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Recently it was revealed that even a single‐component liquid can have more than two liquid states. The transition between these liquid states is called “liquid‐liquid transition”. This phenomenon has attracted a considerable attention because of its counter‐intuitive character and the fundamental importance for our understanding of the liquid state of matter. The connection between the liquid‐liquid transition and polyamorphism is also an interesting issue. In many cases, liquid‐liquid transitions exist in a region which is difficult to access experimentally. Because of this experimental difficulty, the physical nature and kinetics of the transition remains elusive. However, a recent finding of liquid‐liquid transition in molecular liquids opens up a possibility to study the kinetics in detail. Here we report the first detailed comparison between experiments and a phenomenological theory for the liquid‐liquid transition of a molecular liquid, triphenyl phosphite. Both nucleation‐growth‐type and spinodal‐decomposition‐type liquid‐liquid transformation are remarkably well reproduced by a two‐order‐parameter model of liquid that regards the liquid‐liquid transition as the cooperative formation of locally favored structures. This may shed new light on the nature and the dynamics of the liquid‐liquid transition. We also show evidence that this second order parameter controls the fragility of the liquid. We also discuss a possibility of controlling liquid‐liquid transition by spatial confinement. Remaining open questions on the nature of the transition are also discussed.

We performed broadband dielectric measurements for the process of liquid‐liquid transformation in triphenyl phosphite (TPP). According to our dielectric measurements, the static dielectric constant monotonically decreases and the distribution of the relaxation time becomes broader during the liquid‐liquid transformation from liquid I to II. The direct comparison with morphological evolution provides key information on the dynamical and structural evolution during LLT.