Abstract
In this study, we demonstrated a thermally tunable acoustic beam splitter using a poly(vinyl alcohol) poly(N-isopropylacrylamide) hydrogel (PVA-pNIPAM). The nature of PVA-pNIPAM hydrogel offers exceptional temperature-dependent physical properties due to its phase transition around its lower critical solution temperature. The acoustic impedance of the hydrogel can be tuned below, above, or matched to that of water by changing the environmental temperature. An acoustic wave propagating in water can be split into transmitted and reflected components by the PVA-pNIPAM hydrogel slab on varying its angle of incidence. The intensity ratio between the reflected and the transmitted componence can be adjusted by tuning the temperature of the medium. The acoustic beam can be entirely reflected at a temperature corresponding to the matched impedance between hydrogel and water. The beam-splitting behavior was observed for acoustic waves from both a monochromatic wave and broadband pulse source. In addition, the phase of beam split pulses can be reversed by selecting the hydrogel’s operating temperature.
Highlights
The anomalous phase transition in certain liquid or hybrid hydrogel composites can be induced by varying the environmental pH condition [1] or temperature [2]
Based on the similarity in the physical properties between PVA-pNIPAM hydrogel and water, we investigate the feasibility of controlling the underwater acoustic wave propagation by designing an acoustic beam splitter for active acoustic systems
At around 32 ◦C, a crossing point can be found between temperature-dependent acoustic impedance lines in PVA-pNIPAM hydrogel and water, indicating the acoustic impedance of the hydrogel can be matched with DI water to achieve acoustic transparency
Summary
The anomalous phase transition in certain liquid or hybrid hydrogel composites can be induced by varying the environmental pH condition [1] or temperature [2]. The thermal responsive hydrogels commonly transform from a hydrophilic state to a hydrophobic state around its lower critical solution point (LCST). The anomalous phase transition of thermally tunable hydrogels opens a great potential in drug-delivering [4], sensing [5], and micro-robotics [6] fields. Bulk poly(N-isopropylacrylamide) (pNIPAM) composite is one of the commonly studied thermal sensitive hydrogels with a sharp transition in volumetric and mechanical properties around its LCST [7]. The dramatic change in the physical properties was observed in terms of physical volume [8], static elasticity [9], dynamic elasticity [10], density [11], and swelling/deswelling ratios [12]. The reported behaviors and characterizations have resulted in the development of practical applications, such as micropumps [13], small-scale surgery [14], acoustic lens [15], and even 4D additive manufacturing products [16]
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