Coalescence of two viscoelastic droplets connected by a string

摘要

A new type of coalescence of viscoelastic droplets is described and analyzed. A viscoelastic drop of polyisobutene (PIB) containing high molar mass PIB (HPIB), suspended in polydimethylsiloxane, is separated into two droplets connected by a string, which is called "bead-string-bead" (BSB), through repeated elongation and relaxation in a four-roll mill rheometer. Upon flow cessation, the two drops are pulled to approach each other by the string and eventually coalesce. This process exhibits interesting features: The string remains stable like a rod without capillary breakup; the string length and the merging force decay exponentially with time; the string diameter may not change significantly through the approach. The string in BSB is more stable than that of the capillary thinning process of beads on string in jet or fixed-end rheometer. These phenomena are modeled based on the force balance of the viscoelasticity of HPIB systems, viscous drag, and the Laplace force. The model prediction agrees with the experimental observation reasonably well, revealing the viscoelastic nature of the coalescence of two droplets. The characteristic time of drop approach is comparable to that of the pulling stress decay. The condition of keeping a constant diameter in BSB approach creates a status where both drop approach and stress decay exponentially with the same characteristic time, which is comparable to the material relaxation time. This case allows the model to analyze the relationships between critical string diameter and the material parameter as well as the process parameters and to discuss the microscopic images of the BSB process. (C) 2008 American Institute of Physics.