Fluidic-Controlled Slot-Loaded Microwave Reflective Surface for Multistate Phase Control

Abstract - This work presents a fluidic-controlled microwave reflective surface enabling multi-state phase control through slot-coupled fluidic loading. The proposed structure employs a slotted ground plane integrated with a polydimethylsiloxane (PDMS)-based microfluidic channel that can be selectively filled with air, liquid metal (LM), or water to realize discrete reflection phase states without the use of semiconductor switches or bias networks. By positioning the fluidic channels beneath the ground plane, the electromagnetic characteristics of the reflective surface are preserved while enabling controllable impedance loading of the slot region. The symmetric element geometry and slot-loading configuration provide polarization-independent operation. A fully analytical equivalent circuit model (ECM) is developed to describe the interaction between the crossed patch, grounded substrate, and slot-loaded fluidic channel, providing physical insight into the underlying microwave phase-control mechanism and enabling closed-form prediction of the reflection response under different fluidic states. The structure supports three discrete phase states, enabling three-level phase-gradient control for anomalous reflection. A 16 × 16 prototype operating at 15 GHz is fabricated using a PDMS–printed circuit board (PCB) bonding process and experimentally characterized. Measured results demonstrate beam steering and anomalous reflection up to 76°, showing good agreement with the simulated results and confirming the feasibility of fluidic loading as a bias-free technique for implementing reconfigurable microwave reflective structures.