The effect of viscoelasticity on the drop deformation

breakup and coalescence is a challenging topic in polymer processing as well as in drop dynamics which is still mainly based on the analysis of Newtonian fluids.In this study

the viscoelastic liquids used for drops were mainly low molecular weight polyisobutylene (PIB) containing high molecular weight polyisobutylene (HPIB).The mass fractions of HPIB were 0.1%

0.5% and 1% covering polymer concentrations from dilute to entanglement.The matrix was made of polydimethyl siloxane (PDMS)

a Newtonian liquid and the viscoelastic fluid of PDMS with 5% silicone rubber (HPDMS).After iterative elongation and relaxation in a four-roll mill rheometer

a meso-scale viscoelastic drop turned into dumbbell like structure

two beads connected by one string

which was called BSB (bead-string-bead) according to its shape.It is found that the string does not break for a long time and its length decrease exponentially

the two beads at string ends coalescence eventually.During the coalescence process

the string diameter nearly keeps constant below a critical value.This phenomenon is unlike the existing coalescence of Newtonian drops through stages of matrix film formation

drainage

rupture and drop merge.Moreover

BSB coalescence is different from the known phenomenon of beads-on-string in which the viscoelastic string reduces several orders of magnitude and breaks ultimately.A model is provided based on the force balance of BSB.The elastic force from stretched molecules in the string pulls the two beads approach overcoming the viscous drag of matrix

the string is drained into beads under the Laplace force.In case of the string diameter is a fixed value

the model predictions agree with experiments reasonably.Only in this ideal condition

the relaxation time of material in string matches with the time of string length change keeping the string diameter stable.The critical string diameter is related to both microscopic material parameter (the initial stress in string and the relaxation time of fluid) and macroscopic process parameter (viscosity of matrix

droplets diameters

string length) in the model.If the stress in string is too large

the string diameter will increase; if the viscosity of matrix is too high

BSB phenomenon will transfer to unstable beads-on-string.When the viscosity of matrix is infinity

BSB structure can be reduced to the case of filament stretching rheometer.Indeed

both phenomena are dominated by stretched viscoelastic stress relaxation and interfacial tension.BSB phenomenon is inspiring for the basic understanding of the orientation and entanglement state of polymers and the nonlinear drop dynamic in multiphase systems.