Molecular and Nanoparticles vapor deposition methods to create super hydro/oleophobic surfaces

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Abidi & al. / Mor. J. Chem. 3 N°1 (2015) 167-184

167

Molecular and Nanoparticles vapor deposition methods to create super hydro/oleophobic surfaces

Noureddine Abidi*, Luis Cabrales, and Payam Aminayi

Fiber and Biopolymer Research Institute, Department of Plant and Soil Science, Texas Tech University, Lubbock, Texas 79403, USA

*Corresponding author: E-mail address: [email protected]

Received 11 Nov 2014, Revised 24 Nov 2014, Accepted 15 Dec 2014

Abstract

: In this paper, glass slide surfaces were functionalized using nanoparticles vapor deposition (NVD) and molecular vapor deposition (MVD) techniques. The NVD technique allowed the deposition of aluminum oxide (Al₂O₃) nanoparticles on the surface. The MVD technique allowed the deposition of a thin layer of (Tridecafluoro-1,1,2,2,-tetrahydrooctyl)trichlorosilane (FOTS). Nanoparticles deposition increased the surface roughness, leading to higher contact angles when compared to surfaces treated with only FOTS. Fourier Transform Infrared spectra showed the presence of peaks corresponding to fluorocarbon chains and Al2O3 on the functionalized surfaces. Dynamic contact angles of aqueous and organic solutions were measured in order to assess hydrophobic/oleophobic properties. The surface free energies of the samples were calculated using different methods. Dynamic contact angles higher than 150º were obtained for water and other organic liquids for functionalized samples. In addition, glass slides treated with Al2O3 nanoparticles followed by FOTS layer showed low hysteresis when performing advancing and receding contact angles with water and hexadecane, thus, indicating super hydro/oleophobic properties.

Keywords: Nanoparticles, super hydrophobic, super oleophobic, chemical vapor deposition, fluorosilane, molecular vapor deposition, contact angle.

1. Introduction

Surfaces with super hydrophobic and super oleophobic characteristics have attracted special interest in recent years. In nature, numerous surfaces possess hydrophobic characteristics. In general, surfaces are defined as super hydrophobic when the static contact angles are higher than 150º and the contact angle hysteresis’s are low. Typical example of super hydrophobicity is illustrated by the lotus leaves. The hierarchical structures that give the lotus leaves low adhesion to water have been extensively studied [1, 2]. In contrast to the low adhesion properties of the lotus leaves, some rose petals exhibit high water contact angles with high adhesion. The structures of both surfaces have been studied to understand what makes a super hydrophobic surface with low or high adhesion to water [2, 3]. Highly hydrophobic surfaces have many industrial applications (solar panels, architectural glass, heat transfer surfaces, piping, boat hulls, microfluidics, etc.). It is projected that the number of applications will increase as the technology of non-wettable surfaces matures. Generally, there are two

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168 common approaches to produce a super hydrophobic surface: (1) a coating is applied to a rough surface using low surface energy materials, or (2) a low surface energy material is roughened in order to achieve this remarkable property [4]. Several materials are used as low surface energy materials such as silicones [5-7] and fluorocarbons [8-13]. Although super hydrophobic surfaces are very common, surfaces that resist the wetting of low surface tension liquids, such as hydrocarbons, are rare. This type of surfaces are called super oleophobic [1]. Some surfaces with high repellency to low surface tension liquids have been fabricated in the past by combination of low energy materials and surface topography [14-17].

Processes to impart water or oil repellency to substrates are commonly carried out in aqueous phase. However, this process has many disadvantages (disposal of organic solvents, incomplete wetting of high aspect ratio structures, diffusion limited transport of reactants, and poor control of reactant supply) [18]. In this study, a vapor phase treatment was used to impart super hydro/oleophobic properties to glass slides. Chemical vapor deposition (CVD) is a gas phase process where the gas molecules react to create thin films [19]. In the past, CVD was used to create hydrophobic surfaces [20]. A special type of gas phase treatment is the atomic layer deposition (ALD). In ALD, binary sequential gas reactions occur. Each chemical is introduced into the chamber separately after the other reactant is removed. One of the advantages of ALD process is the precise thickness (Angstrom scale) and high uniformity of the coating, which is due to the self-limiting surface reactions used in ALD. Since the process is done under gas phase, the reaction can take place even in the pores of the substrate [21]. Because of the ability of the sequential reaction, the coating thickness is not limited to one layer; hence the thickness is controllable depending on the number of sequences. The ALD was previously used to produce conformal hydrophobic surfaces by creating a “seed layer” on substrates followed by a reaction with non- chlorinated alkylsilanes [22]. The deposition of Al2O3 is a model system for ALD. Al2O3 is deposited by the sequential reaction of tri-methyl aluminum (TMA) and water (H2O) [23].

