Interfacial microrheology and tensiometry in a miniature, 3-d printed Langmuir trough

We introduce a new approach for manipulating and studying complex fluid interfacial systems, that enables simultaneous tensiometry to measure surface pressure and high-performance interfacial shear microrheology. In contrast to existing techniques, we use a miniature Langmuir trough produced by 3-d printing methods, which fits on the stage of a conventional inverted microscope and which allows compression and stretching of the fluid-fluid interfaces at a constant bulk fluid volume. The tensiometry measurement relies on the quantitative imaging of a curved meniscus separate from the flat interface being imaged under the microscope. We first demonstrate that this approach can measure the interfacial tension of simple fluid-fluid and polymer-laden interfaces with an accuracy comparable to the pendant drop apparatus and Wilhelmy plates. We then study the mechanics of an adsorbed protein layer formed at an air-water (a-w) interface by simultaneously measuring its surface pressure and its interfacial viscosity. To perform two-point interfacial microrheology, micrometer-sized probes spread on the interface are tracked to 5 nm precision with very low drift, yielding measurements of interfacial viscosity having a wide dynamic range and high sensitivity (10^-9-10^-1Nsm^-1).