Electrowetting of Ionic Liquids in Solid–Liquid–Liquid Systems - Drops and Bubbles in Contact with Solid Surfaces - page 247

though the activation energy related to k 0 can be reinterpreted to account explicitly
for the viscosity of the liquid ( k 0 = k B T/μλ 3 exp (

−

G/k B T) [57]). By express-
ing the contact line friction coefficient, B , through a diffusion coefficient, D ,the
following estimate for the molecular-kinetic dissipation, D MK , is obtained:

k B T
D

1 k B T

Bu 2

λ u 2 k 0 λ 3 u 2 .

D MK = fu = = = (9)

It is physically clear that both viscous and molecular dissipation should be con-
sidered [58, 59], but most often the hydrodynamic and the molecular-kinetic ap-
proaches are treated as excluding alternatives. Models combining different dissipa-
tion channels tend to be unreliable or intractable [52, 58] and have not gained wider
acceptance.
In this work we used a range of ionic liquids (1-alkyl-3-methyl-imidazolium
tetrafluoro-borates, Rmim.BF 4 , 1-butyl-3-methyl-imidazolium ionic liquids with
various anions, bmim.X, and bmim.BF 4 -water mixtures) to electrowet a Teflon
AF1600-coated electrode, immersed in n -hexadecane. The solid-liquid-liquid sys-
tems showed robust electrowetting behaviour: very large contact angle variations
(up to 130 ◦ ), excellent reversibility and insignificant contact angle hysteresis. Elec-
trowetting curves obtained with DC and AC voltages were similar and overlapping
but lower contact angles could be achieved with AC voltage. The spreading and
retracting of the ionic liquid droplet under electrowetting conditions was very fast
and involved both viscous and molecular dissipation.

B. Materials and Methods
B.1. Insulator

Teflon AF1600 (an amorphous perfluorinated polymer available from DuPont)
was dip-coated on ITO-coated glass slides. AF1600 is a random copolymer of
2,2-bis(trifluoromethyl)-4,5-difluoro-1,3-dioxole and tetrafluoroethylene in a ra-
tio PDD:TFE

=

2:1. It is soluble in perfluorinated solvents (e.g., 3M Fluoroinert
FC-75) and therefore can be coated on various surfaces. It is very popular in elec-
trowetting studies either as a thin hydrophobic layer on top of an insulating coating
[1, 60] or as a hydrophobic insulator [2, 18, 27, 48-50].
In aqueous solutions, at normal pH, Teflon AF1600 acquires spontaneously a
negative charge. The reasons for that are passionately debated but the mechanism
appears to be specific adsorption of OH − ions at the hydrophobic interface [61,
62]. The zeta potential, ζ , as a function of bulk pH for both AF1600 and PTFE
is shown in Fig. 3. The trend is very similar for both polymers and their isoelec-
tric points ( ζ 4). However, the magnitude of the negative

potentials is significantly larger in the case of AF1600. This can be attributed to
the oxygen present in the dioxole monomer (PDD). This oxygen has been detected
by surface spectroscopies, such as Secondary Ion Mass Spectroscopy (SIMS) [18]
and X-ray Photoelectron Spectroscopy (XPS) [63], and therefore it resides in close

= 0) are identical (pH =