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Electrowetting: from basics to applications
Frieder Mugele, Jean-Christophe Baret
To cite this version:
Frieder Mugele, Jean-Christophe Baret. Electrowetting: from basics to applications. Journal
of Physics: Condensed Matter, IOP Publishing, 2005, 17 (28), pp.R705-R774. �10.1088/0953-
8984/17/28/R01�. �hal-02148730�
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J. Phys. Condensed Matter: TOPICAL REVIEW
Electrowetting:
from Basics to Applications
Frieder Mugele
1,*
and Jean-Christophe Baret
1,2
1: University of Twente; Faculty of Science and Technology; Physics of Complex
Fluids; P.O. Box 217; 7500 AE Enschede (The Netherlands)
2: Philips Research Laboratories Eindhoven; Health Care Devices and Instrumentation;
WAG01; Prof. Holstlaan 4; 5656 AA Eindhoven (The Netherlands)
*: corresponding author
phone: ++31 / 53 489 3094; fax: ++31 / 53 489 1096; email: f.mugele@utwente.nl
2
Abstract. Electrowetting has become one of the most widely used tools to manipulate
tiny amount of liquids on surfaces. Applications range from lab-on-a-chip devices to
adjustable lenses or new types of electronic displays. In the present article, we review the
recent progress in this rapidly growing field including both fundamental and applied
aspects. We compare the various approaches used to derive the basic electrowetting
equation, which has been shown to be very reliable as long as the applied voltage is not
too high. We discuss in detail the origin of the electrostatic forces that induce both the
contact angle reduction as well as the motion of entire droplets. We examine the
limitations of the electrowetting equation and present a variety of recent extensions to the
theory that account for distortions of the liquid surface due to local electric fields, for the
finite penetration depth of electric fields into the liquid, as well as for finite conductivity
effects in the presence of AC voltage. The most prominent failure of the electrowetting
equation, namely the saturation of the contact angle at high voltage, is discussed in a
separate section. Recent work in this direction indicates that a variety of distinct physical
effects - rather than a unique one – is responsible for the saturation phenomenon,
depending on experimental details. In the presence of suitable electrode patterns or
topographic structures on the substrate surface, variations of the contact angle can not
only give rise to continuous changes of the droplet shape, but also to discontinuous
morphological transitions between distinct liquid morphologies. The dynamics of
electrowetting are discussed briefly. Finally, we give an overview of recent work aimed
at commercial applications, in particular in the fields of adjustable lenses, display
technology, fiber optics, and biotechnology-related microfluidic devices.
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1. Introduction
Miniaturization has been a technological trend for several decades. What started
out initially in the microelectronics industry has long reached the area of mechanical
engineering, including fluid mechanics. Reducing size has been shown to allow for
integration and automation of many processes on a single device giving rise to a
tremendous performance increase, e.g. in terms of precision, throughput, and
functionality. One prominent example from the area of fluid mechanics are Lab-on-a-
Chip systems for applications such as DNA- or protein analysis, and biomedical
diagnostics [1-3]. Most of the devices developed so far are based on continuous flow
through closed channels that are either etched into hard solids such as silicon or glass, or
replicated from a hard master into a soft polymeric matrix. Recently, devices based on the
manipulation of individual droplets with volumes in the range of nanoliters or less have
attracted increasing attention [4-10].
From a fundamental perspective the most important consequence of
miniaturization is a tremendous increase in the surface-to-volume ratio, which makes the
control of surfaces and surface energies one of the most important challenges both in
microtechnology in general as well as in microfluidcs. For liquid droplets of
submillimeter dimensions, capillary forces dominate [11, 12]. The control of interfacial
energies has therefore become an important strategy to manipulate droplets at surfaces
[13-17]. Both liquid-vapor and solid-liquid interfaces have been influenced in order to
control droplets, as recently reviewed by Darhuber and Troian [15]. Temperature
gradients as well as gradients in the concentration of surfactants across droplets give rise
to gradients in interfacial energies, mainly at the liquid-vapor interface, and thus produce
forces that can propel droplets making use of the thermocapillary and Marangoni effects.
Chemical and topographical structuring of surfaces has received even more
attention. Compared to local heating, both of these two approaches offer much finer
control of the equilibrium morphology. The local wettability and the substrate topography
together provide boundary conditions within which the droplets adjust their morphology
to reach the most energetically favorable configuration. For complex surface patterns,
however, this is not always possible as several metastable morphologies may exist. This
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can lead to rather abrupt changes in the droplet shape, so-called morphological
transitions, when the liquid is forced to switch from one family of morphologies to
another by varying a control parameter, such as the wettability or the liquid volume [13,
16, 18-20].
The main disadvantage of chemical and topographical patterns is their static
nature, which prevents active control of the liquids. Considerable work has been devoted
to the development of surfaces with controllable wettability – typically coated by self-
assembled monolayers. Notwithstanding some progress, the degree of switchability, the
switching speed, the long-term reliability, and the compatibility with variable
environments that have been achieved so far are not suitable for most practical
applications. In contrast, electrowetting (EW) has proven very successful in all these
respects: contact angle variations of several tens of degrees are routinely achieved.
Switching speeds are limited (typically to several milliseconds) by the hydrodynamic
response of the droplet rather than the actual switching of the equilibrium value of the
contact angle. Hundreds of thousands of switching cycles were performed in long term
stability tests without noticeable degradation [21, 22]. Nowadays, droplets can be moved
along freely programmable paths on surfaces, they can be split, merged and mixed with a
high degree of flexibility. Most of these results were achieved within the past five years
by a steadily growing community of researchers in the field [23].
Electrocapillarity, the basis of modern electrowetting, was first described in detail
in 1875 by Gabriel Lippmann [24]. This ingenious physicist, who won the Noble prize in
1908 for the discovery of the first color photography method, found that the capillary
depression of mercury in contact with electrolyte solutions could be varied by applying a
voltage between the mercury and electrolyte. He formulated not only a theory of the
electrocapillary effect but developed several applications, including a very sensitive
electrometer and a motor based on his observations. In order to make his fascinating
work, which has only been available in French up to now, available to a broader
readership, we included a translation of his work in the Appendix of this review. The
work of Lippmann and of those who followed him in the following more than hundred
years was devoted to aqueous electrolytes in direct contact with mercury surfaces or
mercury droplets in contact with insulators. A major obstacle to broader applications was
![Figure 10. Pellat experiment: Electrowetting-induced capillary rise. a) Schematic setup and electric equivalent circuit. b) Frequency dependence of K(w) for DI water, mannitol, and KCl. Reprinted with permission from ref. [40].](/figures/figure-10-pellat-experiment-electrowetting-induced-capillary-2c7i9bse.webp)
