Critical Casimir effect and wetting by helium mixtures

TL;DR

This study measures the contact angle of helium mixture interfaces and finds an unusual increase near the tri-critical point, suggesting a critical Casimir effect influences wetting behavior contrary to typical critical point wetting.

Contribution

It provides experimental evidence of the critical Casimir effect affecting wetting behavior in helium mixtures near the tri-critical point.

Findings

Contact angle increases with temperature near T_t.

The behavior deviates from typical critical point wetting.

Critical Casimir effect causes effective attraction at the interface.

Abstract

We have measured the contact angle of the interface of phase-separated $^{3}$He-$^{4}$He mixtures against a sapphire window. We have found that this angle is finite and does not tend to zero when the temperature approaches $T_t$, the temperature of the tri-critical point. On the contrary, it increases with temperature. This behavior is a remarkable exception to what is generally observed near critical points, i.e. "critical point wetting''. We propose that it is a consequence of the "critical Casimir effect'' which leads to an effective attraction of the $^{3}$He-$^{4}$He interface by the sapphire near $T_{t}$.