Previously, we reported on the use of the MVD and NVD methods to impart super hydro/oleophobic properties to cotton fabric [24]. Textile surfaces are not ideal surfaces. Therefore, in this paper we used glass slides to create super hydro/oleophobic surfaces. The same reactants for ALD of Al2O3 were used to create ALD-like films and to deposit nanoparticles with a method called nanoparticle vapor deposition (NVD). In NVD, a binary sequential reaction was also used, just as in ALD; the only difference was that the second reactant was introduced without removing the first reactant from the chamber. By doing this, aluminum oxide nanoparticles were deposited on the substrate instead of a conformal coating, leading to an increase of the surface roughness.

Then, by using a CVD method, called molecular vapor deposition (MVD), a thin fluorocarbon layer is deposited on the nanoparticles. With the increased roughness of the substrate surface and the addition of the fluorocarbon layer, a material with super hydro/oleophobic properties was obtained.

2. Experimental Section

2.1. Materials

Microscope glass slides from Fisher Scientific (Houston, TX) were used. Distilled water was deionized in a Milli-Q plus system from Millipore (Billerica, MA) to reach a final resistivity of 18.2 MΩ-cm. The following liquids were used to assess the hydrophobic and oleophobic properties of the treated glass slides: hexadecane, tetradecane, dodecane, decane, octane, heptane, hexane, isopropyl alcohol (IPA), diiodomethane, formamide and glycerol. All these chemicals were acquired from Fisher Scientific with the highest purity available.

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169 2.2. Methods

Glass slides were thoroughly washed with ethanol. They were further cleaned with N2-plasma in a microwave plasma system (PLASMAtech Inc Erlanger, KY) with a generator of 2.45 GHZ at 500 W for 120 seconds. The system used for vapor phase deposition was a RPX-550 from Integrated Surface Technologies (Menlo Park, CA). The system consists of 5 containers for chemical reactants with independent controlled temperature, a vacuum pump, and sample chamber. Inside the chamber there is a perforated plate to deliver the gases over the samples. This configuration enhances the deposition of nanoparticles on the substrate. Nitrogen is used to transfer the reactants from the containers to the chamber in vapor phase through heated delivery lines. The chemicals used for all the treatments are the following: water, (Tridecafluoro-1,1,2,2,- Tetrahydrooctyl)trichlorosilane (FOTS), a blend of bifunctional trichlorosilanes, and TMA.

The system chamber was maintained at 55ºC during all the processes. The temperature of the chemical containers was maintained at 40ºC and the transfer lines at 45ºC to avoid condensation. Due to the low vapor pressure of FOTS, the container for this reactant was maintained at 90ºC and the transfer line at 95ºC. The chamber was purged with N2 and vacuumed to 0.1 Torr at the beginning of all processes. Three replications were performed for each treatment and all samples were rinsed with distilled water and air dried before any further testing. Different treatments were applied to glass slides as shown in table 1.

Table 1. Treatments applied to glass surfaces.

