Past exam of the short options paper shared by Prelims and Parts A and B of the physics course of the University of Oxford / 2017 / S12 / 2 / 4 / Solution

In a sufficiently uniform flow the Stokes force is $F=\gamma v$ toward trap 2. Its work over separation $d$ lowers the second effective well by $Fd$. Equal effective free energies require $\gamma vd=k_{B}T\log 2$. Henceat 298 K. The temperature was not specified, so the numerical result scales as $T/298\mathrm{K}$. This approximation treats a weak uniform drag as a constant tilt and neglects changes in barrier shape and hydrodynamic interactions with boundaries.