Microfabricated Deflection Tensiometers: A Novel Technique in Interfacial Tensiometry

Monolayers of phospholipids and fatty acids provide simple model systems for fluid-fluid interfaces in biological systems and microemulsions. Although the Wilhelmy plate technique is the most conventional method for measuring surface pressure, limitations exist when dealing with small samples or enclosed systems. Inspired by the idea of Hu et al [Langmuir, 2003, 19, 100], who related the elastic deformation (deflection) of a teflon wire loop to the surface pressure, we have developed a new technique for microtensiometry. Using a simple design equation derived from elastic beam theory, we microfabricate micro-tensiometers and validate their use by comparing measured surface pressure isotherms for fatty acids and phospholipid monolayers at air-water and fluid-fluid interfaces against simultaneous Wilhelmy plate measurements, with excellent quantitative agreement. Our technique offers a high degree of design flexibility due to control of geometry, materials and surface chemistry, is relatively inexpensive and enables measurements in small or enclosed systems. Finally, minor modifications enable our tensiometers to function as Langmuir micro-troughs for insoluble surfactants.