Contact angles in sequential wetting: pentane on water

In recent experiments, pentane deposited on water has been found to undergo two sequential wetting transitions as the temperature is increased (Bertrand et al., Phys. Rev. Lett. 85 (2000) 1282). At low temperatures (<25°C), pentane does not wet the water surface; rather, the liquid pentane phase, in coexistence with pentane vapor, forms lenses floating on water. At 25°C, there is a first-order wetting transition from discrete lenses to a wetting film of finite thickness (≈100 Å). Heating the system further leads to an increasing film thickness, which ultimately diverges at 53°C, the critical wetting transition temperature. We calculate the contact angles of pentane lenses on the water surface (and those that sit on top of the finite wetting layer) in the temperature range 0–53°C and compute the magnitude of the discontinuity in the temperature derivative of the contact angle at the first-order transition, but are currently unable to monitor how the contact angle vanishes as a temperature of 53°C is approached. For these purposes, we use a modified Cahn theory which incorporates the long-range interaction terms responsible for the occurrence of critical wetting.