High-$Q$ acoustic whispering-gallery modes (WGMs) support narrow size-dependent resonances, but a narrow linewidth alone does not guarantee a high-contrast radiation force because a smooth nonresonant background may remain comparable to the resonant contribution. Using acoustic Mie theory and the radiation stress tensor, we derive an exact decomposition of the in-plane force on a compressible fluid sphere with respect to a selected Mie coefficient. The force separates into a target-independent term and two target-resonant terms proportional to the real and imaginary parts of that coefficient; within an isolated-resonance window, the target-independent term forms a smooth background. Evanescent excitation reweights the incident partial waves: The enlarged tangential wave number preferentially enhances coupling to high-order WGMs, whereas spatial decay reduces the local driving amplitude. Because the target WGM benefits from this preferential coupling while the background does not, their combined action substantially increases the resonant-to-background force contrast. Near resonance, the tangential force exhibits an approximately Lorentzian profile governed mainly by the real-part channel, whereas the normal force exhibits a dispersive profile governed mainly by the imaginary-part channel. These results establish a channel-resolved mechanism for generating high-contrast, size-selective acoustic forces and provide a theoretical basis for resonance-assisted particle manipulation and sorting.

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• Acoustic forces
• Acoustic wave phenomena
• Mie scattering
• Whispering gallery mode resonators