Two identical pockets of viscous fluid trapped beneath an elastic sheet, brought close enough to touch, do not merge instantaneously. A thin neck forms at the contact point and grows. Experiments using synthetic schlieren imaging --- a technique that converts refractive index gradients into thickness measurements --- track this neck growth with high temporal resolution. The early dynamics follow a scaling law controlled not by the viscosity of the trapped fluid but by the bending stiffness of the sheet above it.
This is counterintuitive. The fluid is the substance that must flow to fill the neck, yet the rate-limiting step is the deformation of the elastic boundary confining it. The sheet must unbend locally to accommodate the growing bridge between the two blisters, and the energy cost of that unbending sets the pace. A stiffer sheet means slower coalescence, even with identical fluid underneath. The container governs the merger, not the contents.
A lubrication model captures this relationship analytically. The curvature at the coalescence neck determines the pressure gradient that drives fluid into the growing bridge, and that curvature is set by the elastic sheet's resistance to bending. At short times, the neck width grows according to a power law whose exponent depends on the sheet's bending modulus. The fluid's viscosity enters the problem eventually --- at longer times, as the neck widens and the sheet relaxes --- but the initial dynamics are dominated entirely by elasticity.
The principle generalizes. Whenever two regions of a confined substance begin to merge, the earliest phase of merging may be controlled by the boundary rather than the substance. The thing that separates determines how --- and how fast --- separation ends. Barriers do not merely prevent connection; they regulate the rate of connection even after they have been breached.