Fig. 4. The mechanism of spherical shell formation.

a Schematic representation of the mechanism of spherical shell formation. 1) Fuel-driven activation outside of the droplet. 2) The product diffuses into the droplet. 3) Deactivation inside of the droplet. 4) Precursor diffuses out of the droplet. 5) The droplet´s core is depleted of product. 6) The droplet transitions to a spherical shell. b, c Confocal micrograph timelapse series of an active droplet with a critical radius larger than r_unstable. Over 20 min, the droplet wetted the microreactor’s bottom and transitioned into a spherical shell. The images in b show the XY-plane close to the bottom of the reactor. The images in c show the XZ-projection. The scale bar of all images represents 10 µm. The color scale is given next to the images. Concentration profiles of a large chemically fueled droplet with r > r_unstable (d) before transitioning into a spherical shell, a spherical shell (e), and a small chemically fueled droplet with r <r_unstable (f). The insets show a scheme of the droplet and spherical shell. g The system’s behavior as a function of steady-state concentration fuel and reactor volume. The shaded areas represent the stable state calculated by the model. The red-blue shaded area represents the coexistence of stable droplets and shells. The markers show the phase-separated state of the experimental data. Overlapping data points are shown with an offset. All experiments were performed in triplicate (N = 3). The shell thickness L_shell for a microreactor with a radius of 25 µm as a function of varying deactivation rate constants (h) or precursor diffusion constants (i). Source data are provided as a Source Data file.