Xiaopeng Chen, Shreyas Mandre & James J. Feng
Phys. Fluids 18, 092103 (2006).
Abstract - When a water drop falls onto an
oil-water interface, the drop usually rests for some time before
merging with the water underneath the interface. We report experiments
on this process using water- and oil-based Newtonian liquids and
polymer solutions, with an emphasis on the non-Newtonian effects. We
find that the drop surface is immobilized, and for Newtonian fluids,
the rest time scales with the matrix viscosity. The results are
compared with lubrication models for film drainage. If the surrounding
matrix is a dilute polymer solution, the rest time is identical to that
for a matrix of the solvent alone. Further investigation indicates that
the polymer molecules have been cleared from the film by surface
adsorption. Depending on the fluid properties and drop size, the
drop-interface merging may be completed in one shot or through a
cascade of partial coalescence. Partial coalescence occurs for an
intermediate range of drop sizes; it is arrested by viscosity for
smaller drops and by gravity for larger ones. When either the drop or
the matrix phase is a polymer solution, viscoelasticity is shown to
suppress partial coalescence for smaller drops. This is apparently due
to the inhibition of capillary pinch-off which would otherwise produce
a secondary drop before the merging is complete.