, Texas 79409-1163
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|Author:||McDaniel, Les N.; Romero, Nicholas A.; Cobb, George P.; Coimbatore, Gopal|
|Publication:||The Texas Journal of Science|
|Date:||May 1, 2008|
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