A 3-mirror surface force balance for the investigation of fluids confined to nanoscale films between two ultra-smooth polarizable electrodes

We present a new technique, based on the Surface Force Balance (SFB), for the direct measurement of surface forces between two ultra-smooth and polarizable gold electrode surfaces across thin fluid films. Combining the direct interferometric measurement of surface separation and contact geometry with smooth electrode surfaces has proved challenging in the past, and for this reason, previous measurements with the SFB typically involved two insulating mica surfaces, or one mica surface and one electrode surface, or an alternative less direct measure of the surface separation. Here, we demonstrate that a 3-mirror interferometer can overcome these difficulties: the setup involves two ultra-smooth electrode/mirror surfaces between which the fluid is confined and a third mirror to allow for interferometric detection of the liquid thickness with nanometer resolution and at thicknesses much smaller than the diffraction limit of the light. We conclude with a proof-of-concept measurement across dry nitrogen gas. The technique should prove useful for studying the properties of fluids confined at the nanoscale inside a slit-pore of controlled electrical potential or subject to applied electric fields