Environ. Eng. Res. 2012 December,17(S1) : S49-S52 Research Paper http://dx.doi.org/10.4491/eer.2012.17.S1.S49 pISSN 1226-1025 eISSN 2005-968X Wettability Studies of ITO Substrates Using Binary Mixture of Aniline and Alcohols Jonathan Lalnunsiama1, Vinjanampaty Madhurima1,2† 1Department of Physics, Mizoram University, Aizawl 796004, Mizoram, India 2Department of Physics, Central University of Tamil Nadu, Thiruvarur 610001, Tamil Nadu, India Abstract This paper suggests a method to reduce electronic wastage by controlling the wetting of the substrates to enable reuse of the device in an effort to improve device control. If the devices were cleaned after a given operation, it can be reused. With this preliminary idea, the present investigation reports the concentration dependent wetting of indium tin oxide by aniline-alcohol binary liquid and shows how varying the concentration of the liquids can control the wetting phenomenon. This binary liquid system is an important prototype for the biological hydrogen bond interactions of N-H---O. Surface tension (liquid-liquid), adhesive energy (solid-liquid), surface energy of the substrate and contact angle of the liquids with the substrate are measured by using a contact angle goniometer. Keywords: Bio-micro-electro-mechanical systems, Contact angle, Surface energy, Surface tension, Wettability 1. Introduction polar component results from hydrogen bonding [6]. Carre [7] showed that the variation of the contact angle of a polar/non- Micron sized bio-devices such as bio-micro-electro-mechan- polar liquid on various polymer substrates was dependent on ical systems (bio-MEMS) are plagued by problems regarding the the geometric mean of the dispersion and polar contribution of biological fluid/chemical attaching itself to the substrate due to the liquids and solid surface free energy. adhesive forces. Many techniques are employed to overcome Hence, it was intended to look at the wetting of ITO by the this difficulty by making the surfaces hydrophobic [1]. Some of binary aniline-alcohol system. For studies of hydrogen bond- the techniques including wettability switching by applying elec- ing, the binary mixture of different concentrations of aniline- trical fields, layering with a metal layer such as Au [2], surface alcohols provides suitable prototypes for interactions in larger patterning [3] and surface patterning using lasers [4]. This paper molecules due to the presence of OH and NH2 groups. Alcohols presents wettability switching studies based on the concentra- are one of the simplest organic molecules that undergo inter- tion variation of a binary liquid. The feasibility of concentration and intra-molecular hydrogen bonding [8]. The N-O--H hydro- dependent wettability transitions, which can be used for further gen bond is among the important hydrogen bonds in biological applications in production of reusable MEMS devices, is ex- systems [9] and such bonds are important in the fabrication of plored. bio-microelectronic devices. The molecular interaction between This paper focuses mainly on the wetting of the indium tin ox- lipids and proteins has been studied by dynamic surface tension ide (ITO) substrate by the binary mixture of the aniline-alcohols measurements [10]; the wettability of pyrex glass by binary mix- system. Previously, the influence of an electrostatic field on the tures of different liquids was experimentally studied through ob- wetting property was analyzed through classical electrostatics by servation of the Marangoni wetting effect [11]. Theoretical pre- Kang [5]. This is a useful study since controlling the wetting of dictions of interfacial tension and contact angle in a solid-fluid the substrate by a liquid is of importance in regards to the func- system have been reported elsewhere [12]. From these studies, it tioning and cleaning of electronic and bio-devices. The wettabil- was seen that when one or both of the solid or liquid are polar, ity of the solid surface depends on the surface free energy and their molecular interaction and the orientation of the molecules surface roughness of the solid, along with surface tension of the at the interface play a significant role in predicting solid-vapor liquid since the equilibrium of a drop on the surface is essentially and solid-liquid interaction. In a study undertaken [13] to de- a three-phase equilibrium scene. The surface free energy has two termine the influence of the presence of electrolytes to solvents components, namely, polar and dispersive. The dispersive com- on adhesion, the increase in the contact angle of hydrophobic ponent is due to an instantaneous dipole moment whereas the diiodomethane on polar silicon doped with carbon coated sur- This is an Open Access article distributed under the terms of the Creative Received September 11, 2012 Accepted October 04, 2012 Commons Attribution Non-Commercial License (http://creativecommons. † org/licenses/by-nc/3.0/) which permits unrestricted non-commercial use, Corresponding Author distribution, and reproduction