We elucidate the interparticle force between a pair of particles suspended in a liquid exposed to a standing bulk acoustic wave. A three-dimensional model based on the perturbation technique and tensor integral method is employed to predict the interparticle force by subtracting the time-averaged primary radiation force due to the scattering effect from the time-averaged total radiation force due to combined scattering and re-scattering effects. The results show that irrespective of the sizes of particles at the nodal plane, interparticle force is attractive and symmetric and scales with the product of wavenumber (k) and interdistance (δx) as . By studying the interparticle force between a pair of particles located out of the nodal plane, we unravel that the interparticle force is independent of their positions and depends on the horizontal (parallel to the nodal plane) and vertical interdistances. Our results reveal interparticle force changes from attractive to repulsive at a critical interdistance, attributed to the competition between time-averaged second-order pressure and velocity terms. We found that for a pair of particles parallel to the nodal plane, the interparticle force is independent of their distance from the nodal plane. Considering the total radiation force as the sum of the interparticle force, axial primary force, and drag force, we demonstrate a methodology for experimental quantification of the interparticle force. The interparticle force predicted from the model shows good agreement with experimental data (within 5%). Our study sheds light on interparticle forces that will facilitate more accurate estimation of forces on particles in an acoustic field.
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