Humidity-driven bifurcation in a hydrogel-actuated nanostructure: A three-dimensional computational analysis

Hydrogel-based adaptive structures that respond to specific external stimuli present immense potential for applications in microfluidics, shape-memory devices, artificial muscle and actuators. Using a three-dimensional finite element method, we analyse the humidity-driven bifurcation of a nanostructure, made up of periodically distributed nanoscale rods embedded vertically in a swollen hydrogel layer. The bifurcation manifests as a switching behavior of the nanorods between vertical and tilted states. The use of representative volume element with realistic boundary conditions allows us to fully consider inhomogeneous deformations of the hydrogel. Our computations reveal that at higher initial swelling ratio, the bifurcation behavior of the nanostructure approaches that of the case where homogeneous deformation in the hydrogel is considered. However, large deviation in the behavior may occur between the two at lower initial swelling ratio. We further investigate quantitatively the effects of geometrical and material variations on the bifurcation behavior. It is found that geometric-material parameters can significantly affect the critical switching state and its post-bifurcation behavior, enabling great tunability in the design and application of hydrogel-based adaptive nanostructure.