FIG. 2.

Graphs showing the effect that droplets have on the surfactant abundance on the wall, in the oil, and on the droplet with (a) one surfactant and (b) two surfactants, where the second surfactant specifically coats the device wall. (c) Effect of droplet frequency on the relative wall surfactant coverage. (d) Statistical mean of surfactant wall coverage at different droplet frequencies for systems with one and two surfactants. (e) Effect of adding a silane to functionalise the device wall surface. Droplets, at a frequency of 10 Hz, are added to the system at t = 25 s. Prior to this, the silane coverage of the wall is 95%. The addition of droplets causes the silane wall coverage to decrease to approximately 30%. Adding a second surfactant with an affinity for the wall ameliorates this effect to a certain extent. Relative abundance is the ratio of surfactant present on the wall surface compared to the theoretical maximum surface sites available for surfactant packing. Relative abundance is the equivalent of concentration.