Shape-Programmed Folding of Stimuli-Responsive Polymer Bilayers

Georgi Stoychev†‡, Svetlana Zakharchenko†‡, Sébastien Turcaud§, John W. C. Dunlop§, and Leonid Ionov†*

^† Leibniz Institute of Polymer Research Dresden, Hohe Straße 6, D-01069 Dresden, Germany and Department of Biomaterials

^‡ Technische Universität Dresden, Physical Chemistry of Polymer Materials, 01062 Dresden, Germany

^§ Max Planck Institute of Colloids and Interfaces

ACS Nano, Article ASAP

DOI: 10.1021/nn300079f

Publication Date (Web): April 24, 2012

Copyright © 2012 American Chemical Society

We investigated the folding of rectangular stimuli-responsive hydrogel-based polymer bilayers with different aspect ratios and relative thicknesses placed on a substrate. It was found that long-side rolling dominates at high aspect ratios (ratio of length to width) when the width is comparable to the circumference of the formed tubes, which corresponds to a small actuation strain. Rolling from all sides occurs for higher actuation, namely when the width and length considerably exceed the deformed circumference. In the case of moderate actuation, when both the width and length are comparable to the deformed circumference, diagonal rolling is observed. Short-side rolling was observed very rarely and in combination with diagonal rolling. On the basis of experimental observations, finite-element modeling and energetic considerations, we argued that bilayers placed on a substrate start to roll from corners due to quicker diffusion of water. Rolling from the long-side starts later but dominates at high aspect ratios, in agreement with energetic considerations. We have shown experimentally and by modeling that the main reasons causing a variety of rolling scenarios are (i) non-homogenous swelling due to the presence of the substrate and (ii) adhesion of the polymer to the substrate.

Controlling Nanoscale Friction through the Competition between Capillary Adsorption and Thermally Activated Sliding

Christian Greiner†, Jonathan R. Felts‡, Zhenting Dai‡, William P. King‡, and Robert W. Carpick†*

^† Department for Mechanical Engineering and Applied Mechanics, University of Pennsylvania, 112 Towne Building, 220 South 33rd Street, Philadelphia, Pennsylvania 19104, United States,

^‡ Department of Mechanical Science and Engineering,University of Illinois at Urbana—Champaign, 242 Mechanical Engineering Building, 1206 West Green Street, Urbana, Illinois 61801, United States

We demonstrate measurement and control of nanoscale single-asperity friction by using cantilever probes featuring an in situ solid-state heater in contact with silicon oxide substrates. The heater temperature was varied between 25 and 790 °C. By using a low thermal conductivity sample, silicon oxide, we are able to vary tip temperatures over a broad range from 25 ± 2 to 255 ± 25 °C. In ambient atmosphere with 30% relative humidity, the control of friction forces was achieved through the formation of a capillary bridge whose characteristics exhibit a strong dependence on temperature and sliding speed. The capillary condensation is observed to be a thermally activated process, such that heating in ambient air caused friction to increase due to the capillary bridge nucleating and growing. Above tip temperatures of 100 ± 10 °C, friction decreased drastically, which we attribute to controllably evaporating water from the contact at the nanoscale. In contrast, in a dry nitrogen atmosphere, friction was not affected appreciably by temperature changes. In the presence of a capillary, friction decreases at higher sliding speeds due to disruption of the capillary; otherwise, friction increases in accordance with the predictions of a thermally assisted sliding model. In ambient atmospheres, the rate of increase of friction with sliding speed at room temperature is sufficiently strong that the friction force changes from being smaller than the response at 76 ± 8 °C to being larger. Thus, an appropriate change in temperature can cause friction to increase at one sliding speed, while it decreases at another speed.

