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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