Abstract
A fluid-independent ultrasonic approach for flow determination in microchannels in the harsh environment of an ultra high pressure liquid chromatography (UHPLC) system is presented. Ultrasonic waves in the fluid are excited by separate media surface acoustic waves (SAW) of Rayleigh-Wave type. The LiNbO3 SAW chip being equipped with interdigitated transducers for SAW excitation also marks the bottom of the fluid channel and thus allows for very effective SAW coupling to the fluid. The channel ceiling acts as an acoustical mirror for longitudinal ultrasonic waves propagating through the fluid. To deduce the fluid flow from the ultrasonic transmission after reflection, we employ a combination of time differential phase and time of flight measurements with a two port vector network analyzer. To verify and assign our experimental results, we use an adapted time explicit finite element method. In the simulation, both the piezoelectric single crystal and the fluid are included and we solve the linear Navier-Stokes equation to evaluate the background flow. By changing the ultrasonic propagation direction, we are able to deduce the fluid volume flow over time with very high accuracy, independent of the actual liquid in the channel.
Highlights
In microsystems, where small amounts of fluids are propagating within tiny channels, the flow properties of the fluids are very difficult to monitor and investigate
The flow measurement is investigated with a special focus to high performance liquid chromatography (HPLC)
The interaction of the stationary phase, the mobile phase and the constituents causes the constituents to flow with different speeds through the column
Summary
In microsystems, where small amounts of fluids are propagating within tiny channels, the flow properties of the fluids are very difficult to monitor and investigate. These sensors, are based on direct SAW (mostly Shear Waves) resonance frequency shifts caused by flow dependent pressure or temperature shifts They operate in the range of flows between 10 ml/min and 1000 ml/min with a corresponding significantly lower need in accuracy than our application with a maximum of 2-3 ml/min. In contrast to such more common approaches, our work presented in this article is based on bulk waves in the fluid being excited by the strong interaction with Rayleigh type SAW [12],[13]. The somewhat larger standard error in this case can be explained with the continuous fluid property change and with a significantly higher error of the dwell time measurement with the network analyzer used for the measurement in this work, compared to the pure phase measurement
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