Snake‐Inspired, Nano‐Stepped Surface with Tunable Frictional Anisotropy Made from a Shape‐Memory Polymer for Unidirectional Transport of Microparticles

Abstract

The ventral scales of many snake species are decorated with oriented micro‐fibril structures featuring nano‐steps to achieve anisotropic friction for efficient locomotion. Here, a nano‐stepped surface with tunable frictional anisotropy inspired by this natural structure is presented. It is fabricated by replicating the micro‐fibril structure of the ventral scales of the Chinese cobra (Naja atra) into a thermo‐responsive shape‐memory polymer via hot embossing. The resulting smart surface transfers from a flat topography to a predefined structure of nano‐steps upon heating. During this recovery process, the nano‐steps grow out of the surfaces resulting in a surface with frictional anisotropy, which is characterized in situ by an atomic force microscopy. The desired frictional anisotropy can be customized by stopping the heating process before full recovery. The nano‐stepped surface is employed for the unidirectional transport of microscale particles through small random vibrations. Due to the frictional anisotropy, the microspheres drift unidirectionally (down the nano‐steps). Finally, dry self‐cleaning is demonstrated by the transportation of a pile of microparticles.

Precise Controlling of Friction and Adhesion on Reprogrammable Shape Memory Micropillars

“…Tuning friction in a dynamic, reversible, and controllable way has long been a great challenge in the mechanics and materials community . Now, it is clear that the deformability of micro-structures can provide a way of tuning the friction of contact interfaces, involving stimulus response materials. , Although extensive progress has been made in tunable friction, the locally regulatable surface friction properties obstruct the further development of surface engineering. The SMRM surface can be developed to be a smart device with reprogrammable and adjustable friction behaviors because of its remotely and precisely regulated morphology abilities.…”

“…29−33 For example, to realize an efficient locomotion control for unidirectional transport of microparticles, Wu et al fabricated a nano-stepped surface with tunable frictional anisotropy by replicating the micro-fibril structure of the ventral scales of the cobra into a thermo-responsive SMP via hot embossing. 34 Combining the advantages of gecko seta and creeper root, Tan et al developed a micropillar surface with switchable adhesion based on a kind of graphene/shape memory polymer composites; the micropillar array can be switched between the robust-adhesion state and low-adhesion state by electric heating. 35 Recently, inspired by the special super-wetting performances and microstructure shapes on the lotus leaf and rice leaf, Cheng et al reported a surface that can transit reversibly between the superhydrophobic isotropic and anisotropic wettings through dynamically controlling the shape memory micropillar shape to imitate a lotus-leaflike structure and a rice-leaf-like structure, respectively.…”

“…Till now, some SMPs have been employed as the ideal candidates to develop programmable micropillars for varied functionalities, since those materials can be readily set as a temporary shape through a programming step and recover their original shape in response to an external stimulus. − These smart material systems have been widely used to manufacture tunable friction and tunable wettability surfaces. − For example, to realize an efficient locomotion control for unidirectional transport of microparticles, Wu et al fabricated a nano-stepped surface with tunable frictional anisotropy by replicating the micro-fibril structure of the ventral scales of the cobra into a thermo-responsive SMP via hot embossing . Combining the advantages of gecko seta and creeper root, Tan et al.…”

Programming Anisotropic Functionality of 3D Microdenticles by Staggered‐Overlapped and Multilayered Microarchitectures

“…The hierarchical microtextured surfaces of natural living creatures have attracted considerable scientific attention, with researchers attempting to mimic their various functionalities, [1] such as self-cleaning, [2,3] antifouling, [4][5][6] and drag reduction. [7][8][9][10][11][12] For example, sharkskin can reduce surface drag [7,8] to facilitate fast swimming [4] (Figure 1a) and exhibit hard-on-soft multilayered mechanical properties, to protect itself from predator attacks.…”

Shape Memory Polyurethane as a Drilling Fluid Lost Circulation Material

“…These polymers are set in temporary shape, and when triggered by an external stimulus such as heat, pH, light, electric field, water and magnetic field recovered their original shape. [24][25][26][27][28] For example, thermo-responsive SMP effectively plugged fractures once triggered by formation temperature (≈70 °C). [29][30][31][32] In another study, a thermoset SMP mixed with fibers having wide size distribution increased the fracture plugging efficiency significantly.…”