examples of surface characterization and modification experiences

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Surface Characterization

Jacob Feste

November 20th, 2014

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Abstract

The objective of the experiment was to determine the surface properties, specifically the surface energies and tensions of various materials. The materials tested in this experiment include a glass slide, parafilm, red silicone, polystyrene, and aluminum. In order to determine the surface energies of these materials, each of the materials were subjected to droplets of water and ethanol. The contact angle of these droplets were measured by both a contact angle meter and Tantec’s half angle method. The contact angles measured then give a direct correlation to the surface energies of these materials. The results displayed that red silicone had the largest mean contact angle and polystyrene had the smallest with water, with red silicone, parafilm and aluminum all over 90 degrees. All of the contact angles with ethanol were small (less than 45 degrees), with red silicone at the largest and glass slide with the smallest. These results conclude that red silicone has the lowest surface energy and critical surface tension with parafilm and aluminum low as well and polystyrene had the highest of these properties with glass slide high as well. They also conclude that ethanol has a much lower surface tension than water.

Introduction

Surface properties are very important properties to consider in biomedical applications. The specific surface properties measured in the experiment include surface energy and indirectly, surface tension. The surface energy of a surface formed between two different compounds correlates to the altercations of intermolecular bonds when the surface is formed. It also correlates to the degree of the forces of cohesion, the intermolecular attraction between similar molecules, and adhesion, the intermolecular attraction between dissimilar molecules, between the two compounds. The surface energy of a material is important for factors such as the degree of wetting, or spreading of a liquid over a surface. The lower the surface energy between the material and liquid, the more adhesion the dissimilar molecules have with each other and consequently the more the liquid is able to spread out over the surface, or the more wetting will occur. Wetting is an important aspect of surface interactions and is directly related to surface tension and the interfacial free energy of the surface. It can be generalized that wetting occurs when the surface tension of the liquid is lower than the critical surface tension of the surface it is in contact with. In terms of the interfacial free energy of the surface; the

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higher the interfacial free energy of the surface the more wetting, or spreading, of the liquid will occur, and vice versa. Different biomedical applications may call for different desirable surface energies and degrees of wetting between the surface of the material being used and the liquid it will be in contact with in the body, for instance, blood. In order to determine the surface energy of a created surface between a material and a liquid, measurement of the contact angle of the applied liquid provides one of the simplest and reliable methods of determining this. The contact angle is the angle at which the edge of the liquid makes with the surface it is applied to. The contact angle is a function of the surface free energies of the liquid and the solid. Young’s equation gives the following relationship.

Eq. (1): Γsv = Γsl + Γlvcos(Θ).

Where Γsv is the solid surface free energy, Γsl is the solid/liquid interfacial free energy, Γlv is the liquid surface free energy and Θ is the contact angle of the liquid droplet with the surface. The contact angle can be measured quite simply by methods such as Tantec’s half angle method or by contact angle meter, which makes it a very simple and efficient tool for understanding the surface energy between a liquid and a solid. In correlation to the wetting of the liquid, complete wetting occurs when the contact angle is zero and therefore also has a very high interfacial surface energy with the surface tension of the liquid much lower than the critical surface tension of the surface. With this experiment using water as its primary liquid for testing, this tends to occur with strongly hydrophilic solids. In general, if the solid is less hydrophilic it will exhibit lesser degrees of these properties given and the liquid will form a bead as it is not completely wetted. This property is exhibited when the contact angle is between zero and ninety degrees. A contact angle greater than ninety degrees suggests a hydrophobic solid and almost no wetting. As suggested, the surface properties of different materials vary immensely based on their chemical make-up and properties. Metals generally have a high surface energy and high critical surface tension likely due to their high energy of the metallic bonds. Polymers, on the other hand, are much more varied in these properties and may have either high or low surface energies and are directly determined by their chemical make-up and structure. Polymers that exhibit hydrophilic characteristics with frequent high-energy bonds such as covalent bonds can be suggested to have a high surface energy, and vice versa. Composites will generally have a very high surface energy as the polarity of the cation-anion bonding greatly increases hydrophilicity and the ionic bonding between these molecules contains very high energy.

In this experiment, our goal was to determine the surface properties of various materials including glass slide, parafilm, red silicone, polystyrene, and aluminum. In order to determine these properties each of these materials were tested by using both water and ethanol as the liquid and measuring the contact angle formed. In order to measure the contact angles formed, both a contact angle meter and Tantec’s half angle method were used. While the contact angle meter simply uses specified machine to make this measurement, Tantec’s half angle method is a little more complex in nature. Tantec’s half angle measurement proves to be

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efficient as it eliminates error from the small, random tangent line of the droplet measured. For instance, a droplet may have a very brief, steep slope on its base but rounds off in a way at which this steep slope isn’t a completely accurate measurement of the true contact angle but is comparatively close and hard to measure accurately. The contact angle meter measures this angle. Tantec’s half angle method is a way of verifying the accuracy of this implication. Tantec’s equation is given as:

Eq. (2): Θ = 2* arc tan (H/R)

Where Θ is the solved, not measured, contact angle. H is the height of the droplet and R is the radius of the droplet’s base. Both of these methods are performed in order to give accurate representation of each contact angle. By finding the contact angle of these surfaces, we are able to accurately give estimations of their surface energies. The surface energies of the measured materials may be hypothesized by their chemical properties and structure. The glass slide in this experiment can be suggested to have a high surface energy and therefore a small contact angle. This is due to glass’s identity as a composite. Glass generally has the formula SiO2, but most glass and most likely the one tested in this experiment are generally have sodium carbonate added to increase toughness [1]. The bonding between the anionic oxygen and cationic sodium create a polar, hydrophilic environment and the energy of these bonds are very high. Parafilm is composed of a mixture of paraffin wax and polyolefins [2]. Paraffin wax has a composition anywhere from C20H42 to C40H82 [3], while polyolefins consist of hydrocarbons with double bonded carbons [4]. This composition suggests hydrophobicity. The double bonded carbons are decently high in energy but are few in number compared to the amount of single bonded hydrocarbons. Parafilm can conclude to have a relatively low surface energy. Red silicone has a structure similar to glass but without the cationic sodium, and therefore is a polymer. It consists of alternating silicon and oxygen atoms [5], which are not very polar bonds and not too high in energy as the carbon bonds seen in some of the other tested polymers. It can infer to have a very low surface energy. Polystyrene is made up of hydrocarbon chains with phenyl rings branching off [6]. While the hydrocarbon chains may provide little to surface energy, the phenyl rings each contain three double bonds, and are found at every other carbon in the hydrocarbon chain. With the decently high energy of these double bonds in combination with the very high concentration of these bounds, polystyrene can be suggested to have a very high surface energy. The final material, aluminum, is a post transition metal. Metals such as aluminum generally have a very high critical surface tension and a high surface energy. However, metals are also very smooth and hard and resist adhesion to other molecules such as water. Therefore aluminum will likely have low surface energy in this experiment due to this sense of hydrophobicity. In terms of the liquids tested, both water and ethanol are used. Water is commonly seen in the body, and the suggestions listed regard to their surface energies with water due to polarity or hydrophilicity reasons. Ethanol is much different than water in that it is much more polar due to its hydroxyl group. Therefor it may be hypothesized that the surface energies will be much greater in ethanol than in water, indicated by small contact angles. In regards to the experiment performed, a null hypothesis that each of these surfaces will give the

