Chemistry Reference
In-Depth Information
Fig. 2.8
Addressing a single
droplet with a pipette
Fig. 2.9
Contacting droplets
(150mM/l NaCl in Millipore
water) electrically bymeans of
micro-pipettes poked through
the walls of the PDMS-
micro-device. The pipettes
are equipped as Ag
AgCl
2
electrodes and connected to
a patch-clamp amplifier. The
oil phase consists of squalane
plus 25mM mono-olein. The
channel is 100
/
µ
mwide
problem, since the membranes are objects of minute thickness (about 4.5nm in the
case of mono-olein) and relatively small energy of formation (see above).
Quite remarkably, we found the bilayer membranes to be very stable against
mechanical stresses, such as those exerted on them in micro-fluidic flow. A partic-
ularly striking example is shown in Figs.
2.8
and
2.9
. Glass micro-pipettes can be
inserted through thin walls (100
m) created in the PDMS directly into the droplets.
This is important for establishing ohmic contacts to the aqueous droplet content by
means of standard Ag
µ
AgCl
2
electrodes, as used in electro-physiology. In order to
isolate the droplets hydrodynamically from the inside of the pipettes, the latter were
filled with Millipore water plus 150mM/l of NaCl and a small amount of agarose
to form a gel. Electrical contact was established by inserting a 500
/
m diameter
Ag wire into the pipette, which was chlorided before electrochemically with 3M
KCl. The droplets were pumped through the channel at a rate of about three droplets
per second. As seen from Fig.
2.9
a, b, the membranes survive their motion past the
inserted glass pipettes: coalescence is not induced. We found that the stability of
the membranes against such mechanical stress depended on the type of intruder.
While the glass pipette electrodes did not seem to affect the membranes in any way,
a tungsten wire (100
µ
µ
m diameter, sharpened to a 30
µ
m tip) induced immediate
coalescence.
