Microscopic Structural Precursors of Phase Separation in Binary Lennard–Jones Fluids

Abstract

Phase separation in binary fluids is commonly identified only after concentration fluctuations develop into observable domains, leaving the earliest stages of the instability largely unexplored. Here, equilibrium molecular dynamics simulations were used to determine whether microscopic chemical ordering provides earlier evidence of phase separation in a three-dimensional binary Lennard–Jones fluid. Initially homogeneous mixtures were subjected to interaction quenches, and local coordination contrast, coordination susceptibility, radial-distribution-function contrast and concentration variance were monitored throughout the precursor regime. Persistent local chemical ordering emerged while the system remained macroscopically homogeneous, indicating that molecular neighbourhoods retain measurable signatures of the impending instability. The coordination susceptibility provided the earliest statistically significant detection at , followed by the mean coordination contrast , radial-distribution-function contrast  and concentration variance . This sequence yielded a precursor lead time of up to  for deep quenches, decreasing systematically as the unlike interaction approached the miscibility boundary. The detection hierarchy remained reproducible across independent simulations, with bootstrap uncertainties below 6%. These findings establish local chemical coordination as a physically interpretable and computationally efficient early indicator of phase separation, providing new insight into the microscopic onset of spinodal decomposition in binary fluids.