Revista Brasileira e Ensino e Física, v. 34, n. 3, 3312 (2012) www.sbfisica.org.br Simple measurement of surface free energy using a web cam (Meia simples a energia livre e superfície usano uma web cam) C.M.S Vicente 1,2,3, P.S. Anré 1,3 an R.A.S. Ferreira 2,3 1 Instituto e Telecomunicações, Campus e Santiago, Aveiro, Portugal 2 Centro e Investigação em Materiais Cerâmicos e Compósitos, Universiae e Aveiro, Campus e Santiago, Aveiro, Portugal 3 Departamento e Física, Universiae e Aveiro, Campus e Santiago, Aveiro, Portugal Recebio em 23/8/2011; Aceito em 6/8/2012; Publicao em 21/11/2012 Neste trabalho, escrevemos uma experiência simples e peagógica para meir a energia livre e superfície (SFE) que é um tema ominante no ensino a Física, a nível e estuos e grauação e pós-grauação. A vantagem esta experiência baseia-se na simpliciae os materiais utilizaos, ou seja, proutos e baixo custo e não prejuiciais, como água, glicerol, etileno glicol e propanol, que oferecem uma boa oportuniae para iscutir peagogicamente conceitos básicos, mas relevantes sobre fenómenos e superfície. Como exemplo, meias e ângulo e contacto foram utilizaas para estimar a SFE e a molhabiliae e iferentes superfícies sólias, tais como o viro e o politetrafluoretileno (PTFE, Teflon R ). Palavras-chave: práticas peagógicas, energia livre e superfície, ângulo e contacto. In this paper we escribe a simple an peagogical experiment to measure surface free energy (SFE), which is a mainstream subject to teach unergrauate an grauate level Physics science. Beyon this, the avantage of this work relies on the simplicity of the materials use, namely non-harmful an low cost proucts such as water, glycerol, ethylene glycol an propanol, offering a useful peagogical opportunity to iscuss basic but relevant concepts regaring surface science phenomena. As example, contact angle measurements were use to estimate SFE an the wetting behavior of istinct soli surfaces such as glass an polytetrafluoroethylene (PTFE, Teflon R ). Keywors: peagogical practices, surface free energy, contact angle. 1. Introuction Surface phenomena, namely surface free energy (SFE), is a multiisciplinary topic of interest at unergrauate an grauate levels, combining knowlege of physics, electrical engineering, materials science an thermoynamics. At a liqui-soli interface, the interaction forces are etermine by the cohesion an ahesion forces. The ratio between these two forces permits the etermination of the solis SFE. One way to experimentally etermine the SFE of solis is the measurement of the contact angle between the outline surface tangent of a liqui rop an the surface [1-4]. The contact angle is a measure of a liqui ability to sprea on a surface, which also enables the iscrimination between polar an ispersive interactions. In this work, a short review of the physical aspects that are involve in the contact angle measurements consiering the Owens an Went metho [5] is performe in section 2. Section 3 presents the experimental 1 E-mail: panre@av.it.pt. methoology to peagogically implement contact angle measurements, using an accurate an very simple apparatus making using of non-harmful an cost effective liqui an soli materials, given emphasis to the apparatus implementation. In section 4, measurement of SFE will be pursuit by means of contact angle measurements of liquis on soli surfaces an the obtaine results will be interprete in terms of polar an ispersive contributions of the SFE an the wetting envelope function. At the en, final consierations about the evelope metho will be presente through the comparison of the obtaine results with the ones in the literature. 2. Theoretical backgroun The contact angle is efine as the angle that a liqui rop surface makes when in contact with a soli surface. The contact angle value will epen on the liqui SFE an will initially vary with time as the liqui spreas Copyright by the Socieae Brasileira e Física. Printe in Brazil.