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same contact angle for both liquids, and therefore have the same surface energy, is suitable. However a true hypothesis based on the information presented may be made if this null hypothesis is rejected. If each of these materials are subjected to water droplets, then the hydrophilic surfaces with the high bond energies, glass slide and polystyrene, will exhibit small contact angles between zero and ninety degrees and therefore have the highest surface energies. The hydrophobic materials, red silicone, parafilm, and aluminum, will show large contact angles greater than ninety degrees in water. It can also be hypothesized that if these materials are subjected to both water and ethanol droplets, then the materials will have a much higher surface energy and much smaller contact angle with ethanol than with water due to polarity.

Procedures

Equipment/Supplies and Materials Used

I. Contact Angle Meter

II. Sample Surfaces

a. Glass Slide

b. Parafilm

c. Red Silicone

d. Polystyrene

e. Aluminum

III. Water

IV. Ethanol

V. Syringe

The procedure of this experiment was done to measure and record the contact angle of ethanol and water on the surface listed above. A syringe was first used to drop five droplets of water in a line on the surface of the glass slide. This procedure was done two times for water and twice for ethanol. Each droplet contained 15 microliters of water/ethanol. Using the laboratory materials and the contact angle meter, the left most point of the water/ethanol droplet was aligned with y axis, and the bottom surface of the droplet was aligned with the x axis. The contact angle meter was then also used to measure the height, diameter, contact angle and tantec’s half angle for each water droplet. The contact angle meter was also used to measure the radius, contact angle, and tantec’s half angle of each droplet of ethanol. This

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procedure was repeated for each material surface type. Ten measurements were recorded in total for each ethanol and water droplet on each surface type. All observations and data obtained was subsequently recorded.

ResultsI. Water:

Table 1: Glass Slide Calculations for Height, Diameter, Contact Angle Measurements, and Percent Error Calculations for each Water Droplet along with mean and standard deviation measurents

Glass SlideDroplet Height

(mm)Diameter (mm)

Contact Angle Meter (degrees)

Tantec's 1/2 Angle (degrees)

Percent Error

1 10 36 55 58.11 5.35%2 10 36 55 58.11 5.35%3 10 30 60 67.38 10.95%4 10 38 58 55.52 4.47%5 10 34 50 60.93 17.94%6 10 34 65 60.93 6.68%7 12 24 78 90.00 13.33%8 10 30 70 67.38 3.89%9 10 30 60 67.38 10.95%10 8 30 53 56.14 5.60%

Height (mm) Diameter (mm)Contact Angle Meter (degrees)

Mean 10 32.2 60.4Standard Deviation 0.94280904 4.157990968 8.500980336

Table 2: Parafilm Calculations for Height, Diameter, Contact Angle Measurements, and Percent Error Calculations for each Water Droplet along with mean and standard deviation measurents

ParafilmDroplet Height

(mm)Diameter (mm)



Percent Error

1 14 24 99 98.80 0.21%2 14 24 100 98.80 1.22%3 16 22 97 110.98 12.60%4 16 24 100 106.26 5.89%5 14 22 100 103.69 3.55%6 14 22 95 103.69 8.38%7 16 20 100 115.99 13.79%

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8 12 24 100 90.00 11.11%9 14 24 100 98.80 1.22%10 14 24 98 98.80 0.81%


Mean 14.4 23 98.9Standard Deviation 1.26491106 1.414213562 1.728840331

Table 3: Red Silicone Calculations for Height, Diameter, Contact Angle Measurements, and Percent Error Calculations for each Water Droplet along with mean and standard deviation measurents

Red SiliconeDroplet Height

(mm)Diameter (mm) Contact Angle Meter Tantec's 1/2 Angle

Percent Error

1 22 30 110 111.43 1.28%2 22 30 110 111.43 1.28%3 22 30 110 111.43 1.28%4 22 30 115 111.43 3.21%5 20 30 110 106.26 3.52%6 22 30 110 111.43 1.28%7 22 32 110 107.95 1.90%8 22 30 110 111.43 1.28%9 22 30 115 111.43 3.21%10 22 32 110 107.95 1.90%


Mean 21.8 30.4 111Standard Deviation 0.63245553 0.843274043 2.108185107

Table 4: Polystyrene Calculations for Height, Diameter, Contact Angle Measurements, and Percent Error Calculations for each Water Droplet along with mean and standard deviation measurents

PolystyreneDroplet Height

(mm)Diameter (mm)



Percent Error

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1 10 22 60 84.55 29.03%2 10 30 55 67.38 18.37%3 8 30 50 56.14 10.94%4 10 28 50 71.08 29.65%5 10 30 40 67.38 40.64%6 8 28 45 59.49 24.36%7 6 30 45 43.60 3.20%8 8 30 48 56.14 14.51%9 8 16 62 90.00 31.11%10 8 16 62 90.00 31.11%



Table 5: Aluminum Calculations for Height, Diameter, Contact Angle Measurements, and Percent Error Calculations for each Water Droplet along with mean and standard deviation measurents

AluminumDroplet Height

(mm)Diameter (mm)



Percent Error

1 14 24 99 98.80 0.21%2 14 24 100 98.80 1.22%3 16 22 98 110.98 11.70%4 16 24 100 106.26 5.89%5 14 22 100 103.69 3.55%6 14 22 95 103.69 8.38%7 16 20 100 115.99 13.79%8 12 24 100 90.00 11.11%9 14 24 100 98.80 1.22%10 14 24 100 98.80 1.22%



