Ultra-oleophobic cotton fabric prepared using molecular and nanoparticle vapor deposition methods

Surface & Coatings Technology xxx (2015) xxx–xxx

SCT-20304; No of Pages 9

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Surface & Coatings Technology

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Ultra-oleophobic cotton fabric prepared using molecular andnanoparticle vapor deposition methods

Payam Aminayi, Noureddine Abidi ⁎Fiber and Biopolymer Research Institute, Department of Plant and Soil Science, Texas Tech University, Lubbock, TX 79403, USA

⁎ Corresponding author.E-mail address: [email protected] (N. Abidi).

http://dx.doi.org/10.1016/j.surfcoat.2015.06.0050257-8972/© 2015 Elsevier B.V. All rights reserved.

Please cite this article as: P. Aminayi, N. Abid

a b s t r a c t
a r t i c l e i n f o
Article history:Received 2 May 2015Revised 1 June 2015Accepted in revised form 3 June 2015Available online xxxx

Keywords:Nanoparticle vapor deposition (NVD)Ultra-oleophobicCottonFOTS

Preparation of ultra-oleophobic cotton fabrics usingnanoparticle vapor deposition (NVD) techniques is discussedin this paper. An atomic layer of (Tridecafluoro-1,1,2,2,-Tetrahydrooctyl)trichlorosilane (FOTS) was deposited torender the fabric surfaces oleophobic while preserving the original fabric properties such as strength, comfort,and natural feel. Surfaces were roughened by trimethylaluminium (TMA) nanoparticles, which imparted uniqueoleophobicity to the fabrics with contact angles as high as ~170° and very low hysteresis (nearly zero). Scanningelectron microscopy (SEM) and universal attenuated total reflectance–Fourier transform infrared (UATR–FTIR)were used to investigate the development of the oleophobic coating over the fabric surface. Dynamics of the sur-face were also studied, and desired results, such as very low force to initiate the droplet movement, low work ofadhesion, and low surface free energy, were reported.

© 2015 Elsevier B.V. All rights reserved.

1. Introduction

Surfaces functionalized with fluorinated chemicals exhibiting lowsurface energies have attracted a great deal of attention for applicationsin the modern coating, microelectronic, and biomedical industries [1].These surfaces exhibit a low wettability toward high and moderatepolar liquids. The chemical composition and roughness structure ofthe materials play an important role in obtaining low surface energysubstrates [2]. To modify the chemical compositions, methods such aschemical surface reactions [3] and plasma treatments [4] were used.In order to manipulate the roughness structure of the surfaces, variousapproaches such as laser technology [5] and nanocasting methods[6] were employed. Coating industries are distinctly interested inoleophobic surfaces which can resist mineral oils [1]. However, veryfew reports exist for such surfaces [7]. The oleophobic surfaces have ap-plications in facades, vehicles, solar cells, etc. Examples of such applica-tions are: antireflective surfaces, sunshade glazing, and antireflectivecoating for instrument panels [7].

The complexity of a fibrous structurewetting phenomenon,which isdue to various simultaneous wetting mechanisms such as immersion,spreading, capillary penetration, and adhesion [8], calls for a generaldefinition of repellency. Repellency for fibrous structures, such as fab-rics, can be defined as the tolerance of the fabric to withstand the pen-etration of a liquid in the presence of capillary forces where noexternal pressure other than the weight of the droplet exists [8]. Thereare many examples of natural and artificial surfaces which can repelhigh surface tension liquids. Nevertheless, surfaces which can repel

i, Surf. Coat. Technol. (2015),

low surface tension liquids such as hydrocarbons are neither easy tofind in nature nor easy to manufacture. These types of surfaces arecalled ultra-oleophobic [9]. In order to obtain ultra-oleophobic surfaces(which provide high contact angles above 150° and sliding angles of lessthan 10° [10]), both factors of roughness and chemical composition arerequired [11]. Fluorocarbons can be used as an ultra-oleophobic coating[8], however, obtaining rough surfaces with fluorocarbons is difficult[11]. Numerous methods have been used for the fabrication of ultra-oleophobic surfaces, including electrodeposition [12], electrospinning[13], sputtering [14], hydrothermal reaction [15], and etching [16].However, despite the simplicity, low cost, and great versatility, the mo-lecular vapor deposition (MVD) technique requires experimental stud-ies and developments to fabricate ultra-oleophobic surfaces. In order toachieve roughness in this study, nanoparticles were deposited using theMVD technique.

