Collision characteristics of droplet pairs with the presence of arriving distance differences

Abstract Four collision behaviors are observed for droplet pairs at a T-junction and phase diagrams are constructed by considering the arriving distance difference, the droplet length and the capillary number. Critical conditions for both coalescence and splitting are shown. The main coalescence regime is varied for asynchronous droplet pairs and the cause of the coalescence which occurs while two droplets are pulled apart is confirmed. Movements of the front caps of various droplet pairs are exhibited to show their different degrees of competitions to penetrate into the downstream channel and the advantage that favors coalescence is acquired by comparison.