Time-of-flight thermal flowrate sensor for lab-on-chip applications

TLDR

The study presents a thermal microflowrate sensor that accurately measures liquid flow velocity in microfluidic channels, independent of the fluid’s thermal and physical properties. The sensor uses a heater and temperature‑detector bridges on suspended silicon to generate heat pulses, whose downstream advection is cross‑correlated with injected pseudo‑stochastic signals to determine time‑of‑flight, and a phenomenological model explains the response. Multiple detector bridges eliminate diffusion, enabling accurate, linear flow velocity measurement over more than two orders of magnitude, with experimental data matching finite element analysis and prototype tests confirming feasibility for lab‑on‑chip applications.

Abstract

We describe a thermal microflowrate sensor for measuring liquid flow velocity in microfluidic channels, which is capable of providing a highly accurate response independent of the thermal and physical properties of the working liquid. The sensor consists of a rectangular channel containing a heater and several temperature detectors microfabricated on suspended silicon bridges. Heat pulses created by the heater are advected downstream by the flow and are detected using the temperature detector bridges. By injecting a pseudo-stochastic thermal signal at the heater and performing a cross correlation between the detected and the injected signals, we can measure the single-pulse response of the system with excellent signal-to-noise ratio and hence deduce the thermal signal time-of-flight from heater to detector. Combining results from several detector bridges allows us to eliminate diffusion effects, and thus calculate the flow velocity with excellent accuracy and linearity over more than two orders of magnitude. The experimental results obtained with several test fluids closely agree with data from finite element analysis. We developed a phenomenological model which supports and explains the observed sensor response. Several fully functional sensor prototypes were built and characterized, proving the feasibility and providing a critical component to microfluidic lab-on-chip applications where accurate flow measurements are of importance.