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II. Ethanol:

Table 6: Glass Slide Calculations for Height, Radius, Contact Angle Measurements, and Percent Error Calculations for each Ethanol Droplet along with mean and standard deviation measurents

Glass SlideDroplet

Height (mm) Radius (mm)Contact Angle Meter (degrees)

Tantec's 1/2 Angle (degrees) Percent Error

1 2 7.7 21 29.12 27.89%2 2 8 25 28.07 10.94%3 2 8 25 28.07 10.94%4 2 18 25 12.68 97.15%5 4 10 30 43.60 31.20%6 4 14 30 31.89 5.93%7 4 14 30 31.89 5.93%8 4 14 30 31.89 5.93%9 4 14 30 31.89 5.93%10 4 14 30 31.89 5.93%


Mean 3.2 12.17 27.6Standard Deviation 1.03279556 3.499222136 3.306559138

Table 7: Parafilm Calculations for Height, Radius, Contact Angle Measurements, and Percent Error Calculations for each Ethanol Droplet along with mean and standard deviation measurents

ParafilmDroplet



1 4 14 30 31.89 5.93%2 6 14 40 46.40 13.79%3 6 14 40 46.40 13.79%4 6 14 41 46.40 11.63%5 6 14 40 46.40 13.79%6 6 14 40 46.40 13.79%7 6 14 40 46.40 13.79%8 6 14 40 46.40 13.79%9 6 14 40 46.40 13.79%10 6 14 40 46.40 13.79%

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Mean 5.8 14 39.1Standard Deviation 0.63245553 0 3.212821536

Table 8: Red Silicone Calculations for Height, Radius, Contact Angle Measurements, and Percent Error Calculations for each Ethanol Droplet along with mean and standard deviation measurents

Red SiliconeDroplet



1 6 14 41 46.40 11.63%2 6 14 43 46.40 7.32%3 6 14 42 46.40 9.48%4 5 14 41 39.31 4.31%5 5 14 40 39.31 1.76%6 6 14 42 46.40 9.48%7 4 14 40 31.89 25.43%8 6 14 40 46.40 13.79%9 6 14 41 46.40 11.63%10 5 14 41 39.31 4.31%


Mean 5.5 14 41.1Standard Deviation 0.70710678 0 0.994428926

Table 9: Polystyrene Calculations for Height, Radius, Contact Angle Measurements, and Percent Error Calculations for each Ethanol Droplet along with mean and standard deviation measurents

PolystyreneDroplet



1 4 14 37 31.89 16.02%2 4 14 39 31.89 22.29%3 4 14 39 31.89 22.29%4 4 15 39 29.86 30.60%5 4 14 38 31.89 19.16%6 4 14 38 31.89 19.16%7 4 14 38 31.89 19.16%

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8 4 14 38 31.89 19.16%9 5 14 38 39.31 3.33%10 4 14 37 31.89 16.02%



Table 10: Aluminum Calculations for Height, Radius, Contact Angle Measurements, and Percent Error Calculations for each Ethanol Droplet along with mean and standard deviation measurents

AluminumDroplet



1 5 14 40 39.31 1.76%2 5 14 38 39.31 3.33%3 5 16 30 34.71 13.56%4 5 14 38 39.31 3.33%5 5 14 38 39.31 3.33%6 5 14 38 39.31 3.33%7 5 14 38 39.31 3.33%8 5 14 38 39.31 3.33%9 6 14 40 46.40 13.79%10 6 14 40 46.40 13.79%



Table 11: T-test results of Water/Ethanol Contact Angles on different material surfaces.

Comparison p-values Degrees of Freedom Significance T-criticalwater vs. ethanol on polystyrene 0.00016998

118 95% 1.734

water on glass vs. aluminum 4.40203E-08 18 95% 1.734water on red silicone vs. polystyrene

1.37602E-10 18 95% 1.734

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water vs. ethanol on parafilm 1.60002E-17 18 95% 1.734

Discussion

The results of this experiment accurately reinforce the implications made about the various material’s surface energies and the differences between the water and ethanol. With water, polystyrene had the lowest mean contact angle at 51.7 degrees with the lowest mean height of the water droplet at 8.6mm indicating the most wetting. Glass slide also had a low mean contact angle at 60.4 degrees with a low mean height at 10 mm. Red silicone had the highest mean contact angle at 111 degrees and the highest mean droplet height at 21.8mm. Parafilm and aluminum also had high mean contact angles greater than 90 degrees, with parafilm at 98.9 degrees and aluminum at 99.2 degrees. These results indicate that polystyrene had the highest surface energy and most wetting occur, with glass slide having a high surface energy and wetting as well. With their mean angles under 90 degrees, it can also be implied that these materials are somewhat hydrophilic as well. The results also indicate that red silicone had the lowest surface energy with water, and that little wetting occurred. Parafilm and aluminum also had low surface energies with water and experienced little wetting. With red silicone, parafilm, and aluminums mean contact angles greater than 90 degrees, it can also be suggested that these materials are hydrophobic and therefore experience little adhesion with water. The tests performed with the ethanol droplets gave much different results. Glass slide had the lowest mean contact angle at 27.6 degrees with a mean droplet height of 3.2mm, while red silicone had the highest mean contact angle at 41.1 degrees with a droplet height of 5.5mm. Parafilm, polystyrene, and aluminum had mean contact angles of 39.1, 38.1, and 37.8, respectively. The main conclusion these results support is that ethanol was much different than water in that all of the materials had relatively high surface energies indicated by their low contact angles (all less than 45 degrees) and therefore experienced much more wetting. These results can be primarily explained by the much higher degree of polarity in ethanol than water. This therefor indicates that ethanol has a much lower surface tension than water; a low enough surface tension to cause a high degree of wetting in all of the surfaces while water’s surface tension was low enough to only cause a decent degree of wetting in the glass slide and polystyrene. Multiple t-tests were performed to determine whether or not different variables impacted the contact angle of different surfaces, as illustrated in table 11. The null hypothesis being tested for each of these tests was that the factors had no impact on the contact angle and therefore the contact angles should be the same with the different variables. The first t-test tested whether or not the different liquids, water and ethanol, had an impact on the contact

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angle formed with polystyrene. The p values indicate that they do indeed have an impact as illustrated by the data. The next t-test tested whether or not the contact angle with water on the glass slide vs water on aluminum was the same. The p values also indicate that they were much different. The third t-test tested water on red silicone vs water on polystyrene. Once again, the p values suggest a large difference. Finally, a t-test was performed to determine if water vs ethanol had an effect on the contact angle formed on parafilm. The p values suggest a big difference as well. All of the t-tests performed rejected the null hypothesis. That is, whether the liquid is water or ethanol has an impact on the contact angle, and the same liquid applied to two different surfaces will give different contact angles. The main implication these tests conclude is that no two liquids or solid surfaces will give the same contact angle and therefore the contact angle is impacted by the liquid composition and the solid composition. This is supported by the data given for each material subjected to the different liquids.

