There has been a long-standing interest in lipid bilayers, not least due to their relation to cell membranes. It is, however, only recently that mixed bilayers have attracted a lot of attention. In giant unilamellar vesicles (GUVs) whose membranes contain mixtures of different phospholipids separate domains with different lipid compositions can exist. The underlying physical properties that give rise to this lateral heterogeneity and the properties of these different domains, for example in respect to their spatial and temporal scales, is still not fully understood and quantitative, systematic data on membrane lipid domains is scarce. Nevertheless, confocal imaging, in particular two-photon fluorescence microscopy, has already proven to be a very valuable tool.

We will use multi-photon fluorescence microscopy to investigate the domain formation in a model mixed bilayer, which consists of a lipid mixture with different head groups but the same tail lengths (DPPC/DPPE). We are particularly interested in the size and shape of bilayer domains and whether their properties depend on the preparation procedure or undergo a 'ripening' process (indicating that the domains are non-equilibrium structures).

Dyes with different affinities to the different regions in a mixed bilayer will be used to highlight the domains formed. The use of multi-photon fluorescence microscopy is crucial as the fluorophore (N-Rh-DPPE) has to be excited in the UV range and bleaching should be avoided to be able to detect any ripening i.e. long-term behaviour.

Based on previous work done in COSMIC, high quality results can now be obtained. Subsequent time on the confocal will be used to examine the effects of temperature history on the size, shape and formation of these phase domains in a systematic way and to look at the time-scale of the movement of these domains. Firstly I will compare the areas of the two phases formed in the gel-fluid region and compare this to the phase diagram for the system using the Lever Rule to get some insight into whether the phase separation is equilibrium in nature.