Finite size effects on the phase diagram of a binary mixture confined between competing walls
A symmetrical binary mixture AB that exhibits a critical temperature T<sub>cb</sub> of phase separation into an A- and a B-rich phase in the bulk is considered in a geometry confined between two parallel plates a distance D apart. It is assumed that one wall preferentially attracts A whi...
Guardado en:
| Autores principales: | , , |
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| Formato: | Articulo |
| Lenguaje: | Inglés |
| Publicado: |
2000
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| Materias: | |
| Acceso en línea: | http://sedici.unlp.edu.ar/handle/10915/129677 |
| Aporte de: |
| Sumario: | A symmetrical binary mixture AB that exhibits a critical temperature T<sub>cb</sub> of phase separation into an A- and a B-rich phase in the bulk is considered in a geometry confined between two parallel plates a distance D apart. It is assumed that one wall preferentially attracts A while the other wall preferentially attracts B with the same strength (“competing walls”). In the limit D → ∞, one then may have a wetting transition of first-order at a temperature T<sub>w</sub>, from which prewetting lines extend into the one phase region both of the A- and the B-rich phase. It is discussed how this phase diagram gets distorted due to the finiteness of D: the phase transition at T<sub>cb</sub> immediately disappears for D < ∞ due to finite size rounding, and the phase diagram instead exhibit two two-phase coexistence regions in a temperature range T<sub>trip</sub> < T < T<sub>c</sub>₁ = T<sub>c</sub>₂. In the limit D → ∞ T<sub>c</sub>₁,T<sub>c</sub>₂ become the prewetting critical points and T<sub>trip</sub> →T<sub>w</sub>. For small enough D it may occur that at a tricritical value D<sub>t</sub> the temperatures T<sub>c</sub>₁ = T<sub>c</sub>₂ and T<sub>trip</sub> merge, and then for D < D<sub>t</sub> there is a single unmixing critical point as in the bulk but with T<sub>c</sub>(D) near T<sub>w</sub>. As an example, for the experimentally relevant case of a polymer mixture a phase diagram with two unmixing critical points is calculated explicitly from self-consistent field methods. |
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