Chemical vapor deposition is a technique in which the deposition offunctional materials takes place through a chemical reaction in thevapor phase [17]. Due to the vapor phase reaction, even the pores ofthe substrate can be treated with functional materials [18]. Comparedwith the conventional wet chemistry process, CVD is advantageous inits ability to avoid disposal of organic solvents, incomplete wetting ofhigh porosity structures, limited diffusion of reactants, and poor controlof the reactant supply, among other undesirable occurrences [19]. Re-cently, the scale-up possibility of the CVDmethod has been investigatedand made possible through a number of enhancements and modifica-tions in the process setup, such as the incorporation of dielectric barrierdischarge (DBD) in addition to the atmospheric pressure plasma jet(APPJ). It was shown that fluorocarbon-based coatings can be potential-ly deposited using a movable platform for large-scale deposition appli-cations [20].

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Table 2Surface tensions of liquids at 20 °C [51,52,59,60].

Liquid Surface energya (mN/m)

Hexadecane 27.47Tetradecane 26.56Dodecane 25.35Decane 23.83Octane 21.62Heptane 20.14Hexane 18.40Formamide 58.00Diiodomethane 50.80Glycerol 64.00

a Measured at 25 °C.

2 P. Aminayi, N. Abidi / Surface & Coatings Technology xxx (2015) xxx–xxx

The CVD technique can be categorized as either nanoparticle vapordeposition (NVD) or atomic layer deposition (ALD). The only differencebetween the two techniques is that in NVD, two binary chemicals,which are required for the vapor phase reaction, are present at thesame time. The presence of both of the binary chemicals in the reactionenvironment leads to the formation of nanoparticles which are deposit-ed on the surface. However, in ALD, each of the two chemicals reactswith the substrate in sequential order. Therefore, no nanoparticle isformed on the substrate. As a consequence, the NVD method impartsroughness to the substrate and further enhances its oleophobicity. Inthis study, both techniques have been used for comparison.

2. Experimental section

2.1. Materials

Testfabrics, Inc. (West Pittston, PA) provided scoured, desized, andbleached cotton fabrics. Three fabrics with different specifications(Table 1) were identified as C1, C6, and C9. Liquids (Table 2) used to as-sess the oleophobic properties of the treated cotton fabrics were ac-quired from Fisher Scientific with the highest purity available.

2.2. Methods

In order to remove oils and contaminants, fabrics were treated withamicrowaveN2-plasma (PLASMAtech, Erlanger, KY)with a generator of2.45 GHz for 120 s at 500 W. The N2 flow rate and pressure were set to60ml/min and 25 Pa, respectively. The system used for the vapor phasedepositionwas a RPX-550 from Integrated Surface Technologies (MenloPark, CA, USA). TheMVD system uses up to 5 containers for chemical re-actants, eachwith an independent temperature control. For the purposeof this study, 4 of the containerswerefilledwith the following chemicals:water, (Tridecafluoro-1,1,2,2,-Tetrahydrooctyl)trichlorosilane (FOTS),and a blend of bifunctional trichlorosilanes and trimethylaluminium(TMA). The fifth container was filled with nitrogen. While in the vaporphase, nitrogen was used to carry the reactants from the containers tothe chamber through heated delivery lines. A perforated plate was de-signed inside the chamber which delivered the chemical vapors. The re-action chamber was equipped with a vacuum pump to purge thereaction environment. The temperatures of the chamber and the chem-ical containers were held at 55 °C and 40 °C, respectively, during all theprocesses. The transfer lines temperature was set to 45 °C to avoid con-densation. However, due to the low vapor pressure, 90 °C and 95 °Cwere set for the FOTS container and the transfer line, respectively.

In order to fully compare the ALD and NVDmethods, two sets of ex-periments were designed for each method. The fabrics for all the sam-ples were subjected to N2-plasma pretreatment before the vaporphase reaction. For the ALD method, 3 samples were prepared. For thefirst treatment (T1 treatment), the fabrics were subjected to one FOTSlayer deposition using the MVD instrument. For the second treatment(T2 treatment), a layer of aluminum oxide was deposited. For thethird treatment (T3 treatment), the aluminum oxide deposition wasfollowed by a FOTS layer deposition using the MVD instrument.

For the NVD method, 4 samples were prepared: for the first treat-ment (T4 treatment), trimethylaluminumnanoparticleswere depositedon the fabrics using the MVD instrument. For the second treatment (T5treatment), the deposition of trimethylaluminum nanoparticles was

Table 1Properties of cotton fabrics used in this study.

ID Yarn size Weight Picks Ends

W, Ne F, Ne oz/yd2 g/m2 W F

C1 36 40 3.5 118.7 100 95C6 41 40 3.2 109.3 85 74C9 22 13 6.8 229.0 99 55

Please cite this article as: P. Aminayi, N. Abidi, Surf. Coat. Technol. (2015),

followed by a FOTS layer deposition. For the third treatment (T6 treat-ment), the deposited trimethylaluminum nanoparticles were treatedwith the bifunctional trichlorosilane blend. Lastly, for the forthtreatment (T7 treatment), a FOTS layer was deposited over thetrimethylaluminum nanoparticles which were treated with the bifunc-tional trichlorosilane blend. As a control, T0 treatment represents thesubstrate subjected only to N2-plasma pretreatment.

