Chemistry Reference
In-Depth Information
Fig. 4.5 a
Diffraction pattern of a microfluidic bilayer membrane prepared from a Monoolein-
squalene solution/water at an effective propagation distance of 29.67mm. The Fresnel fringes are
highly visible at high deflection angles. Glue residues at the surface of the self-adhesive kapton
window material produce parasitic features in the image.
b
The profile between the black lines is
extracted from the region of interest (ROI) in
a
. Due to planar sample geometry it can be fitted using
a symmetric phase slit model [
11
], which is characterized by the phase shift
φ
and the thickness
d
.
The state of the membrane is such that there is a lot of solvent remaining between
the lipid monolayers and the visibility of Fresnel fringes is therefore relatively
high. A one-dimensional diffraction profile is extracted by averaging over the region
between the two black lines as shown in Fig.
4.5
a. A least-squares fit together with
the extracted profile is depicted in
4.5
b.
The symmetry of the intensity profile can be explained by the geometry of the
lipid membrane, as mentioned earlier. A pressure gradient
p
across the membrane
induced by the two pumps driving the aqueous fingers will always cause the mem-
brane shift inside the channel. When the pumps are stopped, removing any pressure
difference i.e.
0, the membrane stays at a fixed position and in this position
it reaches a planar orientation in order to minimize its surface tension. Since it is
composed of symmetric monolayers, we can explain the symmetry of the intensity
pattern in the same way, as for parallel beam thinning series by using a symmetric
phase slit approach [
10
]. The membrane thickness extracted from the fitting proce-
dure gives
d
p
=
5nm and is large, as expected due to the interstitial oil between
the lipid monolayers. The fit result of
=
383
.
593 can be used to calculate the path
length of the X-rays through the membrane. According to Eq.
4.2
we get
φ
=
0
.
φ
0
.
593
L
=
=
10
−
7
µ
m
=
54
.
3
µ
m
,
(4.7)
10
4
−
k
δ
8
.
872
×
·
1
.
23
×
which at the same time corresponds to the diameter of the membrane patch, consid-
ering that the contact area of the two fingers leads to a flat circular membrane patch.
This value appears to be quite reasonable for a channel depth of 300
m as also seen
in Fig.
4.4
. It can be seen in Fig.
4.5
b that the fit shows slight deviations from the
actual profile in the central region. It is also not able to catch the slight asymmetry of
µ
