Phase Contrast X-Ray Imaging of Lipid Membranes - Towards Autonomous Soft Matter Systems - page 48

Chapter 4
Phase Contrast X-Ray Imaging of Lipid
Membranes

A propagation based phase contrast X-ray imaging technique is
developed and is used to study the structure and interfacial
properties of microfluidic lipid membranes and the fusion of the
monolayers they consist of.

4.1 Introduction

The investigations of membranes and associated processes in Chaps. 2 and 3 clearly
point to very important phenomena at the nanoscale. For example, electroporation
of lipid membranes is expected to occur via nanoscopic orifices that open up in the
bilayer under the influence of an electric field [ 1 - 3 ]. The intermediate structures of
membrane fusion viz. the hemifusion and stalk formation states, if they exist, are
certainly of nanometer scales. While the combination of a variety of techniques help
to understand many aspects of such phenomena, direct visualization of the nanoscale
structures is still a far away goal.
For light of a given wavelength , diffraction sets the limit on the resolution, d

of observation in far field optical microscopy as d

λ

is
the numerical aperture of the system. Therefore for visible light, resolutions below
a few hundreds of nanometers are not achievable easily. While electron microscopy
is capable of resolving nanostructures, its adaptability to the study of dynamics
and the cumbersome sample preparation involved [ 4 ], limit its usage. Fluorescence
microscopy techniques such as STED and STORM [ 5 , 6 ] can reach resolutions of
down to a few tens of nanometers, but this is still not sufficient to resolve the structural
details of a lipid bilayer, for instance, which is only a few nanometers thick.

∼ λ/ 2 n sin α , where n sin α