Environmental Engineering Reference
In-Depth Information
For each of these particular domains, we have focused mostly on solutions that
have the potential to be applied in magnetic refrigeration. Since all these mecha-
nisms represent
uidic contact thermal switches, we will focus on those which
apply liquids, since they represent a better potential for high heat
fl
fl
uxes compared
to gases.
6.4.1 Electrohydrodynamics
6.4.1.1 Electrowetting
Electrowetting is a process in which the surface tension of the
fl
uid can be
manipulated by an externally applied electric
eld. This kind of process can
in
fl
uence the shape of a particular drop, as well as activate the drop
'
s movement.
The latter we denote as electrocapillary
fl
ow.
Electrowetting as the Manipulation of the Static Thermal Diode Mechanism
The electrowetting effect was
rst described by Lippmann [
88
]. He applied his
ndings in the development of a capillary electrometer and several other applica-
tions. It was not until 1993 that Berge [
89
] suggested that an insulating layer
between the electrodes and the
uid should be applied to prevent the electrolysis.
The electrowetting process can be divided into four main mechanisms [
90
]:
fl
The electrocapillary effect (we will consider this as electrowetting electro-
capillary
fl
ow),
electrowetting on a dielectric (EWOD),
electrowetting on insulator-coated electrodes (EICEs),
(spontaneous) electrowetting on line electrodes (ELEs).
In Fig.
6.15
we show the EICE and EWOD mechanisms. In the EICE
(Fig.
6.15
a) a coating insulating
lm with a thickness from a few
μ
m to about
200
m is deposited on the surface of the electrode. The electrostatic potential is
applied between the electrode and the drop, which is on the insulator. In Fig.
6.15
b
the EWOD electrowetting is provided by the dielectric
μ
lm that is coated on the top
and the bottom of the electrodes. The result is the alternation of the static changes in
the liquid
solid contact angle. With an increase in the voltage, the contact angle is
-
reduced.
An example of an electrowetting EWOD application in magnetic refrigeration is
shown in Fig.
6.16
. In this particular case the Brayton-type magnetic refrigeration
cycle is considered. The electrodes, which are separated (not shown in the
gure),
serve for the alternation of the voltage and thus control the manipulation of the
surface tension of electrolyte liquid drops. The microchannels serve as the
expanded surface for the heating and cooling of the working
fl
uid, which then
fl
ows
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