Viscosity rise of supercritical liquid copper above the Frenkel line and related structural features: a molecular...

We present a comprehensive molecular dynamics and topological data analysis study of liquid copper structure and transport properties at supercritical pressure. Contrary to expectations that viscosity decreases monotonically with temperature approaching 10^-5 Pa·s (Angell 1995Science2671924-35), we observe an anomalous viscosity increase at high temperatures. This behavior correlates with fundamental changes in local atomic topology and medium-range order. Using persistent homology (PH) to characterize 1-dimensional holes (H₁) and 2-dimensional voids (H₂), combined with pair distribution functions, radial distribution functions, and coordination number analysis, we reveal significant structural reorganization between 5000 K and 10000 K at 100 kbar (10 GPa) pressure. Shannon entropy increases by ∼15% for bothH₁andH₂features above the Frenkel line, indicating a transition from the liquid-like to gas-like dynamics while maintaining high interatomic coordination. Our results demonstrate that this transition represents a genuine transition in both transport properties and topological structure in supercritical metallic liquids.