(560c) Optoelectric Trapping: Effect of Electrode Material and Thickness on Light-Induced Electrothermal Flow

Microfluidics is ushering a new era in drug discovery, tissue engineering, and point-of-care diagnostics. By conducting analysis in micro- to nanoscale environments, microfluidic biochips outperform some of the conventional benchtop technologies. However, these advantages come with the added complexity of particle manipulation at these small length scales; hence, since the advent of microfluidics, non-contact particle manipulation methods have been highly sought after. Rapid Electrokinetic Patterning (REP) has received considerable attention for its tunable trapping and translation of particles in a microfluidic chip. REP utilizes a laser-induced toroidal electrothermal flow and particle-electrode interactions for trapping particles on demand. In previously demonstrated applications of REP, a monochromatic infrared laser beam, integrated into an inverted microscope, was used to create the necessary temperature rise required for producing an electrothermal flow. However, usage of a high-intensity laser beam in a REP set up is expensive and time-consuming, restricting the adoption of the technique. The experimental setup will be significantly simplified if REP trapping can be achieved with an incoherent low-intensity light source. To accomplish this objective, we identify the optimum electrode layer so that REP can be performed with minimal optical-intensity. Using theoretical, computational, and experimental approaches, we demonstrate that conventionally used 700 nm Indium tin oxide (ITO) layer is an inefficient electrode choice as more than 92% of the irradiated illumination on the ITO electrodes undergoes transmission without absorption. At a given optical power, the magnitude of the electrothermal flow is controlled by both the electrode material and its thickness. After comparing electrode films of different materials and varying thickness, we find that a 25 nm thick titanium layer is the best electrode choice. This titanium electrode generates an electrothermal flow of the same magnitude as the ITO electrode while requiring only 14% of the light intensity used by ITO. In this presentation, we plan to discuss the importance of the electrode material choice and the associated applications.