Optical Measurement of Wetting by 3He-4He Mixtures Near Their Tri-Critical Point

We have measured the contact angle of the interface of phase-separated 3He-4He mixtures against a sapphire window using an interferometric method in an optical cryostat. Our measurement has been performed at saturated vapor pressure, along the phase separation line and near Tt, the temperature of the tri-critical point. We have found that the contact angle is finite, contrary to what was generally accepted up to now, i.e., complete wetting by the 4He-rich phase. Furthermore, this angle does not tend to zero when the temperature approaches Tt. This is a remarkable exception to “critical point wetting,” a phenomenon which is usually observed near ordinary critical points.