Self-similar Hierarchical Wrinkles as a Potential Multifunctional Smart Window with Simultaneously Tunable Transparency, Structural Color, and Droplet Transport

Smart window has
immense potential for energy savings in architectural and vehicular
applications, while most studies focus on the tunability of a single
property of optical transmittance. Here we explore harnessing dynamically
tunable hierarchical wrinkles 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 prestrained poly­(dimethylsiloxane)
elastomer through sequential strain release and multistep oxygen plasma
treatment. We show that the hierarchically wrinkled elastomer displays
both opaqueness and iridescent structural color. We find that restretching/releasing
the elastomer leads to the reversible and repeatable switch from opaqueness
to transparency, arising from the flattening of large wrinkles (micrometer
scale), while a nonvanishing structural color occurs due to the nondisappearing
small wrinkles (nanoscale). The unique features of combined reversible
large wrinkles and irreversible small wrinkles during hierarchical
wrinkling are well reproduced by corresponding finite element simulation.
The criteria for generating self-similar hierarchical wrinkles is
revealed through a simplified theoretical model and validated by experiments.
In addition to its tunable optical property, we further show its ability
in control of water droplet transport on demand through mechanical
stretching and release. We find that an initially pinned water droplet
on the tilted hierarchically wrinkled surface starts to slide when
the surface is stretched, and becomes pinned again upon strain release.
Such a process is reversible and repeatable. The hierarchically wrinkled
surface could find broad potential applications not only in multifunctional
smart windows with additional features of aesthetics and water collection,
but in microfluidics, design of slippery surfaces, and directional
water transportation.