This paper shows that a C1-C3 near-critical fluid system goes through a Cahn transition(at an IFTC1-C3 = 03m N/m), the latter value being based on pendant drop-measurements on a 2,6-lutidine-water model system and the application of critical scaling principles.

A Cahn transition into a perfect wetting regime is not just a transition from very good wetting to perfect wetting in the capillary sense. This is a consequence of the fact that for near-critical fluids in thermodynamic equilibrium the solid(channel) wall is an essential part of the system that is in thermodynamic equilibrium, leading to a redistribution of phases in the hydraulic channels, whereas the spreading coefficient becomes essentially zero for such a system. When viscous forces are dominant this leads on its turn inside the Pressure-Composion Cl-C3-(Berea) phase envelope to a core-annular ("film-flow") regime for the bigger channels. The smaller ones go upon pressure depletion through a noticeable transition region outside this envelope, causing capillary condensation, that might start in the millidarcy-range. At the Cahn transition inside the envelope relative permeability curves of the Cl-C3(Berea) system straighten. In the filmflow-regime an Einstein emulsion contribution to relative permeability is clearly identifiable at the saturation extremes leading to asymetric straightening of the relative permeability curves.

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