Electrowetting of Ionic Liquids in Solid–Liquid–Liquid Systems - Drops and Bubbles in Contact with Solid Surfaces - page 261

Figure 15. Characteristic spreading ( 2 ) and retraction ( ! ) time, τ , as a function of the viscosity, μ ,
of the Rmim.BF 4 ionic liquid. Adapted with permission from Paneru et al. [51]. Copyright 2011 VSP.

interface, achieved under a given voltage, is easier to establish than to destroy.
This was confirmed in experiments with a glycerol-water mixture of a viscosity
160 mPa s (similar to that of bmim.BF 4 ). The characteristic time was very similar
( τ 20 ms) and identical during spreading and retraction. It should be noted that

the behaviour of ionic liquids near a charged wall is non-trivial [76, 77] and rather
poorly understood.
The above discussion suggests that spreading dynamics is dominated by vis-
cous dissipation. We now consider the influence of the contact line friction. The
dependence, usually considered in wetting dynamics, is the relation between dy-
namic contact angle, θ , and speed of the contact line, u . The instantaneous dynamic
contact angle was calculated from the base area, A , and the fixed droplet volume as-
suming the droplet shape was spherical. The speed of the contact line was calculated
from the derivative of the base area: u

≈

] − 1 d A/ d t . The results
obtained for a bmim.BF 4 droplet spreading under constant DC voltage, exceeding
the saturation voltage, are presented in Fig. 16.
The format of the graphs is chosen so that a linear dependence should be seen if
equations (6) and (8) are obeyed. In line with previous work [58, 91], we find that
each of the models fits only a portion of the velocity dependence. Up to a speed of
about 0.04 m/s, the dynamic contact angle is well described by the hydrodynamic
description. At speeds higher than about 0.01 m/s, the molecular-kinetic description
is more adequate. The slope of the hydrodynamic line is about four times smaller
than the one estimated through equation (6) but this level of discrepancy is rather
common [52]. From a hydrodynamic point of view, the viscosity of both advanc-
ing and receding liquids should be taken into account [54]. However, bmim.BF 4 is
quite viscous and the viscosity ratio, m (

A) 1 / 2

= =[ π

d x/ d t 2 (

= μ HD /μ IL = 0 . 02) is in effect the same

as for a water droplet spreading in air ( m = μ air /μ water = 0 . 022). The molecular