Microrobots that are light and agile yet require no artificial power input can be widely used in medical, military, and industrial applications. As an actuation system to drive such robots, here we report a biologically inspired bilayer structure that harnesses the environmental humidity energy, with ratchets to rectify the motion. We named this actuator-ratchet system the hygrobot. The actuator uses a hygroscopically responsive film consisting of aligned nanofibers produced by directional electrospinning, which quickly swells and shrinks in lengthwise direction in response to the change of humidity. The ratchets based on asymmetric friction coefficients rectify oscillatory bending motion in a directional locomotion. We mathematically analyzed the mechanical response of the hygrobot, which allowed not only prediction of its performance but also the optimal design to maximize the locomotion speed given geometric and environmental constraints. The hygrobot sterilized a trail across an agar plate without any artificial energy supply.

Capillary rise of water in porous cellulose sponges is investigated considering hygroscopic shape evolutions of micropores.

Articles you may be interested inEffect of indium surfactant on stress relaxation by V-defect formation in GaN epilayers grown by metalorganic chemical vapor deposition J. Appl. Phys. 108, 093511 (2010); 10.1063/1.3487955Strain-free bulk-like GaN grown by hydride-vapor-phase-epitaxy on two-step epitaxial lateral overgrown GaN template

No abstract

A cellulose sponge is a mundane porous medium composed of numerous microporous cellulose sheets surrounding macroscale voids. Here, we quantify the capillary rise dynamics of non-aqueous liquids in a sponge using a combination of experiment and theory. Although the classical law of Washburn is obeyed in the early stages, the wet front propagation is no longer diffusive in the late stages and follows a power law, $h\sim t^{1/4}$, with $h$ and $t$ being the capillary rise height and time respectively. The transition of the power law is a consequence of the peculiar heterogeneous pore structure of cellulose sponges. The permeability and driving pressure change at the rise height above which the macro voids can no longer be filled completely due to significant effects of gravity. We rationalize the $t^{1/4}$ law by considering liquid flows along the corners of macro voids driven by capillary pressure of microscale wall pores.

We report significant reduction of threading dislocations in GaN films grown by hydride vapor phase epitaxy on AlN/sapphire templates by employing CrN nanoislands on the AlN. High quality GaN films with very small twist mosaic as well as small tilt mosaic have been grown on the AlN/sapphire templates, which had small tilt but very large twist mosaic. The CrN nanoislands were formed by nitridation of a thin Cr film deposited by sputtering on the AlN/sapphire template, where the AlN/sapphire template was prepared by metal organic vapor phase epitaxy. The full width at half maximum values of x-ray rocking curves from the GaN film with the CrN were 114, 209, and 243arcsec for (0002), (10−12), and (11−20) reflections, respectively, while those of the GaN film without the CrN were 129, 1130, and 1364arcsec, respectively. Evaluation of total dislocation density of the GaN films by plan view transmission electron microscopy revealed that the dislocation density was reduced to 2.7×108 from 6.4×109cm−2 by employing the CrN nanoislands. The CrN nanoislands play a key role in reducing the threading dislocations by masking the propagation of dislocations as well as by bending the dislocations.

GaN epitaxial films are grown on c‐plane sapphire substrates with CrN buffer. The GaN layers show high crystalline quality and smooth surface morphology without being cracked. Selective etching of CrN buffer is performed by wet etching using conventional Cr metal etchant, which results in success ful lift‐off of GaN thick layers. We confirm that the crystalline quality of GaN does not change through the etching process. These results indicate that the chemical lift‐off process using CrN buffer is promising for production of both freestanding GaN substrates and vertical‐structure devices. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Strain-free freestanding GaN layers were prepared by in situ lift-off process using a ZnO buffer as a sacrificing layer. Thin Zn-polar ZnO layers were deposited on c-plane sapphire substrates, which was followed by the growth of Ga-polar GaN layers both by molecular beam epitaxy (MBE). The MBE-grown GaN layer acted as a protecting layer against decomposition of the ZnO layer and as a seeding layer for GaN growth. The ZnO layer was completely in situ etched off during growth of thick GaN layers at low temperature by hydride vapor phase epitaxy. Hence freestanding GaN layers were obtained for the consecutive growth of high-temperature GaN thick layers. The lattice constants of freestanding GaN agree with those of strain-free GaN bulk. Extensive microphotoluminescence study indicates that strain-free states extend throughout the high-temperature grown GaN layers.