中文摘要：

在四辊流变仪中，黏弹性高分子介观液滴经反复拉伸和松弛形成了有细丝相连的两个黏弹液珠，研究了黏弹液珠的聚并过程，依形状叫做BSB（bead-string-bead，液珠-细丝-液珠）聚并。BSB现象与常见的通过滴间液膜破裂实现的液滴聚并过程大不相同。根据界面上的Laplace力、液珠移动时的黏性阻力和细丝中黏弹应力之间的平衡，推导出一个力学模型来描述BSB现象，理论分析与实验结果相符较好。细丝直径的变化和稳定性由过程参数和物料参数共同决定，尤其是液滴的黏弹性有较大影响。这一效应对多相高分子与复杂流体加工过程的基础理解富有启发。

英文摘要：

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 tumed 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 ?