Treatment Description

T0 Control

T1 MVDFOTS

T2 ALDAl2O3

T3 ALD Al2O3 + MVDFOTS

T4 NVD Al2O3 nanoparticles

T5 NVD Al2O3 nanoparticles + MVDFOTS

T6 NVD Al2O3 nanoparticles+ MVD bifunctional trichlorosilane blend

T7 NVD Al2O3 nanoparticles+ MVD bifunctional trichlorosilane blend+ MVDFOTS

Atomic Layer Deposition of Al2O3 (ALD-Al2O3):

In this process, after the initial purging of the chamber, TMA vapor was introduced into the chamber until a pressure of 0.28 Torr was reached. TMA reacts with one or two of the surface hydroxyl groups of the silica of the glass slide to form aluminum-methyl. This forms the initial layer on the glass slide. Then, the chamber was purged with N2 four times and evacuated to 0.05 Torr. Afterwards, water vapor was introduced into the chamber to reach a pressure of 0.3 Torr. This allows water molecules to react with aluminum-methyl group to form a layer of aluminum oxide (Al2O3). This process creates functional groups (-OH) on the surface, which will be used to anchor another TMA and, thus, a second layer is deposited. In this work, this process was repeated 4 times.

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170 Nanoparticles Vapor Deposition of Al2O3 (NVD-Al2O3):

NVD-Al2O3 nanoparticles process was performed as follows: after the initial purging, TMA vapor was introduced to the chamber to reach a pressure of 0.38 Torr. Subsequently, water vapor (without any previous purging) was introduced to reach a pressure of 0.8 Torr. After 30 seconds of reaction, the chamber was evacuated and purged. The process was repeated 4 times.

Nanoparticles Vapor Deposition of Al2O3 (NVD-Al2O3) and Molecular Vapor Deposition of bifunctional trichlorosilane:

NVD-Al2O3 nanoparticles process was performed as follows: after the initial purging, TMA vapor was introduced into the chamber to reach a pressure of 0.38 Torr. Subsequently, water vapor (without any previous purging) was introduced to reach a pressure of 0.8 Torr. After 30 seconds of reaction, the chamber was evacuated and purged. Then, a chemical mixture of bifunctional trichlorosilanes (such as bis-trichlorosilane- ethane, bis-trichlorosilane butane, and bis-trichlorosilane-hexane) [25, 26] was introduced into the chamber to reach a pressure of 0.28 Torr. After 5 seconds, water vapor was introduced into the chamber to reach a pressure of 2 Torr. After 3 seconds, the chamber was evacuated to 0.1 Torr. The purpose of adding the bifunctional trichlorosilanes blend was to increase the wear resistance of the Al2O3 nanoparticles coating [25, 26].

Molecular Vapor Deposition of FOTS (MVD-FOTS):

MVD-FOTS layer was performed as follows: After evacuating the chamber, water vapor was introduced to reach a pressure of 0.08 Torr. Thereafter, the FOTS vapor was introduced to reach a pressure of 0.14 Torr. After 300 seconds, the chamber was purged and vented.

2.3. Characterization

Morphology of the nanocoatings:

Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) were used to investigate the morphology of the coating. SEM and TEM images were acquired with S-3400N and H-7650 models (Hitachi High-Technologies America, Pleasanton, CA), respectively. For the SEM experiments, the samples were coated with gold. SEM images were recorded with the secondary electrons detector at 5KV accelerating voltage. For TEM experiments, the coating on the glass slides containing aluminum oxide nanoparticles was scratched off and a drop of water was placed on it and collected on a copper grid. The drop was left to dry and images were taken.

FTIR analysis of the nanocoatings:

Fourier transform infrared (FTIR) spectra of the samples were recorded using Spectrum-One equipped with a universal attenuated total reflectance (UATR) accessory (Perkin Elmer, Waltham, MA). The UATR-FTIR was equipped with a ZnSe-diamond crystal composite that allows the FTIR spectra collection without any special sample preparation. The UATR accessory has a “pressure arm”, which is used to apply a constant pressure to the samples positioned on top of the crystal to ensure good contact between the crystal and the sample. FTIR spectra were collected at a spectra resolution of 4 cm^-1, with 32 co-added scans over the range from 4000 to 650 cm^-1. The spectra were baseline corrected and normalized to the highest absorption peak. The coatings on glass

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171 slides with treatments T4, T5, T6, and T7 were scratched off and placed on the ZnSe-diamond crystal. A background scan of the clean crystal was acquired before scanning the samples.

