= Phase-dependent mean force of an externally driven sphere pair
{title2=$\langle F_1+F_2\rangle_t\sim9\pi\mu a_1a_2\delta^2\omega\ell_0^{-2}\sin\phi$}
For equal oscillation amplitudes $\delta$, frequency $\omega$, mean separation $\ell_0$ and phase lag $\phi$, the leading mean total force on the fluid is $9\pi\mu a_1a_2\delta^2\omega\sin\phi/\ell_0^2$. Each sphere supplies half. The large-separation calculation expands the <Stokeslet> interaction; reciprocal in-phase and antiphase motions have zero mean force.
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