2.3. MVD and ALD treatment procedures

At the beginning of the process, the chamber was purged with N2

and vacuumed until the pressure of 0.1 Torr was achieved. TMA vaporwas then introduced until a pressure of 0.28 Torr was reached. For theinitial layer in the ALD process, TMA reactedwith one or two of the sur-face hydroxyl groups to form aluminum-methyl, bounded to an oxygenatom. Then the chamber was purged with N2 four times and vacuumedto reach 0.05 Torr pressure. The next layer process was initiated withwater, which reacted with aluminum-methyl group to form a layer ofaluminum oxide (Al2O3) with hydroxyl surface termination under thefollowing reaction [21]:

2Al(CH3)3 + 3H2O → Al2O3 + 3CH4 (ΔH) = −376 kcal.

Water was introduced to the chamber and a pressure of 0.3 Torr wasachieved. This process was repeated four times. Because of the sequen-tial reaction, the coating thickness is not limited to one layer; therefore,the thickness is controllable depending on the number of the Tri-methylaluminum (TMA)/water (H2O) reaction sequences.

In the NVD process, Al2O3 nanoparticles were deposited on the sub-strate with the following procedure: TMA vapor was introduced in thechamber until a pressure of 0.38 Torr was reached. Water vapor wasthen introduced for 30 s and the pressurewas raised to 0.8 Torr. Asmen-tioned earlier, the binary chemicals of water and TMA conducted a reac-tion. Later the chamber was purged. This process was repeated 4 times.

For treatments T6 and T7, in which the MVD bifunctionaltrichlorosilane blend was used to increase the wear resistance ofthe Al2O3 nanoparticles coating [22,23], the same sequences asabove were performed. However, after the first Al2O3 nanoparticletreatment, the chamber was purged. Then, the blend of bifunctionaltrichlorosilanes, such as bis-trichlorosilane-ethane, bis-trichlorosilanebutane, and bis-trichlorosilane-hexane [22,23], was introduced to thechamber until a pressure of 0.28 Torr was reached. A diffusion time of5 s was allowed. This process was performed after each individualAl2O3 nanoparticle treatment (4 times).

As the final layer of the treatments T1, T3, T5, and T7, theMVD FOTSlayer was applied with the following procedure: The chamberwas purged and water vapor was introduced to reach a pressure of1 Torr. Then, a pressure of 0.08 Torr was achieved with the vacuumpump. Thereafter, the FOTS vapor was introduced to reach a pressureof 0.14 Torr. The reaction was allowed to take place for 300 s. Thechamber was then purged and vented. This process was repeated 3times. This final layer is fundamentally a fluorocarbon chain called



Fig. 2. FTIR spectra of a control fabric (T0) and fabric subjected to treatment T7.

Fig. 1. Morphology of (a) untreated cotton fabric, and cotton fabrics subjected to (b) treatment T3, (c) treatment T5, and (d) treatment T7.

3P. Aminayi, N. Abidi / Surface & Coatings Technology xxx (2015) xxx–xxx

(Tridecafluoro-1,1,2,2,-Tetrahydrooctyl)trichlorosilane (FOTS) whichimparts oleophobicity to the substrates. Sampleswere rinsed in distilledwater and air-dried before further testing.

In order to compare the effect of the number of MVD sequences onthe oleophobicity results, two sets of samples were prepared. In oneset of samples, 1 cycle of FOTS deposition was performed, and in anoth-er, 3 layers of deposition were performed.

2.4. Material characterization

The surfacemorphologies of the sampleswere examined using scan-ning electron microscopy (SEM) (S-3400N, Hitachi High-TechnologiesAmerica, Pleasanton, CA). The samples were coated with a gold sputter,and the secondary electrons detector (at 5 kV accelerating voltage) re-corded the SEM images.

The vibrational spectra of the sampleswere recorded using a Fouriertransform infrared (FTIR) accessorizedwith a universal attenuated totalreflectance (UATR) accessory (Perkin Elmer,Waltham,MA). The spectrawere analyzed by Spectrum-One software. A ZnSe-diamond crystalcomposite was employed allowing FTIR spectra to be collected withno special sample preparation. TheUATR accessory employs a “pressurearm,” which applies a constant pressure to the samples situated on thetop of the crystal to guarantee the desired contact between the crystaland the sample. The FTIR spectra resolution was set to 4 cm−1. 32 co-added scans in the range of 4000 to 650 cm−1 were collected and aver-aged to represent the final spectra. The highest absorption peaks wereused to baseline correct and normalize the spectra. A spatula was usedto scratch off the nanoparticle coatings on samples subjected to T4, T5,T6, and T7 treatments to be placed on the UATR accessory crystal. Allsamples were conditioned in a controlled environment (65 ± 2% rela-tive humidity and 21 ± 1 °C) at least 72 h prior to testing.

