The influence of the viscosity ratio on polymer droplet collision in quiescent blends

The coalescence of polymer droplets is studied, in particular the final regime named collision. This phenomenon involves the complex deformation of the interface, a motion driven by the interfacial tension, and is mainly governed by the viscosity ratio. An accurate measurement technique of the collision time has been explored experimentally. In a recent paper [C. Verdier, C. R. Acad. Sci. Paris Série IV, 2000], a particular case of the collision was studied, corresponding to viscosities in the same range. One of the aims of the present work is to analyze collision, and to extend these previous results to a wider range of viscosity ratios. For droplets of equal size, three regimes are obtained, corresponding to power law dependence of the collision time as a function of the viscosity ratio. The flow field shows the importance of the area close to the neck, where most of the flow dissipations occur. By assuming that this flow gives rise to a competition between interfacial energies and shear viscous dissipations, a simple model is presented, based on a previous study [Brochard-Wyart F, de Gennes PG, Advances in Colloid and Interface Science, 1992], and it recovers two of the experimental regimes. The final one can also be predicted using results from Frenkel [J. Phys. USSR, 1945]. This theory is in qualitative agreement with the observed data. This experimental technique is simple and provides a new way to measure interfacial tension, when the viscosities of the two fluids are known. The advantage of the technique is that sample preparation is easy, and that it requires only an optical microscope. (