Electrowetting of Ionic Liquids in Solid–Liquid–Liquid Systems - Drops and Bubbles in Contact with Solid Surfaces - page 257

Ta b l e 2 .
Maximum surface charge, σ max , in various physical situations ( L (e/σ max ) 1 / 2 = is the average dis-

tance between two surface charges)

Phenomenon System σ max L

mC/m 2 nm

Air ionisation Flat insulating surface in dry air 0.03 [75] 78

Electrowetting (SLV) 0.1 M aqueous KCl on AF1600 in air 0.25 [18] 25

Electrowetting (SLL) bmim.BF 4 on AF1600 in hexadecane 0.5 [50] 18

Charge Injection Polystyrene and Chlorinated 1.7 [74] 10

Polyethylene in 10 − 2 M aqueous HCl

Electrokinetics Neutral polymers in aqueous salt solutions 4.8 [73] 6

(streaming potential)
Electrokinetics Teflon AF1600 in 10 − 3 M aqueous KCl 5 [61] 6

(streaming potential
and streaming current)

All of our results are well-described by the Young-Lippmann equation (4). The
solid lines shown in Figs 6-11, are the least-squares fits obtained by using the equa-
tion (4) in the form cos θ = cos θ 0 + α(εε 0 /γd)V 2 , with θ 0 and α taken as fitting

parameters. The factor α is obtained since all quantities in the electrowetting term
are known. The closeness of α to

1

2 is a good indicator of the quality of the exper-
imental data [2, 17, 18]. Thus the Young-Lippmann equation provides a consistent
description of the electrowetting curve as long as .

It is informative to estimate the surface charge density, σ , at the solid surface.
The limiting value of σ is σ max =

| V | | V S |

CV S , and is of the order of 0.5 mC/m 2 (Table 2).
This value is close to the one estimated for Teflon AF1600-aqueous 0.1 M KCl-air
systems [18]. It is about one order of magnitude smaller than values typically en-
countered in electrokinetic studies of polymers surfaces [61, 73] or charge injection
measurements [74]. It is also one order of magnitude larger than the limiting surface
charge possible in air [75].
This rather low surface charge density corresponds nicely with the fact that the
electrowetting curves presented here do not show any unusual or asymmetric be-
haviour with respect to the Young-Lippmann equation [17]. The average distance
between two surface charges, L (Table 2) is much larger than the size of the charge
carrier. In other words, the charges accumulated on the surface can be considered
as randomly distributed point charges.
The experiments with bmim.BF 4 -water mixtures provide another insight into the
mechanism of electrowetting. When changing the composition of the mixture, the
conductive droplet varies from concentrated electrolyte (i.e., ionic liquid) to dilute
electrolyte. The interfacial structure and electrochemical behaviour of dilute and
solvent-free electrolytes are very different [76, 77]. Nonetheless the electrowetting