3312-2 Vicente et al. over the surface. The wettability of a liqui on a soli surface is etermine by the work of ahesion between the liqui an the soli, W a, an the work of cohesion (efine as the work require to separate a unit area of two contacting phases), W c, of the liqui. While the ahesive work contributes to the liqui sprea over the soli surface, the cohesive one offers resistance to it, inucing a liqui contraction. The ahesive work an the cohesive work are relate with the SFE of the liquivapour, γ LV, soli-vapour, γ SV, an soli-liqui, γ SL, interfaces, accoring to Eqs. (1) an (2), respectively W a = γ LV + γ SV γ SL (1) W c = 2.γ LV (2) At equilibrium the energy must be stationary with respect to a shift of the soli-liqui bounary [6], Fig. 1, being given by Young s equation as function of the contact angle, θ γ SV = γ SL + γ LV cos(θ). (3) From a thermoynamic point of view, when a liqui contacts a soli surface on the presence of a vapour phase, the liqui will wet the soli surface if the free energy value require to create a new surface is lower than the energy value of the liqui-vapour interface. The equilibrium spreaing coefficient, S, can be efine as the ifference between the ahesion work W a an the cohesion work W c. The spreaing parameter S is also efine as the ifference between the SFE (per unit area) of the soli surface when ry an wet. For spontaneous spreaing occurs it is necessary that the spreaing coefficient S is negative, accoring to Eq. (4) [5]. S = W a W c = [γ SV ] ry [γ LV + γ SL ] wet. (4) Owens an Went evelope the iea that the SFE at a liqui-vapour an at a soli-vapour interfaces has two contributions, namely the polar (p) an the ispersive () ones, ue to intermolecular interactions [5]. For a small vapour relative contribution, we may assume that γ SV = γ S an γ LV = γ L. Figura 1 - Scheme representing the contact angle (θ) between the vapor (V), liqui (L) an soli (S) phases for a liqui on a soli surface. Therefore, the SFE is compose of two components, the Lifshitz-van er Waals component (ispersive contribution, γ ) an the ipoles an Lewis aci-base component (polar contribution, γ p ). The polar an ispersive contributions for the SFE can be express in the partial Eqs. (5) an (6) for liquis an solis, respectively γ L = γ L + γ p L, (5) γ S = γ S + γ p S. (6) In pure liquis, the interaction between the liqui an the soli can be escribe in terms of reversible work of ahesion [5] ( (γ ) ) W a = 2 S γ L + (γ p S γp L ). (7) From Eqs. (1), (3) an (7), results γ L (1 + cos(θ)) = 2 γ L γ p S γ p L + γs. (8) The previous linear relation written in terms of the inepenent variable (/ p γ / L ) 1/2 an of the epenent variable γ L (1 + cos(θ)) 2, allows us to etermine the square root of the ispersive an polar soli SFE components. When both ispersive an polar SFE are known, we can estimate which are the liquis that will wet the soli surface, through the so-calle wetting envelope function [6]. The wetting envelope function calculation requires the numerical resolution of Eq. (8), replacing the values of the SFE components an the complete wetting conition (θ = 0 ). In this case, a close contour with a quaratic behavior is obtaine in terms of the ispersive an polar components of the liqui. The knowlege of the soli wetting envelope enables easy wettability etermination. Any liqui which polar an ispersive fractions lay within the contour will wet the corresponing soli surface [7]. 3. Methoology γ L 3.1. Experimental apparatus Figure 2 shows a schematic representation of the implemente experimental apparatus constitute of a Web cam equippe with a CCD sensor (Creative R Live Ultra) to recor vieos with a minimum resolution of 640 480 pixels at 30 frames per secon an of a iffuse light source mae from a tungsten lamp. In our particular case, we connecte it through an optical fiber array
Simple measurement of surface free energy using a web cam 3312-3 to rener easier the reirection of the light into the liqui rop. A flat sample of the substrate material is place unerneath a syringe an a rop of liqui is ispense onto the substrate. The syringe couple with a neele of 0.5 mm iameter was use an manually hanle to ispense a small liqui rop on substrates with minimum mechanical impact. The vieo of the rop eposition was acquire using the Arcsoft R Vieo Impression software that is provie with the web cam. The images of the liqui rop were extracte from the vieo files when a static conition of the rop shape was achieve. The selecte test liquis were water, glycerol, ethylene glycol an propanol with analytical-reagent quality, whose ispersive an polar components of the SFE are gather in Table 1 [7-9]. These values will be use to iscuss the accuracy of the metho propose here. The selecte soli surfaces were soalime Glass slies (Normax) an a thin sheet of polytetrafluorethylene (PTFE, Teflon R ). The glass surface preparation inclues three steps: glass slies were cleane in an ultrasonic cleaner, with i) acetone uring 15 minutes; ii) ethanol uring 15 minutes an iii) ry at 60 C uring 10 minutes in an oven. The PTFE surface was cleane with propanol in orer to remove possible hanling contaminations. The Glass an PTFE surfaces were cleane uner an air flux before the experimental measurements. 