FIG. 1.

(a) Graphical representation of the synthetic quorum membrane (SQM, purple dotted line). Aqueous droplets (shaded grey) in a channel move past immobile walls (hashed sections) and their surface area and separation varies as a function of time. To simplify this system, droplets are treated as a membrane, in the same way as the wall. This membrane has an area A_d and is at a mean distance from the wall, D, which depends on the separation of the droplet from the wall, S_d, through the equation $D=\sqrt{\Sigma S_d^2}$. The geometric mean is used to calculate D due to the sinusoidal input associated with droplets moving past the wall surface. D must be a value such that the Fourier number is higher than 2 to allow equilibrium. (b) Surfactant effects seen in droplets due to diffusion and advection (in purple), with hashed sections showing areas of low and high surfactant concentration at the front and back of the droplet respectively (this can also be seen experimentally in analogous systems where there is an increased product concentration at the back of droplets⁶⁴). The relative movement of the oil past the droplet is shown by the big black arrows (from left to right) and internal droplet flow recirculation effects are shown by dashed arrows.