Microfluidic sensing and power generation with coplanar interdigital capacitors

Abstract

Microfluidics is considered as both science and technology that deal with fluids in microchannels or in micro scale space. During the past two decades, microfluidic has been the hot spot in various research areas, e.g., biomedical assay, microchemical system, thermal management of electronic device, micro-electro-mechanical systems, etc., due to the emergences of various microfluidic devices. Transduction and sensing for microfluids therefore attract numerous interests from the diverse fields. Interdigital capacitance comprising of interdigital electrodes (IDEs) and thin insulation film provides a feasible solution for microfluidic transduction. Particularly, it has coplanar configuration and can be easily realized by use of microfabrication processes. Hence, it is of great interest to integrate the interdigital capacitance into a microsystem device for microfluidic transduction. This thesis explores the promising applications of interdigital capacitance for microfluidic generation and sensing. Due to its inherent flexibility, fluidic energy harvester has obtain many attentions. The conventional electrostatic energy harvesters have been proposed to convert the ambient vibration to electric energy, often seen with solid spring-mass configurations. The springmass structure takes full advantages of resonant vibration to give maximum output power at a narrow frequency band; however, it in the meantime brings challenges to be adapted for wideband and low frequency applications. To overcome these challenges, a fluidic electrostatic energy harvester is proposed with employment of interdigital capacitance with a thin PTFE film deposited by sputtering process. Owing to charges embedded inside the dielectric film, no extra charging process is required. When a conductive droplet or ionic liquid marble rolls across the interdigital capacitance, this fluidic energy harvester can output an electric power. Capacitance variation and open circuit voltage of the fluidic energy harvester was determined by finite element method (FEM) simulation when the droplet was at different position with respect to the IDEs. The charges on the IDEs were also checked. It is found that when the IDEs with given dimensions of finger/gap width, the total capacitance variation increases rapidly with the increase of droplet size to a peak value, and then drops as the droplet size keeps increasing, even turning to be negative when the droplet size is big enough to cover more than one pair of fingers. Microfabrication processes have been used to fabricate a prototype of this fluidic energy harvester. Experimental investigations of the fluidic energy harvester were conducted with both mercury droplet and ionic liquid marble. With a 1.2-mm mercury droplet rolling across the electret film of the prototype, a maximum output power was obtained at 0.18 ??W and the peak value of the output voltage was 1.5 V. A semi-empirical model was developed to understand the output waveforms. Several factors influencing the output performance are discussed. This fluidic electrostatic energy harvester is especially suitable for very low frequency vibration up to a few Hz. To explore the sensing capability of interdigital capacitance for microfluids, a microfluidic flow pattern sensor was also proposed and demonstrated, in which the insulation film was made of SU-8 rather than PTFE film. This microfluidic flow pattern sensor operates on capacitance variation corresponding to different flow regime passing across the sensing area. The prototype of the flow pattern sensor composed of the glass substrate, IDEs covered by the thin insulation film, and the PDMS cover, therefore to form a microchannel. Experimental investigation on the microfluidic flow pattern senor was performed by use of deioned water and olive oil. The capacitance variation was characterized corresponding to 3 typical flow patterns, namely, droplet flow, short slug flow, and long slug flow. According to the data of time-dependent capacitance variation, both velocity and size of the flow regime can be determined due to that the sensing length of the sensor is known. The constants specific to each flow pattern are calculated so that the microfluidic flow pattern can be easily identified. This microfluidic flow pattern sensor can be easily integrated into complicated microsystems as all the fabrication processes are compatible. With the demonstrations of fluidic electrostatic energy harvester and microfluidic flow pattern sensor, we may conclude that interdigital capacitance is very promising for microfluidic generation and sensing, due to the coplanar configuration and the capability of non-invasive operation.