Past exam of the mathematics course of the University of Cambridge / 2007 / iii / Paper 78 / 2 / Solution

For a slender viscous bubble, approximate the interface normal by the radial direction and its curvature by $1/a$. Axial curvature and axial derivatives of the leading external motion are smaller by the slenderness factors. The exterior leading Stokes flow is locally radial. Incompressibility gives ${\partial }_r(r{u}_r)=0$, while the kinematic boundary condition gives $u_r(a)=a_t$ to leading order, henceThe radial viscous Laplacian of this $1/r$ velocity vanishes, so exterior pressure is constant to leading order; set its reference value to zero. Its radial normal stress at the interface is $2\mu {\partial }_r{u}_r{|}_a=-2\mu a_t/a$. Inner viscous normal stress is smaller by the viscosity ratio $\lambda$, and the bubble's leading normal stress is $-P$. The interfacial stress balance with variable surface tension, here with constant $\gamma$, consequently givesThese are local leading-order relations: the outer axial flow and tangential traction enter at higher slenderness order. The sign also has a useful check: a uniform cylinder with no imposed excess pressure contracts, $a_t=-\gamma /(2\mu )$, under surface tension.