The variations between the contact angles are a direct result of the composition of both the liquid and the surface. Factors such as the hydrophilicity of the solid, the energy of the bonds in the solid, and the surface tension of the liquid in respect to the lower critical surface tension of the solid have a big impact on the contact angle and surface energy. Polystyrene and the glass slide’s contact angles were likely small and indicating a high surface energy with water due to their hydrophilicity and bond energies as expected. They also had high enough lower critical surface tensions to allow water to spread. Parafilm, red silicone, and aluminum has large contact angles greater than 90 degrees suggesting low surface energy likely due to their hydrophobic nature and smaller lower critical surface tensions in which the water’s surface tension was not low enough to cause much wetting. The chemical make-up of these materials as previously examined suggest this hydrophobic nature and therefore these results were expected. On the other hand, all of the surfaces showed small contact angles less than 45 degrees and therefore high surface energies with ethanol. This is likely due to ethanol being much more polar than water while having a low enough surface tension to surpass all of the materials lower critical surface tensions. This allowed more wetting to occur and is indicated by the small droplet heights found in each of the tests. The strikingly similar small contact angle values were likely due to the ethanol being able to spread a similar amount for each material due to its low surface tension and the high polarity of ethanol allows it to disrupt the bonds of each surface as well, hydrophobic or not. This suggests that the properties of ethanol seemingly overwhelmed the properties of the surfaces to the extent at which the contact angles were almost entirely a factor of the ethanol and not the surface, as indicated by the similar values. As previously identified by Eq. (1) (Young’s equation), the contact angle is a function of the surface energies of the system. The greater the contact angle, the less the surface energy of the solid. This is due to the liquid being able to spread more on higher energy surfaces as the higher energy surfaces have a higher lower critical surface tension for which the liquid to be lower than which allows the spreading. This is illustrated by our results as the surfaces with the chemical make-up indicating high surface energies gave small contact angles while those indicating low surface energies gave high angles. In generally, the more polar or hydrophilic a

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surface is the higher its surface energy. This is due to the molecules being more susceptible to rearrangement than non-polar molecules as polar bonds are easier to break when their polar counterparts are present from another surface. In addition, the energy between the bonds is also an important factor as the energy of a bond that is disrupted is released upon disruption, which applies to the surface energy. Therefore, a material that exhibits a large degree of hydrophilicity in order to allow bond rearrangement, in addition to these bonds being of high energy, would expect to have a high surface energy. On the other hand, a completely non-polar, hydrophobic in which bond rearrangement is nearly impossible in addition to very low bond energies would expect to have a low surface energy. These properties also correlate to surface tension as the hydrophilic, high energy material would have a much higher lower critical surface tension for the liquid to be lower than and spread than would the hydrophobic, low energy material. The critical surface tension of glass is 73 dynes/cm [7], parafilm is 23 dynes/cm [8], red silicone is 24 dynes/cm [9], polystyrene is 64 dynes/cm [10], and aluminum is 500 dynes/cm [11]. The critical surface tension indicates how likely a liquid will spread on a surface, with the highest spreading the most. These known values correlate pretty accurately to the measured contact angles, and glass and polystyrene have high critical surface tension values with low measured contact angles and red silicone and parafilm have low values with high contact angle measurements. Aluminum, on the other hand, follows a different pattern with an incredibly high critical surface tension but with a large contact angle over 90 degrees. While the critical surface tension may be high for aluminum suggesting a small contact angle, this large angle and low surface energy may be a result of the its inherent hydrophobicity. Aluminum is indeed a metal and therefore may be too smooth and hard to allow adhesion. I would use Polystyrene as a cell culture surface because it has the lowest contact angle with water and therefore the highest free surface energy. It also has an intermediate critical surface tension and is not too smooth or hard like aluminum. This is important as polystyrene’s high surface energy allows the cells to spread or wet very efficiently in order to grow and proliferate. Water and ethanol had a large difference in contact angle measurements in which ethanol gave similar and much lower angles. This was previously discussed and likely due to ethanol’s higher polarity and lower surface tension than water. Ethanol’s low surface tension was seemingly low enough for each of the materials critical surface tensions unlike water in which it was allowed more wetting. It also was polar enough to move bonds around in which water couldn’t. Limitations on sessile drop contact angle measurements primarily lie on the principle of its local-only application and its requirement of a liquid to solid interface. It is also limited on the fact that it requires a flat solid surface. An application may call for different geometrical shapes of which this method would only test the flat surface properties of a material. There may be variations in properties between a curved surface of a material and a flat one. There are various ways of characterizing free surface energy. One of these methods is the Fowkes method. The Fowkes method is an efficient, accurate way to measure the surface energy of a nonpolar solid [12]. It uses the equation Γs= Γs

d=0.25 Γl(1+cos Θ)2 and it’s advantage lies in the fact that it simplifies the free surface energy measurements for non-polar solids while being highly accurate in doing so [12]. Another method is the Owens-Wendt method [12]. This method uses

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the simplicity and accuracy of the Fowkes equation but its advantage allows its application to polar solids as well by using two measuring liquids and a few more substitution equations [12]. Thirdly, the Van Oss-Chaudhury-Good method builds on the Fowkes equation even more than the Owens-Wendt method by using three measuring liquids with three more substitution equations to solve the system [12]. It is more difficult to perform than the Owens-Wendt method but its advantage lies in its seemingly perfect accuracy due to its complexity and may also be used for polar solids and polymers [12]. There are also a variety of methods performed to manipulate surface energies. One method reverses the surface tension by applying an electric potential through embedded electrodes [13]. Another method uses laminar flows or UV light to generate irreversible surface tension gradients [13]. A final method applies a very small positive charge, less than 1 volt, causing an electrochemical reaction and creating an oxide layer on a metal which lowers its surface tension [14].

