Microfluidic thermal flow sensor based on phase-change material with ultra-high thermal sensitivity

Abstract

A microfluidic thermal mass flow sensor based on planar micro-machining technology and a phase-change material is designed, fabricated, and characterized. The sensor configuration uses a small patch of vanadium dioxide (VO₂) thin film as the sensing element closely placed in the down streaming direction of the heat source. By operating the VO₂ sensor in the phase transition region, no thermal insulation structure is required due to the ultra-high thermal sensitivity in this region. The characteristic 3-order resistance change, from 290 kΩ to 290 Ω, is observed during the full heating and cooling cycles by using both substrate heating and resistive heating methods. The equivalent maximum temperature coefficient of resistance (TCR) is calculated to be −0.37 K⁻¹ in the cooling cycle and −0.43 K⁻¹ in the cooling and heating cycle, respectively. The sensing operation principle is determined to follow the major cooling curve to avoid falling into minor loops and to secure high TCR. The resistance of VO₂ is monitored under flow rates ranging from 0 to 37.8 μL s⁻¹ with the maximum sensitivity of 1.383%/(μL min⁻¹). The studies presented in this research may enable the application of utilizing nonlinear metamaterial in microfluidic flow sensors with orders of magnitude improvement in sensitivity.