Oscillating Droplets: Chemical Micro-Oscillators - Towards Autonomous Soft Matter Systems - page 64

self-organization. In spatially extended settings, the BZ reaction gives rise to com-
plex propagating wave patterns, which are reminiscent of spatio-temporal patterns
known from other catalytic systems driven far off thermal equilibrium [ 4 , 5 ]. Cou-
pled BZ oscillators have previously been studied in the laboratory with large reactors
connected directly by small channels for controlled mass exchange of bulk solution.
In this case, coupling occurs via all species. In living systems, however, coupling
often occurs through special signalling molecules, as in synaptic communication.
Collections of neural oscillators can access a vast repertoire of coordinated behavior
by utilizing a variety of topologies and modes of coupling, including gap junctions
and synaptic links, which may be either excitatory or inhibitory, depending on the
neurotransmitter involved. In small containers, however, where diffusive coupling
across the container is strong, the BZ reaction containers behave like single homoge-
neous oscillators. It then suggests itself to consider larger ensembles of such coupled
BZ oscillators as analogous to, e.g., systems of firing neurons.
It has been shown before that systems like that can be nicely realized by using
aqueous droplets containing the BZ educts as the oscillators, and coupling them via
an oil phase separating the droplets [ 6 ]. Both the promotor and the inhibitor are
sufficiently hydrophobic to enter the oil phase easily, such that they can diffuse from
one droplet to the other if they are sufficiently close. The delicate balance of promotive
and inhibitory coupling gives rise to a wealth of oscillation patterns [ 7 ]. They strongly
differ from the wave-like patterns which are well-known from continuous systems,
and are as yet not fully understood. In this chapter, we investigate whether we can
distinguish coupling through the bulk oil phase from coupling through a bilayer
membrane (formed between droplets as discussed in Chap. 2 ) , and to investigate
what types of coupling can be achieved solely via the latter.
5.2 Experimental Techniques
Our chemical oscillators are made from incorporating the BZ reaction mixture in
aqueous droplets in an external oil phase of squalane contaning mono-olein as sur-
factant. The droplets are for these experiments are made using a flow focussing
channel geometry in a PDMS microfluidic chip as shown in Fig. 5.1 . In order to
prevent any pre-reaction and the formation of unwanted gaseous bubbles of carbon-
di-oxide, the BZ reaction mixture is separated into two parts and they are combined
on chip. The two parts are created in stock with concentrations as follows: (i) 500mM
sulphuric acid (H 2 SO 4 ) and 280 mM sodium bromate (NaBrO 3 ) (ii) 300-800 mM
malonic acid (C 3 H 4 O 4 ) and 3 mM ferroin (C 36 H 24 FeN 6 O 4 S). The concentration of
the mono-olein in the squalane ranges between 25-100 mM.
As can be seen from the left panel of Fig. 5.1 , the BZ reaction consists of an
autocatalytic cycle in which the species HBrO 2 catalyses its own production via the
reduction of the ferroin catalyst which changes its colour rapidly from red to blue in
response. This is when the inhibitory cycle proceeds, leading to a slow production
of bromine which quenches the autocatalysis. The effect of this is a gradual change