Chemistry Reference
In-Depth Information
The resulting Marangoni stresses due to the asymmetry of the droplet contact with
the substrate propels the droplet forward. Such droplets were also caused to move
on a liquid interface due to the differential adsorption of surfactants on the droplet
interface [
13
]. Another kind of strider is a liquid droplet on a hot plate whose surface
has a ratcheted step geometry [
14
]. These are propelled by the flow of air between
the droplet and the surface caused by the Leidenfrost effect [
15
]. Various other such
examples of striders such as millimetric camphor and phenanthroline disks on liquid
surfaces have also been realised [
16
-
18
].
Finally, swimmers are objects that are completely immersed in a fluid and therefore
have a hydrodynamic flow field associated with their propulsion. Colloidal janus
beads or rods, half covered with platinum, can be propelled by immersing them
in H
2
O
2
[
19
]. The platinum catalyzes H
2
O
2
H
2
O, and the resulting osmotic
gradients from the asymmetry leads to directed motion. An artificial flagellum in
which a red blood cell attached to paramagnetic colloidal beads via DNA linkers
can be propelled by an externally applied time varying magnetic field [
20
]. Finally,
modifications of a surfactant at an oil-water interface, of a droplet, either by chemical
[
21
] or photochemical [
22
] means leads to gradients of interfacial tension around the
droplet, thus setting it in motion. All together, a general principle seems to work in
the realizations of artificial SPPs i.e. polarity together with a dissipative mechanism
leads to motility.
As fas as interactions are concerned, shaken granular media interact by contact and
excluded volume interactions alone. Swimmers have a well defined hydrodynamic
flow field that emerges due to their motion and can cause longe range interactions
mediated by the fluid surrounding them, in addition to excluded volume interactions.
Strider interactions can be quite complicated and depend on the surface/ interfacial
properties significantly.
The squirming mechanism of swimming is very appealing for elucidating the role
of hydrodynamic interactions, since the velocities in the near and far field around
such a swimmer can be well described analytically [
23
,
24
]. This makes such swim-
mers ideally suited for the quantitative study of many open questions regarding the
hydrodynamic effects on their interaction with surfaces and with each other, their
behavior in external flow, and the origins of coupled and collective behaviour [
6
,
7
].
In this chapter, we present a simple model artificial swimmer that consists of
an aqueous droplet moving in an oil 'background' phase. Propulsion arises due to
the spontaneous symmetry breaking caused by the reaction of the surfactant at the
droplet interface with the contents of the droplet. The reaction at the interface leads
to a dynamic instability promoting gradients of interfacial tension around the droplet.
As we demonstrate, this results in propulsion of the droplet due toMarangoni stresses.
We study the behavior of the self propelled droplets in quasi one and two dimensional
environments. Further, we measure the flow fields around such a swimmer and show
that they are similar to those due to squirmers.
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