Positional information-based morphogenesis of surfactant droplet swarms emerging from competition between local and global Marangoni effects.

Positional information is proposed as a key element in morphogenesis, where cells differentiate based on their position in an external morphogen gradient. Here, we demonstrate the position-dependent differentiation of floating surfactant droplets as they self-organize in a pH gradient, using the Marangoni effect to translate gradients of surface-active molecules into motion. We start with a field of surfactant microliter-droplets floating on water, that upon the release of surfactant drive local, outbound Marangoni flows and myelin filament growth. Next, we introduce a competing surfactant based on a hydrolysable amide, which is more surface active than the myelin surfactant and thereby inhibits the local Marangoni flows and myelin growth from the droplets. Upon introducing a pH gradient, the amide surfactant hydrolyses in the acidic region, so that the local Marangoni flows and myelin growth are reestablished. The resulting combination of local and global surface tension gradients produces a region of myelin-growing droplets and a region where myelin-growth is suppressed, separated by a wave front of closely packed droplets – featuring an organization that is reminiscent to the ‘French flag’-patterns emerging from concentration gradients in morphogenesis.