Entangled glycerol hydrogels for wave modulation of Rayleigh-SAW devices

Surface acoustic wave (SAW) devices, particularly of the Rayleigh type, harness nanoscale-amplitude vibrations on a piezoelectric substrate to sense, actuate and process various targets. SAW modulation in selected areas is essential for high-performance devices. Here, we develop a highly entangled glycerol hydrogel as a compact, high-efficiency absorber for wave modulation of Rayleigh-SAW devices. A high glycerol content of 50 wt% ensures strong Rayleigh-SAW attenuation and suppresses dehydration, whereas the dense chain entanglements preserve network stiffness despite inhibited radical polymerization at high glycerol fractions. The highly entangled glycerol hydrogel shows a SAW-attenuation coefficient of 1110 m^-1 (113.5% higher than that of the widely adopted absorber polydimethylsiloxane, PDMS) together with a stiffness of ∼100 kPa, enabling a 52.9% reduction in the characteristic absorber length relative to PDMS. Moreover, it adheres strongly to a LiNbO₃ substrate without surface treatment, delivering an adhesion energy of up to 50 J m^-2 (400% higher than that of PDMS). We also demonstrate two SAW-based microfluidic applications of the highly entangled glycerol hydrogel absorber. Compared with PDMS, the glycerol hydrogel absorber increases the streaming velocity by 36.8% in droplet stirring and the collection rate by 56.9% in particle enrichment. The highly entangled glycerol hydrogel absorber offers a space-efficient route toward further miniaturization and integration of SAW devices.