Electrokinetic Micro Power Generation - Streaming Potential

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Original entry by Hyerim Hwang, AP 225, Fall 2011.

Reference

Myung-Suk Chun, Min Suk Shim, and Nak Won Chioi, "Fabrication and Validation of a Multi-Channel Type Microfluidic Chip for Electrokinetic Streaming Potential Devices", Lab Chip 2006 6, 302-309

Keywords

Microchannel, Electrophoresis, Capillary, Flow, Streaming potential

Introduction

This study fabricates the silicon-glass as well as the PDMS-glass microfluidic chips with the unique features of a multi-channel to elaborate on the applicability of the electrokinetic micro power generation. Besides miniaturizing the device, the key advantage of their microfluidic chip utilization lies in the reduction in water flow rate. Silicon-based microchannel leads to increasing streaming potential and higher external current compared to those of the PDMS-based one and experimental results agree with theoretical results well. It is useful to recognize that a material inducing a higher magnitude of zeta potential has an advantage for obtaining higher power density under the same external resistance.

Figure 1. Development of flow-induced streaming potential along a charged microchannel and the multichannel array circuit with external resistance.
Figure 2. Schematic of the experimental setup with electrokinetic streaming potential device.
Figure 3. Streaming potential versus pressure difference in siliconglass and PDMS-glass microfluidic chips without external resistance.

*Streaming Potential The streaming potential is electrokinetic phenomena studied in the areas of surface chemistry and electrochemistry. It is the opposite electrokinetic phenomenon to electro-osmosis in that it uses motion to produce an electric field. Streaming current is an electric current which originates when an electrolyte is driven by a pressure gradient through a channel or porous plug with charged walls. Adjacent to the channel walls, the charge-neutrality of the liquid is violated due to the presence of the electrical double layer; a thin layer of counterions attracted by the charged surface. The transport of counterions along with the pressure-driven fluid flow gives rise to a net charge transport.

Results

Such electrokinet behavior is basically present due to the electric double layer (EDL), which forms as a result of the distribution of electric charges near a charged surface. Streaming potential occurs owing to the charge displacement in the EDL caused by an external force shifting the liquid phase tangentially against the solid phase. As shown in Figure 1, the counterions in the diffusive part of the EDL are carried toward the downstream and for the applied pressure p and the accumulation of ions sets up an electric field. Here, they first measured the streaming potentials without external resistance generating from the silicon-glass and the PDMS-glass microchannels according to an increase of applied pressure difference. For silicon-glass microchannel, the increasing tendency in streaming potential is more considerable, and accordingly its value of streaming potential is higher than the PDMS-glass on as shown in Figure 4.

Figure 4. Multi-channel: (a) top view of the silicon glass chip, (b) top view of the PDMS-glass chip, (c) cross-sectional view of the silicon-glass chip, and (d) cross-sectional view of the PDMS-glass chip.

Discussion

This study develops a multi-channel microfluidic device which can generate electrokinetic streaming potential and external electric current. Silicon and PDMS chips are designed to have distributor and collector with optimal geometry for achieving uniform flow distribution, the flow channel assembled with hundreds of single microchannels, and a pair of electrodes. Higher magnitude of zeta potential of the silicon surface than that of the PDMS provides a higher streaming potential and in the same chip, the electric power density increases with increasing external resistance. The multi-channel microfluidic chip is a challenging issue in devoloping the streaming potential devices, further studies are necessary to enhance the power density conducting advanced stack system.