Past exam of the mathematics course of the University of Cambridge / 2014 / iii / Paper 73 / 1 / b / i

Write $\mathbf{n}=\mathbf{R}/R$. At fixed geometry, Linearity of Stokes flow makes the translational velocities linear in the couples. With zero forces and couples perpendicular to $\mathbf{R}$, isotropy and reflection symmetry allow only terms proportional to ${\mathbf{G}}_j\times \mathbf{n}$ in each ${\mathbf{U}}_i$. There is no longitudinal term: reflect in the plane containing $\mathbf{R}$ and the common torque axis, taking account of the axial-vector transformation of torque, and use linearity under torque reversal. HenceThe direction of $\mathbf{R}$ can still change; constant separation does not mean stationary centers.

When the couples are equal, a rotation through $\pi$ about their common axis exchanges the identical spheres and reverses the in-plane translational velocities while leaving the axial angular velocities unchanged. Uniqueness of Stokes flow then givesThe midpoint is stationary and the pair can orbit it. The body spin need not equal the angular rate of that orbit. These symmetry statements hold at arbitrary noncontact separation, not just in a distant rotlet approximation.