Contact Angle Measurement for The Surface Characterization of Solids D. P. Subedi Department of Natural Sciences, School of Science, Kathmandu University, Dhulikhel, Kavre. [email protected] Abstract properties of a solid using relatively a very simple approach. The surface energy of a solid can be This paper reports a description of the theory of contact angle measurement and its use for the determination of surface energy determined from the measurement of contact angle of a pure liquid drop on that solid. Contact of solids. The main objective of the work is to present a convenient approach to explain wetting phenomenon using contact angle theory. A brief review of Young’s equation andangle Young- measurement Dupre equation ishas also been presented used followed in the by studyan extended of surface Fowke’s equation energy, wettability and adhesion of low widely used for the calculation of surface energysurface of solids. energy The two liquidmaterials. model has In been this applied paper, to determine the theorythe surface of energy contact of angle measurement and an polycarbonate and low density polyethylene. important model for the determination of surface energy will be discussed. Key words: Contact angle, surface energy, surface characterization, wettability, Young’s equation, Fowke’s model Theory of Contact Angle Measurement Introduction Theory of Contact Angle Measurement Studies concerning the interaction between a solid and liquid require reliable experimental values Over the years a large number of techniques have been developed Studies concerning the interaction between a solid and liquid to probe different aspects of the physicsof and contact chemistry angle of andrequire surface reliable experimental energy. The values quantitative of contact angle evaluation and surface of the wetting of a solid by a surfaces, however, only a few have found liquidwide application is made in in termsenergy. Theof thequantitative contact evaluation angle !of. theFig. wetting 1 shows of a solid the by schematic a diagram for the contact basic surface science and applied surface analysis [1]. The choice liquid is made in terms of the contact angle θ. Fig. 1 shows the of the technique depends upon the type ofangle the characterization and the interfacial schematic tensions diagram for at the the contact three angle phase and the boundary. interfacial tensions to be made. Among the most widely used methods are X-ray at the three phase boundary. photoelectron spectroscopy (XPS) and Fourier transform infra- �γ"$# red spectroscopy (FTIR) are used to study the surface chemical lv composition. Similarly, scanning electron microscopy (SEM) and ! "#$%&! atomic force microscopy (AFM) are used to investigate the surface morphology of the material in atomic scale. These methods '()*(+! require relatively expensive equipments, skilled technicians and �Ø# �γ!"# �γ!$# sl sophisticated techniques to interpret data. [2-3]. Measurement of sv ,%-(+! surface energy of the solid can also provide a good understanding ! of the surface properties of a solid using relatively a very simple approach. The surface energy of a solid can be determined from Fig. 1. Schematic diagram of the contact angle and interfacial the measurement of contact angle of a pure liquid drop Fig.on that 1. Schematictensions of the diagram three surfaces of the at contact the three-phase angle boundary and interfacial tensions of the three surfaces at the three-phase boundary solid. Contact angle measurement has been used in the study of The most important relation regarding the contact angle is the surface energy, wettability and adhesion of low surface energy Young’s equation that relates the contact angle θ, liquid surface materials. In this paper, the theory of contact angle measurement The most importanttension relation γ , solid regardingsurface energy the γ , solid-liquid contact surfaceangle tensionis the γ Young’s as equation that relates the and an important model for the determination of surface energy l s sl ! expressed in Eq.(1) [4]. will be discussed. contact angle , liquid surface tension �l, solid surface energy �s, solid-liquid surface tension �sl as expressed in Eq.(1) [4]. //1// (1) For the convenience of understanding the wetting phenomenon, Eq. (1) can be rearranged as shown in Eq. (2). !" " The Himalayan Physics Contact Angle Measurement for The Surface Characterization of Solids Vol. II May 2011 γ θ = θ γγ …………… (1) wetting process. 