Photograph: Saskia van Lijnschooten.

Abstract

Imagine a drop of water lying on a surface, pulled into a ball by surface tension. With electricity it is possible to change the shape of the drop and cause it to flatten out. This is electrowetting, a physical phenomenon which has aroused great interest in recent years as it has found new applications. Here we will describe the phenomenon and some of its applications, particularly the application to electric paper – a thin, flexible surface for the display of video and text – a long awaited technological aim.

The physics of wetting

Go into the kitchen and do a small experiment: place a drop of water on a smooth, clean glass surface (a plate, for example), and another drop on a Teflon frying pan or on greased baking paper. You will be able to see the difference in the behavior of the drops: on the glass the drop flattens out whereas on the Teflon or the greased paper it turns into a ball. We say the drop wets the glass, whereas on a hydrophobic (“water hating”) surface such as Teflon the wetting is only partial (see figure 1).

Figure 1: Left: A drop of water on a surface of high polarity, such as glass or metal. The drop flattens out and there is much wetting. Photograph: Betty Winter. Right: A drop of water on a surface with low polarity (hydrophobic), such as Teflon or greased baking paper. The drop draws itself into a ball and there is little wetting. Photograph: Vincent Kwong, UBC.

The spherical shape of a drop is a result of intermolecular forces between the molecules of which it is made. A molecule located within the drop is equally attracted in all directions by the molecules surrounding it, and so the total force exerted on it is zero. However, a molecule located near the drop surface is attracted only by its inner neighbors, and so feels a resultant force in the direction of the neighboring molecules (figure 2). This effective attraction of a molecule on the surface creates “surface tension”. Surface tension is a physical quantity measured in units of force per unit length, or equivalently in units of energy per unit area, and it expresses the amount of energy necessary to enlarge the surface by one surface unit. Since the sphere has the lowest surface area per given volume, it is easy to understand that this is also the state with the lowest surface energy, and that is what causes the drop to take on a spherical shape.

Figure 2: The molecules within the drop are equally attracted in all directions, whereas the molecules on the surface of the drop are attracted inward to their neighbors – and that is the source of surface tension.

In different fluids intermolecular forces possess different character and intensity. In organic fluids, such as oil, the attractive forces are a result of momentary electric polarization of the electrons. This polarization creates a non uniform distribution of electrons in the molecules, and as a result a mutual attraction is created between every two molecules, similar to the attraction between two magnets with opposite polarity. The forces responsible for the attraction are called van der Waals forces, after the 19^th century Dutch scientist. The surface tension between oil and air resulting from these forces is about 20 millijoule per square meter (mJ/m²). Water is a fluid with many special characteristics resulting from the large permanent dipole of water molecules and their intermolecular hydrogen bonds. Among other things this leads to the relatively high value of surface tension between water and air: 72 mJ/m². In mercury, which is a metallic liquid at room temperature, the attractive forces are a result of the free conduction electrons as in solid metals, and the surface tension reaches 485 mJ/m².

Surface tension at the interface of two materials depends on their mutual properties, and not just on one of them. For example, the surface tension of a water drop in air is different from the surface tension of that same drop in an oil medium. Therefore, when we place a liquid drop on a surface, the behavior of the drop depends not only on characteristics of the liquid, but also on characteristics of the material of which the surface is made. In general, we can say that if the polarization of the material making up the surface (that is, the ability of the electric charge in a molecule to distribute so that an electric dipole is created) is higher than the polarizability of the liquid, total wetting will take place, as in the case of a water drop on glass or metal. In other cases, as in the case of Teflon or a greasy surface partial wetting will take place (see the different wetting states in figures 1 and 3).

The noted British physicist Thomas Young, working at Cambridge University, found in 1805 that the contact angle θ (the angle created between the outer surface of the liquid and the surface on which it lies, see figure 3) depends on three surface tensions: the surface tension between the liquid and the solid surface γ_SL, between the surface and the air γ_SG, and between the liquid and the air γ_LG. The Young equation is expressed using the cosine of the angle θ as follows

Figure 3: Different degrees of wetting. On the left there is much wetting and the contact angle is small. On the right little wetting and the contact angle is large. If we write force equations according to the drawing we obtain the Young equation.

