Bandgap formation and tuning in stimuli-responsive hydrogel-based periodic structures
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Over the past two decades, researchers have studied elastic/acoustic metamaterials and phononic crystals to enable certain structural properties that are not found in ordinary materials. One such property is the formation of a bandgap (a frequency range in which wave propagation is forbidden) to attenuate structural vibrations or noise over a desired frequency band. Metastructures, which are finite periodic structures with specified boundary conditions, can exhibit bandgap formation via geometry and/or material periodicity (Bragg scattering-based) or via resonating unit cells (locally resonant). The efforts presented in this thesis focus on the former approach in stimuli-responsive hydrogels which can achieve reprogrammable geometric and material periodicity through periodic swelling. Hydrogels are soft materials primarily composed of a network of hydrophilic polymers and water molecules. Many hydrogels have been made stimuli-responsive that can swell or shrink upon exposure to external triggers such as light radiation, relative humidity, temperature, etc. Because of their biocompatibility, hydrogels have been used extensively in many biomedical applications such as artificial organs, cell culture scaffolds, wound dressings, and drug carriers. Furthermore, hydrogels exhibit less internal damping compared to commonly used soft materials, offering a potential for structural dynamics and wave propagation applications. This work intends to study the use of hydrogels as metamaterials for bandgap formation and tuning due to their stimuli responsive behavior. This thesis first utilizes the transfer matrix method to perform the band structure (dispersion) analyses on unit cells of periodic hydrogel structures. In addition, finite structure frequency response analyses are performed using the finite element method. These models are then experimentally validated for hydrogel-based periodic cantilever beam configurations conducted on leucohydroxide (concentration is directly proportional to periodic swelling) based photo-responsive hydrogels. The effects of various parameters, such as unit cell length ratio and leucohydroxide concentration on bandgap and its tuning are further explored and experimentally validated. Both model simulations and experimental results suggest that increase in unit cell length ratio reduces both the center frequency and width of the bandgap, whereas higher leucohydroxide concentrations result in wider bandgaps at lower center frequencies.