How do two water droplets collide in the space? Suppose two balls coming towards each other with the same speed, collide head on, in the space (to neglect other external forces). After collision they will move along opposite directions.
Then replace the balls with two big water droplets. As we are in the space, water droplets are shaped as spheres. This time how should I imagine the situation at and after collision?
 A: I agree completely with what Marco wrote and almost made this a comment instead. But I think is sufficient to be supplemental answer.
Water or other fluids experience ram force and experience pressure above atmospheric during collisions, even at the boundary with the air. That is the way that fluids transmit force or impact (or even weight: a gas in a box weighed in a vacuum will add its weight via a pressure gradient inside the box). In this case if we assumed perfect symmetry, and negligible surface tension effects (ie high-speed collision), then the end result will be a higher pressure blob that “explodes” in every direction. With liquids, there is nothing privileged about the axes of collision when there is no net final momentum. Pressure is non-directional. The pressure would not last and here, unlike impacts with surfaces, we might even make the case that the outer edges are atmospheric. (It becomes academic eventually as pressure head and kinetic head transition so rapidly).
If they went slowly enough (and again perfect symmetry), they would form into one mass and vibrate, not not moving in translation or rotation. The vibration is viscously dampened and tends asymptotically toward zero. The viscous damping would provide more heat than surface tension would, but still not much.
If the blobs were both rotating the same direction, then we wouldn’t be able to assume an irrotational fluid and the coalescing blob would be rotating and would continue to. If they collided at high but differing speeds (or high speed and different masses) then the explosion would be skewed to obey conservation of momentum. Similarly if they were moving quickly with net angular momentum then the explosion would have droplets which rotate.
A: It depends upon the speed of impact relative to the surface tension of water. If the drops came together sufficiently slowly in a head-on collision then they would coalesce and become one drop with the combined momentum of the two original drops, the KE of the drops being converted to heat. If the drops collided at an extremely high speed, they would splatter into a mist of much smaller droplets. Between those two extremes there is a spectrum of possibilities combining combinations of the two outcomes with varying weightings.
