Langmuir monolayer characterization via polymer microtensiometers

• Device designed to have linear deflection response to surface pressure.
• High sensitivity and embedded read-out system.
• No adjustable or fit parameters required for result interpretation.
• Good correlation of results with independent reference measurements.

A semi-rigid, semi-flexible SU-8 polymer microdevice was designed to measure changes in interfacial tension at an air–water interface. The suspended microtensiometer enclosed a clean air–water interface, with an insoluble surfactant on the exterior. The difference in surface tension between the inside and the outside of the device, called the surface pressure, caused the 850 μm by 3 mm device to deflect. Finite element simulations were performed to predict device behavior prior to fabrication. Finished devices were tested in a Langmuir trough during multiple compression and expansion cycles using large area changes and slow compression speeds. Shorter experiments subjecting the interface to rapid local monolayer concentration variations were also performed. A platinum Wilhelmy plate was used as an independent surface pressure measurement. The microtensiometer had a theoretical resolution of 0.02 mN m−1.