Experiments have shown that when a miniature semiconductor diode floats in an aqueous solution and an alternating electric field is applied, (1) the diode rectifies the alternating potential induced between the electrodes, and (2) the resulting pulsating DC potential gives rise to an electro-osmotic flow in the vicinity of the diode and associated force that orients the diode along the electric-field direction, and propels the diode through the water. The propulsive force on the diode is along the electricfield direction and can be toward the cathode or the anode end, depending on the precise nature of the surface charge on the diode and the chemical composition of the solution. It has been proposed to exploit this phenomenon in developing microscale and nanoscale pumps and motors for diverse purposes. The proposal is not as radical as it might first seem: Electro-osmosis in applied DC electric fields has been used to pump liquids in microfluidic devices.

Electro-osmotic Speeds of two switching diodes of different sizes were measured as functions of the magnitude of the applied AC electric field.
In one family of potential applications, microscopic diodes would be incorporated into microdevices suspended in liquids so that the microdevices could be moved through the liquids by applying AC electric fields. There should be no impediment to extrapolation of diode propulsion to the microscale and below, inasmuch as results of both experiments on macroscopic diodes and theoretical calculations indicate that the electro-osmotic speeds of diodes do not depend strongly on their sizes (see figure).

In another family of potential applications, microscale and nanoscale pumps and mixers would be embedded at selected locations in walls and would be activated by application of global AC electric fields to effect active local control of flows. Diode-based microscale and nanoscale pumps and mixers would thus enable achievement of an unprecedented degree of complexity and performance in microfluidic applications in general, and more specifically, in manipulation (especially, separation) of biomolecules in laboratory-on-a-chip applications. In some cases, separations might be enhanced by use of combinations of AC-electric-field-actuated diode electro-osmotic pumping and DC-electric- field-actuated electrophoresis.

In yet another family of potential applications, diodes would be embedded in microscopic gears, wheels, propellers, and the like, wherein they would be positioned and oriented so that upon application of AC electric fields, the resulting electro-osmotic flows would produce torques that would cause these objects to rotate. The feasibility of this concept was demonstrated in an experiment on a macroscopic (≈1.8-cm-diameter) rotor with diodes mounted on its periphery.

In a variation on the basic theme of these and other potential applications, diode propulsion units would be made to emit or respond to light. (In experiments, light-emitting diodes suspended in the water in the presence of a suitably strong AC electric field both propelled themselves electro-osmotically and emitted light.) In another variation, regulation of speed would be achieved through exploitation of electrical characteristics of diodes or of microcircuitry integrated with diodes: For example, in an experiment on Zener diodes, the electro-osmotic speed remained approximately constant when the magnitude of the AC field was increased beyond a point corresponding to the Zener voltage.

This work was done by Orlin D. Velev of North Carolina State University for the Air Force Research Laboratory. For more information, download the Technical Support Package (free white paper) at www.defensetechbriefs.com/tsp under the Electronics/Computers category. AFRL- 0020

This Brief includes a Technical Support Package (TSP).
Miniature Semiconductor Diodes as Pumps and Motors

(reference AFRL-0020) is currently available for download from the TSP library.

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This article first appeared in the August, 2007 issue of Defense Tech Briefs Magazine.

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