Electrowetting has become one of the most widely used tools for manipulating tiny amounts of liquids on surfaces. Applications range from 'lab-on-a-chip' devices to adjustable lenses and new kinds 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 contact angle reduction and 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?are 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 give rise not only 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, fibre optics, and biotechnology-related microfluidic devices.

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Electrowetting: from basics to applications

Advanced functional polymer membranes

We describe devices in which optics and fluidics are used synergistically to synthesize novel functionalities. Fluidic replacement or modification leads to reconfigurable optical systems, whereas the implementation of optics through the microfluidic toolkit gives highly compact and integrated devices. We categorize optofluidics according to three broad categories of interactions: fluid–solid interfaces, purely fluidic interfaces and colloidal suspensions. We describe examples of optofluidic devices in each category.

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Developing optofluidic technology through the fusion of microfluidics and optics

The realization of miniaturized optofluidic platforms offers potential for achieving more functional and more compact devices. Such integrated systems bring fluid and light together and exploit their microscale interaction for a large variety of applications. The high sensitivity of compact microphotonic devices can generate effective microfluidic sensors, with integration capabilities. By turning the technology around, the exploitation of fluid properties holds the promise of highly flexible, tunable or reconfigurable microphotonic devices. We overview some of the exciting developments so far.

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Integrated optofluidics: A new river of light

The trend towards miniaturization in optical imaging, diagnostics and lab-on-a-chip technology is creating a demand for sophisticated microlenses. A new type of smart liquid microlens has been developed that differs from most current devices in that it is self-focusing. The key component is a stimuli-responsive hydrogel integrated into a microfluidic system and acting as a container for a liquid droplet. The hydrogel simultaneously senses stimuli and actuates a change in droplet shape — and hence focal length. Stimuli can include biological and chemical agents and physical parameters. At this micrometre scale, pinned liquid-liquid interfaces are used to attain stable devices, and response times of ten to a few tens of seconds. Lenses can have virtually any focal length and are readily integrated into arrays. Despite its compactness, the human eye can easily focus on different distances by adjusting the shape of its lens with the help of ciliary muscles1. In contrast, traditional man-made optical systems achieve focusing by physical displacement of the lenses used. But in recent years, advances in miniaturization technology have led to optical systems that no longer require complicated mechanical systems to tune and adjust optical performance. These systems have found wide use in photonics, displays and biomedical systems. They are either based on arrays of microlenses with fixed focal lengths2,3,4,5, or use external control to adjust the microlens focal length6,7,8,9,10,11,12. An intriguing example is the tunable liquid lens, where electrowetting or external pressure manipulates the shape of a liquid droplet and thereby adjusts its optical properties. Here we demonstrate a liquid lens system that allows for autonomous focusing. The central component is a stimuli-responsive hydrogel13 integrated into a microfluidic system and serving as the container for a liquid droplet, with the hydrogel simultaneously sensing the presence of stimuli and actuating adjustments to the shape—and hence focal length—of the droplet. By working at the micrometre scale where ionic diffusion and surface tension scale favourably14, we can use pinned liquid–liquid interfaces to obtain stable devices and realize response times of ten to a few tens of seconds. The microlenses, which can have a focal length ranging from -∞ to +∞ (divergent and convergent), are also readily integrated into arrays that may find use in applications such as sensing, medical diagnostics and lab-on-a-chip technologies15,16,17,18,19.

Adaptive liquid microlenses activated by stimuli-responsive hydrogels

The meniscus between two immiscible liquids can be used as an optical lens. A change in curvature of this meniscus by electrowetting leads to a change in focal distance. It is demonstrated that two liquids in a tube form a self-centered lens with a high optical quality. The motion of the lens during a focusing action was studied by observation through the transparent tube wall. Finally, a miniature achromatic camera module was designed and constructed based on this adjustable lens, showing that it is excellently suited for use in portable applications.

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Variable-focus liquid lens for miniature cameras

A variable focus lens comprising a first fluid (A) and a second, non-miscible, fluid (B) in contact over a meniscus. A first electrode (2) separated from the fluid bodies by a fluid contact layer (10), and a second electrode (12) in contact with the first fluid to cause an electrowetting effect whereby the shape of the meniscus is altered. The fluid contact layer has a substantially cylindrical inner wall.

Variable focus lens

Development and applications of very high flux microfiltration membranes

The meniscus between two immiscible liquids of different refractive indices can be used as a lens. A change in curvature of this meniscus by electrostatic control of the solid/liquid interfacial tension leads to a change in focal distance. It is demonstrated that two liquids in a tube form a self-centred variable-focus lens. The optical properties of this lens were investigated experimentally. We designed and constructed a miniature camera module based on this variable lens suitable for mobile applications. Furthermore, the liquid lens was applied in a Blu-ray Disc optical recording system to enable dual layer disc reading/writing.

Stein Kuiper

Papers

Variable-focus liquid lens for miniature cameras

Variable focus lens

Development and applications of very high flux microfiltration membranes

Electrowetting-Based Variable-Focus Lens for Miniature Systems

Resistless patterning of sub-micron structures by evaporation through nanostencils