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Abstract:
This thesis presents experiments on lipid bilayer membranes and non-equilibrium phenomena
in active emulsion droplets. In the first part, we outline a concept of self assembled
soft matter devices based onmicro-fluidics, which use surfactant bilayer membranes
as their main building blocks. Membranes form spontaneously when suitable waterin-
oil emulsions are forced into micro-fluidic channels at high dispersed-phase volume
fractions and are remarkably stable even when pumped through the micro-fluidic channel
system. Their geometric arrangement is self-assembling, driven by interfacial energy
and wetting forces. The ordered membrane arrays thus emerging can be used to build
wet electronic circuitry, with the aqueous droplets as the ’solder points’. Furthermore, the
membranes can serve as well-controlled coupling media between chemical processes taking
place in adjacent droplets as is shown for the well-known Belousov-Zhabotinski reaction.
We also investigate the dynamics of the fusion of vesicles with bilayer membranes.
The particular process that we study is the fusion mediated by the SNARE-proteins embedded
in the membranes. It is shown that the electrostatic repulsion between the membranes,
due to the charged lipids that comprise them, blocks their fusion. Under such
conditions, the conformational change of the membrane protein Synaptotagmin-1, under
the influence of Ca2+ binding, restores membrane fusion. Thus we show in vitro, for the
first time, the massive increase in the membrane fusion due to Ca2+ triggering, as is the
case in vivo. Further, we present a propagation based x-ray phase contrast imaging to
study structure and interfacial properties of ultrathin model membrane systems.
A scheme of active self-propelled liquid micro-droplets which closely mimics the locomotion
of some protozoal organisms, so-called squirmers, is presented. In contrast to
other schemes proposed earlier, it is demonstrated that locomotion paths of the swimmers
are not self-avoiding, since the effect of the squirmer on the surrounding medium
is weak. Our results suggest that not only the velocity, but also the mode of operation
(i.e., the spherical harmonics of the flow field) can be controlled by appropriate variation
of parameters. We have studied experimentally the collective behaviour of such
self-propelling liquid droplets. We find strong polar correlation of the locomotion velocities
of neighboring droplets, which point to the formation of ordered rafts. This shows
that pronounced textures, beyond what has been seen in simulations so far, may show
up in crowds of simple model squirmers, despite the simplicity of their (purely physical)
mutual interaction. As such, the self propelled droplets are not restricted by the
classical equilibrium constraints such as the fluctuation dissipation theorem. We build a
correlation ratchet, which relies on a broken detailed balance, to demonstrate a passive
rectification scheme of a population of the swimmers. Finally, we study the collective
dynamics of a population of swimmers when they are confined to a single dimension, in
a setting similar to the well studied single-file diffusion. It is shown that when the short
time dynamics of the swimmer are ballistic, a transition to a diffusive behaviour is seen at
the long times and when the short time dynamics are diffusive, the long time dynamics
follow an anomalous diffusion law, as predicted by theory.
