Figure 5.

Schematic describing the process of “tree-ring growth.” As a saturated lipid/unsaturated lipid binary-mixture bilayer cools, the temperature drops below the phase transition temperature (A; figure for a simple two-phase system as taken from a DOPC/DPPC phase diagram in the literature (39)). With cooling, nucleated l_β domains begin to appear (B (38)). Subsequent drops in temperature result in further phase separation, but only between lipids in the l_d phase, as lipids in the l_β domains are kinetically trapped due to the domain immiscibility. Thus, each incremental layer of the l_β domain has a subtly different composition and therefore different mechanical properties. Experimentally, the temperature ramp is smooth and continuous; hence, the composition of the domains will follow the solidus curve, and the composition of the melt will follow the liquidus curve.