Continuous Electrowetting Research Articles

Long Life Electrochemical Diodes for Continuous Electrowetting

The rate of electrochemical reactions in some systems varies with the polarity of the overpotential on the working electrode, introducing diode-like behavior at the electrode/electrolyte interface. However, with repeated bipolar cycling, the electrochemical current damages the electrodes. We have connected electrochemical diodes in series with opposing polarities to reduce the diode current while charging a capacitive circuit. We have previously used this capacitive circuit arrangement to actuate aqueous droplets continuously using the electrowetting (EW) effect. In this study, the performance of electrochemical diodes under repeated voltage cycles is investigated. Aluminum and titanium electrodes in contact with three electrolyte solutions (0.1 M citric acid, 0.1 M Na2SO4 and 0.1 M NaOH) are employed. The diode behavior of electrochemical systems with single diodes and diode pairs is compared. A coefficient of continuous electrowetting (CEW) actuation (referred to as actuation coefficient) is introduced. Actuation coefficient varies between zero (no actuation) and one (equivalent to grounded droplet actuation at the same voltage. Experimental results how that titanium maintains higher actuation coefficients (0.6 −0.8) than aluminum (0.6). The actuation coefficients of titanium increase over the first trials and remain stable under alternating −50 V and +50 V inputs for 2000 trials (experiment duration).

Continuous electrowetting via electrochemical diodes

We describe a novel method for droplet transport combining electrowetting on dielectric (EWOD) and the diode-like behavior of valve metals to achieve unique actuation performance. While traditional EWOD droplet transport requires switching of voltage between multiple electrodes, our method, which we term continuous rectified electrowetting, utilizes a simple single electrode and a DC voltage to move a 50 μl droplet 28 mm with velocities up to 32 mm s(-1).

A Unified Velocity Model for Digital Microfluidics

This article presents a unified model for the velocity of discrete microdroplets. Simple algebraic expressions for steady-state droplet velocities are presented, as well as exact and approximate transient solutions. Specific results in terms of known experimental parameters are derived for the cases of electrowetting on dielectric (EWOD), dielectrophoresis (DEP), continuous electrowetting (CEW), and thermocapillary pumping (TCP). Model predictions are shown to agree with previously published theoretical and experimental results, giving fluid velocities for a broad range of applications in digitized microfluidics. A relative comparison of the model's predictions for EWOD, CEW, DEP and TCP is also presented.

Liquid loading of silicon-based cantilevers using electrowetting actuation for microspotting applications

An array of silicon-based cantilevers, so-called Bioplume, that can actively be loaded with liquid solutions was designed and fabricated for microspotting applications. Each cantilever incorporates a slit and a reservoir for liquid storage, and droplet formation occurs by a direct contact deposition technique. The loading scheme, based on the use of continuous electrowetting, was implemented either by patterning metallic electrodes inside the channel or by doping the silicon tip with phosphorus. Fabricated devices were tested using dimethyl sulfoxide (DMSO) and water–glycerol solutions. Application of a dc voltage between the liquid to be loaded and the conducting part of the cantilevers resulted in a liquid rise inside the channel. Voltages as low as 30 V were used to load the cantilevers and matrices of more than 50 droplets were successfully patterned with a single load. The diameter of achieved droplets ranged from 10 to 20 μm.

A surface-tension driven micropump for low-voltage and low-power operations

In this paper, we first report a micropump actuated by surface tension based on continuous electrowetting (CEW). We have used the surface-tension-induced motion of a mercury drop in a microchannel filled with an electrolyte as actuation energy for the micropump. This allows low voltage operation as well as low-power consumption. The micropump is composed of a stack of three wafers bonded together. The microchannel is formed on a glass wafer using SU-8 and is filled with electrolyte where the mercury drop is inserted. The movement of the mercury pushes or drags the electrolyte, resulting in the deflection of a membrane that is formed on the second silicon wafer. Another silicon wafer, which has passive check valves and holes, is stacked on the membrane wafer, forming inlet and outlet chambers. Finally, these two chambers are connected through a silicone tube forming the complete micropump. The performance of the fabricated micropump has been tested for various operation voltages and frequencies. We have demonstrated actual liquid pumping up to 70 /spl mu/l/min with a driving voltage of 2.3 V and a power consumption of 170 /spl mu/W. The maximum pump pressure is about 800 Pa at the applied voltage of 2.3 V with an operation frequency of 25 Hz.

Surface-tension-driven microactuation based on continuous electrowetting

This paper describes the first microelectromechanical systems (MEMS) demonstration device that adopts surface tension as the driving force. A liquid-metal droplet can be driven in an electrolyte-filled capillary by locally modifying the surface tension with electric potential. We explore this so-called continuous electrowetting phenomenon for MEMS and present crucial design and fabrication technology that reduce the surface-tension-driving principle, inherently powerful in microscale, into practice. The key issues that are identified and investigated include the problem of material compatibility, electrode polarization, and electrolysis, as well as the micromachining process. Based on the results from the initial test devices and the design concept for a long-range movement of the liquid-metal droplet, we demonstrate a liquid micromotor, an electrolyte and liquid-metal droplets rotating along a microchannel loop. Smooth and wear-free rotation of the liquid system is shown at a speed of /spl sim/40 mm/s (or 420 r/min along a 2-mm loop) with a driving voltage of only 2.8 V and little power consumption (10-100 /spl mu/W).

Continuous electrowetting effect

We introduce a new electrowetting effect, continuous electrowetting (CEW), and show its advantages for applications to displays and other electro-optic devices. We demonstrate experimentally, by using CEW, fast and reversible electrowetting flow on the theoretically predicted scale of ∼10 cm/s for ∼1-V driving voltage.

Electrowetting optical switch

Continuous electrowetting (CEW) and refractive index matching have been used as the physical basis of an optical switch for multimode fibers. This switch has low cross talk (∼−30 dB), fast response times (∼20 ms), low operating voltage (∼1 V), and low power consumption (∼1 μW).