Contact angle measurements were conducted on a FTA 1000 instru-ment (First Ten Angstroms, Portsmouth, VA). Static and dynamic contactangle results were collected using the liquid deposition system equippedwith a 26-gauge needle. The liquid deposition system deposited dropsbetween 5 and 8 μL on the surfaces and, a computer-based image


analysis software used the Laplace fitmethod to calculate the contact an-gles. Table 2 indicates the surface tension of the liquids used for thisstudy to calculate the hysteresis. In order to perform dynamic measure-ments, the advancing anglesweremeasured by bringing the needle closeto the sample surface and pumping the test liquid out for 20 s until a dropsize of ~30 μl is achieved. The receding angles were measured bypumping the liquid at the same rate until the drop detached from theneedle or the whole amount of liquid was back in the syringe. The aver-age of threemeasurements on each samplewas used for the calculations.

3. Results and discussions

3.1. Scanning electron microscopy

Fig. 1 shows the SEM micrograph of the fabric surfaces subjected totreatments T7 and T0 (untreated fabric). This figure clearly illustrates



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Fabric C1 Fabric C6 Fabric C9

Fabric C1Fabric C6Fabric C9






Fabric C1 Fabric C6 Fabric C9

a) b)

c) d)

e) f)

g) h)

Fig. 3. (a–b) Glycerol, (c–d) ethylene glycol, (e–f) formamide, and (g–h) methylene iodide contact angles on C1, C6, and C9 fabrics subjected to T1, T3, T5, and T7 treatments. (a, c, e, g)samples are subjected to one cycle of FOTS and (b, d, f, h) samples are subjected to three cycles of FOTS.


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Table 3Variance analysis: Effect of treatments and substrates (fabrics) on the glycerol contact an-gle for the samples subjected to T5 and T7 treatments with 1 and 3 cycles of FOTSdeposition.

Parameter Df⁎ F⁎ Probability Contact anglea (°)

Intercept 1 7868.33339 0.000001Treatment 7 388.593662 0.000001Fabric 2 0.570227513 0.569180Fabric × treatment 14 1.30088212 0.242051T5 (4TMA + 1FOTS)

C1 164 aC6 153 aC9 163 a

T7 (4TMA + L + 1FOTS)C1 169 aC6 169 aC9 165 a

T5 (4TMA + 3FOTS)C1 169 aC6 168 aC9 164 a

T7 (4TMA + L + 3FOTS)C1 169 aC6 169 aC9 166 a

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


the increased roughness of the T7 fabric in comparisonwith theuntreat-ed cotton surface (T0 treatment). It is important to note that samplestreated with the ALD method (T1, T2, and T3) do not demonstrate no-ticeable roughness.

3.2. FTIR results

In Fig. 2, the spectra of the control (treatment T0) and the fabric sub-jected to treatment T7 are displayed. The penetration depth of the infra-red beam in the UATR technique is ~1.66 μmat 1000wavenumbers [24,25]. Since the thickness of the coatings in the ALD method does not ex-ceed a few nanometers, vibration attributed to the cellulose substratewere the only vibrations present in the FTIR spectra of cotton fabric sub-jected to T1, T2, or T3 treatments, and no additional vibrationswere ob-served. Fig. 2 shows the FTIR spectra of the fabrics subjected totreatments T0 and T7. The peaks at 880 cm−1 and 706 cm−1, whichare present in the spectra of the samples subjected to T4 and T7 treat-ments (containing Al2O3 nanoparticles), are ascribed to Al–O stretchingand O–Al–O bending, respectively [26,27]. The presence of peaks in therange of 1300–1800 cm−1, with a stronger absorption at 1580 cm−1, in-dicates the presence of impurities, such as carbon, in the aluminumoxide coatings [28]. The intensity of FTIR signals are enhanced withthe increase in the surface area [29]. Consequently, the functionalgroups ascribed to FOTS and bifunctional trichlorosilane layers on nano-particles obtained from T7 treatment were detected in the FTIR spectraand presented in Fig. 2. The peak at 1239 cm−1 is assigned to C–Fstretching, and the peaks at 1145 cm−1 and 1212 cm−1 are attributedto symmetric and asymmetric CF2 stretching [30]. The absence of the

Fig. 4. Glycerol drop on fabrics C1 (left), C6 (center), and C9 (right) subjected to


mentioned peaks in the sample subjected to T3 treatment, in whichthe FOTS layer is not present, confirms the absence of the aforemen-tioned FOTS layer (graph not shown). As formerly explained, the pres-ence of the bands at ~880 cm−1 and ~706 cm−1 in the spectrum ofsample subjected to T7 treatment is ascribed to the vibration of func-tional groups of Al2O3 nanoparticles.

