Biomedical Engineering Reference
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Figure 14.
Saturation voltage,
V
S
, and viscosity,
μ
, as a function of the number of carbon atoms,
n
C
, in the alkyl chain of the imidazolium cation for tetrafluoroborate ionic liquids. Adapted with
permission from Paneru
et al.
[51]. Copyright 2011 VSP.
curves retain their shape and behaviour—Figs 10 and 11, with minor changes re-
flecting the variation in material parameters.
D.2. Electrowetting Saturation
All ionic liquids studied electrowet in accordance with the Young-Lippmann equa-
tion and saturate—under DC voltage—at about the same contact angle (
50
◦
).
Therefore the saturation voltage,
V
S
, is very different. For the homologous tetraflu-
oroborates (shown in Fig. 8), the saturation voltage,
V
S
, drops linearly with the
length,
n
C
, of the alkyl chain of the imidazolium cation—Fig. 14.
As
n
C
increases, lower voltages produce the same contact angle change. How-
ever, the advantage of using imidazolium ionic liquids with longer side chains may
be outweighed by the logarithmic increase in viscosity (Fig. 14).
We have shown [2] that the saturation contact angle,
θ
S
, for solid-liquid-vapour
systems is often correctly estimated by equation (5). In the present case, for
the solid-ionic liquid-hexadecane system, the zero-interfacial-tension hypothesis is
written as:
∼
γ
S
/
HD
γ
IL
/
HD
.
cos
θ
S
=
(11)
The values estimated through (11) range between 72
◦
and 83
◦
(with the exception
of dmim.BF
4
,44
◦
) and are very different from the values found in our experi-
ments:
θ
S
=
15
◦
(when using AC voltage).
Therefore the zero-interfacial-tension hypothesis cannot be used directly for solid-
liquid-liquid systems. Intriguingly, the value estimated through the above equation,
approximates reasonably well the inflection point on the AC electrowetting curve
[49] (Fig. 7). The inflection point may indicate a transition between different charg-
ing mechanisms and therefore would reinforce the status of equation (11) as esti-
48
◦
(when using DC voltage) and
θ
S
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