Flow Velocity Gradient Sensing Using a Single Curved Bistable Microbeam

Abstract

An approach for gas flow velocity measurement at two different closely located points using a simple device incorporating only one thermal sensing element, is introduced and the feasibility to determine different velocities is experimentally demonstrated. An electrostatically actuated initially curved bistable microbeam heated by an electric current and convectively cooled by airflow is switched between two stable locations through the snap-through (ST) and snap-back (SB) buckling mechanisms. The velocities of an air flow at these positions are obtained by measuring the critical ST and SB values of the actuation voltage. In the experiments, our $500~\mu \text{m}$ long, $2~\mu \text{m}$ wide single-crystal Si beam with $\approx \, 2.5\,\mu \text{m}$ nominal initial elevation demonstrated sensitivity of $S_{\text {ST}} \approx 0.25$ V/(m/s) and $S_{\text {SB}} \approx 0.84$ V/(m/s) at the ST and SB points, respectively. In the present device, the distance between the two measurement points is $\approx 10~\mu \text{m}$ . Our experimental results indicate that the suggested approach can be used for the velocity gradient measurements. The sensing principle relying on a single bistable sensing element opens new opportunities for measurement of gas flow velocity and velocity gradients at scales significantly smaller than the state-of-the-art multi hot-wire sensors.