Hydro/oleophobic properties assessment:

Dynamic contact angle measurements were performed with a FTA 1000 instrument (First Ten Angstroms, Portsmouth, VA). For static and dynamic contact angles, a drop of a liquid (between 5 and 8 µL) was placed on the glass slide surfaces. A 26 gauge needle was used for this purpose. Laplace fit method was used to calculate the contact angles. Table 2 summarizes the surface tension values of the liquids used for testing. Isopropyl alcohol (IPA) and water solutions were prepared by volumetric proportion. For example, the H2O:IPA 9:1 solution was prepared by adding 10 ml of IPA to 90 ml of deionized water. Advancing and receding contact angles were measured to calculate contact angle hysteresis. In order to measure these angles, the needle was brought close to the sample surface and the test liquid was pumped out for 20 seconds until the drop reached a size of approximately 30µl. Then, the liquid was pumped in at the same rate until the drop detached from the needle (hydrophobic) or the whole amount of liquid was back in the syringe (super hydrophobic). Three replications of contact angle measurements were performed per sample.

Table 2. Surface tensions of liquids at 20ºC (*Measured at 25ºC) [27-30].

Liquid Surface energy (mN/m)

H₂O 72.80

IPA 21.30

H2O:IPA 9:1 42.00*

H2O:IPA 8:2 33.00*

H2O:IPA 7:3 27.50*

H₂O:IPA 6:4 25.40*

Hexadecane 27.47

Tetradecane 26.56

Dodecane 25.35

Decane 23.83

Octane 21.62

Heptane 20.14

Hexane 18.40

Formamide 58.00

Diiodomethane 50.80

Glycerol 64.00

3. Results and Discussion

3.1. Scanning Electron Microscopy:

Figures 1-a and 1-b show SEM micrographs of untreated glass slide surface (treatment T0) and functionalized glass surface (treatment T7) respectively. In contrast to untreated glass slide, the glass slide with treatment T7 exhibits remarkable roughness. The TEM micrograph of the same treatment shows the presence of

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172 semispherical particles of different sizes (between 10 and 200 nm) (Figure 1-c). These results illustrate the deposition of nanoparticles of different sizes over the samples treated using the NVD method (treatments T4, T5, T6, and T7). However, glass slides with treatments T1, T2, and T3 exhibit surfaces very similar to the control (Figure not shown).

Figure 1-a. SEM micrograph of untreated glass slide surface (T0).

Figure 1-b. SEM micrograph of glass slide with treatment T7.

Figure 1-c. TEM micrograph of the coating on the glass slide with treatment T7.

3.2. Fourier Transform Infrared Spectroscopy:

The FTIR spectra of the glass slides with treatments T1, T2, and T3 did not show any additional vibrations in addition to the vibrations originating from the glass substrate. However, the FTIR spectra of the glass slide with treatment T4 (NVD-Al2O3) show additional vibrations (Figure 2-a).The vibration 3413 cm^-1 is attributed to OH groups on the aluminum oxide nanoparticles [31, 32]. FTIR absorptions of impurities in aluminum oxide coatings, such as carbon, have been reported to show between 1300-1800 cm^-1, with a stronger absorption at

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173 1580 cm^-1 [31]. The vibrations 666 cm^-1and 860 cm^-1 are assigned to O-Al-O bending and Al-O stretching respectively [33, 34].

Figure 2-a. FTIR spectra of untreated glass slide (T0) and glass slide with treatment T4 (NVD-Al2O3

nanoparticles).

Figure 2-b. FTIR spectra of glass slides with treatments T5 (NVD-Al2O3 nanoparticles + MVD-FOTS layer) and T6 (Al2O3 nanoparticles + MVD bifunctional trichlorosilane blend).

Figure 2-b shows the FTIR spectra of glass slides with treatments T5 and T6. The high surface area of nanoparticles increases the signal of surface groups in FTIR [35]. Thus, the functional groups attributed to FOTS and bifunctional trichlorosilane layers on nanoparticles were detected with infrared spectroscopy. The vibrations 2920 and 2850 cm^-1are attributed to CH2 asymmetric and symmetric vibrations respectively [36].

