A modified bulge test for in-situ study of ionic permeation properties of membranes under continuously tunable, uniform pressure

The bulge test is a well-known material test to measure the mechanical properties of metal plates, thin films, and membranes. Also, two different experimental setups are needed to apply pressure and make a measurement. In this work, we describe a modified bulge test to both apply pressure and measure the electrical and ionic permeation properties of membranes in situ. A membrane, clamped at its periphery, with a circular window for measurement, is sandwiched between two liquids. The liquids serve dual purpose by facilitating the application of differential pressure and thus stress, by controlling the extent of immersion of the membrane in the liquid below the membrane, as well as enabling measurement of electrical and mass percolation properties. This was achieved with a stepper motor, a load cell, and a microcontroller. Relevant mathematical models are developed and discussed. Nafion was used to test and validate this approach, using electroimpedance spectroscopy in a 2-electrode configuration with gallium on both sides and in a 3-electrode configuration with electrolyte on one side and gallium on the other. Frequency-dependent response was modeled using equivalent circuits. The resistance of Nafion increases with the depth of immersion and therefore applied pressure. For Nafion in the 2-electrode configuration, conductivity was calculated to be ∼6.4 × 10-3 S/cm at the equilibrium position, where stress on the membrane is zero. This value matches well with existing literature values for partially hydrated Nafion. Also, it was observed that the response is symmetric about the equilibrium position.