IV. CONCLUSIONS

Our LB investigation indicates that confinement can substantially alter the thread breakup process from the bulk. These changes include not only changes in the rate of thread breakup, but also qualitative mechanistic changes in thread breakup process such as the suppression of satellite droplet formation and even the Rayleigh-Plateau instability itself. In general, the thread breakup becomes a complex admixture of capillary and end-pinch instabilities for "confined threads" (Λ 1.9) so that the nature of the thread perturbation magnitude and type (random versus impulsive "taps") can have a large influence on the final morphology. We note that these changes in the kinetics of capillary breakup, even under weak confinement conditions (tube radius less than ten times that of thread and greater than 2.5 thread radii), have practical implications for surface tension and other property measurements based on observations of the kinetics of thread breakup in confined geometries.

A comparison between thread breakup in tube and parallel plate geometries indicate that confinement has a similar effect for both these geometries. Of course, greater confinement, as measured by 1/ Λ, was required to achieve the same relative effect on the slowing down of the thread breakup kinetics. This trend is natural given that the tube involves confinement along two (orthogonal) directions, while confinement occurs only along one direction in the parallel plate geometry. Our comparison also reveals some unique characteristics of parallel plate confinement. Specifically, the confinement parameter Λ_plate can be less than 1 if we allow the "threads" to deform under confinement to form fluid strips. These "ribbons" are effectively extended tubular plugs in a direction orthogonal to the plane substrate, while in the inplane direction the boundary fluid ribbon remains free to undulate as an unconfined fluid thread. Since the Rayleigh-Plateau instability does not exist in two-dimensional systems [50], it is not surprising that these plug-like and ribbon-like aspects combine to create highly persistent extended structures. Such structures have been observed in the processing relevant contexts of immiscible polymer blends subjected to a Couette flow [20] and in thin phase separating blend films [49]. Of course, this is a completely different kind of "stabilization" phenomenon than the slowing down of thread breakup due to hydrodynamic lubrication forces. Fluid-substrate thermodynamic ("wetting") interactions can be expected to have a large influence on the stabilization of both ribbon and confined thread structures and further measurements and simulations are needed to understand the interplay between geometrical and thermodynamic boundary interaction effects in these structures.

Flexible boundaries, such as those arising from surrounding fluid threads, can apparently influence thread breakup. In some cases, we find that this type of constraint leads to enhanced stability, while in other cases stability is diminished. A wide range of flexible boundary types is evidently possible (e.g., threads in supported and free-standing films, etc.) and investigation of this type of confinement is needed since many new features apparently arise for this type of confinement.