Keywords: contact angle, wetting, surface tension, measurement, new instrumewnt, smartphone, coating, surface energy, interfacial tension
Summary:In developing new coatings, membranes or surface engineering, it is often required to measure contact angle, or sliding angle. Also, in many applications, surface tension of liquids or solid surface energy play an important role, e.g. adhesion of coatings, biofouling, drug discovery, spreading of pesticides on plant leaves, or in petrochemical industry. As such, accurate and facile measurement methods for contact angle, and surface tension, etc. are needed to enable R&D, quality control, or field tests. Drop shape analysis (DSA) methods using image processing and solution of Laplace equation are the most popular measurement methods for the above properties. This is so as they require small amount of sample liquid, also DSA method is accurate, and convenient due to automation. Drop shape methods are also a convenient way to determine solid surface energy through contact angle measurements, e.g. either using an equation of state (e.g. Neumann’s equation of state), or acid-base approach. From their advent in 80’s computerized DSA systems have kept the original system design, i.e. a droplet is positioned between a camera connected to a lens, and a light source. The camera is then connected to an external computer and monitor. This arrangement lends itself to a bulky system that is bench mounted. Also due to existence of many different components the costs of such systems can be high. Industry demand for on-site (field) measurements has very recently compelled a few manufactures to offer mobile versions of such instruments, but full functionality is not available, and the instrument is still tethered to a computer. In this talk we present a compact design for liquid surface tension measurement, and solid surface tension measurement through contact angle that can be held on palm of a hand (see Fig. 1). This novel method is based on a smartphone (mobile-phone) fitted to a very compact hardware for droplet positioning either on a surface (for contact angle or solid surface energy measurements) or imaging a pendent drop for DSA to find surface tension of a liquid. As such, we have been able to create an instrument that is more economical compared to current systems; also it is compact, and can be mobile for field work (see Fig. 2). It is shown that the accuracy of our method can be 0.001% with ideal synthetic drop profiles (750 synthetic droplets representing a wide range of surface tension values were used) . The performance of this instrument was also compared by experimentation with a high-end commercial surface tension measurement instrument. We used various liquids (from high to low surface tension), and show that our instrument and the developed methodology can provide surface tension and contact angle measurements as precise and accurate as the best in the current commercial instruments [1,2]. The results of measuring solid surface energy for various polymeric surfaces using both approaches of equation state, and acid-base method will also be presented. Finally, our latest development regarding implementation of ADSA-D method on a smartphone platform will also be discussed.