Laser-Induced Fluorescence Thermometry for Joule Heating in AC Electrokinetic Chips

AC electrokinetic manipulation of particles and fluids are important techniques in the development of lab-on-a-chip technologies. Most of these systems involve planar microelectrode geometries, generating high strength electric fields. When these fields are applied to a dielectric medium Joule heating occurs. Understanding electrothermal heating and monitoring the temperature in these environments is critical for temperature-sensitive investigations including biological applications. Additionally, significant changes in fluid temperature when subjected to an electric field will induce electrohydrodynamic flows, potentially disrupting the intended microfluidic profile. This work investigates heat generated from the interaction of AC electric fields and water at various electrical conductivities (from 0.92–390 mS/m). The electrode geometry is an ITO electrode strip 20 μm wide and a grounded, planar ITO substrate separated by a 50 μm spacer with microfluidic features. Laser Induced Fluorescence (LIF) is used to measure the experimental changes in temperature. A normalization procedure that requires a single temperature-sensitive dye, Rhodamine B (RhB), is proposed to reduce uncertainty. The experimental electrothermal results are compared to theory and computer simulations.