Droplet motion electrically controlled

The movement of small droplets on a substrate is governed by surface-tension forces. A technique that can tune the surface tension of robust oxide substrates for droplet manipulation could open up many applications.
Frieder Mugele is at the MESA+ Institute for Nanotechnology, Physics of Complex Fluids, University of Twente, Enschede, 7500 AE, the Netherlands.

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Many plants and insects control their interactions with raindrops and other forms of ambient humidity using surface patterns of hydrophobic and hydrophilic regions1. In artificial devices, the ability to switch between such patterns enables the complex manipulation of droplets for applications including biomedical lab-on-a-chip systems2,3, optofluidic lenses4 and displays5, and energy-harvesting systems6. A method called electrowetting-on-dielectric (EWOD) is arguably the most mature and versatile tool for achieving such functionality7. However, its commercial success has been limited, because the required hydrophobic dielectric (electrically insulating) surfaces gradually degrade. Writing in Nature, Li et al.8 report an approach to tune the wettability of chemically robust hydrophilic surfaces by electrically controlling the adsorption and desorption of molecules called surfactants. If this technology is successful, it might help to turn some previous EWOD-based systems into reliable devices.

The wettability of a solid surface for a particular liquid is determined by the chemical properties of the materials involved. If molecules of the liquid and the solid strongly attract each other, the liquid will cover as much of the solid surface as possible. As a result, there will be a small contact angle — the angle between the liquid surface and the solid surface at the point at which these surfaces meet. In the case of water, such a solid (for example, clean glass or silicon oxide) is hydrophilic. By contrast, if the attraction between water and the solid is weak (for instance, in the case of the non-stick pan coating polytetrafluoroethylene), the surface is hydrophobic and water will bead off.

For hydrophobic surfaces, the tension (force per unit length) at the interface between the solid and the liquid is larger than that at the interface between the solid and the surrounding gas (Fig. 1). For hydrophilic surfaces, the opposite applies. At equilibrium, the contact angle adjusts itself in such a way that the difference between the solid–liquid and solid–gas interfacial tensions is balanced by the horizontal component of the liquid–gas interfacial tension.

Figure 1 | Droplet manipulation. a, In a method called electrowetting-on-dielectric7 (EWOD), a droplet is placed on a micrometre-thick hydrophobic layer. The solid–liquid interfacial tension (force per unit length) is larger than the solid–gas interfacial tension. The difference between these tensions is balanced by the horizontal component of the liquid–gas interfacial tension. As a result, the contact angle between the solid and the liquid is large. An applied voltage generates an electric force that causes this angle to decrease. b, Li et al.8 report a technique dubbed electro-dewetting, in which a droplet is placed on a nanometre-thick hydrophilic layer. The solid–liquid interfacial tension is smaller than the solid–gas interfacial tension and the contact angle is small. The droplet contains charged molecules called surfactants that have a hydrophobic tail and a hydrophilic head. Under an electric current, surfactants are adsorbed on the hydrophilic layer. These regions are rendered hydrophobic, which causes the contact angle to increase.

Tuning the wettability requires the balance between these surface-tension forces to be manipulated. In EWOD, this is achieved by applying a voltage between a droplet on a thick, hydrophobic dielectric layer and an electrode that is positioned underneath the layer. This voltage generates an electric force that, along with the solid–gas interfacial tension, pulls on the droplet and thereby reduces the contact angle (Fig. 1a). The combination of EWOD and patterned electrodes allows for complex droplet operations such as transport, splitting, merging and mixing2,3.

The success of EWOD crucially depends on the stability and the chemical inertness of the dielectric layer. Almost two decades of applied research have focused on optimizing these layers, based on the principle that they should be hydrophobic and as thin as possible, but also should block any voltage-induced electric current that would degrade performance. This has led to layers of polytetrafluoroethylene-like fluoropolymers being a gold standard in the field. Notwithstanding impressive successes, the intrinsically high tension of any interface between a hydrophobic layer and water makes such surfaces prone to adsorption of solutes and to other degradation processes on continued exposure to water. This limitation has become the main bottleneck for the commercialization of the technology.

Li and colleagues avoid this inherent problem of EWOD by using a hydrophilic silicon oxide surface that has an intrinsically small solid–liquid interfacial tension. They tune the wettability of this surface using electrically controlled, reversible adsorption of surfactants (Fig. 1b). These molecules consist of a hydrophobic tail and a hydrophilic head. Their adsorption on the hydrophilic surface reduces the solid–gas interfacial tension and thereby increases the contact angle. For this reason, the authors refer to their approach as electro-dewetting, in contrast to electrowetting and, therefore, EWOD.

Unlike EWOD, in which the dielectric layer blocks any electric current, electro-dewetting relies on passing a current through the liquid and a nanometre-thin silicon oxide layer to an underlying electrode. Depending on the direction of the current, charged surfactants are transported either towards or away from the solid surface, inducing surfactant adsorption or desorption, respectively. The authors demonstrate that this technique can be applied to a remarkably wide range of liquids and surfactants, as long as the concentration of these molecules is within a specific range of conveniently low values. Efficient droplet manipulation is also shown for some highly saline buffer solutions that are commonly used in biotechnology.

Li and colleagues use electro-dewetting in conjunction with patterned electrodes, and demonstrate lateral movement of droplets and the basic droplet operations of lab-on-a-chip systems. They find that these manipulations can be carried out even more easily than when using EWOD, despite somewhat slower response times for the droplets and a smaller range of accessible contact-angle variations.

The main promise of the authors’ approach is to deliver a robust and versatile droplet-manipulation platform. Although the results presented show a remarkable degree of versatility, challenges remain. For instance, the surfactants tend to adsorb to the solid surface and increase the contact angle even without an applied electric current. But Li et al. show that this adverse effect can be suppressed by adjusting the liquid’s composition (for example, its pH) depending on the type of surfactant that is used. Given the wide range of surfactants that are available, it seems plausible that suitable material combinations can be found that maximize the electro-dewetting efficiency and that minimize possible interference from other solutes such as proteins, for many applications.

Another challenge is that the required electric current will drive electrochemical reactions that could gradually degrade the droplet-manipulation platform and the associated liquids. Stringent tests will need to be carried out after hundreds, thousands or even millions of adsorption–desorption cycles to fully evaluate the robustness and versatility of electro-dewetting.

Li and colleagues’ work might also have implications for fundamental research. Standard wetting theories9 are equilibrium theories that are based on energy minimization. However, the need for a permanent electric current in electro-dewetting demonstrates that the microscopic origin of this mechanism requires some intrinsically non-equilibrium processes that remain to be identified. This concept could therefore offer opportunities for controlling interfacial adsorption even beyond wettability alteration.

Nature 572, 445-446 (2019)

doi: 10.1038/d41586-019-02451-5


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