Breakup of a fluid thread in a confined geometry: droplet-plug transition, perturbation sensitivity, and kinetic stabilization with confinement

We investigate the influence of geometrical confinement on the breakup of long fluid threads in the absence of imposed flow using a lattice Boltzmann model. Our simulations primarily focus on the case of threads centered coaxially in a tube filled with another Newtonian fluid and subjected to both impulsive and random perturbations. We observe a significant slowing down of the rate of thread breakup ("kinetic stabilization") over a wide range of the confinement, Lambda=R-tube/R(thread)less than or equal to10 and find that the relative surface energies of the liquid components influence this effect. For Lambda<2.3, there is a transition in the late-stage morphology between spherical droplets and tube "plugs." Unstable distorted droplets ("capsules") form as transient structures for intermediate confinement (Lambdaapproximate to2.1-2.5). Surprisingly, the thread breakup process for more confined threads (Lambdaless than or equal to1.9) is found to be sensitive to the nature of the intial thread perturbation. Localized impulsive perturbations ("taps") cause a "bulging" of the fluid at the wall, followed by thread breakup through the propagation of a wavelike disturbance ("end-pinch instability") initiating from the thread rupture point. Random impulses along the thread, modeling thermal fluctuations, lead to a complex breakup process involving a competition between the Raleigh and end-pinch instabilities. We also briefly compare our tube simulations to threads confined between parallel plates and to multiple interacting threads under confinement.