Electrostatics and the Lab on a Chip

At the heart of the laboratory on a chip are chemical or biochemical probes designed to detect specific molecules or reaction products. For these probes to perform their function, a microfluidic "plumbing system" is needed to accept samples, then manipulate, dispense, and distribute tiny liquid volumes on the chip. The most promising mechanisms for handling such small quantities of liquid -from microliters down to tens of picoliters -are all electrostatic, e.g., electrocapillarity, electroconvection, electrophoresis, electro-osmosis, and dielectrophoresis. Despite years of study of these effects, the body of existing work is an imperfect guide to their effective exploitation in microfluidic applications. Attempts to harness electrostatic forces in structures <100 microns in size often encounter unanticipated behavior. Such surprises can be attributed to dramatic changes in the relative influences of the various competitive forces (e.g., viscous shear, surface wetting, and capillarity). In this paper, some of the more promising avenues of research on electric-fieldmediated microfluidics are examined. To explicate why and when electrostatic forces can be advantageous in the lab on a chip, Trimmer’s bracket notation is invoked to perform an investigation of the scaling laws for microfluidic systems. The methodology facilitates examination of the effects of device size on throughput, processing time, temperature rise, and other important measures of system performance.