Modeling the coulometric data for on-chip flow rate detection as a first order decay problem in a microfluidic device

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.