Characterizing the pressure field in a modified flow cytometer quartz flow cell: A combined measurement and model approach to validate the internal pressure

Abstract

We incorporated an ultrasound transducer into a flow cytometer to activate microbubbles passing the laser interrogation zone (J. Acoust. Soc. Am. 126, 2954–2962, (2009)). This system allows high throughput recording of the volume oscillations of microbubbles, and has led to a new bubble dynamics model that incorporates shear thinning (Phys. Med. Biol. 58, 985–998 (2013)). Important parameters in the model include the ambient microbubble size, R0, driving pressure, PA, and the shell parameters χ and κ, the shell elasticity and viscosity, respectively. R0 is obtained by calibrating the cytometer. Pressure calibration is difficult because the flow channel width (<200µm) is too small to insert a hydrophone. The objective of this study was to develop a calibration method for a 20-cycle, 1 MHz transient pressure field. The pressure field propagating through the channel and into water was compared to a 3-D FEM model. After validation, the model was used to simulate the driving pressure as input for the bubble dynamics model, leaving only χ and κ variables. This approach was used to determine the mechanical properties for different bubbles (albumin, lipid, and lyzozyme shells). Excellent fits were obtained in many cases, but not all, suggesting heterogeneity in microbubble shell parameters.