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Journal of Science Natural Science

THE THERMODYNAMIC CONTACT ANGLE IN THE PREWETTING PHASE OF A BINARY BOSE-EINSTEIN CONDENSATE MIXTURE

Pham Duy Thanh, Nguyễn Văn Thụ

Published Sep 30, 2025• 0 citations
Publisher

Abstract

To investigate the prewetting state of a Bose-Einstein condensate (BEC) mixture adsorbed on an optical hard wall at zero temperature, we consider a system in which condensate 1 coexists with condensate 2 with the chemical potential held fixed. Using the double parabola approximation (DPA) and solving the coupled Gross-Pitaevskii equations, we derive an analytical expression for the thermodynamic contact angle based on Youngs equation. Although the resulting formula is not algebraically simple, it reveals that complete wetting can occur before the bulk coexistence point is reached, indicating the presence of a continuous (second-order) prewetting transition. Our findings are in agreement with theoretical predictions in the literature and emphasize that the wetting film thickness grows smoothly without a discontinuous jump. This work provides useful guidance for the design of experiments aimed at observing wetting phenomena in Bose-Einstein condensates.

DOI
10.18173/2354-1059.2025-0035

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To investigate the prewetting state of a Bose-Einstein condensate (BEC) mixture adsorbed on an optical hard wall at zero temperature, we consider a system in which condensate 1 coexists with condensate 2 with the chemical potential held fixed. Using the double parabola approximation (DPA) and solving the coupled Gross-Pitaevskii equations, we derive an analytical expression for the thermodynamic contact angle based on Youngs equation. Although the resulting formula is not algebraically simple, it reveals that complete wetting can occur before the bulk coexistence point is reached, indicating the presence of a continuous (second-order) prewetting transition. Our findings are in agreement with theoretical predictions in the literature and emphasize that the wetting film thickness grows smoothly without a discontinuous jump. This work provides useful guidance for the design of experiments aimed at observing wetting phenomena in Bose-Einstein condensates.
In this study, we determine the interface position of a binary Bose-Einstein condensate mixture in the prewetting phase, confined by a hard wall, at absolute zero temperature. An approximate analytical result is obtained by employing the double-parabola approximation (DPA) to the quartic potential energy density within the Gross-Pitaevskii (GP) theory. The interface position depends on the control parameter and the chemical potential ratio. Our analytical result shows good agreement with numerical results, which indicates that the DPA is a reliable method. This study provides insights for experiments focused on observing the wetting transition of gases in the Bose-Einstein condensate phase.
An ultralow-temperature binary mixture of Bose-Einstein condensates adsorbed at an optical wall can undergo a wetting phase transition in which one of the species excludes the other from contact with the wall. Interestingly, while hard-wall boundary conditions entail the wetting transition to be of first order, using Gross-Pitaevskii theory we show that first-order wetting as well as critical wetting can occur when a realistic exponential optical wall potential (evanescent wave) with a finite turn-on length $\ensuremath{\lambda}$ is assumed. The relevant surface excess energies are computed in an expansion in $\ensuremath{\lambda}/{\ensuremath{\xi}}_{i}$, where ${\ensuremath{\xi}}_{i}$ is the healing length of condensate iii. Experimentally, the wetting transition may best be approached by varying the interspecies scattering length a12{a}_{12}a12​ using Feshbach resonances. In the hard-wall limit, $\ensuremath{\lambda}\ensuremath{\rightarrow}0$, exact results are derived for the prewetting and first-order wetting phase boundaries.
We identify the adsorbed index as an experimentally accessible order parameter for the wetting phase transition in dilute ternary Bose–Einstein condensates (BECs). By tracking the nucleation and growth of a surfactant film of the third component at the interface between two immiscible condensates, we show that the adsorbed index exhibits a logarithmic divergence with the chemical-potential ratio in the prewetting phase. Numerical Gross–Pitaevskii calculations are consistent with analytical predictions from the double-parabola approximation, confirming the reliability of the approach. Unlike the conventional surfactant layer thickness, which becomes ill-defined near the nucleation point, the adsorbed index remains well defined throughout the entire prewetting-wetting regime and is directly proportional to the number of condensed atoms in the surfactant component, a quantity that is readily measurable in ultracold-atom experiments and tunable via Feshbach resonances. Our results offer a realistic opportunity to observe wetting transitions in multicomponent quantum gases.
In previous work within Gross-Pitaevskii (GP) theory for ultracold gases, wetting phase transitions were predicted for a phase-segregated two-component Bose-Einstein condensate (BEC) adsorbed at an optical wall. The wetting phase diagram was found to depend on intrinsic atomic parameters, being the masses and the scattering lengths, and on the extrinsic wall boundary condition. Here we study wetting transitions in GP theory without an optical wall in a setting with three phase-segregated BEC components instead of two. The boundary condition is removed by replacing the wall with the third component and treating the three phases on an equal footing. This leads to an unequivocal wetting phase diagram that depends only on intrinsic atomic parameters. It features first-order and critical wetting transitions, and prewetting phenomena. The phase boundaries are computed by numerical solution of the GP equations. In addition, useful analytic results are obtained by extending the established double-parabola approximation to three components.
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.
The interfacial profiles and interfacial tensions of phase-separated binary mixtures of Bose-Einstein condensates are studied theoretically. The two condensates are characterized by their respective healing lengths ${\ensuremath{\xi}}_{1}$ and ${\ensuremath{\xi}}_{2}$ and by the interspecies repulsive interaction KKK. An exact solution to the Gross-Pitaevskii (GP) equations is obtained for the special case ${\ensuremath{\xi}}_{2}/{\ensuremath{\xi}}_{1}=1/2$ and K=3/2K=3/2K=3/2. Furthermore, applying a double-parabola approximation (DPA) to the energy density featured in GP theory allows us to define a DPA model, which is much simpler to handle than GP theory but nevertheless still captures the main physics. In particular, a compact analytic expression for the interfacial tension is derived that is useful for all ${\ensuremath{\xi}}_{1},\phantom{\rule{0.28em}{0ex}}{\ensuremath{\xi}}_{2}$, and KKK. An application to wetting phenomena is presented for condensates adsorbed at an optical wall. The wetting phase boundary obtained within the DPA model nearly coincides with the exact one in GP theory.