in any medium, provided the original work is E-mail: [email protected] properly cited. Tel: +91-4366-220311 Fax: +91-4366-225312 ©Copyright The Korean Society of Environmental Engineers 49 http://www.eer.or.kr Jonathan Lalnunsiama, Vinjanampaty Madhurima 55 44 50 42 45 40 ) 40 2 38 θ) 35 36 30 34 25 32 Contact angle( 20 30 Methanol Surface energy (mJ/m Methanol 15 28 Ethanol Ethanol 10 Isopropanol 26 Isopropanol Butanol Butanol 5 24 0102030405060708090 100 110 0102030405060708090 100 110 Mole fraction of alcohols in aniline Mole fraction of alcohols in aniline Fig. 1. Variation of contact angle with mole fraction of alcohols in Fig. 2. Variation of surface tension with mole fraction of alcohols in aniline over indium tin oxide substrate. aniline. faces with an increase in electrolyte concentration demonstrat- 3. Results and Discussion ed the weakening of attractive interaction on the surface, thus suggesting a decreased adhesion to Si-doped diamond like car- Wetting by the various concentrations of the aniline alcohol bon coatings by the addition of electrolytes for all polarities of system over ITO substrate was studied. The contact angle made the surrounding solvent. This can be utilized for the cleaning of by a drop of liquid when it interacts with the substrate is mea- hydrophobic material from a polar surface with increasing the sured using a goniometer and the corresponding surface ener- concentration of the electrolyte [13]. gies and surface tension are calculated using the program of the goniometer. Greater wetting is observed in the alcohol rich re- gion rather than in the aniline rich region. For the calculation of 2. Materials and Methods surface energy, shown in Table 1, the test liquids used are water, glycerol and diiodomethane. The contact angles made by the Aniline and alcohols of purity >99.9% analytical reagent (AR) aniline-alcohols binary system over the entire concentration re- grade, were fractionally distilled and the mid fraction was stored gion on the substrate was measured and is reported in Fig. 1. The over 4A molecular sieves. Binary mixtures of these systems are surface tension variation with concentration is shown in Fig. 2. made at different mole fraction concentrations. The substrates Wetting of the aniline-alcohol system demonstrates a dramatic were sonicated for 30 min in a 50 W ultrasound sonicator filled change in contact angle with a concentration lower than that of with distilled water, dried in a hot-air dryer at 110oC and cooled aniline. This rapid wetting is observed over ITO at ~10% concen- to room temperature before use. Contact angle measurements tration of aniline in methanol, ~30% in ethanol, ~50% in isopro- were made using the M/s Rame-Hart contact angle goniometer panol, and ~50% in butanol. This is due to the fact that the total model MB3000 (Rame-Hart, Succasunna, NJ, USA), which con- energy of the binary system (surface tension) at this particular sists of a volume controlled syringe placed over the substrate concentration matches with the surface energy of ITO. As the holder, and the charge-coupled device (CCD) camera which concentration is further decreased, the total energy of the liquid takes the drop image and advanced drop image software for is greater than that of the substrate, and hence wetting is rapid. the processing of data. This software was used to measure the Complete wetting is also observed for isopropanol and butanol. contact angle between the liquids and the substrate, the surface Alcohols form clusters due to intermolecular hydrogen bond- energy of the liquids (surface tension) and the surface energy of ing [15]. Hydrogen bonded liquids, including methanol, are used the substrate using the multi-liquid tool of the goniometer. The extensively in the manufacturing of MEMS devices [16]. The experiment was conducted at ambient temperature. The contact clustering behavior diminishes with increasing the chain length. angle between substrates and liquids are related by the Young’s Although the exact size of cluster I alcohols varies, rough esti- equation [14]. mates of such sizes have been previously made. A graphical rep- YY− resentation based on the main results of [17, 18] is given in Fig. 3. cosθ = SV SL YLV Where, Y = solid-vapor interfacial tension, Y = solid-liquid SV SL Table 1. Surface energy of indium tin oxide interfacial tension, and YLV = liquid-vapor interfacial tension. 2 Contact angles were measured using diiodomethane, water and Surface energy (mJ/m ) Substrates glycerol as reference liquids by obtaining the snapshot of the ses- Polar Dispersive Total sile drop over the substrate and the software of the goniometer. Indium tin oxide 0.46 30.60 31.06 http://dx.doi.org/10.4491/eer.2012.17.S1.S49 50 Wetting Studies of ITO 7 more than that for smaller alcohols (methanol and ethanol). At these concentrations where there exists rapid wetting, it can be surmised that the aniline destroys the alcohol clusters, thus lead- 6 ing to an effect on the surface tension and hence causing rapid wetting.