Fig. 1. (a) Unit of the water microchannel structure. (b) Side view of the unit. (c) Schematic of the metamaterial absorber.

Fig. 2. Effect of changing structural parameters on absorption. (a) Width of bottom annular water channel. (b) Width of top square ring water channel. (c) Height of PLA bottom layer. (d) Height of top square ring water channel.

Fig. 3. (a) Unit cell of the designed devices, which are named Model_I, Model_II, Model_III, Model_IV, Model_V, and Model_VI. (b) Simulated absorptivity spectrum of different devices from Model_I to Model_VI.

Fig. 4. (a) Strategies for improving absorption bandwidth. (b) The overview of the reflection and interference theory model.

Fig. 5. Simulated vector distributions of (a) power loss densities, (b) electric fields, and (c) magnetic fields of the microwave absorber at 37.00 GHz (top), 58.50 GHz (middle), and 86.03 GHz (bottom).

Fig. 6. Normalized impedance of the proposed device.

Fig. 7. (a) Fabricated sample; (b) comparison of the absorption spectra of calculated, measured, and simulated water MMA.

Fig. 8. Absorptivity spectra at different temperatures.

Fig. 9. Simulation and theoretical calculation of absorption spectra of water-based metamaterial absorbers with oblique incidence: (a), (c) TE mode; (b), (d) TM mode.