The detection of acoustically-induced cavitation in water, blood plasma and whole blood in vitro, by means of a heated thermocouple array
My thesis is composed of two parts. A method for the detection of bubbles using thermal sensors was developed and tested in the first part, and the measurement of intensity thresholds for cavitation in liquids is emphasized in the second. A heated thermocouple array for use as a bubble detector has been developed in this study. The array is a multi-junction probe with a heater situated close to the array. When the heated thermocouple array is placed in a sonicated liquid at a temperature below that of the array, heat losses from the sensor will be modified (increased or decreased) by the presence of gas bubbles close to the site of an individual junction by virtue of the different thermal conductivities of gases and liquids, and because of ultrasound scattering by the bubbles. The sudden appearance of gas bubbles in a liquid close to a heated thermocouple junction should therefore lead to temperature fluctuations whose magnitude will depend on the size and position of the bubbles relative to the ultrasound transducer and to the detector. The output of each thermocouple was fed to an amplifier. This circuit also compensated for room temperature changes and processed the sensor signal to give a digital reading of temperature on digital display. This detecting circuit also called a multi-point sensor was constructed by Assaf, Watmough and Robertson (1986). The heated thermocouple array was first tested in liquid flowing down a pipe which demonstrated its ability to detect single bubbles over a range of sizes. Bubbles generated by ultrasound in liquids (water, blood plasma, and whole blood) were detected by the means of the heated thermocouple array. The intensity thresholds for cavitation in liquids in vitro has been measured using the heated probe and also by a doppler probe for comparison. The dependence of the intensity thresholds in dissolve gases has been considered. The construction of multi-junction probes by argon arc welding is also described.