Bubble interference in ultrasonic PMMA particle sizing: Mechanisms and quantitative analysis

Abstract

Building upon the acoustic scattering theories for both solid elastic particles and gas bubbles, we established a Monte Carlo physical model of a solid-bubble mixed system to quantitatively analyze bubble-induced interference effects to ultrasonic attenuation in micron-sized particle suspensions. Systematic numerical simulations evaluated spectral distortions and their impact on polymethyl methacrylate (PMMA) particle size inversion, focusing specifically on bubble size and gas-solid mixing ratio effects. The results demonstrate that even trace bubble concentrations significantly alter system acoustics. For instance, at a gas-to-solid mixing ratio of 0.01 %, the presence of 10 μm bubbles produced particle size inversion errors up to 15 % with the particle swarm optimization (PSO) algorithm. This observation underscores the necessity of quantifying bubble artifact interference in ultrasonic particle sizing, particularly due to their resonance scattering effects near characteristic frequencies. To experimentally validate the acoustic behavior, we performed experiments using through-transmission ultrasonic attenuation spectroscopy (1–8 MHz) combined with real-time microscopic imaging under controlled bubble generation. In a pure PMMA suspension, ultrasonic sizing gave a number-median radius of 20.3 μm, within 2.53 % of optical imaging. When bubbles were introduced (increasing voltage from 3 to 5 V), significant disturbances appeared in the ultrasonic signals. We found that when particles are smaller than bubbles (R_p/R_b < 1), sizing errors rise sharply (R² = 0.98), whereas for R_p/R_b ≥ 1 errors remain within ±5 %.