Special Heat Transfer Mechanisms: Active and Passive Thermal Diodes - Magnetocaloric Energy Conversion: From Theory to Applications - page 233

Fig. 6.17 Examples of the micro fl uidic electrocapillary fl ow

relatively high electrical conductance of the uid system, which can lead to high

currents and interactions between the neighbouring channels.
The application of the electrowetting principle in capillary

fl

ow seems to be a
much better solution for magnetic refrigeration. Figure 6.17 c shows the application
of the EWOD principle for the capillary

fl

fl ow of the droplet. In this particular case

the electrode s array patterns can be created by photolithography on, e.g. the glass

substrate [ 92 , 94 ]. A constant ground potential is applied to the droplet of the
polarizable and/or conductive liquid, which is embodied between the bottom plate
of electrodes and a top plate. On the bottom, the electrodes are covered by the
insulating layer in order to prevent the electrical current from passing through the
droplet. Both surfaces on the top and bottom are covered by the hydrophobic
coating. Usually, the

'

ller oil (e.g. silicone oil) surrounds the droplet; however, this
is not necessary in all applications. There are two aims of using the ller liquid. One

'

is to support the droplet s motion mechanism and the other is to prevent the

evaporation of the droplet [ 92 ]. The oil

- water surface tension gradient induces the

fl ow of the droplet.

An example of applying the EWOD principle or the manipulation of a drop s

movement in magnetic refrigeration is shown in Fig. 6.18 . In this particular case,
two drops are applied with their movement, which depends on a certain thermo-
dynamic process in the magnetic refrigeration (e.g.

'

the Brayton refrigeration