The initial coalescence of miscible drops

When two drops of different miscible liquids are brought into contact, their coalescence speed is governed by the liquid having the weaker surface tension. Marangoni waves propagate along the drop with the stronger surface tension. We present surface profiles and propagation speeds of these waves, from experiments with a pendent water drop coalescing with a flat ethanol surface or with a sessile drop of ethanol. We find in the former case that the capillary-Marangoni waves along the water drop show self-similar character when measured in terms of arc length along the original surface. The coalescence of two liquids of different viscosities is also studied. For large viscosity difference, mobility is confined to the lower viscosity fluid and a sharp corner forms where the two liquids meet along the free surface. The coalescence speed of a water drop with a much more viscous liquid is nearly independent of the strength of the viscosity difference.