<p>We investigate acoustophoretic migration of a microparticle suspended in an immiscible coflowing liquid system inside a microchannel actuated with bulk acoustic waves. A three-dimensional acoustic model with hard-wall boundary conditions is developed to resolve resonance modes of the two-liquid system and to compute the acoustic radiation force (ARF) using perturbation theory and a tensor-integral formulation. Simulations quantify how density and sound-speed contrasts across the liquid-liquid interface alter the resonance frequency, pressure nodal location, and ARF magnitude. In experiments with coflow system with matched impedances, the ARF field resembles that of a single-phase medium, rendering particle migration largely insensitive to the interface position. Experiments using PEG-Dextran coflows confirm stable interface and show that a single particle consistently moves toward the pressure nodal plane under resonant actuation. Incorporating the 3D-computed ARF into transient two-phase simulations yields particle trajectories in excellent agreement with measurements. Together, our numerical-experimental study provides new insight into acoustically driven particle transport in layered microflow systems and establishes a predictive basis for acoustophoretic manipulation in multiphase microfluidics.</p>