Instability driven reconfigurable soft materials: mechanics and functionality

Abstract

Mechanical instability, a deformation mode involving abrupt switching between two distinct equilibrium structural configurations, has historically been viewed as a failure mechanism in engineering and materials science. Since the pioneering work in harnessing spontaneous buckling for surface micro-patterning in 1998, tremendous research interest has focused to utilize, rather than avoid, buckling instability in soft materials at small scale for achieving unique properties and multifunctionality. The benefit of small-scale bucking instability in soft materials and structures lies in the reversible dynamic tunability of the buckled structural or surface configuration in response to different external stimuli, which enables the coupling of structural or surface reconfiguration with dynamically tunable properties, such as mechanical, optical, wetting, and electrical properties. In this dissertation, I explore the fundamental mechanics and functionality of surface-based buckling and hierarchical wrinkling on substrates in multifunctional opto-electronic devices and smart windows. I will first explore the benefits of classical plate buckling in soft materials. The challenge lies in the intrinsic indeterminate characteristics of buckling in terms of its buckling orientation, which could lead to geometric frustration and random global structures. To address this challenge, I introduce cuts-based geometrical imperfection to guide the deterministic buckling in arrays of parallel active polymeric plates on rigid substrates. After introducing patterned cuts, the originally random phase-shifted buckling transits to a prescribed buckling with controllable phases. The design principle for cut-directed deterministic buckling in plates is revealed through both mechanics model and finite element simulation. By harnessing cut-directed buckling for controllable contacts and interactions in buckled parallel plates, I demonstrate the array of parallel plates as a multifunctional platform for selectively steering the electronic and optical pathways on demand, as well as the potential application in design of mechanical logic gates. I then explore the hierarchical wrinkling of thin films on soft substrates via sequential wrinkling for design of a potential multifunctional smart window with combined structural color and water droplet transport control. The self-similar hierarchical wrinkles with both nanoscale and microscale features are generated on a pre-strained poly(dimethylsiloxane) (PDMS) elastomer through sequential strain release and multi-step oxygen plasma treatment. I exploit the criteria for generating self-similar hierarchical wrinkles through both simplified theoretical model and experiments. I show that the hierarchically wrinkled elastomer displays both opaqueness and iridescent structural color. I further show its ability in control of water droplet transport on demand through mechanical stretching and release. I further extend the study of self-similar hierarchical wrinkling to the dynamic wetting behavior of multiscale self-similar hierarchical wrinkled surfaces on PDMS substrates through combined plasma and ultraviolet ozone (UVO) treatment. The generated surface structure shows an independently controlled dual-scale roughness with level-1 small-wavelength wrinkles resting on level-2 large-wavelength wrinkles, as well as accompanying orthogonal cracks. By tuning the geometry of hierarchical wrinkles, I explore the small degree of wetting anisotropy in hierarchical wrinkled surfaces, defined as the contact angle difference between the parallel and perpendicular directions to the wrinkle grooves through both experimental characterization (confocal fluorescence imaging) and theoretical analyses. I find that the measured larger apparent contact angle than the theoretically predicted Wenzel contact angle is attributed to the three-phase contact line pinning effect of both wrinkles and cracks, which generates energetic barriers during the contact line motion. I reveal that the observed small degree of wetting anisotropy in the hierarchical wrinkled surfaces arises from the competition between orthogonal wrinkles and cracks in the contact line pinning.