ON THE PINCH-OFF OF A PENDANT DROP OF VISCOUS-FLUID

The pinch-off of a drop of viscous fluid is observed using high-speed digital imaging. The behavior seen by previous authors is observed her e; namely, the filament that attaches the drop to the orifice evolves into a primary thread attached to a much thinner, secondary thread by a slight bulge. Here, we observe that the lengths of the primary and s econdary threads are reproducible among experiments to within 3% and 1 0%. The secondary thread becomes unstable as evidenced by wave-like di sturbances. The actual pinch-off does not occur at the point of attach ment between the secondary thread and the drop. Instead, it occurs bet ween the disturbances on the secondary thread. After the initial pinch -off, additional breaks occur between the disturbances, resulting in s everal secondary satellite drops with a broad distribution of sizes. T he pinch-off of the thread at the orifice is similar to that at the dr op with one main difference: there is no distinct secondary thread. In stead, the primary thread necks down monotonically until wave-like dis turbances form, resulting in pinch-off at multiple sites in between. T he speed of the tips of the retreating, secondary threads after pinch- off are reported and discussed in the context of various scaling laws. (C) 1997 American Institute of Physics.