## 2 Materials and Methods
### Experiments Configuration  
This study used the adjacent method for current injection, maintaining a consistent input voltage of 5.0 V for the voltage-to-current control module. The current output amplitude was 5.0 mA throughout the experimental procedures [29; 30; 31]. The adjacent method involves injecting current through adjacent pairs of electrodes, facilitating the measurement of voltage differentials across other pairs. This approach is commonly employed in EIT experiments, enabling the acquisition of impedance measurements that are effectively utilized for image reconstruction. The selection of specific parameters, such as the 5 V input voltage and the 5.0 mA current, aligns with established practices in EIT experimentation, striking a balance between signal strength and safety considerations. Tank electrodes, crucial components in the experimental setup, were constructed of Ag/AgCl with a size of 10 mm. Located strategically around the tank were a total of 16 electrodes, following the adjacent method for current injection. This configuration contributed to the effective measurement of impedance variations. This differential imaging uses reference data to create images of a homogeneous medium without any objects inside at various research frequencies. The image reconstruction method used in this study was based on the Jacobian method, a widely recognized and accepted approach to EIT imaging. To implement the Jacobian method, we used the open-source reconstruction software EIDORS [32; 33]. This software ensures accurate and efficient image reconstruction by applying the principles of EIT. The focus on frequency-dependent properties suggests that the reconstruction process may involve considering in-phase signals, which are essential for capturing the impedance characteristics of materials at different frequencies. The study comprised a meticulously designed series of experiments aimed at dissecting the frequency-dependent contrast discernibility within EIT. This examination is particularly relevant when applied to objects with variant electrical properties, as demonstrated by the combination of acrylic resin and copper. In the following, we present the outline of specific experiments configured for this investigation, as illustrated in Figure 1.  
In Experiment I, a copper object was used in a homogeneous environment of 0.9% NaCl brine solution. The objects were systematically repositioned one by one at specific locations, denoted as experiments I.1, I.2,..., I.6. The primary objective of this experiment was to explore and identify the empirical frequency range that enhances the contrast of conductive materials in the context of EIT. For Experiment II, the focus shifted to exploring the influence of adjacent conductive materials on the frequency-dependent contrast behavior of copper. A copper object, identical to the one used in Experiment I, was placed in the same homogeneous environment with 0.9% NaCl brine solutions. In addition, an electrically conductive material, aluminum, was introduced nearby. The systematic repositioning of objects, labeled as experiments II.1, II.2,... II.6, aimed to understand how the presence of adjacent conductive materials affects the empirical frequency range for copper contrast in EIT. Figure 1: Experimental configurations with a diameter of the phantom of 160 mm. The diameter of objects (copper, aluminum, and acrylic resin) is 20 mm. Experiments I.1 to I.6: surveys on a copper object in a saltwater environment. Experiments II.1 to II.6: surveys on copper and aluminum objects in a saltwater environment. Experiments III.1 to III.6: surveys on copper and aluminum objects in a heterogeneous environment (ground meat environment). Experiments IV.1 to IV.6: surveys on acrylic resin objects in a saltwater environment. Experiments V.1 to V.6: surveys on copper and acrylic resin objects in a ground meat environment. In Experiment III, the experimental setup involved swapping the positions of copper and aluminum objects from Experiment 2.