3.3. Dynamic contact angle

Ultra-oleophobic surfaces have the advantage of getting cleanedwithout much effort by just running the liquid over the surface. Dueto the low surface energy of these substrates, the dust or particles onthe surface have more attraction to the running liquid than to thesolid surface itself. This characteristic allows the surface to be cleanedof dust or particles by the rolling liquid. This surface property is termedself-cleaning or the “lotus effect” [31]. By definition, with contact angleshigher than 150°, an ultra-oleophobic surface is achieved [32]. Fig. 3shows the average contact angles of C1, C6, and C9 fabrics subjected toT1, T3, T5, and T7 treatments for different liquids presented in Table 2.In order to adhere to the goal of this study, the graphspertaining to sam-ples for which complete wetting has occurred are intentionally elimi-nated. T5 and T7 treatments exhibited the best results. The effect ofthe FOTS layer coverage on the substrate has also been investigated bypreparing two sets of samples, onewith 3 sequences of FOTS depositionand another with only 1 deposition cycle. As shown in these figures, anumber of the ALD treatments resulted in contact angles of zero,denoting that a complete wetting has occurred. The reason can be at-tributed to an incomplete coverage of the surface by oleophobicchemicals which allows capillary forces to act on the liquid, leading toa complete absorption.

The wetting of T5 and T7 treatments was observed for the liquidswith low interfacial tension. The wetting of the nanoparticle coatingsby the low interfacial tension liquids can be justified byWenzel's equa-tion.Wenzel [33] proposed an equationwhich can describe the effect ofroughness on the contact angle, called roughness factor:

r ¼ Ao

Arð1Þ

where Ar is the actual surface including the roughness, and Ao is the ob-served surface area. Wenzel presented this equation as:

r ¼ Ao

Ar¼ cos ′θ

cosθð2Þ

where ′θ is themeasured contact angle on the rough surface and θ is thecontact angle on the smooth surface. This equation shows that whenθ N 90°, there is an increase in the contact angle along with the increaseof the surface roughness. However, when b 90°, there is a decrease inthe contact angle with an increase in the surface roughness [34–36]. Ifthe surface roughness is higher than a specific amount (this amount de-pends on the interfacial tension of the selected liquid), it is difficult forthe liquid to fill the roughness of the surface. Thus, the air remainstrapped under the liquid droplet. Since the contact angle of liquid onair is 180°, the oleophobicity of the droplet on the surface composited

T7 treatment with four layers of TMA and three layers of FOTS deposition.



Fig. 5. Advancing and receding hexa-decane contact angles of fabric C1 subjected to T7treatment. This graph shows the low hysteresis of the T7 nanoparticle coating.


of air and solid increases. Cassie and Baxter [37] have described thisstate using Young's equation as below:

cos θCB ¼ f s cos θY þ f v cos θLV ¼ f s cos θY− 1− f sð Þ ð3Þ

where θCB is the apparent contact angle, and fs and fv are the fractions ofsolid and vapor in contact with the liquid in a way that fs + fv = 1. Theincrease in surface roughness results in a decrease in the fs, and as a con-sequence, results in a significant increase in θCB. TheWenzel model andCassie–Baxter model can overlap in some cases, but as a general rule,surfaces with mild roughness follow the Wenzel model while surfaceswith moderate to high roughness follow the Cassie–Baxter model. Onoleophobic surfaces with contact angle of ~100° and moderate rough-ness r ~ 2, both states can co-exist [35].

To investigate the effect of the number of FOTS cycles of depositionon FOTS layer coverage and its impact on the contact angles, two differ-ent treatments were prepared: onewith three sequences of FOTS depo-sition and another with one sequence of FOTS deposition applied on thesurface. The analysis of the variance in the contact angle results of thenanoparticle coatings (treatments T5 and T7) did not show a statistical-ly significant difference between the two sets of samples. In addition, nosignificant difference was reported between the T5 and T7 treatments.Therefore, the addition of bifunctional trichlorosilane did not have anef-fect on the contact angles. As an example, Table 3 shows the analysis ofvariance of the effect of the treatments on the glycerol contact angle.Fig. 4 shows the glycerol contact angle on C6 fabric corresponding tothe three sets of treatments.

