Drop deformation, breakup, and coalescence with compatibilizer

The effect of copolymers on the breakup and coalescence of polybutadiene ~PB! drops in polydimethylsiloxane ~PDMS! is studied using a four-roll mill flow cell. Copolymers are produced at the interface by a reaction between functionalized homopolymers. They reduce the interfacial tension and thus enhance breakup; they also inhibit coalescence of drops. Under the conditions of our experiments, the latter effect is much more significant than the former. For example, the addition of copolymer sufficient to reduce the interfacial tension by only 3% relative to the bare interface value is found to reduce the critical capillary number Cac for coalescence by a factor of 6. The critical capillary number for coalescence in the absence of copolymer is also measured for the first time. It is found to scale with the drop radius a as Cac;a 20.8260.03 and with the viscosity ratio l as Cac;l . © 2000 American Institute of Physics. @S1070-6631~00!01203-4#