Electrowetting of Ionic Liquids in Solid–Liquid–Liquid Systems - Drops and Bubbles in Contact with Solid Surfaces - page 245

undergoing multiple-step manipulation on a single chip [34]. The ambient oil sig-
nificantly reduces biofouling in microfluidics and is of major importance in this
field [14, 34]. Static and transient capacitance measurements have demonstrated
convincingly the presence a wetting film of oil [14]. Berry and co-workers stud-
ied the electrowetting of aqueous droplets containing salt and surfactant in alkanes
on an amorphous fluoropolymer surface [22-24]. Electrowetting experiments have
been performed with mercury in salty water on mica [35]. Staicu and Mugele [36]
studied the dynamic entrapment of an oil film between an aqueous droplet and the
insulating polymer layer. The film is unstable and breaks into droplets. The same
effect provides opportunities to optimize the design of electrowetting display pixels
[11].

A.4. Conductive Liquids

Ionic liquids are a new class of solvents made widely available in recent years [37-
40]. These are organic salts with relatively low melting points (usually

100 ◦ C)
due to the different size of the ions. Their fluidity and good thermal stability are
attractive properties. The volatility of ionic liquids is extremely low (they can be
probed with vacuum-based techniques, e.g., X -ray photoelectron spectroscopy [41,
42]) and therefore exclusively suitable for applications where evaporation is unde-
sirable. Many ionic liquids can be synthesized and this chemical diversity offers
numerous options to tailor their properties [43]. On the other hand minor com-
ponents, such as water or halides, and other additives can affect their properties
considerably. The surface tension of ionic liquids is intermediate between alka-
nes and water, i.e., similar to that of polar molecular solvents [44]. The toxicity
of ionic liquids is largely unexplored [45]. Ionic liquids are conductive though
their conductivity is somewhat reduced by their viscosity (typically 50-500 times
higher than the viscosity of water). The concentration of ions is naturally very high
though ionicity can vary widely [46]. Ionic liquids are often stable within a wide
range of potentials [47] and this makes them excellent electrowetting agents. We
showed that ionic liquids can electrowet fluoropolymer surfaces in air, though not
very efficiently [48]. More recently, we have focussed on the solid-liquid-liquid
electrowetting of ionic liquids on a fluoropolymer surface in ambient hexadecane
[49-51].

A.5. Wetting Dynamics

One major advantage of electrowetting over other actuation mechanisms is that it
is fast. The kinetics of spreading and retraction of the conductive droplet is of key
importance to achieve controlled manipulation. In the electrowetting experiment
(Fig. 1), when voltage is switched on, the wettability of the solid substrate is im-
proved and the droplet spreads until the macroscopic contact angle reaches its final
value. When the voltage is switched off, the original wettability is restored, and the
droplet retracts back to its initial position. During the transition between these two