Determination of Contact Angles and Surface Tensions with the Quartz Crystal Microbalance

A method based on the quartz crystal microbalance (QCM) for measuring the sessile contact angles and surface energies of liquid-air and liquid-liquid interfaces is described. The method involves measurement of the frequency change accompanying the introduction of a small liquid droplet to the center of a vibrating quartz resonator, which comprises an AT-cut quartz crystal sandwiched between two gold electrodes. If the density and viscosity of the liquid are known, the contact angle between the droplet and the gold substrate surface can be determined directly. QCM measurements of contact angles formed between aqueous droplets and gold surfaces modified with various organosulfur monolayers having different surface energies agree with sessile contact angles determined by optical goniometry. Furthermore, the QCM method can be used to measure the contact angle formed between an aqueous droplet and the QCM surface when both are submerged under an immiscible solvent such as hexadecane. In this case, the frequency change relies on the differences in the densities and viscosities of the water droplet and the fluid displaced by the droplet at the surface. The dependence of the contact angle on the concentration of surfactant in the aqueous droplet provides for determination of the critical micelle concentration for aqueous phases in contact with air or an immiscible organic fluid. These measurements can be performed under conditions where contact angles cannot be measured readily, such as in the presence of opaque media or in the case of two liquids having similar refractive indexes.