Angle-Independent Reflectors: Flexible, Angle-Independent, Structural Color Reflectors Inspired by Morpho Butterfly Wings

1. Kyungjae Chung¹, 
2. Sunkyu Yu², 
3. Chul-Joon Heo³, 
4. Jae Won Shim³, 
5. Seung-Man Yang³,
6. Moon Gyu Han⁴, 
7. Hong-Seok Lee⁴,
8. Yongwan Jin⁴, 
9. Sang Yoon Lee⁴, 
10. Namkyoo Park², 
11. Jung H. Shin^1,*

Article first published online: 2 MAY 2012

DOI: 10.1002/adma.201290105

The image shows a schematic representation of close-packed multilayer reflecting columns with the same periodicity but with random variations in location, both in horizontal and vertical dimensions, that form the Morpho-mimetic thin-film structural color reflectors described in the manuscript by J. H. Shin and co-workers, on page 2375. Overlaid are photographs of an actual Morpho butterfly, a 6-inch diameter Morpho-mimetic thin film that demonstrates its color, brightness, and flexibility, and images of cyan, green, and red ‘Morpho butterflies’ created from photos of Morpho-mimetic thin films with corresponding colors.

Designing biomimetic pores based on carbon nanotubes

1. Rebeca García-Fandiño^a,^b and 
2. Mark S. P. Sansom^a,¹

1. Published online before print April 16, 2012, doi:10.1073/pnas.1119326109

1. Biomimetic nanopores based on membrane-spanning single-walled carbon nanotubes have been designed to include selectivity filters based on combinations of anionic and cationic groups mimicking those present in bacterial porins and in voltage-gated sodium and calcium channels. The ion permeation and selectivity properties of these nanopores when embedded in a phospholipid bilayer have been explored by molecular dynamics simulations and free energy profile calculations. The interactions of the nanopores with sodium, potassium, calcium, and chloride ions have been explored as a function of the number of anionic and cationic groups within the selectivity filter. Unbiased molecular dynamics simulations show that the overall selectivity is largely determined by the net charge of the filter. Analysis of distribution functions reveals considerable structuring of the distribution of ions and water within the nanopores. The distributions of ions along the pore axis reveal local selectivity for cations around filter, even in those nanopores (C0) where the net filter charge is zero. Single ion free energy profiles also reveal clear evidence for cation selectivity, even in the C0 nanopores. Detailed analysis of the interactions of the C0 nanopore with Ca²⁺ions reveals that local interactions with the anionic (carboxylate) groups of the selectivity filter lead to (partial) replacement of solvating water as the ion passes through the pore. These studies suggest that a computational biomimetic approach can be used to evaluate our understanding of the design principles of nanopores and channels.

Liquid-crystal-mediated self-assembly at nanodroplet interfaces

• J. A. Moreno-Razo,
• E. J. Sambriski,
• N. L. Abbott,
• J. P. Hernández-Ortiz
• & J. J. de Pablo

• Affiliations
• Contributions
• Corresponding author

Nature

 

485,

 

86–89

 

(03 May 2012)

 

doi:10.1038/nature11084

Received

 

10 October 2011 

Accepted

 

19 March 2012 

Published online

 

02 May 2012

Technological applications of liquid crystals have generally relied on control of molecular orientation at a surface or an interface^1, 2. Such control has been achieved through topography, chemistry and the adsorption of monolayers or surfactants^2, 3. The role of the substrate or interface has been to impart order over visible length scales and to confine the liquid crystal in a device. Here, we report results from a computational study of a liquid-crystal-based system in which the opposite is true: the liquid crystal is used to impart order on the interfacial arrangement of a surfactant. Recent experiments on macroscopic interfaces have hinted that an interfacial coupling between bulk liquid crystal and surfactant can lead to a two-dimensional phase separation of the surfactant at the interface⁴, but have not had the resolution to measure the structure of the resulting phases. To enhance that coupling, we consider the limit of nanodroplets, the interfaces of which are decorated with surfactant molecules that promote local perpendicular orientation of mesogens within the droplet. In the absence of surfactant, mesogens at the interface are all parallel to that interface. As the droplet is cooled, the mesogens undergo a transition from a disordered (isotropic) to an ordered (nematic or smectic) liquid-crystal phase. As this happens, mesogens within the droplet cause a transition of the surfactant at the interface, which forms new ordered nanophases with morphologies dependent on surfactant concentration. Such nanophases are reminiscent of those encountered in block copolymers⁵, and include circular, striped and worm-like patterns.