When it comes to textile surfaces, surface roughness, heterogeneity,and the adsorption of a liquid with a consequent change in surface en-ergy render the wetting phenomena complicated. In such surfaces, themeasured contact angle, after the liquid drop advances, is usually great-er than the contact angle after the droplet recedes from a previouslywetted surface. The difference between the advancing and recedingcontact angles is known as hysteresis. In other words, hysteresis can

Table 4Critical angle required to initiate a dropmovement over the surface, calculated from force of drtiate a droplet movement over the surface, calculated from advancing and receding contact an

Critical angle

Treatment Avg. wa(mm) ma (mg) sin α

T7: C1-4TMA-L-1FOTS 1.0 21.7 0.003T3: C1-4ALD-1FOTS 1.4 21.7 0.188

a m is the droplet mass and w is the solid–liquid contact width.b Advancing contact angle and receding contact angle.


be defined as the change in the surface energy [38] or the roughnessof the surface [8] which results in a change in the adsorption of liquidon a solid surface. As is observed in Fig. 5, the advancing and recedingcontact angles of the C1 fabric subjected to T7 treatment reflected avery low hysteresis. In contrast, T3 treatment on the same fabric result-ed in high hysteresis (figure not presented). Any defects or heterogene-ities, even at the molecular scale (for example, differing lengths of themolecule chain), will result in some degree of hysteresis in the advanc-ing and receding contact angles measurement [31]. The shape of the tipof themolecule chains emerging from the surface also influences the ad-vancing and receding angle. The hysteresis of the samples was mea-sured using hexa-decane with the procedure mentioned previously.

Knowing the hysteresis of the surface is critical in interpreting nu-merous surface characteristics, such as the required force to move adroplet on the surface, by taking advantage of the thermodynamicequations. By measuring the surface hysteresis, MacCarthy et al. [39,40], proposed a relationship which estimates the required force tomove a droplet on a solid surface:

F � γLV cos θR− cos θAð Þ ð4Þ

where cos θA and cos θR are the advancing and receding contact angles,respectively. Table 4 compares the forces of hexa-decane dropletmove-ment on C1 fabrics subjected to T3 and T7 treatments.

In order to prevent the droplet from rolling on the surface, a balanceshould bemaintained between the gravity and the surface tension. Oncethe surface tension force which keeps the droplet from moving on thesurface becomes smaller than the gravity acting on the droplet, the liq-uid droplet starts to roll on the surface. The gravitational force on thedroplet becomes larger when the surface is tilted to the point that at acertain angle, the gravitational force overcomes the surface tensionforce. This so called critical angle, α, can be calculated by equatingthese two forces:

yLV cos θr− cos θað Þ ¼ mgw

sin α ð5Þ

where m is the droplet mass and w is the solid–liquid contact width.This equation proposes that for a given mass of a droplet, smaller con-tact angle hysteresis will lead to an easier droplet roll off with a smallertilting angle [35,36]. The critical angles (α)were calculated for C1 fabricssubjected to T3 and T7 treatments (Table 4) based on the w and mvalues as well as the force of droplet movement. Notably, in order tomove a droplet on a surface with contact angle smaller than 140°, alow hysteresis (b4°) is required. However, it is difficult to obtain lowhysteresis on surfaces which exhibit small contact angles. On theother hand, liquid droplets will easily roll on surfaces with high contactangles even with a hysteresis higher than 5°. This is explained by thesmall solid–liquid (w) contactwidth. Contact angle hysteresis can be in-creased by inhomogeneity and contamination of the surface such as oilor dust [41].

3.3.1. Work of adhesion and surface free energyThe estimation of the solid surface energy can be achieved by placing

different standard liquids with known interfacial tension on the solidsubstrate until the wetting condition is met. The surface tension of

oplet movement,w andm, using hexa-decane as the test solution, and force needed to ini-gles of hexa-decane.

Force of droplet movement

α (°) Avg. adv. CAb (θ) Avg. rec. CAb (θ) Force (mN/m)

0.17 160 159 0.610.84 116 93 28.2



Table 7Force of attraction of different liquids over samples subjected to T7 treatment.

Tested liquids Fabric Force of attraction (dynes/cm2)

Glycerol C1 6.34 × 107

C6 7.27 × 107

C9 1.28 × 108

Ethylene glycol C1 1.30 × 108

C6 1.79 × 108

C9 2.02 × 108

Formamide C1 7.37 × 107

C6 8.91 × 107

C9 8.26 × 107

Table 6Surface free energy of C1 fabrics, calculated using Owens–Wendt and Van Oss methods.

