npsm 새물리 New Physics : Sae Mulli

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New Phys.: Sae Mulli 2024; 74: 251-256

Published online February 29, 2024 https://doi.org/10.3938/NPSM.74.251

Copyright © New Physics: Sae Mulli.

Detection of Water Droplets in Oil-Filled Dielectric Tubes by Microwave Near Field Imaging

Hyeri Song, Hanju Lee*

Department of Physics, Jeju National University, Jeju 690-756, Korea

Correspondence to:*hlee8001@jejunu.ac.kr

Received: September 20, 2023; Revised: November 29, 2023; Accepted: December 5, 2023

This is an Open Access article distributed under the terms of the Creative Commons Attribution Non-Commercial License(http://creativecommons.org/licenses/by-nc/3.0) which permits unrestricted non-commercial use, distribution, and reproduction in any medium, provided the original work is properly cited.

Detecting impurities within fluids is a crucial technology in various industries such as chemical plants, equipment manufacturing, and automotive systems. In this study, we conducted research on detecting water droplets in dielectric tubes filled with oil using microwave near-field imaging. We visualized the distribution of microwave near-fields in oil-filled dielectric tubes containing water droplets located at different positions. The measurement results showed that the microwave near-field distribution of oil-filled dielectric tubes changed depending on the position of the water droplets, and the change was maximized when the microwave electric field is parallel to the length direction of the tubes. From the experimental results, we showed that the near-field imaging technology can be applied for detecting impurities inside fluid-filled tubes, showcasing its potential usefulness in impurity detection.

Keywords: Microwave, NDT&,E, Microwave imaging

Technology to control and utilize the material in the fluid state can be considered highly important in various industries and medical fields[1]. Materials in the fluid state typically exhibit high fluidity, allowing substances from the external environment to mix within the fluid material. In such cases, various contaminants can exist within the fluid material, and these contaminants can lead to impurity issues and even undesired chemical reactions. Therefore, in industries utilizing fluids, inspection techniques detecting impurities within the fluid are crucial to ensure material reliability[1].

The detection of impurities within fluids is mainly based on destructive testing, such as direct contact between the fluid and inspection equipment, or non-destructive methods like optical measurements[2,3]. Destructive testing techniques often involve inserting cameras or sensors into the fluid material to detect impurities, allowing for direct observation and relatively accurate inspection results[2]. However, these methods can potentially damage the sample and introduce additional contaminants from the inspection equipment. On the other hand, optical-based non-destructive testing offers the advantage of inspecting samples without causing damage. However, it cannot be used to detect impurities within opaque fluids or when impurities are optically transparent[3].

As an alternative to overcoming the limitations of conventional inspection techniques, technologies utilizing microwaves have been actively developed[4-13]. Microwaves are electromagnetic waves with wavelengths ranging from 1 mm to 10 cm, exhibiting excellent penetration properties in dielectric materials. Due to this high penetration capability, microwaves can reach and interact with impurities in fluids. Therefore, it can be anticipated that if spatial variations in dielectric constants caused by impurities within fluids can be detected using microwaves, the impurities within fluids could be non-destructively identified. Indeed, a new technique known as Thermo-Elastic Optical Indicator Microscopy (TEOIM) has been reported for detecting the position and size of air bubbles within dielectric tubes filled with water using microwave near-field imaging[12]. TEOIM technology measures the distribution of microwave electromagnetic fields based on indicator optical microscopy, offering advantages such as high measurement throughput and resolution, along with the ability to capture real-time changes in the near-field distribution.

In this study, we presented a new approach for non-destructive detection of impurities within dielectric tubes using TEOIM-based microwave near-field imaging. We visualized microwave near-field distributions of oil-filled dielectric tubes, in which water droplets with various size were inserted into the tubes at different positions. By visualizing the microwave near-field structure, we showed that the presence, size, and position of water droplets can be determined. The experimental results demonstrated that TEOIM-based microwave near-field imaging technology can be employed for non-destructive detection of impurities within fluids.

Figure 1 illustrates the measurement system of the TEOIM. The TEIOM system consists of two main components: the system for generating and applying microwave signals (frequency: 10 GHz) to the sample, and the system for measuring the optical signal from the indicator. The microwave system includes a signal generator for generating microwave signals, an amplifier to amplify the generated microwave signal, and an open-ended waveguide antenna for irradiating microwaves onto the sample. The microwave signal generated by the signal generator is amplified to 30 dBm through the amplifier, and then applied to the sample using a TE-mode open-ended waveguide antenna. The waveguide antenna is designed to be rotatable, allowing for the adjustment of the electric field direction of the microwave applied to the sample. The optical measurement system consists of a light source, circular and linear polarizer, an optical indicator, zoom lens, and a CCD camera, resembling a conventional polarized light optical microscope setup. During measurement, light generated by the light source is polarized into left-circular polarization state by a circular polarizer, enters the indicator, is reflected, then passes through a linear polarizer. The intensity of the transmitted light is measured using a CCD camera connected to a zoom lens.

