Figure 1: Salt water drops coalescing in silicone oils.

Salt water drops (μ_in=1.0 mPa s, ρ=1,070 kg m⁻³, A=2 mm) coalescing in silicone oil pictured 1.0 ms after contact, with (a) μ_out=0.49 mPa s, and (b) μ_out=48 mPa s. Despite the large difference in μ_out, the neck radii are nearly the same. The only difference is that capillary waves are visible in the less viscous outer fluid⁹. Scale bar: 500 μm. (c) Neck radius versus time for salt water drops coalescing in silicone oils of different viscosities. In these fluid combinations, 38 mN m⁻¹<γ<40 mN m⁻¹. The neck radius does not depend on the outer-fluid viscosity, even when it is 48 times more viscous than the liquid inside the drops.