Phase Contrast X-Ray Imaging of Lipid Membranes - Towards Autonomous Soft Matter Systems - page 58

Fig. 4.6 Left Image series of the “zipper-like” effect, which occurs during the transformation from
a bulk film to a bimolecular membrane from top to bottom . Right The migration of solvent and
the formation of a bilayer membrane leads to a change in the opening angle between the lipid
monolayers at the start of the lipid bilayer as shown

the first order minima. The phase contrast image in Fig. 4.5 a reveals structures which
are related to residues of glue on the self-adhesive kapton foil. These can induce
parasitic scattering, which contributes to the diffraction pattern of the membrane and
leads to deviations from the theoretical intensity profile, as obtained in the extracted
profile. In the absence of glue on the kapton surface, i.e. in previous experiments
where bulged BLMs [ 10 ] were investigated, such features were not observed.
Next, we look at the zipping process of bilayer formation as discussed in Chap. 2 .
A sequence in Fig. 4.6 , shows the phase contrast images of the zipping process. A
droplet of oil (marked by the black arrow), traverses rapidly, forming a molecular
bilayer in its wake. The final state of the bilayer is characterised by the opening
angle at the Plateau-Gibbs border, and as discussed in Chap. 2 , this depends on the
interfacial tension at the oil-monoolein-squalane interface. As we found previously
in Chaps. 2 and 5 , this opening angle is found to be 50 ◦

∼ from the phase contrast

images.
Figure 4.7 shows the transition region between the Plateau-Gibbs border (PGB)
and the bimolecular region of a lipid membrane, which has recently formed as
described above. We now seek to quantitate the thickness variations during this
process. The fresnel fringes close to the PGB are much more strongly pronounced
than in the thinner regions. Profile 9 in Fig. 4.7 b is extracted from a position in
Fig. 4.7 a, where a domain of residual organic solvent just migrates towards the PGB.
Figure 4.7 cshows profile 10 , which is extracted from a ROI in the transition region.
Here the fringe visibility is already poor.