Casimir force in a critical film formed from an electrolytic solution - art. no. 041605

We have studied the thickness of vapor adsorbed films of the critical binar y liquid mixture acetic acid plus nonane adsorbed onto a silicon wafer subs trate as a function of temperature near the critical temperature. This crit ical him possesses opposite boundary conditions (+ -) at its two surfaces a nd, due to the dissociation of acetic acid, both the electrostatic force an d the dispersion force affect the adsorbed film thickness. On approaching t he critical temperature T-c, an increase in the film thickness L is observe d, implying that the sign of the universal Casimir amplitude Delta (+-) is positive, consistent with theoretical predictions. However, we find quantit ative discrepancies in the value of Delta (+-) and the form of the critical Casimir pressure scaling function theta (+-) compared with previous experi mental results. We attribute these discrepancies to the complex nature of t he critical system studied in this experiment.