These bands are more pronounced in the treatment T6 due to the high content of aliphatic short chains in the bifunctional trichlorosilane blend. The vibration 1239 cm^-1 is attributed to C-F stretching and the vibrations 1212 and 1145 cm^-1are assigned to asymmetric and symmetric CF₂ stretching respectively [37]. The vibration 1030 cm^-1 in treatment T6 and 1065 cm^-1 in treatment T5 are attributed to Si-O stretching [38]. The vibrations

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174 880 and 706 cm^-1 in treatment T5 and 875 and 720 cm^-1 in treatment T6 are assigned to functional groups of Al2O3 nanoparticles as mentioned previously. The FTIR spectrum of treatment T7 (not shown) is a composite spectrum of the FTIR spectra of treatments T5 and T6.

3.3. Water contact angle measurements:

In general, surfaces with a contact angle below 90º are considered to be hydrophilic. If contact angles are greater than 90º but lower than 150º, the surface is classified as hydrophobic. Superhydrophobic surfaces have a water contact angle greater than 150º [39]. Figure 3 shows the average water contact angles of glass slides with different treatment as illustrated in Table 1.

Figure 3. Effect of treatments on the water contact angles. Results were arranged by increasing contact angles.

Treatments T0 (control), T4, T6, and T2 result in hydrophilic surfaces. Treatments T1 and T3 gave water contact angles higher than 90º (indicating hydrophobic surfaces). Only treatments T5 and T7 result in glass slides with contact angles higher than 150º and, thus, fall in the super hydrophobic category. Table 3 shows the analysis of variances of the effect of treatment on the water contact angles. These results indicate that treatments T0, T4, and T6 are not statistically different and lead to hydrophylic surface. Treatment T2 results also in hydrophilic surface. Treatments T1 and T3 lead to hydrophobic surfaces. Finally, there is no statistical difference between treatments T5 and T7. These treatments create super hydrophobic surfaces. In conclusion, the best treatment conditions which lead to super hydrophobic properties are based on NVD of Al2O3 followed by the MVD of FOTS.

Using the water contact angle values listed in table 3, additional information can be obtained by performing different calculation. The thermodynamic equilibrium of a drop of a liquid on a flat solid surface was discussed by Young [40]. The contact angle θ of a liquid droplet can be determined using equation 1:

(1)

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175 Where γ_SL,γSV and γ_LV are the interfacial free energy per unit area of the solid-liquid, solid-gas, and liquid-gas interfaces, respectively. Wenzel proposed a model describing the contact angle for a rough surface (θ*) [41]:

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Where r is the roughness factor defined as the ratio of the actual area to the geometric area and θ contact angle for smooth solid surface (untreated glass slide in this case). When the surface is chemically or geometrically heterogeneous, the model of Cassie and Baxter assumes that the cosine of the drop contact angle on a mixed surface is approximate to the linear combination of the cosines of contact angles of the surface components [42]. Considering a surface composed of two different materials, the equation is as follows:

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Where f1 and f2 represent the surface area fractions of surfaces 1 and 2, respectively. If f2 represents the fraction area of trapped air in a rough surface and assuming that the contact angle on a smooth air surface pocket to be 180º, the equation 3 can be modified to:

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Where f represents the remaining area fraction or the liquid-solid interface [41].

Table 3. Variance analysis: effect of treatment on the water contact angle.

Parameter df F Probability Contact angle, degree^a

Intercept 1 23837.55 0.000001

Treatment 7 1878.90 0.000001

T0 18 a

T4 18 a

T6 22 a

T2 44 b

T1 108 c

T3 106 c

T5 167 d

T7 167 d

Error 64

df, degrees of freedom; F, variance ratio, ^a Values not followed by the same letter are significantly different with α=5% (according to Newman-Keuls tests).

The average water contact angle (θ*) of roughened glass slides with treatments T5 and T7 is 167^o while the average water contact angle (θ) of smooth glass slide with treatment T0 is 18^o. Using these values, we determine the value of f to be 0.013. This means that for these samples, around 98.7% of the surface is covered by air pockets and the liquid is only making contact with the solid surface in ~1.3% of the area. The roughness of glass slides with treatments T5 and T7 imparts a super hydrophobic property by trapping air underneath the droplet [43]. In some studies, it has been shown that when a surface has nanoscale roughness, super hydrophobicity can be achieved [42]. Using equation 2, the roughness factor r is determined to be 3.50 for these samples. It was reported previously that an increase in roughness factors leads to an increase in the contact angle up to a certain value that is dependent on the surface geometry [44-46].