Owens–Wendt method Van Oss method

Treatment γsd γs

h γs

(mN/m)γLW γ+ γ− γs

(mN/m)

T7 0.00427 0.00184 0.00612 0.00509 0.00004 0.00239 0.00571T5 0.02413 0.00456 0.02869 0.02694 0.00030 0.00491 0.02936

Fig. 6.Work of adhesion Wa.


liquids can also be measured using different methods, such as capillarytube [42], Du Nuoy ring [43], Wilhelmy plate [44], and pendent drop[45]. When a droplet is resting on an ideally homogenous, smooth,flat, and non-deformable surface, the equilibrium contact angle can bedefined by Young's equation [46]:

γlv cos θ¼ γsa−γslð6Þ

In which γlv is the surface tension of the liquid, γsa is the surface en-ergy of the solid in air, and γsl is the interfacial tension of the solid inequilibrium with the liquid. Based on this equation, with the assump-tion that γsa is equal to γs, the surface tension of the liquid should begreater than the surface tension of the solid in order for the wetting tooccur. It is still possible to completely wet the surface with contact an-gles higher than 0°, if an amount of force is applied on the liquid [47].

The Eq. (6) became controversial among scientists because the sur-face tension of a solid is not well defined. γsv, which is the interfacialtension of a solid in equilibrium with the saturated vapor of liquid, isnot equivalent to γsa, which is the surface energy of a solid in air.Harkins and Livingstone [48] discovered that Young's equation can becorrect when the surface has adsorbed a layer of liquid's vapor. They re-alized that the γsv value is smaller than the γsa value, so they proposed anew term, πe, to show the difference of these two tensors:

γsv ¼ γsa− πe: ð7Þ

By combining Eq. (6) with Eq. (7), Young's equation becomes:

γsa ¼ γlv cos θþ γsl þ πe: ð8Þ

On the other hand, Dupre [49] defined a new term calledwork of ad-hesion,Wa, which is the work necessary to separate liquid from solid:

Wa ¼ γlv þ γsa−γsl: ð9Þ

In his theory, he explained that when a liquid is separated from asolid, the solid–liquid phase is completely destroyed, and two newsolid-air and liquid-air are formed (Fig. 6).

By combining Eqs. (8) and (9), the equation below is obtained:

Wa ¼ γlv 1þ cos θð Þ þ πe: ð10Þ

Under certain conditions [47], we can ignore πe. So, Eq. (10) can besimply written as Young–Dupre's equation:

Wa ¼ γlv 1þ cos θð Þ: ð11Þ

Table 5Work of adhesion of liquids with different surface energies for treatment T5 onfabric C1.

Liquid Work of adhesion (mN/m)

Glycerol 1.1Ethylene glycol 2.1Formamide 1.1Methylene iodide 1.0Hexa-decane 3.7Tetra-decane 3.3Do-decane 0.5Decane 9.8


This equation means that the work of adhesion can be calculated byknowing the surface tension and the contact angle of a liquid. Therefore,the solid surface energy is not required to calculate the work of adhe-sion. Using the surface tension of the probe liquids [50,51] (Table 2),the work of adhesion, Wa, was calculated using Eq. (11) for differenttreatments. The results show that treatments T7 and T5 have lowerwork of adhesion compared to treatments T1 and T3. The best result,which is presented in Table 5, was achieved by the T5 treatment on fab-ric C1.

There is no known direct method to calculate the surface energy ofsolids. Several approximation methods were proposed to estimate thesolid surface energy using contact angles, such as Berthelot's approxi-mation, Fowkes [52] approximation, acid–base approximation, and geo-metric and harmonic mean approximation [53–55]. However, theOwens and Wendt [56] approximation method seems most suited forultra-oleophobic surfaces. This equation is the general form of theFowkes relationship in which both dispersion and hydrogen-boundingforces operate:

1þ cosθ ¼ 2γdSγ

dL

� �0:5γLV

!þ 2

γhSγ

hL

� �0:5γLV

!: ð12Þ

The surface energy can be calculated using Eq. 12 bymeasuring con-tact angles of two different liquids (water and methylene iodide wereused in this study) on solid substrates. By solving simultaneous equa-tions of each liquid, γS

h and γSd can be obtained. The sum of these two

components is equal to the total surface energy of the solid. Table 6shows the surface free energy results for C1 fabrics with different treat-ments calculated using this method. For comparison, the van Oss [57]method was also employed to check the accuracy of the Fowkes meth-od. The results of the vanOssmethod (Table 6) is in agreementwith the

Methylene iodide C1 7.37 × 107

C6 9.31 × 107

C9 6.90 × 107

Hexa-decane C1 1.31 × 108

C6 1.58 × 109

C9 5.63 × 108

Tetra-decane C1 1.31 × 108

C6 8.07 × 108

C9 6.89 × 108

Do-decane C1 2.09 × 107

C6 1.56 × 109

C9 1.94 × 109

Decane C1 3.98 × 108

C6 1.94 × 109

C9 4.99 × 108



20 21 22 23 24 25 26 27 28

Interfacial Tension

-1.2

-1.0

-0.8

-0.6

-0.4

-0.2

0.0

0.2

0.4

0.6

0.8

1.0

Cos

θ

y = 7.2405 - 0.3085*x

Fig. 7. Zisman plot of fabric C1 subjected to T7 treatment for calculating the critical surfacetension.


