DEFORMATION AND BREAKUP MECHANISMS OF SINGLE DROPS DURING SHEAR

The behavior of single drops of isotropic hydroxypropylcellulose solut ions immersed in a polydimethylsiloxane matrix subjected to a constant shear rate was investigated using a rheo-optical technique. Our main results concern the nonstationary deformation and breakup mechanisms o f drops characterized by large capillary numbers. The deformation of t hreads follows a pseudoaffine deformation for Ca/Ca-crit larger than 2 .5 and above a certain strain. The end pinching mechanism occurs at a specific scaled strain, which includes the initial drop size and the a pplied shear rate. Capillary instabilities develop when the thread dia meter reaches a critical value, d(r), inversely proportional to the ap plied shear rate and independent of the initial drop diameter. The tim e necessary for the total rupture of a thread depends on the initial d rop diameter, the applied shear rate, and the critical thread diameter d(r). The droplet size resulting from the final rupture of a thread i s found to be half that of the critical size, below which a drop defor ms into a stable ellipsoid. (C) 1998 The Society of Rheology. [S0148-6 055(98)00906-7].