At terminal velocity, gravitational force equals Stokes drag: mg=6πηrv
For a sphere with mass m=34πr3ρ, this gives v=9η2r2ρg
Therefore, v∝r2
When 64 drops coalesce: 64×34πr13=34πr23, so r2=4r1
New terminal velocity: v1v2=(r1r2)2=16
v2=16×10=160 cm/s