Casimir force in a critical film formed from an electrolytic solution

We have studied the thickness of vapor adsorbed films of the critical binary liquid mixture acetic acid plus nonane adsorbed onto a silicon wafer substrate as a function of temperature near the critical temperature. This critical film possesses opposite boundary conditions $(+-)$ at its two surfaces and, due to the dissociation of acetic acid, both the electrostatic force and the dispersion force affect the adsorbed film thickness. On approaching the critical temperature $T_c,$ an increase in the film thickness L is observed, implying that the sign of the universal Casimir amplitude ${\Delta }^{+-}$ is positive, consistent with theoretical predictions. However, we find quantitative discrepancies in the value of ${\Delta }^{+-}$ and the form of the critical Casimir pressure scaling function ${\vartheta }^{+-}$ compared with previous experimental results. We attribute these discrepancies to the complex nature of the critical system studied in this experiment.