From the equation we can see that the higher the surface tension between the liquid and the air (γ_LG), the closer the contact angle will approach 90 degrees. In this state the drop has the form of half a sphere so that the area between air and liquid is minimal. A necessary condition to achieve a state of a perfectly spherical drop (θ=180°, a state with no wetting at all) is a greater surface tension between the surface and liquid than between the surface and the air (γ_SL < γ_SG), and in addition the constraint γ_SL -γ_SG =γ_LG. Note that in the opposite case of complete wetting (θ=0°), when γ_SG -γ_SL = γ_LG and also γ_SG is greater than γ_SL, the liquid spreads uniformly over the surface and creates a monomolecular layer. All this shows the great importance of the relative magnitude of the various surface tensions involved.

Certain insects (such as the “water strider”, figure 4) make use of surface tension in order to “float” and to move quickly on the water surface. The “attraction” of water molecules one to another bears the weight of the insect. As a result of its slight weight the water strider does not sink, but only causes slight curvature of the water surface around its feet. A similar phenomenon may be seen in a simple experiment: place a needle or a small coin on the water surface in a glass. If they are placed with care, surface tension will keep them afloat (figure 5).

Figure 4: A water strider floats and moves quickly over the water’s surface using surface tension.

Figure 5: A coin being borne up by surface tension in a glass of water.

Electrowetting

Some decades after Young’s discoveries, in 1875, a French physicist named Gabriel Lippmann investigated effects of electrocapillarity which laid the basis of modern electrowetting. Lippmann worked at the Sorbonne at the end of the 19th and beginning of the 20th century, and was Marie Curie’s doctorate supervisor. He later received the Nobel Prize for physics in 1908 for his work on color photography. For more details on Lippmann's work and electrowetting in general, see Refs. 1-2. Based on his findings, a term due to electric polarization was added the Young equation. This generalized equation is called today the Young-Lippmann equation:

In this equation V is the electric voltage and C the electric capacitance per unit area in the region of contact between the metal and the drop. The electric charges in the liquid are free to move, and so with the operation of the voltage the positive and negative charges are concentrated in different locations in the drop. The forces operating on the charges within the liquid cause the contact region between the drop and the metal to widen, and cause a certain flattening out of the drop (Figure 6). The equation describes the way the percentage of wetting increases (the contact angle shrinks) with the increase in electric voltage. Thus by means of electric voltage the amount of wetting of the drop may be precisely controlled.

In addition to the theoretical description of the phenomenon, Lippmann used it to invent a particularly sensitive electrometer, which measured tiny electric voltages by means of their effect on swings in surface tension between sulfuric acid and liquid mercury within a glass tube. This was an important invention, which was later used for the first measurements of electric voltage in heart operation, known to us today as an electrocardiogram (ECG). Lippmann invented this in 1872, three years before completing the theoretical description of the phenomenon!

Figure 6: Top: A liquid drop on an insulated surface with a high contact angle. Bottom: electrowetting of the surface. Operation of voltage between the drop and the electrode changes the distribution of electric charge in the drop and significantly decreases the contact angle. The polarity of voltage in the drawing is arbitrary, and in both directions electrowetting will occur.

The validity of the Young-Lippmann equation has been checked in experiments with many materials, and it gives a fairly good description for a large number of systems. Nonetheless it was discovered that in extreme cases the equation does not hold; for example, it does not predict the saturation of the contact angle under the operation of high voltages (Figure 7). This saturation phenomenon, as well as other phenomena which are not completely understood, provide a fertile ground of scientific research today.

Figure 7: Measurement of the contact angle as a function of electric voltage as compared to the theoretical prediction from the Young-Lippmann equation. At voltages higher than 35 volts (depending on the system) the contact angle is saturated, in contradiction to the theoretical prediction. The picture is from the University of Cincinnati website.

In order to increase wetting and to control it, various salts are dissolved in water, providing a source of ions for electric charges moving freely in the liquid. In Lippmann’s time the operation of a voltage between the metal and salt water gave rise to electrochemical reactions, similar to those within an electric battery. Over time these reactions decrease the number of free charges in the liquid and so electrowetting persisted only for a short time.

A few years ago a French physicist named Bruno Berge managed to overcome this problem by adding a thin layer of insulating material between the metal surface and the water drop (Figure 6). The insulating layer has two important characteristics: it is an electric insulator, and it is made of hydrophobic material, so that there is very little wetting of water on in. Therefore the contact angle may be controlled without electrical conduction between the salt water drop and the metal. This method is called “electrowetting on dielectric”, or by its initials EWOD and it is a technological breakthrough because the electrowetting thus created is stable for a long period of time.

