Growth of solid domains in model membranes: quantitative image analysis reveals a strong correlation between domain shape and spatial position.

The nucleation and growth of solid domains in supported bilayers composed of a binary mixture of equimolar 1,2-dioleoyl-sn-glycero-3-phosphocholine (DOPC) and 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) have been studied using combined fluorescence microscopy and AFM. We have found that the formation of the DPPC-enriched solid domains occurs by a combination of homogeneous and heterogeneous nucleation and that the nucleation density is directly proportional to the cooling rate. Furthermore, during cooling the shape of the domains evolve from compact to a branched morphology. This suggests that the growth is controlled by the diffusion of DPPC from the liquid phase toward the solid domain interface. In the late stages of the growth, we observe that the size and overall shape of the domains depend on the position of the nucleation points relative to the surrounding nucleation point positions. To analyze this effect, the nucleation points were used as generators in a Voronoi construction. Associated with each generator is a Voronoi polygon that contains all points closer to this generator than to any other. Through a detailed quantitative analysis of the Voronoi cells and the domains, we have found that their area, orientation, and asymmetry correlate and that the correlation becomes stronger for larger domains. This means that the spatial distribution of the nucleation points regulate the domain shape.