Conclusion

The goal of this experiment was to identify the relative surface energies of a glass slide, parafilm, red silicone, polystyrene, and aluminum. In order to achieve this, droplets of both water and ethanol were separately induced on each material and the contact angle of these droplets measured using a meter and Tantec’s half angle method for accuracy. Water and ethanol were used in order to determine how the composition of the liquid droplet used effects the surface energy of the system. By measuring the contact angle we are able to obtain a relative surface energy of the surface due to the contact angle being a function of surface energy. It was hypothesized that the glass slide and polystyrene would have the smallest contact angle and therefore the highest surface energy with water. This was due to the chemical composition of the two: the glass being a polar, hydrophilic composite material with high bond energies due to the cation-anion bonds and the polystyrene being hydrophilic with a large multitude of high energy double bonds. Parafilm, red silicone, and aluminum were all foreshadowed to have large contact angles greater than 90 degrees due to their hydrophobic natures of their chemical composition. It was also hypothesized that ethanol droplets would produce much smaller contact angles and therefore much higher surface energies for each of the materials than water. This was implied due to the higher polarity of ethanol from its hydroxyl group and its lower surface tension than water. All of the objectives were met in this experiment and all of the hypothesized outcomes were proven to be true from the data. To first provide that the contact angle is indeed effected by both the solid surface composition and the composition of the liquid droplet, multiple t-tests were performed with different variables involving the solid and liquid composition as illustrated in table 11. These t-tests all rejected the null hypothesis and prove that both the solid and liquid composition have an impact. The data also supports the implications made on how the composition impacts the contact angle as

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illustrated by tables 1-10 above. When subjected to the water droplets, polystyrene had the lowest mean contact angle at 51.7 degrees with the glass slide low as well at 60.4 degrees. Both of these also showed a good degree of wetting proved by their low mean droplet heights. These properties support that polystyrene and the glass slide had the highest surface energy with water. On the other hand, red silicone had the largest mean contact angle at 111 degrees with parafilm and aluminum large as well at 98.9 degrees and 99.2 degrees, respectively. These materials also had relatively large droplet heights as well and therefore primarily beaded. These angles over 90 degrees support their hydrophobic compositions. In terms of the liquid composition, our data also supports the hypothesis that ethanol will exhibit smaller contact angles and give higher surface energies than water due to its chemical composition. The mean contact angles were low for each material, all lower than 45 degrees and following a similar order of size to that of the water droplets but much more similar in value. It can therefore be concluded that the surface energy of a surface is directly impacted by both the liquid being administered and the solid surface. More specifically, the more polar and the lower the surface tension of the liquid the smaller the contact angle and the higher the surface energy as a liquid is able to enable bonds to rearrange much more efficiently with a higher polarity and lower surface tension. In addition, materials with hydrophilic or polar compositions, containing bonds with high energy, and having a larger lower critical surface tension will give smaller contact angles and therefore have higher surface energies and vice versa. While the experiment seemed to be performed pretty accurately, there is always the possibility of error. The main possibility for error would likely lie in the focusing of the machine. The glass slide and polystyrene tests weren’t as focused as the other three tests for water which could give inaccurate measurements. Human error was also a strong possibility because the counting of very small boxes at a large extent could easily be miscounted and skew the data slightly. However, the use of ten droplets for each material helped eliminate much of this possibility for error. This experiment would need a large amount of error to give inaccurate conclusions of which was most likely not the case and therefore the conclusions in this experiment likely provide to be accurate implications. Understanding the surface properties of a material is very important for biomedical applications and the increasing discovery of these properties may lead to endless applications.

References

[1] "Is Glass a Polymer?" Is Glass a Polymer? N.p., n.d. Web. 03 Dec. 2014.

[2] "Parafilm® Barrier Films - Comparison of Properties." Parafilm M® Laboratory Barrier Film. N.p., n.d. Web. 03 Dec. 2014.

[3] "Paraffin." Princeton University. N.p., n.d. Web. 03 Dec. 2014.

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[4] The Editors of Encyclopædia Britannica. "Polyolefin (chemical Compound)."Encyclopedia Britannica Online. Encyclopedia Britannica, n.d. Web. 03 Dec. 2014.

[5] The Editors of Encyclopædia Britannica. "Silicone (chemical Compound)."Encyclopedia Britannica Online. Encyclopedia Britannica, n.d. Web. 03 Dec. 2014.

[6] The Editors of Encyclopædia Britannica. "Polystyrene (chemical Compound)." Encyclopedia Britannica Online. Encyclopedia Britannica, n.d. Web. 03 Dec. 2014.

[7] "The Critical Surface Tension of Glass." - The Journal of Physical Chemistry (ACS Publications). N.p., n.d. Web. 03 Dec. 2014.

[8] Verlag, Vincentz. "Dynamics of Surfactant Enhanced Spreading." European Coatings Journal (1998): n. pag. European-coatings.com. Web. 03 Dec. 2014.

[9] Andriot, M. "Silicones in Industrial Applications." N.p., n.d. Web.

[10] Nueman, Micheal R. An Introduction to Biomaterials. 2nd ed. N.p.: n.p., n.d. Print.

[11] "Composite Material." Uotechnology. N.p., n.d. Web.

[12] Żenkiewicz, M. "Methods for the Calculation of Surface Free Energy of Solids." AMME. N.p., n.d. Web.

[13] Desai, Tejal, Sangeeta Bhatia, and Mauro Ferrari. BioMEMS and Biomedical Nanotechnology Volume III Therapeutic Micro/Nanotechnology. Boston (MA): Springer, 2007. Print.

[14] "Researchers Control Surface Tension to Manipulate Liquid Metals." NC State News Researchers Control Surface Tension to Manipulate Liquid Metals Comments. N.p., n.d. Web. 03 Dec. 2014.

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Surface Wetting Modifications via Small-Scale Topography and Nanomaterial Applications

Jacob Feste

University of Arkansas, Biomedical Engineering, [email protected]

mailto:[email protected]

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Abstract

The goal of this experiment is to determine how changes in surface topography and surface chemistry influence the wettability and SFE of a surface. Glass, Teflon, and aluminum were subjected to contact angle measurements using primarily-polar water and primarily-dispersive DIM. The glass samples included bare glass, sandblasted glass, and sandblasted glass treated with FAS-17. The aluminum samples included sandblasted aluminum and sandblasted aluminum treated with FAS-17. The Teflon samples included bare Teflon and sanded Teflon. The glass samples increased in wettability and SFE upon sandblasting while decreasing significantly and becoming hydrophobic upon sandblasting and FAS-17 treatment. The sandblasted aluminum samples resulted in significantly increased wettability and SFE upon FAS-17 treatment. The Teflon samples, on the other hand, decreased in wettability and SFE upon sanding.

