Chemistry Reference
In-Depth Information
Fig. 6.1
The three classes of SPPs
a
Shaken granular media, is comprised of an asymmetric unit
vibrated on (or between two) plates resulting in a quasi two dimensional motion (
black lines
in right
image represent object paths)
b
Striders, which move on a surface via modification of the contact
properties of the SPP and surface
a
Swimmers, which move in a surrounding fluid, create well
defined hydrodynamic flow fields (
black lines
) around them. In all figures,
black arrows
indicate
direction of motion
and Volvox belong to a class of swimmers referred to as squirmers and are driven by
tangential and/or radial deformations of the cell surface [
5
-
7
].
Avariety of artificial self propelled particles (SPPs) have also been developed over
the years. In the context of the present work, these can be classified as: shaken granular
media, striders, and swimmers as depicted in Fig.
6.1
. As the name suggests, shaken
granular media consists of beads and rods agitated by mechanical shaking. These
were among the first to be studied in the context of self propelled objects. Yamada,
Hondou, and Sano did pioneering experiments [
8
] in demonstrating non-Brownian
motion of axisymmetric polar objects vibrated between two plates. Asymmetries
in the granular unit, either in the form of an asymmetric weight distribution [
9
]or
asymmetric frictional properties [
10
,
11
] lead to a directed motion under agitation.
Striders are such SPPs that move on a surface or at an interface and the motion
mechanism relies on the presence of a surface. Self propelled liquid droplets on sur-
faces have also received considerable attention. The first [
12
] involves a chemically
modified substrate causing variations in the contact angle of a droplet on the surface.
