Alloy-induced elastic heterogeneity driving subharmonic acoustic generation in nanofluids

Subharmonic generation in acoustically driven fluids provides a sensitive probe of nonlinear wave–matter interactions and material heterogeneity. Here, we report a systematic experimental investigation of one-half subharmonic generation in nonlinear acoustic cavities containing monometallic (Au), bimetallic (Au–Pt), and trimetallic (Au–Pt–Pd) noble-metal nanofluids. Nanoparticles synthesized via a microwave-assisted citrate reduction route were characterized using UV–Vis (ultraviolet–visible) absorption spectroscopy, x-ray diffraction, and electron microscopy to confirm alloy formation, crystallinity, and nanoscale heterogeneity. Ultrasonic measurements performed in variable-length resonant cavities over the 1–3 MHz range reveal robust subharmonic responses, with lower subharmonic thresholds observed in the multimetallic nanofluids than in the monometallic reference, and with the trimetallic system exhibiting the strongest overall nonlinear response. Independent second-harmonic growth measurements yield a relative acoustic nonlinearity parameter β′, increasing from β′ = 3.340 ± 0.054 for Au–Pt to β′ = 5.790 ± 0.062 for Au–Pt–Pd, corresponding to up to a threefold enhancement relative to water under identical conditions. Correlating nonlinear acoustic signatures with optical and structural metrics, we attribute the enhanced nonlinearity and reduced parametric thresholds to alloy-induced elastic heterogeneity, defect-mediated scattering, and interfacial damping. These results establish multimetallic nanofluids as tunable media for nonlinear acoustics, with potential relevance for contrast-enhanced ultrasound, acousto-optic modulation, and reconfigurable acoustic materials.