It might be rare that droplets of different fluid land on top of each other, but what happens when they do? This image shows exactly that happening.
These images, captured by the Splash Lab, show what happens when liquids of different densities land on top of each other. In the images, the red liquid is a 1:1 mixture of water and glycerol (whose density is six times that of water) while the blue drops are just plain water (with a tiny quantity of colouring). The top droplet was released form six inches above the lower, which was sat, stationary, on a hydrophobic surface. The Splash Lab describes what's happening:
The [top] row depicts a droplet of water impacting a sessile water-glycerol droplet. Upon impact, the less viscous water deforms more quickly and wraps around the more viscous glycerol mixture. The droplets spread and coalesce, producing the beautiful gradient in the rebounding jet. The [bottom] row depicts a droplet of glycerol mixture impacting a sessile water droplet. Again, the water droplet experiences greater deformation upon initial impact. A similar spread and coalescence is shown with the absence of a jet in the considered time. A jet similar in magnitude and shape to that created by the water on glycerol mixture impact is formed shortly after the last image in the sequence.