It has been known for a century that pressure will affect the conductivity of electrolyte. The first person to study the influence of ultrasonic pressure change on electrolyte conductivity was Fox in 1946. In recent years, Jossinet and Cathignol have aroused interest in this phenomenon. A study conducted by Zhang and Wang applied ultrasonic electrification effect to biomedical imaging. The change of conductivity σ caused by pressure wave with amplitude Δ p can be written as:
Where K 1 is the interaction constant, and its value is about 0.01-0.1%MPa-11.7 in 0.9% sodium chloride solution. According to the formula (1), the voltage V(t) measured between two electrodes placed in a uniform current field with amplitude I0 is:
Where S(x, y, z) is the sensitivity distribution of the electrode, P(t) is the time-varying pressure field, and dv is the differential volume element. The sensitivity distribution of electrode pairs can be found from the lead field theory, which was originally developed for ECG. The guiding field (sensitivity distribution) of a pair of electrodes has exactly the same shape as the current field generated by the current passing through the pair of electrodes. As can be seen from Equation (2), the voltage signal is:
In the experimental device shown in fig. 2, a 540 kHz transducer is placed in the following form, and its focusing band is located between a pair of tin electrodes. Its diameter is 1 0cm, and its f/ number is1. It is driven by Panametrics 5077PR rectangular wave pulse/receiver through matching network. The width and depth of the focused beam at the focal point are about 3 mm and 20 mm, respectively. The pulse echo reflected on the flat air/water interface is shown in Figure 3.
The tin electrode is a tin strip (3mm× 33mm) on the printed circuit board. Their long axes are parallel to the beam axis. Low-frequency current is injected through a user-customized voltage-current converter (VCC). Waveforms are generated by HP33 120A arbitrary waveform generator connected to VCC.
The voltage signal is measured by a pair of insulated stainless steel electrodes exposed at the top. These two electrodes are placed in the plane formed by the current injection electrodes so that their intersection lines are perpendicular to the ultrasonic axis. Fig. 4 shows the geometry of the experimental device.
The voltage signal is fed into a customized differential amplifier with an analog high-pass filter, which separates the ultrasonic modulation signal from a large number of low-frequency signals. The gain of the differential amplifier is 20dB and the bandwidth is 100 ~ 1000 kHz. The signal is further amplified in EN 1400AP amplifier. The gain of EN 1400AP is 37dB, and the bandwidth is 0. 15 ~ 300 MHz. The signal is sampled and digitized (8 bits) with the digital oscilloscope of 1td 1002, and sent to the computer for further processing.