An improvement in a PVT apparatus, which improvement includes a fluid path model having a fluid path and a sampling section, wherein the depth of the fluid path within the sampling section is substantially uniform and is less than about 100 .mu.m. The apparatus may be configured to provide a micron-scale slim tube which may be used in a manner similar to a conventional slim tube apparatus. A method for characterizing an interfacial property of a dispersion such as, for example, miscibility of a first fluid and a second fluid as a function of pressure. The method includes collecting sets of transmittance images representing transmittance of electromagnetic radiation through the dispersion, generating values of a general interfacial property factor from the transmittance images, and characterizing the interfacial property using the values of the general interfacial property factor.