Integrated electrochemical velocimetry for microfluidic devices

We present a new electrochemical velocimetry approach with direct electrical output that is capable of complete device-level integration. The steady reduction rate of a reversible redox species at an embedded microband working electrode is monitored amperometrically. Only one working electrode of arbitrary width is required; all three electrodes, including counter and reference electrodes, are integrated on-chip for complete miniaturization of the sensor. Experimental results are complemented by a theoretical framework including a full 3D electrochemical model as well as empirical mass transfer correlations and scaling laws. When the sensor is operated in the convective/diffusive transport controlled mode, the output signal becomes a predictable function of velocity in two distinct regimes: (i) in the low velocity regime, the signal is directly proportional to flow rate, and (ii) in the high velocity regime, the signal scales as the cube root of the mean velocity. The proposed velocimetry technique is applicable to all practicable pressure-driven laminar flows in microchannels with known cross-sectional geometry.