Avanish Mishra

Optoelectrical microfluidics as a promising tool in biology

Noncontact robotic particle grippers with trapping, manipulation, and release functions are highly desired in cell biology and microfluidics. Optoelectric techniques combine optical and electrokinetic effects to create thousands of such individually addressable traps. By projecting reconfigurable light patterns, these techniques can concentrate molecules, as well as manipulate, sort, and electroporate cells in a programmable manner. We describe the underlying physical mechanisms and discuss applications in biology and future prospects of these devices.

Dynamic optoelectric trapping and deposition of multiwalled carbon nanotubes

In the path toward the realization of carbon nanotube (CNT)-driven electronics and sensors, the ability to precisely position CNTs at well-defined locations remains a significant roadblock. Highly complex CNT-based bottom–up structures can be synthesized if there is a method to accurately trap and place these nanotubes. In this study, we demonstrate that the rapid electrokinetic patterning (REP) technique can accomplish these tasks. By using laser-induced alternating current (AC) electrothermal flow and particle–electrode forces, REP can collect and maneuver a wide range of vertically aligned multiwalled CNTs (from a single nanotube to over 100 nanotubes) on an electrode surface. In addition, these trapped nanotubes can be electrophoretically deposited at any desired location onto the electrode surface. Apart from active control of the position of these deposited nanotubes, the number of CNTs in a REP trap can also be dynamically tuned by changing the AC frequency or by adjusting the concentration of the dispersed nanotubes. On the basis of a calculation of the stiffness of the REP trap, we found an upper limit of the manipulation speed, beyond which CNTs fall out of the REP trap. This peak manipulation speed is found to be dependent on the electrothermal flow velocity, which can be varied by changing the strength of the AC electric field.

Optoelectric patterning: Effect of electrode material and thickness on laser-induced AC electrothermal flow

Rapid electrokinetic patterning (REP) is an emerging optoelectric technique that takes advantage of laser‐induced AC electrothermal flow and particle‐electrode interactions to trap and translate particles. The electrothermal flow in REP is driven by the temperature rise induced by the laser absorption in the thin electrode layer. In previous REP applications 350–700 nm indium tin oxide (ITO) layers have been used as electrodes. In this study, we show that ITO is an inefficient electrode choice as more than 92% of the irradiated laser on the ITO electrodes is transmitted without absorption. Using theoretical, computational, and experimental approaches, we demonstrate that for a given laser power the temperature rise is controlled by both the electrode material and its thickness. A 25‐nm thick Ti electrode creates an electrothermal flow of the same speed as a 700‐nm thick ITO electrode while requiring only 14% of the laser power used by ITO. These results represent an important step in the design of low‐cost portable REP systems by lowering the material cost and power consumption of the system.