Rapid Electrokinetic Patterning: Theory and Applications

Avanish Mishra, Purdue University

Abstract

Microfluidics is ushering a new era in biological analysis and point-of-care diagnostics. By performing desired analysis in picolitre to nanolitre volumes, microfluidic devices outperform some of the conventional benchtop technology. However, these advantages come with the added challenge of particle manipulation in a microscale environment. Rapid Electrokinetic Patterning (REP) is an emerging particle manipulation method. REP can dynamically trap and manipulate micro- to nanoparticles by modulating electrothermal flow with optical patterns. In the standard configuration, a REP chip is made of indium tin oxide (ITO) electrodes, and a laser beam is used as the optical source. The electrothermal flow in REP is driven by temperature rise induced by laser absorption in the thin electrode layer. In this work, ITO is proved to be a non-optimal electrode choice as more than 92% of the irradiated laser on the ITO electrodes is transmitted without absorption. A 25 nm titanium electrode is showed to generate comparable electrothermal flow while using 86% less laser power than a 700 nm ITO electrode. Based on the above results, a projector-REP setup is prepared which uses low-intensity optical patterns from a digital projector for trapping particles. Ability to program colloidal assemblies in desired spatial patterns and orientation remains a significant roadblock to the development of micro- and nanoparticle-based devices. By shaping electrothermal microvortices via optically generated patterns from a digital projector, assembly of particles in complex shapes is demonstrated. In future, a projector-REP based optoelectrical particle printing system can permanently deposit particles such as nanotubes and nanowires in desired patterns on a substrate at high-throughput. REP can also be used to measure and apply ultra-small forces. However, the magnitude of the radial trapping forces in REP is not well understood. By analyzing the trajectory of trapped particles with subpixel resolution, REP traps are showed to be harmonic in nature. Results showed that the REP traps have trap stiffness on the order of femtonewton/µm, which is three orders of magnitude lower than optical tweezers. This low trap stiffness allows REP to achieve sub-femtonewton force resolution. Apart from characterizing the trap stiffness and improving the REP setup, new applications are also demonstrated. The trapping behavior of swimming bacteria was studied, and the effect of REP trapping environment on the membrane integrity of the cells was probed. At the standard REP conditions, the trapped swimming cells were barely affected for first four minutes in the trap. At present, optical tweezers with laser intensity in excess of 5×10 6 W/cm2 are used for trapping motile bacteria. Such a high-intensity radiation poses the risk of opticution of the trapped cells. REP uses considerably lower light intensity (5×104 W/cm 2) and may emerge as a useful tool for trapping swimming cells.

Degree

Ph.D.

Advisors

Wereley, Purdue University.

Subject Area

Mechanical engineering|Nanotechnology

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