INTRODUCTION

Piezoelectric materials enable direct conversion of mechanical energy into electrical signals, offering unique potential for self-powered sensing systems. Polyvinylidene fluoride (PVDF), particularly in its β-phase, is widely used for flexible piezoelectric applications due to its strong dipole alignment and mechanical resilience. In this work, a novel droplet-driven piezoelectric microfluidic sensor is developed, integrating PVDF-Cs₂AgBiBr₆ composite nanofibers within a microchannel to enable simultaneous energy harvesting and viscosity/pressure sensing. The design leverages hydrodynamic stresses from flowing droplets to induce cyclic deformation in the piezoelectric layer, eliminating external power requirements and enabling real-time fluid characterization in lab-on-chip environments.

Lead-free double perovskite Cs₂AgBiBr₆ nanocrystals were synthesized via a hot-injection process under inert conditions to ensure high purity and phase stability. These nanocrystals were incorporated into PVDF through electrospinning to form composite nanofibers, enhancing β-phase formation and thus improving piezoelectric performance. The microfluidic chip was fabricated using standard photolithography and e-beam deposition, consisting of gold–chrome electrodes patterned on silicon substrates, a PVDF-Cs₂AgBiBr₆ active layer, and a PDMS microchannel bonded atop. The T-shaped channel geometry, optimized with a main-to-branch width ratio (w₁/w₂) of 2:1, ensured stable droplet generation and uniform stress distribution. Electrical signals induced by alternating air-liquid droplet flow were measured in real-time.

Structural characterization via XRD confirmed the coexistence of PVDF and Cs₂AgBiBr₆ phases without degradation. FTIR analysis revealed a significant increase in electroactive β- and γ-phases, with the β-phase content rising to ~86% in the composite versus 78% in pure PVDF. The device produced cyclic voltage peaks corresponding to droplet-induced deformation and relaxation. Under optimized flow rates (air: 750 µL/min; liquid: 100 µL/min), the system generated output voltages up to 1.8–2.8 V, demonstrating efficient mechanical-to-electrical conversion. Viscosity-dependent testing using six different liquids (sunflower oil, olive oil, canola oil, glycerol, silicone oil) revealed a clear correlation between viscosity and voltage response. Medium-viscosity liquids (~500 mPa·s) yielded maximum voltages (>2 V), whereas low-viscosity (<100 mPa·s) and high-viscosity (~1000 mPa·s) fluids produced lower outputs due to insufficient or dampened deformation, respectively. This non-linear yet predictable relationship allows the device to quantitatively sense fluid viscosity based solely on electrical response.