Acoustic Interaction Force Between Two Identical Compressible Particles Trapped in an Acoustofluidic Nodal Plane

We derive a closed-form expression for the acoustic interaction force between two identical compressible particles trapped in an acoustofluidic nodal plane. Working in the long-wavelength scattering regime for an ideal fluid, we solve the coupled scattering problem within a fifth-order multipole expansion. The resulting force law extends the Silva–Bruus theory through explicit finite-separation corrections, while recovering it in the weak-density-contrast and large-separation limits. In the close-range regime, the interaction force retains the characteristic inverse-fourth-power dependence on interparticle distance, with a prefactor renormalized by repeated rescattering. Comparison with previous numerical calculations and experiments shows excellent agreement with ideal-fluid numerics in the weak-density-contrast regime, but substantially larger deviations from several experiments, indicating that viscosity, acoustic streaming, and hydrodynamic interactions remain important for quantitative prediction.