Fowkesmethod. In VanOssmethod, solid surface energy is composed ofthree different components, γLW, γ+ and γ−, and the total surface ener-gy can be calculated by:

γs ¼ γLW þ 2ffiffiffiffiffiffiffiffiffiffiffiffiffiγþγ−

p: ð13Þ

Therefore, the contact angle of three liquids is required for the vanOss method. Water, formamide, and glycerol were used for this test.

3.3.2. Force of attractionBy equating the work of adhesion and the work of separation, the

maximum force of attraction can be achieved [47]. Knowing the forceversus distance relationship of attraction and repulsion, the attractiveforce, Fa, and the repulsive force, Fr, at equilibrium, can be written as:

Fa ¼ Fað Þedx

� �3

ð14Þ

Fr ¼ Frð Þedx

� �8

ð15Þ

where x represents the separation distance and d represents the equilib-rium distance. An estimation of the term d can be achieved by calculat-ing the distance between the centers of the liquidmolecules [47]. Underequilibrium condition where (Fa)e = (Fr)e, the work of adhesion can bewritten as:

Wa ¼Z∞d

Feð Þ dx

� �3

dx−Z∞d

Feð Þ dx

� �8

dx ð16Þ

Table 8Calculated critical surface tension (mN/m) based on Zisman plots.

Treatment Fabric Critical surface tension (mN/m)

T7:4TMA + L + 1FOTS C1 20.2C6 23.1C9 19.0

T5:4TMA + 1FOTS C1 22.4C6 21.6C9 23.3

T3:4ALD + 1FOTS C1 24.7C6 24.7C9 24.7

T1:1FOTS C1 23.7C6 24.3C9 24.7


which is equal to:

Wa ¼ Fed2−

d7

� �: ð17Þ

By knowing the work of adhesion and d, Fe = F= Fr can be derived.To estimate the distance between the centers of the molecules, the

specific gravity (SG) and molecular weight (MW(g/mol)) of the liquidscan be used. By using Eq. (17) for different liquids, the force of attractioncan be calculated, as shown in Table 7. Factors such as the interfacialtension of liquid and the natural characteristics of the substrate, suchas surface geometry or porosity, polarity, and chemical composition, af-fect the absorption of solid–liquid interface. Temperature and pressureas well as other environmental conditions also affect the adsorption ofliquids [8].

Zisman [34] reported that if the surface tensions of a family of liquidsversus the cosine of their contact angles are plotted, the data will fallalong a straight line. Hence, they described the critical surface tension,γc, to be the surface tension value atwhich cos θ=1 (or θ=0°). Zismanproposed that only liquids lower than this value for the surface tensionhave the ability to wet the surface [8]. Others have equated this criticalsurface tension to the surface energy of solids. Because of the similarityof the system studied by Zisman to the study of oleophobic surfaces, theresult will be very useful for the development of oil-repellent finishes[47].

Zisman also used the critical surface tension principle to calculatethe surface energy of a condensed monolayer of low-energy materialson polished platinum or glass. He concluded that the wettability of asurface is in direct relation with the nature and the packing structureof the final layer of the solid and is not related to the underlyingatoms [38]. Fox and Zisman [58] also reported that the critical surfacetension can be different among different liquid types, hence they cannotbe a measure of the surface energy of a solid. Fig. 7 shows the Zismanplot for fabric C1 subjected to T7 treatment. Liquids from decane family(hexa-decane, tetra-decane, do-decane, decane and octane, in descend-ing order of interfacial tension magnitude) were applied on each fabricuntil completewetting (θ=0°) occurred. Table 8 shows the critical sur-face tension for different fabrics with different treatments.

4. Conclusion

In this study, rough surfaceswere obtained throughnanoparticle de-position in a vapor phase reaction.We have shown that the addition of aFOTS layer, as thefinal layer, to the roughened surfaces resulted in ultra-oleophobic substrates. This was demonstrated in the ultra-oleophobicproperties requirements through contact angles as high as ~170° andhysteresis values as low as ~1°. The critical angle required to roll thedroplet on the surface was shown to be very low on the nanoparticleroughened surfaces. Moreover, very low work of adhesion and force ofattraction values were obtained for samples roughened with the nano-particles. Very low critical surface tensions were also measured using aZisman plot for our fabrics.

Acknowledgments

The authors would like to thank the National Science FoundationMajor Research Instrumentation program (CBET 0821162) and theTexas Department of Agriculture/Food and Fibers Research Grant Pro-gram (NFR-11-03) for providing the financial support for this project.Texas Tech University manuscript number T-4-675.

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Ultra-oleophobic cotton fabric prepared using molecular and nanoparticle vapor deposition methods

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