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176 Figure 4. Advancing and receding water contact angles: (a) glass slide with treatment T7 showing low hysteresis (1^o), (b) glass slide with treatment T1, showing high hysteresis (31^o), and (c) Sessile volume.

Hysteresis of the surfaces was measured using the procedure mentioned previously. Figure 4 shows the advancing and receding contact angles of the glass slide with treatments T1 and T7. For treatment T1, as the volume increases, the advancing contact angle remains stable. Once the sessile volume starts decreasing, a reduction of the contact angle occurs until it reaches a plateau. This sample presents a hysteresis value of 31º as indicated in table 4. For treatment T7, the advancing and receding water contact angles remain the same when the volume of the droplet starts decreasing. This sample exhibits a very low hysteresis value (1º) as indicated in table 4. It was reported that the lower the hysteresis, the smaller the tilting angle necessary to roll off a droplet standing over the surface [47].

Table 4. Force needed to start moving a droplet standing over the glass surface with different treatments calculated from advancing and receding contact angles of water.

Treatment Advancing contact angle, degree

Receding contact angle, degree

Hysteresis contact angle, degree

Force (mN/m)

T1 116 85 31 38.25

T3 112 85 27 33.61

T5 167 164 3 0.95

T7 168 167 1 0.27

The determination of the hysteresis (difference between the advancing and receding contact angles) allows us to calculate the force needed to start moving a droplet over a solid surface [47]. McCarthy et al. proposed the following equation [7, 48]:

(5)

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177 Where F is the critical line force per unit length of the droplet perimeter, γ_LV is the surface tension of the liquid.

θA and θR are the advancing and receding contact angles respectively. Table 4 shows the calculated values from equation 5. The results indicate that droplets standing on glass slide with treatments T5 and T7 need a very small force to start moving when compared to a droplet standing on glass slide with treatments T1 and T3. The low hysteresis of the glass slide with treatments T5 and T7 coupled with their water contact angle higher than 150º fulfill the requirements of super hydrophobicity as discussed previously.

3.4. IPA/Water contact angle measurements:

Solutions were prepared from water and IPA by volumetric proportions (H2O:IPA = 9:1, 8:2, 7:3, 6:4). The contact angles of the solutions were measured and the results are reported in Figure 5 as a function of the surface tension (from table 2).

Figure 5. Contact angles of glass slides with treatments T1, T3, T5 and T7 using H₂O:IPA solutions with different surface tension.

The results show that glass slides with treatments T5 and T7 lead to surfaces having high contact angles when the surface tension value are high (>40 mN/m). However, as the surface tension approaches 30 mN/m, there is a sharp decrease in the contact angles.

3.5. Hydrocarbon contact angle measurements:

Contact angles of linear saturated hydrocarbons (with different number of carbons) were measured (Figure 6).

The surface tension of the hydrocarbons decreases as the number of carbons decreases (Table 2). Treatments T1 and T3 do not show any significant changes in contact angles regardless of the type of hydrocarbons. The contact angles remain below 60^o.

For glass slide with treatment T5, the contact angles are in the following order: Hexadecane

>Tetradecane>Dodecane>Decane> Octane > Heptane > Hexane. For glass slide with treatment T7, the contact angles are in the following order: Hexadecane = Tetradecane = Dodecane>Decane> Octane = Heptane >

Hexane. The difference between treatments T5 and T7 is the MVD of bifunctional trichlorosilane blend in treatment T7.

The advancing and receding contact angles of hexadecane on the glass slide with treatment T3 are reported in figure 7. As the sessile volume increases, the advancing contact angle is stable around 78º. However, as the

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178 droplet recedes there is a decrease in the contact angle until it reaches 37º when the droplet detaches from the neddle. This sample shows an average hysteresis value of 41º for the hexadecane (Table 5).

Figure 6. Contact angles of different linear hydrocarbons.

Table 5. Force needed to start moving a droplet of hexadecane over glass slide surface calculated from advancing and receding contact angles.