Nomenclature

θ ,θγ= Measured contact angle (degrees)

θ¿= True contact angle (degrees)

γsl= Solid-liquid interfacial energy/surface tension (mN/m)

γlv= Liquid-vapor interfacial energy/surface tension (mN/m)

γsv= Solid-vapor interfacial energy/surface tension or SFE (mN/m)

γsvd = Dispersive component of γsv (mN/m)

γlvd = Dispersive component of γlv (mN/m)

γsvp = Polar component of γsv (mN/m)

γlvp= Polar component of γlv (mN/m)

r= Surface roughness parameter (unitless)

f = Areal surface fraction (unitless)

Introduction

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The objective of this experiment is to perform a variety of surface modifications on various materials and observe their surface wettability in order to determine how the modifications impact their surface characteristics. Surface wettability is observed when a liquid substance is applied to a solid surface. When a drop of liquid is applied to a solid surface, the liquid will arrange itself in a specific geometry that minimizes the total energy of the solid-liquid interaction [2]. The energy of the solid-liquid system is dependent on factors such as attractive/repulsive intermolecular interactions, electrostatic contributions, and polar contributions [1]. Therefore, surface wettability is primarily influenced by the chemical composition of the liquid and the surface, of which these factors reside. While these factors are too small to be observed, the resulting geometry gives implications about the influence that these factors have on the total energy of the solid-liquid-vapor interaction. The geometry of a liquid applied to a surface may be analyzed to characterize surface wettability by measuring the contact angle between the two phases. The contact angle is given as θγ in the following figure:

Figure (1): Contact angle and surface tension relationships for a solid-liquid system [3].

By measuring the contact angle, other parameters such as surface tension may be determined. Surface tension may be described as the amount of work needed to increase the surface area of the interface, and is dependent on the sum of the cohesive forces [4]. According to figure (1), surface tension arises between the solid and liquid (γsl), liquid and vapor (γ lv), and solid and vapor (γsv). The contact angle measurement and surface tension values are related for a variety of physical properties. For instance, the “force” balance at the solid-liquid-vapor interface in the x-direction is given by:

Equation (1): γsv=γ sl+γlv cos θγ

Furthermore, the contact angle may be described in terms of the interfacial surface tensions via the Young equation [6]:

Equation (2): cosθ γ=γ sv−γ slγlv

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These relationships may be used to calculate the surface tensions and free energies involved in the system, and therefore the influences of the cohesive forces. However, the relationships given by equation (1) and equation (2) involve many surface tension variables, of which are usually unknown. These equations also assume a perfectly smooth and symmetrical solid surface while not accounting for the inherent roughness involved in every surface [5]. The surface free energy (SFE), or the surface tension between the solid and vapor interaction ( γsv), thermodynamically represents the wetting behavior of the system and is defined as the reversible work required to create a new unit of surface area. The Young equation requires a known solid-liquid interfacial energy (γsl) in order to calculate an estimated SFE, of which fails to consider other important interactions. Interactions such as dispersive, polar, ionic, hydrogenic, and other interactions may be accounted for by considering γsl as a function of the other two interfacial energies (γsv∧γlv) [5]. Owens and Wendt accounted for both dispersive and polar interactions to generate the following relationship:

Equation (3): γsl=γ sv+γ lv−2(√γ svd∗γlvd +√γ svp∗γ lvp )By applying this relationship to the Young equation, the contact angle relationship is given as:

Equation (4): (1+cosθγ ) γlv=2(√γsvd∗γlvd +√γ svp∗γ lvp ) Finally, the SFE of a solid may be calculated by measuring the contact angles of two different liquids and applying them to equation (4). A primarily-dispersive liquid and a primarily-polar liquid are desired for simpler calculations [5]. In addition, the contact angle itself may be analyzed in order to give general assumptions about the surface. By applying water to a solid surface, the resulting contact angle may be measured in order to estimate the hydrophilicity/hydrophobicity and polarity of the solid material. Hydrophilic materials are polar, “water-loving” materials that only require a small amount of energy to form new surfaces with water. These materials will have a high SFE with water. Hydrophobic materials are nonpolar and act to avoid water, requiring a large amount of energy to form new surfaces [7]. Therefore, a contact angle between 10o and 90o indicates a polar, hydrophilic material with significant degree of interfacial surface formation, with an angle less than 10 o considered superhydrophilic. On the other hand, a contact angle between 90o and 150o indicates a nonpolar, hydrophobic material with little surface formation, with an angle greater than 150 o

considered superhydrophobic [5].

The previous equations relate the contact angles of solid-liquid-vapor interactions to the surface tensions of the system for a perfectly smooth and solid surface. These equations may be utilized in order to determine estimated values such as SFE. Such values may be considered only as estimates as the equations only account for interfacial interactions, however fail to account for the inherent roughness involved in every surface. Surface roughness has an impact on the wettability of all surfaces and may be modified in order to produce a surface with desired surface characteristics. There are currently two models that may be used to account for

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surface roughness, the Wenzel model and the Cassie-Baxter model [8]. The Wenzel model accounts for the complete wetting of roughened surface and is given as:

Equation (5): cosθ¿=rcosθ

with the surface roughness parameter, r, being greater than one for a roughened surface and increasing with roughness. The surface roughness parameter is given as the ratio of roughened surface area to the projected surface area. The true contact angle, θ¿, is then given by relating this parameter to the contact angle of a perfectly smooth surface composed of the same material, or θ [5]. This model ultimately accounts for surface roughness via an increase in surface area, represented by the surface roughness parameter. The Cassie-Baxter model, however, accounts for surface roughness in a different fashion. Instead of considering the gaps that result from roughness as increased surface area, this model considers the gaps as thermodynamically stable air-filled valleys. The Cassie-Baxter model considers the surface as a composite material composed of the surface material and the air-filled valleys, with the impact of roughness accounted for by the ratio of wetting surface area to the total surface area [5]. The Cassie-Baxter relationship is given as:

