Preparation and characterization of cationic nanofibrillated cellulose from etherification and high-shear disintegration processes

Oat straw cellulose pulp was cationized in an etherification reaction with chlorocholine chloride. The cationized cellulose pulp was then mechanically disintegrated in two process steps to obtain trimethylammonium-modified nanofibrillated cellulose (TMA-NFC). The materials thus obtained were analyzed by elemental analysis, X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD), scanning electron microscopy (SEM) and other techniques. A higher nitrogen content of TMA-NFC samples was found by XPS analysis than by elemental analysis, which indicates that the modification occurred mainly on the surface of cellulose fibrils. XPS also confirmed the existence of ammonium groups in the samples. SEM provided images of very fine network structures of TMA-NFC, which affirmed the positive effect of ionic charge on mechanical disintegration process. According to XRD and SEM results, no severe degradation of the cellulose occurred, even at high reaction temperatures. Because of the different properties of the cationic NFC compared to negatively charged native cellulose fibers, TMA-NFC may find broad applications in technical areas, for instance in combination with anionic species, such as fillers or dyes. Indeed, TMA-NFC seems to improve the distribution of clay fillers in NFC matri

Evolutionary prisoner's dilemma games with optional participation

Competition among cooperators, defectors, and loners is studied in an evolutionary prisoner's dilemma game with optional participation. Loners are risk averse i.e. unwilling to participate and rather rely on small but fixed earnings. This results in a rock-scissors-paper type cyclic dominance of the three strategies. The players are located either on square lattices or random regular graphs with the same connectivity. Occasionally, every player reassesses its strategy by sampling the payoffs in its neighborhood. The loner strategy efficiently prevents successful spreading of selfish, defective behavior and avoids deadlocks in states of mutual defection. On square lattices, Monte Carlo simulations reveal self-organizing patterns driven by the cyclic dominance, whereas on random regular graphs different types of oscillatory behavior are observed: the temptation to defect determines whether damped, periodic or increasing oscillations occur. These results are compared to predictions by pair approximation. Although pair approximation is incapable of distinguishing the two scenarios because of the equal connectivity, the average frequencies as well as the oscillations on random regular graphs are well reproduced.Comment: 6 pages, 7 figure

Carrier relaxation in GaAs v-groove quantum wires and the effects of localization

Carrier relaxation processes have been investigated in GaAs/AlGaAs v-groove quantum wires (QWRs) with a large subband separation (46 meV). Signatures of inhibited carrier relaxation mechanisms are seen in temperature-dependent photoluminescence (PL) and photoluminescence-excitation (PLE) measurements; we observe strong emission from the first excited state of the QWR below ~50 K. This is attributed to reduced inter-subband relaxation via phonon scattering between localized states. Theoretical calculations and experimental results indicate that the pinch-off regions, which provide additional two-dimensional confinement for the QWR structure, have a blocking effect on relaxation mechanisms for certain structures within the v-groove. Time-resolved PL measurements show that efficient carrier relaxation from excited QWR states into the ground state, occurs only at temperatures > 30 K. Values for the low temperature radiative lifetimes of the ground- and first excited-state excitons have been obtained (340 ps and 160 ps respectively), and their corresponding localization lengths along the wire estimated.Comment: 9 pages, 8 figures, submitted to Phys. Rev. B Attempted to correct corrupt figure

The implant material, Ti6Al7Nb: surface microstructure, composition and properties

The excellent biocompatibility of titanium and its alloys is intimately related with the properties of the surface in contact with the biological environment, and therefore it is closely connected with the stable, passivating oxide layer that forms on its surface. In the present paper, the oxide layer on the alloy Ti6Al7Nb has been characterized using X-ray photoelectron spectroscopy, scanning Auger microscopy and pH-dependent lateral force microscopy. The alloying elements Al and Nb are incorporated in the oxide layer and detected in their most stable oxidized form, as Al2O3 and Nb2O5. Their distribution in the oxide reflects the underlying α-β microstructure, with enrichment of Al in the α- and of Nb in the β-phase (determined by electron microprobe). Friction measurements (lateral force microscopy) indicate slightly different, pH-dependent, lateral forces above the α- and β-phase structures that point to small local variations in surface charges. © 1999 Kluwer Academic Publisher

Coevolutionary Dynamics: From Finite to Infinite Populations

Traditionally, frequency dependent evolutionary dynamics is described by deterministic replicator dynamics assuming implicitly infinite population sizes. Only recently have stochastic processes been introduced to study evolutionary dynamics in finite populations. However, the relationship between deterministic and stochastic approaches remained unclear. Here we solve this problem by explicitly considering large populations. In particular, we identify different microscopic stochastic processes that lead to the standard or the adjusted replicator dynamics. Moreover, differences on the individual level can lead to qualitatively different dynamics in asymmetric conflicts and, depending on the population size, can even invert the direction of the evolutionary process.Comment: 4 pages (2 figs included). Published in Phys. Rev. Lett., December 200

Onboard Evolution of Understandable Swarm Behaviors

Designing the individual robot rules that give rise to desired emergent swarm behaviors is difficult. The common method of running evolutionary algorithms off‐line to automatically discover controllers in simulation suffers from two disadvantages: the generation of controllers is not situated in the swarm and so cannot be performed in the wild, and the evolved controllers are often opaque and hard to understand. A swarm of robots with considerable on‐board processing power is used to move the evolutionary process into the swarm, providing a potential route to continuously generating swarm behaviors adapted to the environments and tasks at hand. By making the evolved controllers human‐understandable using behavior trees, the controllers can be queried, explained, and even improved by a human user. A swarm system capable of evolving and executing fit controllers entirely onboard physical robots in less than 15 min is demonstrated. One of the evolved controllers is then analyzed to explain its functionality. With the insights gained, a significant performance improvement in the evolved controller is engineered