1 cos s sl For the convenience of understanding the wetting phenomenon, Calculation of Surface Energy Eq. (1) can be rearranged as shown in Eq. (2). Another important contribution in the field of surface analysis was € made by Dupre [5]. According to this equation, work of adhesion γ s −γ Cosθ − sl ……………… (2) W between a solid (s) and a liquid (l) can be expressed as, γ A l W …………………… (3) A = γ s + γ l − γ sl From Eq. (2), the value of Cosθ is determined by the relative magnitude of the three quantities namely, the surface tension of This work of adhesion WA is the reversible work done in separation € of unit area of solid/liquid interface and equals to one half of the the liquid (γl), surface energy of the solid (γs) and the interfacial € work of cohesion when the arrangement of molecules in the surface tension between the solid and the liquid (γsl). A perfect wetting means the maximum value of Cosθ which is achieved when the region is the same as the bulk. Combining equation (1) and (3), contact angle is zero. There are two possibilities to inrease the value gives the Young-Dupre equation which is represented by Eq. (4). W ……………… (4) of Cosθ, either by reducing the surface tension of the liquid or by A = γ s ()cosθ + 1 increasing the surface energy of the solid. For example, we consider the case of contact angle of water (surface tension of 72.8 dynes/ The work of adhesion WA can be evaluated from Eq. (4) using accessible quantities γ and θ. Equation (4) is the starting point for cm at 25ºC ) on a polyethylene which has a surface free energy of€ l 2 about 25 mJ/m . The contact angle of water on this solid is about interpretation of all contact angle data. After the evaluation of WA, 90º. In this case, the liquid will not wet the solid and we say that the surface energy of the solid γs can be determined by further use the surface is hydrophobic and has a poor wettability. If the surface of other models. tension of water is lowered by mixing surfactant in it keeping the solid surface unchanged, the contact angle will decrease i.e. water Fowke’s equation spreads on the surface of the solid . This phenomenon is very Fowke [6] proposed an approach to the problem of separating common in practice such as; the cleaning of oily dishes by soap- the surface energy of solid into its components. According to this water and use of detergent in washing clothes. Mixing detergents approach, the total surface energy of a solid is expressed as the to spray pesticides uniformly over the surface of plant leaves is sum of the various components as expressed in Eq. (5), d p ind H ……………… (5) another common example. γ = γ + γ + γ + γ It is also possible to modify the solid- surface by grafting of hydrophilic functional groups to reduce the contact angle with where, the subscripts d, p, ind and H refer to dispersion, polar, induced and hydrogen bond forces respectively. Finally it was pure water drop. We can interpret the wetting phenomena in terms€ of the force of cohesion and adhesion. For a high surface tension suggested that the equation can be expressed with only the two liquid drop placed on a solid surface of low surface energy, the dominant terms, force of attraction between the liquid molecules is stronger than d H …………………… (6) γ = γ + γ the force of attraction between the liquid molecules and the atoms γ AB = γ p + γ ind + γ H in the solid and hence the liquid forms a drop and there is a definite …………… (7) value of contact angle. However, if the surface tension of the liquid € is lowered or the surface free energy of the solid is increased, the Where AB refers to the Acid- base interaction. force of attraction between the liquid molecules and the atoms in € the solid becomes more than the force of attraction between the In order to calculate only the total surface energy, it is enough liquid molecules and hence the liquids starts to spread on the solid to know the contact angle with only one test liquid. But, if we surface. In other words, the lower the surface tension of the liquid would like to determine not only the total surface energy but also or the higher the surface energy of the solid, better will be the its associated polar and dispersion components we must measure //2// The Himalayan Physics D. P. Subedi May 2011 Vol. II contact angles with at least two test liquids. If we want to know commonly used polymers calculated from the Fowke’e extended the components related to acid base interaction, it is necessary to model. The data corresponding to the first four rows in the table measure the contact angle with three test liquids. Therefore, in were obtained from literature [ 7] whereas the data in the last practice of surface energy calculation, one liquid, two liquid and two rows were determined by the author [3,8]. It is interesting to three liquid models are used depending upon the required depth of note that the polar component of surface energy is very small in the analysis. In this paer we will present only the two liquid model comparision to the dispersion component of the surface energy of which is the most widely used method.