= Reversible scattering of two spheres in simple shear
For a smooth noncontact pair encounter in deterministic zero-inertia <Stokes flow>, streamwise reflection changes the sign of simple shear and time reversal changes it back. Uniqueness then pairs incoming and outgoing trajectory branches, with equal transverse separation at infinity. Hydrodynamic transverse motion during the encounter need not vanish; its net scattering displacement does. Contact, Brownian forcing and inertia can break this argument.
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