On-chip detection of flow rates in microfluidics is critical to lab-on-chip technology. Thermal sensing techniques although widely used are limited by temperature rise of the fluid and issues involving integration inside a microchannel. Electrochemical methods present a reliable alternative. Among the widely used electrochemical techniques, Coulometry measures the charge consumed in a redox reaction at electrode surface. In present work, constant potential coulometry at 2V was performed using an electrochemical workstation on Titanium-Platinum microelectrodes (micropatterned on Si wafer substrate) submerged in a moving 0.1 M NaCl electrolyte solution. The sensing platform consists of Si wafer plasma bonded to a PDMS (poly-dimethyl siloxane) microchannel on top of the electrodes. For a constant applied voltage, the rate of decay of current with time is observed to be a function of the flow rate (flux of ions per unit time). As the flow rate increased, the ions available at the electrode surface increased per unit time leading to slower decay rates. The model equation for current-time curve was obtained as I = αe-(t/τ); where α = 0.0002 ± (1% of 0.0002) and τ = 0.312 to 0.797 for electrolyte (0.1 M NaCl) flow rates ranging from 0 to 200 μL/min. The sensitivity and 3σ resolution of the flow sensor are 0.10 sec/(μL/min)/mm2 and 3.64 μL/min respectively. This work models the coulometric current-time curve as a first order decay problem.
We present a successful fabrication and characterization of a capacitive micromachined ultrasonic transducer (CMUT) with SU-8 as the membrane material. The goal of this research is to develop a post-CMOS compatible CMUT that can be monolithically integrated with the CMOS circuitry. The fabrication is based on a simple, three mask process, with all wet etching steps involved so that the device can be realized with minimal laboratory conditions. The maximum temperature involved in the whole process flow was 140°C, and hence, it is post-CMOS compatible. The fabricated device exhibited a resonant frequency of 835 kHz with bandwidth 62 kHz, when characterized in air. The pull-in and snapback characteristics of the device were analyzed. The influence of membrane radius on the center frequency and bandwidth was also experimentally evaluated by fabricating CMUTs with membrane radius varying from 30 to 54 μm with an interval of 4 μm. These devices were vibrating at frequencies from 5.2 to 1.8 MHz with an average Q-factor of 23.41. Acoustic characterization of the fabricated devices was performed in air, demonstrating the applicability of SU-8 CMUTs in airborne applications.
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