Applications

Electrowetting has a number of interesting applications which have recently been developed. They are all based on the fact that it is possible using an external electric field, with no mechanical parts, to control movement or quick change (hundredth of a second) between a number of states of the system. It is important that systems can be miniaturized to scales of less than a millimeter and still be controlled with great precision using a miniscule amount of energy for a long period of time. Applications from recent years include transport of liquids for purposes of changing the characteristics of optical conductors and creating optical switches, cooling of electronic circuits by transport of cold drops across them, transport of micro-drops for purposes of printing, suction of liquids in microtubes, and lab-on-a-chip applications for analysis of the chemical composition of liquids, particularly physiological liquids (blood, urine…) by means of transport of drops of the fluid to examination cells on the chip. There they are mixed with other chemical materials and undergo various optical measurements, all at miniscule length scales of less than one millimeter.

Variable Lenses

An interesting and important application of electrowetting which has seen great development recently is the creation of an optical lens with a varying focal length. In this application one small drop of water in placed in a sealed glass cell filled with oil. The drop, which is a few millimeters in size has a nearly perfect spherical shape, and so it can serve as a lens (figures 8 and 9). By using only a few dozen volts the shape of the drop may be changed in a hundredth of a second and thus its focal length may be adjusted as required. In this way it is possible to photograph objects in a wide range from a few centimeters to infinite length while preserving focus. Varying focal length has been realized up to now using a system of lenses moving in relation to each other. The new application enables increased miniaturization, lower sensitivity to mechanical faults, and is already in use in the miniscule digital cameras found in mobile phones (figure 10). Similar uses are being developed for new generation DVD devices.

Figure 8: A water drop in air can focus light like a lens. As early as the 17th century the English scientist Stephen Gray used a water drop as a lens for a microscope he built. Photograph: Gadi Fishel.

Figure 9: left: A schematic description of the section of an optical lens consisting of a water drop in an oil medium. Here electric voltage is not turned on and the spherical drop diffuses the light due to the refractive index of oil which is greater than that of water (Snell’s law). Right: Here electric voltage is turned on, and under its influence the water drop becomes concave in the oil and focuses the light passing through it.

Figure 10: left: Archetype of a miniscule electrowetting lens manufactured by Philips Co. Right: Objects photographed by a camera with an electrowetting lens with varying focal length. Above: An object focused at a distance of 50 cm. Below: An object focused at a distance of 2 cm.

Electronic Paper

Another promising use of electrowetting is in the development of electronic paper (e-paper). E-paper is a display surface very similar to paper in terms of the reader’s sensation, which can display varying content as does a computer or television screen, just as if it had been “printed anew” before our very eyes.

These characteristics are expected to transform e-paper in the future to a substitute for regular paper in many areas. Instead of newspapers and books we can carry with us only a thin flexible surface, as large as we wish, on which we can read the entire daily paper or a book before bed. Among the paper’s illustrations, video articles and film clips might also appear. Turning a page of the book, leafing through the newspaper or watching a film would be done on the e-paper in a way reminiscent of internet surfing today.

We must note here that there is a great deal of research effort invested in the subject of manufacture of e-paper, with preliminary successes based on a number of different technological directions. However, we will focus here only on an innovative development based on electrowetting technique, which has recently been made at the Philips Corp. research laboratories in The Netherlands. Company researchers succeeded in creating very thin display surfaces of a size suitable for use as electronic device screens such as mobile phones or Pocket PCs. The reaction time of the display surfaces they manufactured is sufficiently quick to display films.

These display surfaces are unique in that they reflect light, as opposed to displays which emit light and which are familiar to us from television and computer screens. Reading and watching a light-reflecting display is similar to looking at and reading regular printed paper, which we see by means of natural or artificial surrounding light that shines on the page and is reflected to our eyes. In this method the eye is not exposed to bright light emitted from the screen, but to light arriving from the natural light of the environment, which the reader can adjust at will. Therefore a light-reflecting display is less tiring for the eye and healthier than a light-emitting display. An additional advantage is in looking at the screen in strong lighting conditions such as daylight, where a light-emitting display has difficulty competing with the sunlight intensity, whereas a light-reflecting display makes use of it.

