Chemistry Reference
In-Depth Information
Fig. 5.9
Formation of a target pattern in a 'island' or 'peninsula' type of droplet network.
Left
A
schematic of a hexagonal arrangement of droplets which form an 'island'. At the outer edges of
the structure, the excitatory and the inhibitory components are lost to the oil phase (shown by the
arrows
), while in the center, they are concentrated leading to the formation of pacemaker center.
Right
A target pattern develops within a 'peninsula' of droplet oscillators. The excitation and wave
pattern are shown in
blue
for easy visualization. The scale bar is 500
µ
a similar structure and is connected by a narrow bridge of one or two droplets to
a neighbouring 'island'. As we discussed before, the inhibitory (bromine) and the
excitatory (BrO
·
2
) components of the BZ reaction readily diffuse into the external
oil phase, where they are trapped by the surfactant. Therefore, at the edges of the
island, the oscillatory droplets lose their inhibitory and excitatory components to
the external oil phase. However, at the center of the island, the concentration of the
BZ species increases since they come in from all sides. Depending on the relative
concentrations of the inbitory and excitatory components, the center droplet can
therefore either 'turn off' (i.e. oscillations are inhibited) or trigger an oscillation. For
the malonic acid concentration of 500 mM together with the other concentrations as
described in the experimental section, we find that an oscillation is triggered in the
central droplet as can be seen in the right panel of Fig.
5.9
. This trigger from the
central droplet then propagates outward as a target wave throughout the 'island'. If
it is a 'peninsula' the connecting bridge can couple the wave to the neighbouring
'island' as well. This pattern then repeats periodically.
Next, we sought if we could induce the pacemaker patterns by confining the
oscillators droplets with a channel made of PDMS. In such a scenario as shown in
Fig.
5.10
, the PDMS walls of the channel, in addition to the oil phase act as sinks for
the BZ reaction species. Therefore, we expect that along the length of the channel,
multiple pacemaker centers form, each triggering target waves. That this is indeed
the case, can be seen in the right panel of Fig.
5.10
. The triggering of a wave from a
core can be see in the image sequence shown. In addition, a wave can be seen coming
in from the right of the images, clearly triggered by a pacemaker upstream in the
channel. Indeed there were also waves coming in the from the left side, but not are
not shown here.
Next, for the same concentrations, we looked at a very large network of hexago-
nally packed droplet oscillators, such that the edges are too far away from the cores
to have a significant impact. This is shown in Fig.
5.11
. In such a case, we see the
spontaneous emergence of travelling waves across the network. Indeed, it may be
