Rheol Acta (2006) 45: 505–512 DOI 10.1007/s00397-006-0087-1 ORIGINAL CONTRIBUTION Sergio Caserta Marino Simeone Stefano Guido Received: 22 July 2005 Accepted: 19 January 2006 Published online: 9 March 2006 # Springer-Verlag 2006 A parameter investigation of shear-induced coalescence in semidilute PIB–PDMS polymer blends: effects of shear rate, shear stress volume fraction, and viscosity Abstract In this work, drop coa- lescence of polymer blends under shear flow in a parallel flow apparatus was investigated by optical sectioning microscopy. In each experiment, shear rate was set at values low enough to avoid any break-up phenomena. The time evolution of the drop size distri- bution was determined by motorized sample scanning and iterative acqui- sition of stacks of images along sam- ple depth. Drop size and location in the acquired images was found by automated image analysis techniques. A systematic experimental campaign to investigate the effects of shear rate (in the range 0.1–0.5 s -1 ), volume fraction (2.5–10%), and viscosity of the two phases (3–63 Pa s) at different viscosity ratio (0.1–2.3) was carried out. By comparing data from different experiments, it was found that at any strain value, the average drop size decreases monotonically with the shear stress, calculated as the product of shear rate and matrix viscosity. Furthermore, the coalescence rate slowed down with increasing viscos- ity ratio. Overall, these results provide an extensive set of data, which can be used as a benchmark for modeling shear-induced coalescence in polymer blends. Keywords Coalescence . Polymer blends . Morphology . Shear flow . Optical microscopy . Image Analysis Introduction The investigation of drop coalescence in polymer blends under controlled flow conditions has attracted much interest in the literature, both from the experimental and the theoretical side. Indeed, it is well known that coa- lescence plays an essential role on the flow-induced evolution of the drop size distribution, which is a key feature for the final properties of products obtained by industrial processing of polymer blends in the molten state. From the theoretical standpoint, drop coalescence during flow was modeled (Chester 1991) by considering an external, macroscopic flow field and an internal micro- scopic one, corresponding to the draining of the interven- ing film among two colliding drops. The external flow defines the mutual force and the interaction time between the two colliding drops and the collision frequency C (Smoluchowski 1917). In simple shear flow, in the case of equal-sized particles of diameter d, the collision frequency is proportional to the shear rate γ : ðÞ and to the squared volume fraction of the dispersed phase, which is expressed as the number of drops per unit volume n C / γ   d 3  n 2 (1) The internal flow can be used to give an estimate of the time needed to drain the liquid film between the drops. By Paper presented at the Annual Meeting of the European Society of Rheology, Grenoble, April 2005. S. Caserta (*) . M. Simeone . S. Guido Laboratori “Giovanni Astarita,” Dipartimento di Ingegneria chimica, Università degli Studi di Napoli Federico II, P. le V. Tecchio 80, Napoli 80125, Italy e-mail: scaserta@unina.it