Possibility of Continuous Wetting Transition at the Liquid-Vapour Interface of the Binary Liquid Mixture Cyclohexane + Acetonitrile

We have measured the contact angle θ for a lens of liquid L 2 suspended at the liquid L 1 -vapour interface in a mixture of cyclohexane and acetonitrile. A power law θ ∝ | T - T cw | ν with T cw = (72.2 ± 0.05) °C is obeyed just below the critical bulk temperature T c = 75.61 °C. We obtain ν = 0.98 ± 0.08, whereas an attempt to conform the data to the 1st-order transition law θ ∝ | T - T cw | 1/2 leads to T w = (71.99 ± 0.05) °C and leaves data for the smallest angles unaccounted for. The mean-field prediction θ ∝ | T - T cw | 3/2 for critical wetting in 3D systems with long-ranged forces gives a reasonable fit, however the power law is observed only within an asymptotic region.