Abstract
Smectic liquid crystal films a few molecular layers thick that are freely suspended in air are used as a model system to study the coalescence of fluids in two dimensions. High-speed video microscopy is used to observe the coalescence of islands, which are thicker, disk-shaped regions of the film, in a process driven by the line tension associated with edge dislocations along the island boundaries and limited by viscous dissipation in the liquid crystal and in the surrounding air. The early time growth of the bridge connecting the merging islands reveals much slower dynamics than predicted by Hopper's classical hydrodynamic model of coalescence of two infinitely long, fluid cylinders in vacuum, a discrepancy proposed to be due to significant dissipation in the background film and in the air that is not included in Hopper's theory. At late times, the elliptical merged island relaxes exponentially to a circular shape, at rates that are described quantitatively by a model originally developed for the evolution of fluid domains in Langmuir films.
Highlights
The coalescence of two fluid objects and the converse process, the breakup of fluid drops, are simple, beautiful examples of singular physical phenomena involving the divergence of a physical quantity
We describe experiments using high-speed video microscopy to observe the coalescence of pairs of flat, diskshaped fluid smectic islands embedded in thinner background films of the same liquid crystal (LC) material, in events driven by the line tension along the island boundaries
While we find that the observed functional form of the bridge width dynamics is similar to the Hopper model, in agreement with the results of Shuravin et al [21], our experiments yield measured characteristic growth rates that are only about half the Hopper prediction, contradicting Shuravin’s claim of overall agreement with Hopper’s model
Summary
The coalescence of two fluid objects and the converse process, the breakup of fluid drops, are simple, beautiful examples of singular physical phenomena involving the divergence of a physical quantity. The coalescence dynamics were characterized by measuring the width of the bridge connecting the merging islands as a function of time Analysis of such events is based on treatment of the islands as incompressible, viscous disks of fixed thickness much smaller than their radius, embedded in the background film and in Saffman-Delbrück contact with the surrounding air. While we find that the observed functional form of the bridge width dynamics is similar to the Hopper model, in agreement with the results of Shuravin et al [21], our experiments yield measured characteristic growth rates that are only about half the Hopper prediction, contradicting Shuravin’s claim of overall agreement with Hopper’s model This indicates that the dissipative effects of both the surrounding air and the background film must be included at short times
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