Oscillating fronts produced by spinodal decomposition of metastable ordered phases

Abstract

Mesophase–particle mixtures are emerging functional material systems whose structure, dynamics and formation through phase transitions are poorly understood. In this paper, novel phase transitions in a system of coupled conserved and non-conserved order parameters – a mixture of liquid crystals and nanoparticles – are studied via phase-field modelling, under conditions where an intermediate phase, which is unstable to composition fluctuations, can be formed between the initial supercooled phase and the new stable phase. This paper analyzes the formation, stability and evolution of this intermediate phase and how it impacts the transformation of an isotropic phase when it is quenched with single and double temperature jumps to form a calamitic nematic phase. It is found that the intermediate phase forms through a front-splitting mechanism and grows for some time, but eventually decays through a spinodal decomposition starting at the interface and propagating to the bulk, producing a moving oscillating front. It is found that this phenomenon can be triggered solely by the presence of the interface even in the absence of composition fluctuations in the bulk. Spinodal decomposition initiated by thermal fluctuations also generates a moving oscillating front (because the intermediate phase was formed by a moving front). As a consequence, the velocity of the moving front changes from a small value at short time (interface-induced spinodal), to a large value at some finite time (moving bulk spinodal). The effect of a double quench was also analyzed and it was found that the oscillating front can be confined to a region close to the interface (“mushy” region), and the isotropic phase (originally of infinite extension) can be confined to a region close to the oscillating front, generating a very atypical phase morphology.