Equation (6): cosθ¿=fcos (θ+1 )−1

where f is the areal surface fraction described as the ratio of wetting surface area to total surface area and is less than or equal to one for all surfaces [5]. The relationships given by these models highlight the significant impact that surface roughness has on wettability. According to these models, by modifying the surface roughness of a material it is possible to modify its surface wettability. Surface wettability may be modified by changes in either surface topography or surface chemistry. Surface topography may be modified by removing material from the surface (top-down) or adding material to the surface (bottom-up) [5]. Surface chemistry modification is similar to the bottom-up modification method, however surface chemistry modification involves material addition by the attachment of molecules through covalent bonds. Surfaces may be chemically modified at specific sites and with molecules of desired chemical properties in order to produce a surface with desired surface characteristics. This modification, referred to as chemical functionalization, can be a difficult and complex procedure to perform due to its high specificity and sensitivity. Self-assembling structures allow for easier surface chemistry modifications due to their ability to self-organize. Thin films called self-assembled monolayer films (SAMs) are formed by dispersing the chemically active material in a solvent and applying it to the substrate (surface) in the liquid phase. The SAM molecules will then self-organize over time due to chemisorption and van der Waals forces [9]. This process results in a thin film covalently bound to the surface with binding dependent on the chemical composition of the film and the applied surface. This experiment will perform top-down surface topography modifications via sandblasting and surface chemistry modifications via SAM application in order to determine the influence that these modifications have on surface wettability. The simultaneous effects that both of these modifications have on surface wettability will also be determined by applying nanoparticle films to sandblasted materials.

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Finally, the impact that surface roughness has on wettability at the micro/nanoscale will be further explored by sanding a material and observing changes in wettability. The contact angles for each trial will be measured using two different liquids in order to calculate SFE.

Materials

Substrate materials:

Glass slides (bare and sandblasted) Teflon sheets (bare only) Aluminum alloy plate (bare and sandblasted)

Surface modification materials:

SNOWTEX ST-PS-M colloidal SiO2 nanoparticles (20%-by-weight solution) FAS-17 and denatured ethanol 220 grit sandpaper

Liquids for contact angle and surface energy measurements:

Deionized water Diiodomethane (DIM)

Materials for sample preparation:

Acetone Isopropyl alcohol Deionized water Several 50 mL and 800-1000 mL beakers

Equipment:

Sonicator Scale (with .1 g resolution) Hotplate (with magnetic stirrer) Timer Tweezers and diamond scribers CA goniometer with camera Optical microscope

Samples Tested (one of each):

Bare aluminum Bare glass Bare Teflon

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Sandblasted aluminum Sandblasted glass Sanded Teflon SiO2 nanoparticles on sandblasted aluminum FAS-treated sandblasted glass

Procedures

SiO2 Nanoparticle Deposition on Sandblasted Al:

1. Dilute 25g of SNOWTEX with 25g of deionized water in a 50 mL beaker to produce a 10%-by-weight SiO2 nanoparticle solution.

2. Stir for 20 minutes with a magnetic stirrer.3. Carefully dip a sandblasted aluminum sample into the nanoparticle solution for

several seconds using tweezers to deposit a nanoparticle film on the sample surface.

4. After dipping, place the sample on a 300oC hot plate for several seconds to evaporate any moisture in the nanoparticle films.

FAS-17 Treatment of Sandblasted Glass:

1. In a 50 mL beaker, add 0.3 g of FAS-17 and 14.3g of denatured ethanol to make a solution of FAS-17 and denatured ethanol for a solution with 1.6 mmol of FAS-17 per mole of solution.

2. Submerge the sandblasted glass sample in the FAS-17 solution for 10 minutes to deposit an FAS-17 film on the sample surface.

3. Afterward, place on a 130°C hotplate for 10 minutes to evaporate any remaining solvent.

Teflon Roughening:

1. Use 220 grit sandpaper to roughen a 1”x1” area of Teflon sheet. 2. Rub in random directions for 20 sec. 3. Use a diamond scribe to mark the back side of these samples.

Water Contact Angle Measurements:

1. Contact angle measurements should be performed on all samples using deionized water as the probe liquid.

2. Using the goniometer, drop a ~1 µL deionized water droplet onto the sample surface.

3. Take 3 measurements at different locations on each sample for averaging purposes.

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4. Repeat each step using ~2 µL diiodomethane (DIM) droplets for all samples except the SiO2 nanoparticles on sandblasted aluminum.

Note: Gloves should be worn at all times when handling samples, and take special care to not touch or otherwise damage the sample surfaces.

Note: The Teflon samples are fairly malleable; if a sample will not sit flatly on the goniometer stage, it can usually be flattened by clamping one end of the sample with tweezers and “torqueing” the free end until the sample is flat.

Cleaning Procedure for Teflon:

1. Sonicate for 5 mins in isopropyl alcohol.2. Rinse with deionized water and dry with nitrogen.

Cleaning Procedure for the Remaining Samples:

1. Sonicate for 20 mins in acetone 2. Sonicate for 10 mins isopropyl alcohol 3. 3. Rinse with deionized water and dry with nitrogen

Results

Figure (2): Average water contact angle measurements for each sample including standard deviations.

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Figure (3): Average DIM contact angle measurements for each sample including standard deviations.

Figure (4): Average water and DIM contact angle measurements for each sample for comparison.

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Figure (5): Surface free energy (SFE) calculations for each sample.

Figure (6): Microscope images from left to right: glass (10x), sandblasted glass (10x), FAS-17 treated sandblasted glass (10x).

Figure (7): Microscope images from left to right: sandblasted aluminum (10x), FAS-17 treated sandblasted aluminum (10x), FAS-17 treated sandblasted aluminum (50x).

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Figure (8): Microscope images (10x) of bare Teflon (left) and sanded Teflon (right).

Discussion

The results of this experiment successfully illustrate how the chemistry and roughness of a surface impact a surface’s wettability and surface free energy. According to figure (2), the hydrophilic materials include bare glass with an average contact angle of 53.4o, sandblasted glass with an average contact angle of 16.4o, sandblasted aluminum with an average contact angle of 77.1o, and FAS-17 treated sandblasted aluminum with an average contact angle of 11.9o. The hydrophobic materials include FAS-17 treated sandblasted glass with an average contact angle of 127.2o, bare Teflon with an average contact angle of 107.6o, and sanded Teflon with an average contact angle of 137.5o. The hydrophilic materials are considered polar materials due to their high wettability with primarily-polar water molecules while the hydrophobic materials are considered nonpolar materials. The primarily-dispersive, nonpolar DIM droplets were also applied to each surface with the resulting contact angles measured in order to calculate the SFE for each surface. The application of DIM accounted for the dispersive

interactions (γlvd=50.8mN

m,γlvp=0 mN

m ) as opposed to the polar interactions accounted for by

water application (γlvd=21.8mN

m,γlvp=51mN

m ). Together, the contact angle measurements for

the different liquids allow the presence and magnitude of each interaction and on each of the surfaces to be estimated. The average contact angle measurements using DIM droplets are given by figure (3) with the average contact angle measurements for both water and DIM droplets given by figure (4). All but the two most hydrophilic materials, sandblasted glass and FAS-17 treated sandblasted aluminum, gave a decrease in contact angle measurements when DIM was applied as opposed to water. An average decrease was expected due to a smaller

liquid-vapor surface tension for DIM (γlv=50.8mNm ) compared to water (γlv=72.8

mNm ).

