Fluid Independent Flow Determination by Surface Acoustic Wave Driven Ultrasonic Techniques

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.