Atomistic simulation of crystal-melt coexistence interfaces under tangential flow

Fluid motion is ubiquitous in solidification, yet its atomistic influence on the immediate solid-liquid interfacial region remains difficult to quantify. Here, we develop an atomistic simulation framework for maintaining solid-liquid coexistence interfaces under imposed tangential melt flow. The protocol combines equilibrated FCC(100) crystal-melt coexistence systems with a flow boundary condition, profile-unbiased thermostatting, and solid-phase momentum control, allowing the imposed flow, thermal fluctuations, and solid reference frame to be treated separately. The method is demonstrated for a Lennard-Jones (LJ) model system and an embedded-atom-method Ni system. For each system, dynamically stable flowing coexistence states are identified at three imposed flow velocities by requiring statistically stationary interfaces with no sustained melting, crystallization, overheating, or rigid translation of the solid. Fine-grained density, velocity, and temperature profiles show that the two phases remain distinguishable, the imposed flow is measurable through a central-liquid plateau velocity, and the streaming-velocity-subtracted kinetic temperature remains controlled. The flowing interfaces reveal a measurable flow dependence of liquid-side interfacial layering. For both LJ and Ni FCC(100) interfaces, the liquid-side density-maximum spacing dm increases over the sampled velocity range, indicating that tangential flow further modifies the known equilibrium spacing relaxation of FCC(100) interfacial liquids. In the LJ system, this increase occurs together with a decrease in the coarse-grained density width δρ, showing that flow can sharpen the mean density transition while relaxing the wavelength of liquid-side density oscillations. These results establish an atomistic platform for studying flow-modulated density layering, interfacial structure, and nonequilibrium response at crystal-melt interfaces.