Figure 1. (Color online) Measurement system of TEOIM. The microwave signal is generated by a signal generator, and then, it is amplified by a power amplifier to 30 dBm. The amplified signal is transmitted to open-ended waveguide through coaxial cables, and it is radiated to a sample from the waveguide. The optical indicator is monitored by a polarized light microscope system consist of a circular polarizer, linear polarizer, and the CCD camera.

In TEOIM system, the indicator plays a role in converting microwaves into optical signals and is structured with a material that effectively absorbs microwaves coated on a glass substrate. In this study, we used a soda-lime glass substrate (10 0mm by 100 mm by 1 mm) coated by ITO (Indium Tin Oxide, thickness: 200nm) thin film as the optical indicator. To prevent the influence of reflected light from the sample, the optical indicator was attached to an optically opaque alumina substrate (thickness: 1mm). The sample was prepared using a silicon rubber tube (inner diameter: 1mm), in which one end of the tube was sealed to prevent fluids leakage during measurements. The silicon rubber tube was filled by oil, and then, water droplets in the oil-filled tube were created using a syringe injection method. The prepared samples were then affixed to the alumina substrate with the attached indicator. Finally, the alumina substrate attached with the optical indicator and samples was placed 5 cm away from the waveguide antenna.

When microwaves are applied to the sample, microwave near-field is generated around the vicinity of the sample, and the near-field distribution varies depending on the electromagnetic properties and spatial distribution of the sample. If the indicator is placed near the sample, the microwave near-field is absorbed by the microwave-absorbing layer of the indicator and converted into heat, which is then transmitted to the substrate material of the indicator. In the case of microwave-absorbing layers with high conductivity, like ITO thin film, the magnetic component of the microwave near-field is strongly absorbed. As a result, the distribution of the heat transferred to the substrate is identical to the distribution of the magnetic microwave near-field. The generated heat is conducted through the substrate of the optical indicator, resulting in the formation of thermal stress, and the distribution of this thermal stress can be measured through the photo-elastic effect of the substrate.

In TEOIM, the distribution of thermal stress can be visualized by measuring the intensity variation of light using a CCD camera, and then applying the following equations[13]:

β1(x,y)=βcos2θ=Iφ=π4,MWONIφ=π4,MWOFF2
β2(x,y)=βsin2θ=Iφ=0,MWOFFIφ=0,MWON2

where, I is the intensity of light measured by the CCD camera, φ is the angle between the linear polarizer and the direction perpendicular to the horizontal plane, MWON and MWOFF are the states with and without microwave application, θ is the angle between the principal axis of thermal stress and the direction perpendicular to the horizontal plane, β1 and β2 are the linear birefringence coefficients induced by normal and shear stress in the sample, respectively. By spatially differentiating the values of these parameters, the distribution of heat source density can be calculated from the following equation[5,13] :

q(x,y)=C22β2xy+2β1x2+2β1y2

where q(x,y) is the heat source density of in the indicator, x and y are the spatial coordinates of the image pixels, and C is a constant dependent on the wavelength of incident light and the optical properties of the indicator.

To investigate the changes in microwave near-field distribution according to the property of fluid within the tube, we visualized microwave near-field of tubes filled with air, oil, and water, respectively. Additionally, to explore the microwave near-field distribution of the samples according to the orientation of the applied microwave electric field, the direction of the microwave electric field was aligned vertically (Fig. 2(a), vertical configuration) and horizontally (Fig. 2(b), horizontal configuration) with respect to the length direction of tubes. Figures 2(c–h) show the measured microwave near-field distributions in vertical and horizontal configurations for dielectric tubes filled with air, oil, and water. From the measurement results, in the case of tubes filled with air (Fig. 2(c–d)), it can be observed that in the vertical configuration, microwaves are concentrated around the center of the waveguide, while in the horizontal configuration, the near-field is distributed along the length direction of the tube. For tubes filled with oil (Fig. 2(e–f)), similar distributions are observed in both configurations. However, in the horizontal configuration, the near-field intensity is stronger compared to that in tubes filled with air. On the other hand, in the case of tubes filled with water (Fig. 2(g–h)), it can be observed that the near-field is distributed along the length direction of the tube in both configurations. Particularly, the structural characteristics of this near-field are more distinctly evident in the vertical configuration.