Treatment Advancing contact angle, ^o

Receding contact angle, ^o

Hysteresis contact

angle, ^o Force (mN/m)

T1 74 38 36 14.12

T3 78 37 41 16.07

T5 169 163 6 0.77

T7 167 161 6 0.72

The advancing and receding contact angles of hexadecane on glass slide with treatment T7 are exhibited in figure 7. The advancing contact angle is 167º and the receding contact is 161º. The hysteresis contact angle for hexadecane for this sample is 6º. Furthermore, the low hysteresis contact angles of glass slides with treatments T5 and T7 (Table 5) coupled with their high contact angles (> 150º) fulfill the requirements of super oleophobic surfaces as discussed previously. Table 5 summarizes the results of the force to start moving a droplet of hexadecane on the glass slide surfaces. The force was calucalted using equation 5. A droplet of hexadecane standing on glass slide with treatements T5 and T7 needs a very small force (less than 1 mN/m) or small tilting angle to start rolling off due to the low hysteresis value of the surface. In contrast, a droplet standing over glass silde with treatments T1 and T3 need a force that is 15 times higher.

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179 Figure 7. Advancing and receding hexadecane contact angles: (a) glass slide with treatment T7 showing low hysteresis (4^o), (b) glass slide with treatment T3 showing high hysteresis (51^o), and (c) Sessile volume.

3.6. Work of adhesion and surface free energy:

Fowkes showed that the sum of different intermolecular interactions leads to the surface tension [49]:

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In which is the surface tension and index d, i, p, h, ad and e are dispersion, induced dipole-dipole, dipole- dipole hydrogen bonding, bonding, acceptor-donor and electrostatic index interaction, respectively. All of the above elements are either dispersive elements or nondispersive elements. Fowkes divided the total surface free energy into two components: polar (γ^p) and dispersive (γ^d) components. Where γ^p shows approximation of the sum of all of the nondispersive intermolecular interaction surface tensions [50]. This approximation leads to equation (7):

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The surface tension components of the liquids (water, formamide, diiodemethane, and glycerol) used to calculate the work of adhesion and the surface energy [51] are presented in table 6. Table 7 shows the contact angle values of the liquids used to calculate the surface energy and the work of adhesion. Glass slides with treatments T5 and T7 exhibit contact angles > 160º regardless of their dispersive and polar components.

From the results of the contact angles listed in table 7, the work of adhesion can be calculated. The work of adhesion between a liquid and a solid surface is given by the following equation [53]:

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By combining equations (1) and (8), we derive equation (9), which is defined as the work of adhesion [50]:

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The work of adhesion of water, formamide, diiodemethane, and glycerol for treatments T1, T3, T5, and T7 were calculated (Table 8). The work of adhesion of the test liquids on glass slides with treatments T5 and T7 is almost 50 times lower than the work of adhesion on glass slides with treatments T1 and T3.

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180 Table 6. Surface tension components of liquids [30, 52].

Liquid γ^d (mN/m) γ^p (mN/m)

H₂O 21.8 51.0

Formamide 39.0 19.0

Diiodomethane 50.8 0.0

Glycerol 34.0 30.0

γ^d= surface tension dispersive component and γ^p= surface tension polar component.

Table 7. Contact angle for test liquids.

Contact angle (degrees)

Liquid T1 T3 T5 T7

H₂O 108 106 167 167

Formamide 92 97 169 169

Diiodomethane 96 90 168 167

Glycerol 112 109 168 169

Table 8. Work of adhesion.

Work of adhesion (mN/m)

H2O 50.30 52.73 1.87 1.87

Formamide 56.17 51.11 1.07 1.07

Diiodomethane 45.49 50.80 1.11 1.30

Glycerol 40.03 43.16 1.40 1.18

Average 48.00 49.45 1.36 1.35

3.7. Critical surface tension:

The critical surface tension is the theoretical surface tension that a liquid needs in order to have a contact angle of 0 (cosθ=1) over a particular surface. In order to obtain the critical surface tension of spreading for a solid surface a Zisman plot should be constructed. To construct this plot, liquids with different surface tensions are selected and then their advancing contact angles (θ) are measured. Then, by plotting the cosθ as a function of the liquid surface tension, an approximated linear relationship is obtained, which is called Zisman plot. By extrapolating the linear approximation to zero, where cosθ=1, the critical surface tension, γc is obtained [14].

