We employ combined smoothed particle hydrodynamics and molecular dynamics simulations to systematically investigate the evolution of microjetting from tin targets with sinusoidal surface perturbations under twice-shock loading. Results show that the initial scaled amplitude $k_0{h}_0$ critically controls the bubble morphology after the first shock and the subsequent coupling behavior of second-shock ejecta: a smooth single bubble forms at $k_0{h}_0=0.25$; for $0.50\le k_0{h}_0\le 1.20$, a single microspike splits the bubble into two sub-bubbles, leading to two second jets that first diverge and later merge under strong mode coupling; for $1.40\le k_0{h}_0\le 2.00$, two microspikes divide the interface into three sub-bubbles, producing three independently growing microjetting with markedly weaker coupling. To predict the second-shock ejecta mass across this wide $k_0{h}_0$ range, an approach is introduced that treats the postshock bubble interface as a multimode perturbation. Through Fourier modal decomposition and incorporation of intermode coupling, a predictive model is developed that shows good agreement with simulation results. The effectiveness of the computational approach is further validated against existing second-shock microjetting experiments. This work elucidates the physical linkage between initial surface conditions, shock-induced interface morphology, and jet-coupling dynamics, providing a quantitative framework for the prediction of ejection under multiple-shock loading.

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• Richtmyer-Meshkov instability
• Molecular dynamics