Sling effect in collisions of water droplets in turbulent clouds
  We describe and evaluate the contribution of sling effect into the collision rate of the same-size water droplets in turbulent clouds. We show that already for Stokes numbers exceeding 0.2 the sling effect gives a contribution comparable to Saffman-Turner contribution, which may explain why the latter consistently underestimates collision rate (even with the account of preferential concentration). 
Gregory Falkovich and Alain Pumir