Surface Dynamics at Photoactive Liquid Crystal Polymer Networks

Abstract

creating insulation under cold conditions and emitting evaporable liquid when it is warm. [1][2][3] Peacock feathers, wings of certain butterflies, and plants exhibit color by means of structural color to communicate attraction or, inversely, repulsion. Often, the colors originate from periodic structures on or under a surface, thus benefiting from the interplay of light and diffractive effects.A surface is the outermost layer of matter and therefore is primarily perceived by humans upon touch or vision. Surfaces provide perceptible information by not only shape, color, and/or reflective properties but also how the matter feels upon touch, e.g., smooth or rough. Welldocumented static structures have been reported for haptic applications. [4] Studies have suggested that humans can distinguish changes in topographical dimensions down to nanometers. [5] Additionally, a surface is the first contact between objects of matter, either of the same object or that of another origin. Surfaces facilitate man-machine interactions, [6] either by touch (smooth, rough, heat conductive, etc.) or by vision (scattering, specular reflection, printed information, etc.). [7][8][9] In relation to this function, tribological properties such as friction, stick, and adhesion coefficients also find application in the field of haptics, where robotic manipulation is of importance. In contrast to nature, the surface structures in modern man-made technology are often still static. Mimicking the lotus flower, surfaces with artificially formed micrometer-sized structures that are produced by lithography or micromachining have been made, and they indeed show improved cleaning under rainy conditions. However, one expands the surface function when these structures are made dynamic, rather than static, for example, the surface would be able to remove dirt under dry conditions, as well. In the case of topographies for haptic use, man's sensitivity could be enhanced when the corrugations are moving with a frequency tuned to the maximum sensitivity of humans. However, only a few publications have described dynamic structures to enhance human-surface interactions, which could even help visually disabled people. Moreover, in an advanced technology such as microfluidics, the low Reynolds number prohibits turbulent mixing of fluid components. Mixing can be induced by surface topographical structures in the microchannels to increase the contact area and the contact time between the reagents. The microfluidic functionality would be enhanced if the topography of its inner walls could be switched in a dynamic way, e.g., to decide on the exact location and time that switching The field of advanced and responsive soft materials is at the edge of a new era. After several decades during which liquid crystals generated new functions for information displays and could solve many problems in emerging fields such as (tele)communications, this material system is being utilized to reach out to completely new application fields with functions that can take over biologica...

Light-Fueled Nanoscale Surface Waving in Chiral Liquid Crystal Networks

“…This fact together with a constant area under AFM profiles upon illumination (see Figures 3 c and S5 ) indicates that light-driven inversion of topography proceeds with an insignificant change in volume, unlike previously reported dynamic surfaces based on cholesteric networks with fingerprint structures. 10 In our case, operation of azobenzene cross-linker pulls the strands of polymer network along the alignment direction and reduces the order parameter due to nonmesogenic character of cis -Azo-1, which results in coherent force generation and therefore in the inversion of surface topography.…”

Photoswitching between Water‐Tolerant Adhesion and Swift Release by Inverting Liquid Crystal Fingerprint Topography

“…Subsequent opposite topographical responses occur in planar and homeotropic areas: the homeotropic orientation areas contract along the LC director and planar areas expand in the thickness direction, leading to the inversion of the surface topography. [29,30] Subsequently, the adhesive homeotropic areas become topographically lower than the nonadhesive planar areas upon UV light stimulation, resulting in the switch of the dynamic coating from adhesive to nonadhesive.…”

“…Owing to the azobenzene crosslinkers, the order parameter of the light-sensitive LCN coating can be reduced by the transformation of planar azo trans-isomers to bent cis-isomers. [29] Upon reduction of the order parameter, uniaxially aligned LC domains would shrink along with the LC director and expand in the perpendicular direction. [33] In the case of present fingerprint LCN coatings, topographical deformation occurs due to different deformation behavior of the alternating areas.…”

Functional Liquid Crystal Polymer Surfaces with Switchable Topographies

“…[ 7–12 ] Switchable physical or chemical properties offer possibilities to regulate the function on demand with external stimuli. [ 13–18 ] Among various parameters, surface topography is a key physical factor that can affect the functions of surfaces. Surface morphing, also referred to as a switch in topographies, would affect the performance of surfaces.…”

“…To date, various materials have been investigated to fabricate switchable surfaces with switchable topographies, such as hydrogels, [ 23,24 ] silicones, [ 17,25 ] and liquid crystal (LC) networks/elastomers. [ 13,26,27 ] The various materials that are known to deform their surfaces exhibit both unique advantages and shortcomings. With hydrogels, the shrinking/swelling mismatch is often used to induce shape change of the hydrogels.…”