Compared to the contact angle measurements with water, the DIM measurements for bare glass decreased only slightly from 53.4o to 49.9o. The liquid-vapor dispersive forces of DIM are

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slightly more than twice that of water and similar in value to the liquid-vapor polar forces present in water, while DIM contains no liquid-vapor polar forces. Therefore, the polar interactions decrease by almost twice the amount of which the dispersive interactions increase from water to DIM application. With similar contact angles for both liquids, bare glass likely experiences almost twice the amount of dispersive interactions than it does polar interactions, while containing a significant degree of both. This assumption may be further supported by the sandblasted glass results. The sandblasted glass was more hydrophilic than bare glass and gave a significant contact angle increase from water to DIM application of 16.4o to 39.2o. The significant increase in polar interactions, indicated by the smaller contact angles measured for water, are likely due to the roughened surface having a larger amount of surface area exposed. With a larger surface area, polar interactions at the surface increase as more surface is available for these interactions. The dispersive interactions increased only slightly upon the sandblasting of glass, suggesting that the polar interactions for this surface increase by a larger magnitude than the dispersive interactions as roughness increases. When the sandblasted glass was treated with FAS-17, the resulting surface became hydrophobic with large contact angles of 127.2o for water and 105.7o for DIM. While the Wenzel model likely describes the contact angle changes due to roughness for the other two glass samples, the Cassie-Baxter model likely explains the large contact angles resulting in treatment. The application of the FAS-17 film likely results in thermodynamically stable air-filled valleys that significantly increase the contact angles measured. These valleys are likely formed due to the sandblasted glass material being polar enough to form covalent bonds with the FAS-17 film upon immediate contact and at the tip of the ridges. This results in a thin layer of film lying directly above the surface of the treated material, of which shields a large degree of the polar and dispersive forces involved in the material. The surfaces of the different glass samples are given by figure (6). The smooth nature of the bare glass supports a strong presence of dispersive forces while the roughness seen in the sandblasted glass supports an increased surface area of which polar forces may interact. The sandblasted aluminum samples resulted in a different pattern upon treatment. The sandblasted aluminum sample resulted in slightly hydrophilic contact angles of 77.1o for water and 46.9o for DIM. The dispersive interactions for this material are likely similar to its polar interactions as the contact angles decrease from water to DIM is similar to the relative decrease in the total liquid-vapor interaction from water to DIM. The sandblasted aluminum treated with FAS-17 resulted in highly hydrophilic contact angles of 11.9o for water and 25.1o for DIM. The strong polar forces outweigh the dispersive forces upon treatment as the contact angle decreases and polarity increases significantly upon treatment. The initial polar forces before treatment were not strong enough to allow covalent bonding of the film at the surface unlike the sandblasted and treated glass sample. Instead, the film molecules likely bind at random sites within the sandblasted ridges to increase overall roughness and therefore give a decrease in contact angles, suggested by the images in figure (7). Both of the aluminum samples, therefore, likely follow the Wenzel model. The Teflon samples resulted in hydrophobic contact angles of 107.6o for bare Teflon with water and 137.5o for sanded Teflon with water. The DIM contact angle measurements were much less with a contact angle of 70.1o for bare Teflon and

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89.3o for sanded Teflon. These results indicate small polar and dispersive interactions for these sample. The increase in contact angles upon sanding may be explained by the Cassie-Baxter model. By sanding the Teflon, small ridges are formed of which are small enough to form air-filled valleys between them, decreasing surface roughness and resulting in larger contact angles. The very small sizes of the ridges are further supported by the images in figure (8). Finally, the SFE measurements for each sample are given by figure (5). These values generally represent the strength of the interactions involved in each sample as previously discussed, increasing with decreasing contact angles. Sandblasted aluminum treated with FAS-17 had the smallest contact angles and the largest SFE of 77.97808 mN/m while sandblasted glass treated with FAS-17 had the largest contact angles with the smallest SFE of 6.856499 mN/m. In conclusion, surface wettability may be largely altered via modification. By increasing surface roughness, the surface wettability may follow the pattern of the Wenzel model and increase, or roughness may result in air-tight valleys and follow the pattern of the Cassie-Baxter model with a decrease in wettability. Similarly, SAM treatment may significantly increase or decrease wettability based on its binding formations.

References

[1] G. Ozin, A. Arsenault, Nanochemistry: A Chemical Approach to Nanomaterials, 1st ed., RSC Publishing, 2005.

[2] S. Hartland, Surface and Interfacial Tension: Measurement, Theory, and Applications, (2004).

[3] Lotfi, M., M. Naceur, and M. Nejib. Cell Adhesion to Biomaterials: Concept of Biocompatibility. N.p.: INTECH Open Access, 2013.

[4] Agrawal, Abhinandan. "Definition of Surface Tension." Definition of Surface Tension. MIT, n.d. Web. 30 Nov. 2015.

[5] Zou, Min. "Surface Wetting Modifications Through Small-Scale Topography and Nanomaterials." NUE Lab Module. University of Arkansas, n.d. Web. 30 Nov. 2015.

[6] C.M. Mate, Tribology on the Small Scale, 1st ed., Oxford University Press, 2008.

[7] "The Water Race: Hydrophobic & Hydrophilic Surfaces." National Nanotechnology Infrastructure Network, n.d. Web. 30 Nov. 2015.

[8] L. Gao, T.J. McCarthy, X. Zhang, Wetting and Superhydrophobicity, Langmuir. 25 (2009) 14100-14104.

[9] Y. Song, R.P. Nair, M. Zou, Y. Wang, Superhydrophobic surfaces produced by applying a self assembled monolayer to silicon micro/nano-textured surfaces, Nano Research. 2 (2009) 143- 50. doi:10.1007/s12274-009-9012-0

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