Extraordinary Wetting Phase Diagram for Mixtures of Bose-Einstein Condensates

The possibility of wetting phase transitions in Bose-Einstein condensed gases is predicted on the basis of Gross-Pitaevskii theory. The surface of a binary mixture of Bose-Einstein condensates can undergo a first-order wetting phase transition upon varying the interparticle interactions, using, e.g., Feshbach resonances. Interesting ultra-low-temperature effects shape the wetting phase diagram. The prewetting transition is, contrary to general expectations, not of first order but critical, and the prewetting line does not meet the bulk phase coexistence line tangentially. Experimental verification of these extraordinary results is called for, especially now that it has become possible, using optical methods, to realize a planar “hard wall” boundary for the condensates.

Figure 1

(a) Partial wetting. The interface between the two condensates makes a finite contact angle with the wall. (b) Complete wetting. A macroscopic layer of condensate 2 intrudes between the wall and condensate 1.

Figure 2

Wetting phase diagram in the plane of surface field $({\xi }_1-{\xi }_2)/{\xi }_2$ and relative interaction strength $K$. At bulk two-phase coexistence ($K>1$), a first-order phase boundary separates partial wetting from complete wetting, for ${\xi }_1/{\xi }_2>1$. For ${\xi }_1/{\xi }_2<1$, the roles of the condensates are interchanged and one may use “drying” in place of “wetting”. The phase boundary is parabolic near the triple point (1, 1).

Figure 3

Inflation of a wetting film of condensate 2 at the surface of condensate 1. Normalized order parameters, $f_i\equiv F_i/\sqrt{{\rho }_{0i}}$, are shown for two surface states with the same grand potential, at first-order wetting ($K=1.5$, ${\xi }_1/{\xi }_2=2$).

Figure 4

The phase diagram for prewetting at ${\xi }_1/{\xi }_2=4$, in the space of relative interaction strength $K$ and bulk field $\sqrt{P_1/P_2}-1$. The bulk phases “pure 1”, “pure 2”, and “mixed” coexist at the triple point $T$. Bulk two-phase coexistence of the pure condensates is along the line $P_1=P_2$, on which the first-order wetting transition $W$ is indicated. The second-order prewetting phase boundary intersects the bulk transition at $W$. The line with arrows indicates a possible approach to complete wetting.