Due to these advantages, e-paper has the potential to serve in future a variety of display needs, and perhaps even to replace computer and television screens in use today. However, the achievements of the current development are intended for use as display screens for small electronic instruments only, and much additional research effort and technology is required before it will be possible to buy e-paper at the nearest store to replace a computer screen.

Pixel based on electrowetting

In every display screen (CRT, plasma, LCD….) the picture is made up of a large number of basic units called pixels (picture elements). The characteristics of a single pixel determine the characteristics of the entire screen – its size, reaction time, and the spectrum of hues it can create or reflect. First we will explain how a monochromatic pixel unit works based on the electrowetting technique, as shown in figure 11. The bottom of the cell is made of white light-reflecting material which is also an electric insulation, and its upper cover is made of transparent material. The cell is filled with transparent water and a bit of insoluble oily liquid painted with opaque paint. An electric potential difference is applied between an electrode in contact with the bottom of the cell and a needle (another electrode) connected to the cell side. The cell has two wetting states. Without applying voltage, the layer of oil wets the entire bottom of the cell and its opaque color reflects light with the color of the oil. The liquid layer of oil is under the water since the surface tension between the water and the insulator is higher than the surface tension between the oil and the insulator. This is the state with the lowest surface energy.

With the application of the electric voltage the free charges in the water will be attracted to the appropriate electrodes and the water will wet the bottom of the cell while pushing the oil to the corner of the cell. In this state the cell will become almost completely transparent, and light will be reflected from the white bottom. That is, in both states of the cell there is full reflection of light but in two different colors (for example blue – the color with which the oil has been dyed and white – as the color of the cell bottom), with the possibility of going form one state to another very quickly, and repeating the process hundreds of thousands of times with no decrease in performance. This is how the monochromatic pixel works.

Figure 11: Side view of two monochromatic pixel states based on electrowetting. On the right the reflected light is blue (the color of the layer of oil) with no electric voltage, and on the left the light is white (the color of the insulating layer) with application of the voltage.

In recent years, as a result of intense research, this cell has been miniaturized to a size of about 160 microns (0.16 millimeters). This size enables good picture resolution. Similarly, the dimensions of the basic cell are small enough so that force of gravity operating on the liquids is negligible in comparison to the forces of surface tension, and so inversion of the cell or a change in its spatial direction do not disturb its performance. The range of voltages in which the cell operates today is 15-20 volts; this has been decreased in recent years from values of over 200 volts. The technological aim is to decrease it to less than ten volts, an aim that is achievable by choice of suitable materials or a decrease in thickness of the insulator layer between electrode and water.

Most of the contemporary and future uses of the cell as a pixel in a picture are based on building a large two-dimensional array of pixels, with the possibility of individual control of each of the pixels by means of electric voltage (similar to a plasma or LCD screen). A typical transition rate between two wetting states of a water drop in electrowetting is measured in milliseconds. This reaction time enables quick change between static pictures as well as presentation of video films, whose proper display requires a refresh time of about 25 pictures per second.

Colored pixel

From the principle of the monochromatic cell it is easy to explain the principle of its generalization to a colored pixel. This may be done in two ways: the first and more traditional method is to connect a trio of cells in red, green and blue colors (RGB), which are the optical primary colors (resulting from characteristics of the human eye), and by controlling the brightness of each cell to create a unit with the desired color (figure 12). The general picture looks good although the resolution using this method is three times lower than the corresponding resolution on a monochromatic screen, because a colored unit is three times as big as a monochromatic unit.

Figure 12: A colored electrowetting pixel composed of three monochromatic pixels in the optical primary colors (RGB).

A different approach, also taken by Philips Co., is to manufacture a basic colored unit with three layers of colored oil, each in one of the three primary colors of ink (i.e., the three optical primaries for subtractive mixing): cyan, magenta and yellow (CMY). The unit is constructed of two cells placed one on top of the other (figure 13). The top cell has a layer of yellow oil at its top surface. The bottom cell contains two layers of oil: magenta at its top surface and cyan at its bottom. Again, gravity does not play a role on the “inverted” oil since the forces of surface tension for such tiny liquid layers are far stronger. With this method there is no loss of resolution since each pixel is capable of creating the entire spectrum of colors on its own. The final color of the pixel, as with color printing, is determined by control of the surface area taken up by the colored oil at each layer by means of electrowetting.

