Special Heat Transfer Mechanisms: Active and Passive Thermal Diodes - Magnetocaloric Energy Conversion: From Theory to Applications - page 240

6.4.2.2 Ferrofluid Thermal Diode that Applies the Magnetically
Induced Fluid Flow

This kind of principle can be driven by a combination of temperature and magnetic

eld gradients. Since this principle can also be applied for magnetocaloric fl uids, it

is described in detail in the chapter on magnetocaloric fl uids. Another principle is to

manipulate the ferro fl uid fl ow by a magnetic eld. One of such solutions is pre-

sented in Fig. 6.22 .

6.4.2.3 Ferro uid Thermal Diode that Acts as the Thermal

Contact Switch

fl

Manipulation of the ferro eld can lead to migration

of the drop to the desired heated or cooled surface. This mechanism is similar to
magnetowetting, where the surface tension of the ferro

fl uid drop fl ow with a magnetic

fl uid drop is manipulated by

the magnetic eld.

6.4.2.4 Ferro fl uid and Magnetowetting Principle

The magnetowetting principle is similar to the electrowetting principle; however,
here the manipulation of the surface tension of the ferro fl uid drop is, in most cases,

based on the application of a magnetic eld [ 110 , 115 119 ]. This can lead to an

-

ef uid switch. According to Nguyen

[ 110 ], there are different principles for magnetowetting: magnetowetting in a uni-
form magnetic

cient application of the thermal contact ferro fl

eld; magnetow-
etting with a diamagnetic droplet and the magnetically controllable surface of
magnetic nanotubes; the sliding motion of a ferro

eld; magnetowetting in a nonuniform magnetic

fl uid drop; and the rolling motion

of a ferro uid covered by hydrophobic particles. Figure 6.23 shows examples of

electrowetting with a uniform and nonuniform magnetic

fl

eld. In this case the

dashed lines show the initial state of the ferro fl uid drop. When applying the uniform

eld over the drop, its surface tension will change from the angle h 0 to the
angle h . If the drop was placed between two surfaces, such a manipulation could
lead to the migration of the drop from the bottom to the upper surface and vice
versa. The non-uniform

magnetic

uid drop, will lead to better
wetting ( h 0 [ h ). Also in this case, the drop can act as a thermal contact switch, or
when placed between two surfaces, both being manipulated with the magnetic

eld, when applied to the ferro fl

eld,

such a drop would transport heat from one plate to another.
The magnetowetting principle can also be associated with the Rosensweig
instability [ 101 , 108 110 ], which presents a multiple-peaked structure (like a

-

hedgehog) of the ferro fl uid when subjected to a magnetic eld.