Fig. 1. Trapping wells as proposed in ref. 32.† The shown trapping well pair allows to trap, merge, and mix droplets from two droplet streams. When a droplet reaches the entrance of a trapping well (i.e. point J₁ or J₂) and the respective trapping well does not yet contain a droplet, the droplet should flow into the trapping well. Therefore, the flow rate into the trapping well has to be larger than the flow rate into the bypass channel, i.e. Q_trap > Q_bypass. Furthermore, a trapped droplet should stay in the trapping well and, hence, must not be squeezed into the other trapping well (i.e. P_trap1 − P_trap2 must not exceed the Laplace pressure) and must not be squeezed through the gaps downstream (i.e. P_trap1 − P_down must not exceed the Laplace pressure). On the other hand, when the trap already contains a droplet, following droplets should enter the bypass channel. Therefore, a trapped droplet has to decrease the flow rate into the trapping well so that the flow rate into the bypass channel gets larger, i.e. Q_trap > Q_bypass.