Figure 13: A colored electrowetting pixel with layers of oil in different colors (CMY) placed above each other.

Advantages of the electrowetting display

The electrowetting screen has particularly low energy consumption, since the only energy required is in the transition between states. As long as the picture is static there is no movement of charges and no consumption of energy, because the color is created by external light, which is reflected. Compared to the light-emitting LCD-based video screens, the electric consumption of an electrowetting screen is indeed about five times lower.

An additional advantage of the electrowetting screen over the LCD display is the wide viewing angle, which results from the fact that light is reflected in all directions. Light is emitted from a regular LCD screen primarily in the direction of the center and the picture cannot be viewed from a large side angle, a well known problem with every laptop screen. However with the electrowetting screen it is possible to look from the side and see a high-quality picture.

We must note here that an additional technology for manufacture of light-reflecting e-paper, which is not based on electrowetting, is called Electrophoretic Display. With this technology the pixel is a microscopic capsule containing black and white particles moving in opposite directions under the influence of an electric field, so that two color states may be displayed. This technology is already in use for signs and thin flexible display surfaces, but it is incapable of displaying video because its reaction time is about a second. (A commercial electronic book product using this technology was recently put on the market by Sony Co.).

A colored light-reflecting screen based on electrowetting has many advantages in comparison with accepted technologies. It is very thin, suitable in speed to video display, does not limit the viewing angle and is very economic with electricity.

These advantages are summarized in a table comparing display based on electrowetting and principal other technologies.

In summary, electrowetting is a fascinating physical phenomenon. Although it was first discovered in the 19^th century, the phenomenon is still not fully understood, and it is of great contemporary interest in light of its new and varied applications. In this article we have focused on its application as electronic paper, an application which may soon become an inseparable part of our reading and watching experience of written text, pictures and graphic-visual information, and may take the place of the regular computer screen, the book or newspapers (see figures 14 and 15).

Figure 14: Realization of the use of electronic paper for reading a newspaper. This is an archetype of Plastic Logic Co. based on microcapsule technology.

Figure 15: An example of Liquavista Company’s electrowetting screen in a small electronic device.

Most probably that you are reading this article on paper which has been produced in an industry which consumes many millions of tons of wood and thousands of tons of ink each year, with grave ecological implications. It is interesting to consider when we will no longer require these resources and will be able to read the morning newspaper by means of pixels of oil, water and a little electricity….

Acknowledgments: The authors are indebted to Judy Kupferman and Val Parsegian for their contributions to the English edition of the article. We also thank Roy Beck, David Bergman, Yoram Burak, Michael Cogan, Moshe Deutsch, Haim Diamant, Daniel Harries, Uri Nevo, Adrian Parsegian, Elie Raphael, Shimon Reich, Adi Shafir, Michal and Moshe Siman-Tov, and Yoav Tsori for their helpful comments.

Interesting websites on electrowetting and e-paper:

• http://www.liquavista.com/ - website of the Liquavista, a Philips Co. daughter company that develops display surfaces based on electrowetting.
• www.varioptic.com – website of Varioptic - a French company which manufactures variable optical lenses focused by electrowetting.
• www.eink.com – a company which manufactures electronic paper based on electrophoretic display.
• www.ececs.uc.edu/devices/NDL_Research.html - website of an electrowetting research laboratory at University of Cincinnati (Ohio, USA).
• www.ee.duke.edu/research/microfluidics - website from a lab at Duke University (North Carolina, USA) showing videos of transport of micro-drops by means of electrowetting.

References:

1. F. Mugele and J.-C. Baret, "Electrowetting: from basics to applications", Journal of Physics: Condensed Matter 17, R705–R774 (2005)
2. C. Quilliet and B. Berge, "Electrowetting: a recent outbreak", Current Opinion in Colloid & Interface Science 6, 34-39 (2001)
3. R. A. Hayes and B. J. Feenstra, "Video-speed electronic paper based on electrowetting", Nature 425, 383-385 (2003)
4. B. Comiskey, J. D. Albert, H. Yoshizawa and J. Jacobson, "An electrophoretic ink for all-printed reflective electronic displays", Nature 394, 253-255 (1998)