Chemistry Reference
In-Depth Information
Clearly, there won't be any gain over conventional 'top-down' devices unless the
self-assembled soft matter structures attain length scales below 50nm or so. In the
present study, we still keep far away from this scale, in order to facilitate the produc-
tion and observation of the structures and processes at this very preliminary stage.
The present chapter rather aims at identifying basic mechanisms and conceptional
elements which appear promising, but are still to be scaled down considerably. It
should be noted, however, that one of the major potential problems of the concept,
the instability of emulsions against coalescence, has been found to fade away as size
is reduced. Some results pointing in this direction will be briefly discussed.
2.2 Experimental Techniques
A few key notes about the microfluidic setup (Fig.
2.2
) used for the experiments in
this chapter are presented here. Microfluidic step emulsification devices [
12
,
13
],
formed in poly-dimethyl-siloxane (PDMS, Dow Chemicals) by standard soft litho-
graphic techniques, are used with an inverted fluorescence microscope (Olympus,
IX81). Flow through the microfluidic devices are controlled by home built syringe
pumps. Electrodes, made from glass micropipettes are used to electrically connect
the microfluidic device to a patch clamp amplifier (HEKA, EPC10), which is used
to both excite and measure currents. Micropipettes are pulled using a micropipette
pulle (Sutter Instruments) from borosilicate glass capillaries with an outer diameter
of 1 mm and inner diameters of 500
µ
m. The micropipette tip is then broken to cre-
ate an opening of
m. The capillary is then filled with an agarose gel (1wt%) of
150 mM NaCl electrolyte and sealed at the back after the insertion of a chlorided
silver wire.
∼
1
µ
2.2.1 Patch Clamp Amplifier
We use the patch clamp amplifier in a voltage clamp mode, so that a fixed voltage is
applied across the membrane. All the currents reported here are due to the resistive
contributions of the membrane. The lock-in method, a commonly used technique
to measure capacitance
C
m
, is used to measure the capacitance of the lipid bilayer.
Briefly, a sinusoidal wave excitation
V
m
(
is applied across the membrane and the
current response
I
m
is measured. The membrane is excited by a sinusoidal voltage
t
)
V
m
(
t
)
=
V
0
sin
(ω
t
)
(2.1)
with an angular frequency
ω
. The response of a membrane system consists of a
resistive current
I
R
=
V
m
(
t
)/
R
m
, as well as a capacitive current
I
C
=
C
m
dV
m
(
t
)/
dt
.
At the peak of the sine wave, where
dV
m
(
0, only a resistive current is
obtained and
R
m
can be derived. The contributions to
R
m
from electrodes and buffer
t
)/
dt
=
