Chemistry Reference
In-Depth Information
Fig. 2.1
Sketch of a concept combining top-down lithographic design of liquid channel structures
(
left
,
grey
pattern) with complex gel emulsions forming self-assembled droplet and membrane
arrangements within these channels (
right
, shown in a two-step magnifying glass style to bridge the
differences in length scales). The active components can be amphiphilic molecules or complexes
residing in the bilayer membranes forming between adjacent droplets, and carefully chosen contents
of the droplets themselves
of the desired geometry to subtly 'convince' the soft components to assemble the
way one would like them to as envisaged in Fig.
2.1
.
The concept which thus emerges, and on which we will dwell in the present
chapter, is as follows. We create a system of microfluidic channels, compartments,
posts, orifices and so on, on length scales which are well manageable by conven-
tional top-down lithography. This system is then filled with soft molecular materials,
which self-assemble within the prescribed geometry, forming well-defined structures
down to much smaller scales. To be more explicit, we envisage the use of mono-
disperse water-in-oil emulsions, with a small volume fraction of the oil phase and
a suitable surfactant (e.g., a lipid) for stabilization. This type of emulsions, where
the continuous phase is very dilute, are commonly called gel-emulsions [
9
]. In an
externally predefined channel geometry, the water droplets will form well-oriented
crystalline arrangements [
10
,
11
]. Under suitable conditions, the interfaces of every
two adjacent droplets will form molecular surfactant bilayers. These can serve, by
means of dispersion forces and wetting, as nanoscopic tweezers for holding active
components, such as ion channels or smaller molecules with non-trivial electronic
properties. For molecular electronic circuitry, the water droplets take the role of the
'solder points'.
It is thus inherent to the concept outlined above that we are using self-assembly
processes on two completely different scales: the scale of the droplets within the
prescribed geometry of the micro-fluidic channel structures, and the nanoscopic scale
of the membrane thickness and the functional molecular building blocks. On both
scales, self-assembly is then purely driven by wetting forces. As will be discussed
below, these forces can not only be used for maintaining certainmembrane structures,
but also for their controlled manipulation.