Figure 2. (Color online) (a–b) Illustration of measurement configurations: (a) vertical configuration; (b) horizontal configuration. (c–h) Microwave near-field distribution of dielectric tubes measured in the vertical and horizontal configurations. (c–d) Microwave near-field distribution of air-filled tubes measured in the vertical (c) and horizontal (d) configurations. (e–f) Microwave near-field distribution of oil-filled tubes measured in the vertical (e) and horizontal (f) configurations. (g–h) Microwave near-field distribution of water-filled tubes measured in the vertical (e) and horizontal (f) configurations.

These experimental results suggest that the microwave electric field strongly interacts with the fluid within the tube in the vertical configuration, in which the microwave electric field is oriented parallel to the length direction of the tube. This directional dependence implies that the fluid within the tube exhibits anisotropic polarization characteristics, causing the spatial distribution of the fluid to extend predominantly along the tube's length[12]. Additionally, in the case of tubes filled with water, the pronounced changes in the microwave near-field compared to tubes filled with oil can be attributed to the significantly higher dielectric constant of water (εr=80) compared to that of oil (εr=3)[12,14], resulting in a larger polarization effect within the tube filled with water. From these experimental results, it is evident that the distribution of the microwave near-field varies depending on the type of fluid within the tube, and this variation can be used to detect the presence and type of fluid within the tube.

To further investigate the variations in microwave near-field distribution when two fluids with different dielectric constants coexist within the same tube, a water droplet of 3 mm length was inserted into a tube filled with oil. Figures 3(a–d) show the microwave near-field distribution measured in both vertical (a–b) and horizontal (c–d) configurations. From the measurement results, it can be observed that in the vertical configuration, the near-field distribution is similar to the distribution without the water droplet[12]. In contrast, in the horizontal configuration, a strong concentration of the microwave near-field was appeared in the region where the water droplet was located. These results indicate that the water droplet within the tube strongly interacts with the microwaves, particularly in the horizontal configuration. This directionality of interaction can be explained by the fact that the water droplet forms a cylindrical structure within the tube. In such a cylindrical structure, the polarization along the length direction increases compared to the radial direction of the tube. As a result of this anisotropic polarization, when the direction of the microwave electric field aligns parallel to the length direction of the water droplet, strong interaction occurs, leading to the formation of a pronounced microwave near-field[12]. These experimental results indicate that even when fluids with different dielectric constants coexist within the same tube, the microwave near-field distribution within each fluid changes according to their dielectric constants. Therefore, it can be concluded that from variations in the near-field distribution, one can detect impurities within fluids.

Figure 3. (Color online) (a) Illustration of a sample structure in the horizontal configuration. A water droplet was injected at the center of the oil-filled dielectric tube. (b) Microwave near-field distribution of the oil-filled dielectric tube containing a water droplet measured in the horizontal configuration. (c) Illustration of a sample structure in the vertical configuration. (d) Microwave near-field distribution of the oil-filled dielectric tube containing a water droplet measured in the vertical configuration.

The most important advantage of TEOIM-based microwave near-field imaging is its ability to rapidly measure the wide-ranging microwave near-field distribution. Based on this advantage, various studies have been reported for imaging of the electromagnetic defects in materials and fluid distributions within tubes[6,12,13]. Therefore, one can expect that by using TEOIM, the presence, size, and position of impurities within fluids can be measured in a short measurement time. To validate this idea, the changes in the near-field distribution were measured when the position of a water droplet inside a tube filled with oil was altered. Figures 4(a–b) show the microwave near-field distribution in the horizontal configuration for different water droplet positions. From the measurement results, one can see that even when the water droplet deviates from the center of the waveguide, a strong microwave near-field is still observed around the water droplet. Furthermore, in cases where multiple water droplets are present within a single tube as shown in Fig. 4(c), a strong microwave near-field appears at the position of each water droplet. This result indicates the TEOIM can measure the positions of multiple impurities, even when numerous impurities are present at the same time. Consequently, from these experimental results, it can be concluded that TEOIM-based microwave near-field imaging can rapidly detect the positions of impurities within a wide region, even when multiple impurities are positioned arbitrarily.

Figure 4. (Color online) (a–b) Microwave near-field distribution of water droplets at different positions. (c) Microwave near-field distribution in a tube with multiple water droplets.

In this study, a new approach for non-destructive detection of impurities within dielectric tubes using TEOIM-based microwave near-field imaging was presented. We visualized microwave near-field distributions of oil-filled dielectric tubes, in which water droplets with various size were inserted into the tubes at different positions. From the experimental results, it was observed that the presence of water droplets within the oil-filled dielectric tubes led to changes in the distribution and intensity of the microwave near-field. By analyzing the change of microwave near-field structure, we showed that the presence, size, and position of water droplets can be determined. Through this study, it was demonstrated that TEOIM-based microwave near-field imaging technology can be employed for non-destructive detection of impurities within fluids.

This research was supported by the 2023 scientific promotion program funded by Jeju National University.

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