3.2. Experimental ata analysis The LB-ADSA (Low Bon Axisymmetric Drop Shape Analysis) program (http://bigwww.epfl.ch/emo/ ropanalysis/) was use to fit the rop image profile an to measure the contact angle values. The LB- ADSA is base on the perturbation solution of the axisymmetric Laplace equation. It is thus suite to rops that are uner the force of gravity on a horizontal substrate [10], as in the present case. 4. Results an Discussion Figure 3 exemplifies photographs of one rop of water, glycerol, propanol an ethylene glycol on the top of a PTFE surface. For each liqui an soli surface 100 ifferent measurements were performe so that the average contact angle is gathere in Table 2 for all the selecte experimental conitions. For propanol eposite on glass, the contact angle values are very close to 0, rener ifficult an accurate etermination of the contact angle value, an therefore, were not consier in the calculation performe below. The fact that the contact angle value is 0, inicates a complete wetting of the surface. In orer to etermine the ispersive an polar components of the surface energy for the Glass an for the PTFE soli surfaces, Eq. (8) was applie to the contact angle values in Table 2, yieling to the ata in Fig. 4. Figura 3 - Photographs of the rop of a) water, b) glycerol, c) propanol an ) ethylene glycol forme on the surface of PTFE. Tabela 2 - Average contact angle values ( ) measure for rops of water, glycerol, propanol an ethylene glycol forme on the top of PTFE an glass surfaces. The experimental errors are within 5%. Surface Test liquis water glycerol propanol ethylene glycol PTFE 89.6 86.8 30.0 76.6 Glass 34.9 38.3 0.0 35.1 Figura 2 - Scheme of the experimental set-up use to acquire contact angles images. Tabela 1 - Surface free energy ispersive (γ L ) an polar (γp L ) components (mn.m 1 ) for selecte contact angle test liquis at 24 C. γ p L water [8] 21.8 51.0 glycerol [8] 34.0 30.0 propanol [9] 12.30 8.63 ethylene glycol [10] 30.9 17.4 Figura 4 - Variation of the γ L (1 + cos(θ))/2 epenent variable with the (γ p L /γ L )1/2 inepenent one for Glass an PTFE surfaces. The soli lines represent the ata best linear fit (r > 0.99). A linear epenence between γ L (1 + cos(θ))/2 an (/ p γ L ) 1/2 was observe, enabling the etermination of γs an γ p S values for each soli surface, using
3312-4 Vicente et al. Eq. (8). The linear fitting parameters, namely the slope (m) an intercept (b) were m = 7.01 ± 0.83 an b = 2.61 ± 0.94 (mn.m 1 ) 1/2 an m = 2.79 ± 0.13 an b = 3.28 ± 0.12 (mn.m 1 ) 1/2 for the Glass an the PTFE surfaces, respectively. The soli SFE values were calculate yieling to the results in Fig. 5. In orer to valiate the propose methoology, the SFE values for Glass an PTFE reporte in the literature [11, 12] are also shown in Fig. 5 for comparison purposes. Figura 5 - Surface free energy values for Glass an PTFE. Data from the literature [11,12] is also inclue. We can observe that the experimental results of SFE for Glass an PTFE obtaine by this metho are consistent with previous reporte values [11, 12]. However, in the present stuy, polar contributions are more significant than that observe in previous stuies [11, 12]. This ifference can be ascribe to a less homogeneous surface of the substrates. The stuy of the surface roughness an non-homogeneity lies beyon the scope of the present work. The knowlege of the contribution of the polar an ispersive SFE components allows the etermination of the so-calle surface wetting envelope. The polar an ispersive components of the liqui for which the contact angle is 0 are calculate an the polar fraction was plotte against the ispersive fraction, resulting in a close contour which is calle the wetting envelope. The calculate SFE values were use to etermine the wetting envelope function of glass an Teflon R (Fig. 6). The envelope function escribes the wetting of water, glycerol, ethylene glycol an propanol on the selecte soli surfaces. All the use liquis have a partial wetting of the PTFE surface because the respective values of the SFE lie outsie the PTFE close contour. On glass, water an glycerol present a partial wetting, while propanol completely wets the glass surface with values lying within glass contour, as experimentally observe. Figura 6 - Envelope functions for Glass an PTFE (soli lines). The squares represent the polar an isperse contributions of the surface free energy for selecte liquis. In general, soli surfaces with high values of SFE (constitute by covalent, ionic or metallic bons) are completely wet by most of the liquis with low SFE, while solis with low SFE (bon by Van er Walls forces or in special cases, by hyrogen bons) ten to be ifficult wette by those liquis, allowing only a partial wetting conition [13]. Some exceptions to this empirical rule were explaine ue the contribution of ispersion forces for the SFE [6]. In general, liquis such as water an alcohols (ethylene glycol, glycerol an propanol) have structural an ynamic properties that are influence by intermolecular bons namely hyrogen bon [14, 15]. The number of hyrogen bons that a single liqui molecule can participate eterminates the cohesion properties of liquis conitioning the SFE values [14]. This fact can be explaine on a simplistic way by the ifferent contact angle values an the liquis spreaing tenency on a common substrate. For PTFE an others fluorinate polymers it is well known that surface properties, in particular SFE, are relate with weak molecular forces an the existence of C-F bons from CF 2 groups [16, 17]. The properties of these surfaces epen not only on the coverage by the fluorocarbons but also on the egree of orer of the surface [18, 19]. Moreover, glassy surfaces present a stronger polar character ue to the existence of polar bons that are responsible for a strong interaction with molecular groups of liquis, justifying its hyrophilic character [20]. 5. Conclusion In conclusion, we have presente a simple scheme to measure the contact angle between low cost an non-harmful liqui samples an istinct soli surfaces, which can be can be a useful peagogical approach to stuy surface free energy relate topics. The presente work stimulate iscussion an allows an unerstaning
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