Microscopic mechanisms of Rayleigh–Taylor instability at copper/helium interface

The Rayleigh–Taylor (RT) instability at a copper/helium interface is investigated at the microscopic scale through theoretical analysis and molecular dynamics simulations. The perturbation growth across three copper states (liquid, semi-solid, and solid) under varying initial amplitudes and accelerations is characterized by state-dependent theoretical analysis. For liquid copper, a mechanical model incorporating viscosity and surface tension is developed to describe RT instability growth across linear and nonlinear stages. While viscosity and surface tension inhibit perturbation growth, larger initial amplitudes lower the critical acceleration for instability onset. For semi-solid copper, RT instability is triggered above a critical acceleration threshold of 4 × 1011 m/s2, independent of initial amplitude. Interfacial shear heating induces localized melting, forming a molten film that drives fluid-like spike elongation. In solid copper, dislocation nucleation at stress concentrations leads to the rapid proliferation of stacking faults in the perturbed region. Interfacial shear induces localized melting, and instability is triggered once the molten region loses shear resistance. Throughout this process, dislocation-mediated plasticity governs solid-state evolution. These findings establish quantitative stability criteria for state-dependent RT instability, providing atomic-scale insights into the ultrafast interfacial evolution that remains challenging to observe experimentally in metals under extreme conditions.