Optimizing electrode thickness and material for laser-induced electrothermal flow

In this article, the material dependence of electrothermal flow is studied to optimize an optoelectric particle manipulation technique called Rapid Electrokinetic Patterning (REP). REP utilizes a heat source to produce temperature gradients, which, in this study, are induced by the application of a 1064 nm cw laser. The simultaneous application of an uniform AC electric field assemble colloids on the surface of an electrode that can be moved by translating the laser. The temperature gradients produced from the heat source are crucial in REP as the higher the temperature gradient the faster the electrothermal flow. Thus, optimization of temperature gradients would enhance REP performance with lower laser power. In this study, material properties of the electrode that produces the temperature gradient in REP are investigated to understand the absorption of the radiation from the laser. Analytical, computational and experimental aspects of the phenomenon were studied. It was found that the analytical model and the computation model could be used to select the optimum material and thickness of the electrode to produce the highest temperature gradient per watt of laser power. Furthermore, the computational model can be used to determine the flow velocity of the electrothermal flow accurately. With this study material selection of the electrode can be calibrated and customized for specific needs when conducting REP. It was found that among the materials tested (ITO, titanium, and nickel), the best material that produced the highest temperature gradient was titanium followed by nickel and ITO. When comparing the three materials with approximately the same thicknesses, it was observed that the maximum velocity magnitude produced by titanium and nickel electrodes were almost 80 times higher than the ITO electrodes