Using the contact angle values of linear saturated hydrocarbons, the critical surface tensions were calculated (Table 9). A representative Zisman plot of sample T5 is shown in figure 8. The results indicate that glass slides with treatments T5 and T7 have higher critical surface tension values, which is in agreement with H2O:IPA solutions and linear hydrocarbons results. As the surface tension of the test liquid decreases, surfaces with treatments T5 and T7 are wetted at higher surface tension values than samples with treatments T1 and T3.

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181 Table 9. Results of the surface free energy and critical surface tension.

Treatment γc(mN/m) γS(mN/m) γSd

(mN/m) γSp

(mN/m)

T1 12.55 17.11 14.53 2.58

T3 13.94 17.67 14.95 2.71

T5 18.83 0.35 0.29 0.03

T7 19.22 0.35 0.34 0.01

γc= critical surface tension from Zisman plots. γS= Surface free energy calculated from Wu method. γSd

= Surface free energy dispersive component calculated from Wu method. γ_S^p= Surface free energy polar component calculated from Wu method.

Figure 8. Zisman plot: glass slide with treatment T5 using contact angles of linear hydrocarbons.

Fowkes proposed a relationship between solid-liquid interaction taking into account only the dispersive component. In addition to Fowkes approximation, there are several other approximation methods to estimate the solid surface energy using contact angles, such as Berthelot’s approximation, acid-base approximation, geometric and harmonic mean approximation [50, 53, 54]. Wu used harmonic mean to combine the dispersive and polar components and using equation (1), he obtained the following equation (10) [50].

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Using Wu method, the surface free energy and its polar and dispersive components of each sample were calculated (Table 9). The results show that glass slides with treatments T5 and T7 have very low surface free energy values according to Wu method [55]. Taking into account these results and the critical surface tension results, we can conclude that the wetting of these super hydro/oleophobic surfaces is related only to the surface tension value of the liquid and not to its polar and dispersive components.

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182 Recently, a new approach to determine the apparent solid surface free energy was proposed by Chibowski [56].

This method allows the evaluation of the energy from the advancing and receding contact angles from one liquid as given in equation (11).

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Table 10 summarizes the apparent solid surface free energy for water and hexadecane. As discussed by the proponents of this method, the results of the apparent solid surface free energy are dependent on the test liquid [56]. Treatments T5 and T7 lead to surfaces having very low solid surface free energy as compared to treatments T1 and T3. These results are in agreement with the results of the surface free energy determined using Wu method (Table 9). It is important to notice that the values of γStot

of glass slide with treatments T1 and T3 using water as test liquid are similar to γS values obtained using Wu method.

Table 10. Results of apparent solid surface free energy from advancing and receding contact angles using equation (11).

Apparent solid surface free energy γStot

(mN/m)

H2O 13.59 16.43 0.78 0.67

Hexadecane 38.54 35.30 0.43 0.64

4. Conclusions

Nanoparticle deposition method allows the creation of a rough surface by a vapor phase reaction. The addition of a FOTS layer to the roughened surface resulted in a super hydro/oleophobic substrate. The requirements for super hydrophobic properties were fulfilled by a high water contact angle (> 150º) and a low hysteresis value of advancing and receding contact angles. The evaluation of super oleophobic properties were satisfied by hexadecane contact angle higher than 150º and low hysteresis value. The work of adhesion and the critical surface tension values were calculated. Very low work of adhesion values were obtained for samples treated with nanoparticles vapor deposition method and with FOTS layer. The surface free energy results were obtained using Wu and Chibowski method. The results of the surface free energy from Wu method are in agreement with the results of Chibowski method. All the results lead to the conclusion that the wetting of these super hydro/oleophobic surfaces is related only to the surface tension of the liquid, regardless of their dispersive and polar components.

Acknowledgments

The authors would like to thank the National Science Foundation Major Research Instrumentation program (CBET 0821162) and the Texas Department of Agriculture/Food and Fibers Research Grant Program (NFR-11- 03) for providing the financial support for this project.

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