Ultrasound propagation in liquids is highly influenced by its attenuation due to viscous damping. The dissipated energy will be partially absorbed by the liquid due to its dynamic viscosity as well as its bulk viscosity. The former results in the generation of a flow that is called acoustic streaming, and the latter is associated with the vibrational and rotational relaxation of liquid molecules. Measuring the ultrasonic wave attenuation due to the bulk viscosity is presented as a novel method in this article. Poly(Nisopropylacrylamide) (PNIPAM) microgels, which are soluble in several solvents such as water, were used as acousto-responsive markers in water, which upon absorption of ultrasonic energy undergo a volume phase transition due to the breakage of their hydrogen bonds. Thus, they become insoluble in water, and due to shrinking, their optical density increases. As a result, their agglomeration can be seen as a turbid medium. We managed to visualize the ultrasonic energy absorption due to the bulk viscosity using the turbidity since the excess acoustic energy on top of the absorbed energy for the translational motion of liquid is spent to break the hydrogen bonds between PNIPAM and water. In addition, to quantify the turbidity phenomenon, the total energy required for breaking hydrogen bonds in the solution is calculated, and its evolution, according to the input power intensity, is quantified by image processing. The effect of viscosity by changing the microgel concentration was investigated, and it is shown that an increasing microgel concentration increases the acoustic energy absorption rate much greater than its dynamic viscosity. Therefore, the bulk viscosity, as the responsible parameter for this increase, is measured directly from the energy of broken hydrogen bonds. The results show that at low solution concentration (0.2 wt %) the bulk viscosity is in the same order of magnitude as its dynamic viscosity. Increasing the concentrations to 1 and 5 wt % increases the bulk viscosity and consequently the structural relaxation time by 1 and 2 orders of magnitude, respectively.
Sparse spiral phased arrays are advantageous for many emerging air-coupled ultrasonic applications, since grating lobes are prevented without being constrained to the halfwavelength element spacing requirement of well-known dense arrays. As a result, the limitation on the maximum transducer diameter is omitted and the aperture can be enlarged for improving the beamforming precision without requiring the number of transducers to be increased. We demonstrate that in-air imaging, in particular, benefits from these features, enabling large-volume, unambiguous and high-resolution image formation. Therefore, we created an air-coupled ultrasonic phased array based on the Fermat spiral, capable of transmit, receive and pulse-echo operation, as well as 3D imaging. The array consists of 64 piezoelectric 40-kHz transducers (Murata MA40S4S), spanning an aperture of 200 mm. First, we provide an application-independent numerical and experimental characterization of the conventional beamforming performance of all operation modes for varying focal directions and distances. Second, we examine the resulting imaging capabilities using the single line transmission technique. Apart from the high maximum sound pressure level of 152 dB, we validate that unambiguous high-accuracy 3D imaging is possible in a wide field of view (±80 • ), long range (20 cm to 5 m+) and with a high angular resolution of up to 2.3 • . Additionally, we demonstrate that object shapes and patterns of multiple reflectors are recognizable in the images generated using a simple threshold for separation. In total, the imaging capabilities achieved are promising to open up further possibilities, e.g. robust object classification in harsh environments based on ultrasonic images.
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