The Nature of Solar Polar Rays

We use time series observations from the SOHO and Yohkoh spacecraft to study solar polar rays. Contrary to our expectations, we find that the rays are associated with active regions on the sun and are not features of the polar coronal holes. They are extended, hot plasma structures formed in the active regions and projected onto the plane of the sky above the polar coronal holes. We present new observations and simple projection models that match long-lived polar ray structures seen in limb synoptic maps. Individual projection patterns last for at least 5 solar rotations.Comment: 10 pages, 5 PostScript figures. Fig.1 is in color. The paper is also available at http://www.ifa.hawaii.edu/users/jing/papers.htm

Scaling and biomechanics of surface attachment in climbing animals.

Attachment devices are essential adaptations for climbing animals and valuable models for synthetic adhesives. A major unresolved question for both natural and bioinspired attachment systems is how attachment performance depends on size. Here, we discuss how contact geometry and mode of detachment influence the scaling of attachment forces for claws and adhesive pads, and how allometric data on biological systems can yield insights into their mechanism of attachment. Larger animals are expected to attach less well to surfaces, due to their smaller surface-to-volume ratio, and because it becomes increasingly difficult to distribute load uniformly across large contact areas. In order to compensate for this decrease of weight-specific adhesion, large animals could evolve overproportionally large pads, or adaptations that increase attachment efficiency (adhesion or friction per unit contact area). Available data suggest that attachment pad area scales close to isometry within clades, but pad efficiency in some animals increases with size so that attachment performance is approximately size-independent. The mechanisms underlying this biologically important variation in pad efficiency are still unclear. We suggest that switching between stress concentration (easy detachment) and uniform load distribution (strong attachment) via shear forces is one of the key mechanisms enabling the dynamic control of adhesion during locomotion.This is the final version of the article. It first appeared from the Royal Society via http://dx.doi.org/10.1098/rstb.2014.002

Rate-dependence of 'wet' biological adhesives and the function of the pad secretion in insects.

Many insects use soft adhesive footpads for climbing. The surface contact of these organs is mediated by small volumes of a liquid secretion, which forms thin films in the contact zone. Here, we investigate the role of viscous dissipation by this secretion and the 'bulk' pad cuticle by quantifying the rate-dependence of the adhesive force of individual pads. Adhesion increased with retraction speed, but this effect was independent of the amount of pad secretion present in the contact zone, suggesting that the secretion's viscosity did not play a significant role. Instead, the rate-dependence can be explained by relating the strain energy release rate to the speed of crack propagation, using an established empirical power law. The 'wet' pads' behaviour was akin to that of 'dry' elastomers, with an equilibrium energy release rate close to that of dry van-der-Waals contacts. We suggest that the secretion mainly serves as a 'release layer', minimising viscous dissipation and thereby reducing the time- and 'loading-history'-dependence of the adhesive pads. In contrast to many commercial adhesives which derive much of their strength from viscous dissipation, we show that the major modulator of adhesive strength in 'wet' biological adhesive pads is friction, exhibiting a much larger effect than retraction speed. A comparison between 'wet' and 'dry' biological adhesives, using both results from this study and the literature, revealed a striking lack of differences in attachment performance under varying experimental conditions. Together, these results suggest that 'wet' and 'dry' biological adhesives may be more similar than previously thought.This is the final version of the article. It first appeared from RSC via http://dx.doi.org/10.1039/C5SM01496

Dynamic biological adhesion: mechanisms for controlling attachment during locomotion.

The rapid control of surface attachment is a key feature of natural adhesive systems used for locomotion, and a property highly desirable for man-made adhesives. Here, we describe the challenges of adhesion control and the timescales involved across diverse biological attachment systems and different adhesive mechanisms. The most widespread control principle for dynamic surface attachment in climbing animals is that adhesion is 'shear-sensitive' (directional): pulling adhesive pads towards the body results in strong attachment, whereas pushing them away from it leads to easy detachment, providing a rapid mechanical 'switch'. Shear-sensitivity is based on changes of contact area and adhesive strength, which in turn arise from non-adhesive default positions, the mechanics of peeling, pad sliding, and the targeted storage and controlled release of elastic strain energy. The control of adhesion via shear forces is deeply integrated with the climbing animals' anatomy and locomotion, and involves both active neuromuscular control, and rapid passive responses of sophisticated mechanical systems. The resulting dynamic adhesive systems are robust, reliable, versatile and nevertheless remarkably simple. This article is part of the theme issue 'Transdisciplinary approaches to the study of adhesion and adhesives in biological systems'.This study was supported by a research grant by the Biotechnology and Biological Sciences Research Council (B/R017360/ 1) grant to David Labonte (Imperial College), and European Union’s Horizon 2020 research and innovation programme under the Marie Skłodowska-Curie grant to Walter Federle (Univ Cambridge) agreement no. 642861 to W.F

Functionally different pads on the same foot allow control of attachment: stick insects have load-sensitive "heel" pads for friction and shear-sensitive "toe" pads for adhesion.

Stick insects (Carausius morosus) have two distinct types of attachment pad per leg, tarsal "heel" pads (euplantulae) and a pre-tarsal "toe" pad (arolium). Here we show that these two pad types are specialised for fundamentally different functions. When standing upright, stick insects rested on their proximal euplantulae, while arolia were the only pads in surface contact when hanging upside down. Single-pad force measurements showed that the adhesion of euplantulae was extremely small, but friction forces strongly increased with normal load and coefficients of friction were [Formula: see text] 1. The pre-tarsal arolium, in contrast, generated adhesion that strongly increased with pulling forces, allowing adhesion to be activated and deactivated by shear forces, which can be produced actively, or passively as a result of the insects' sprawled posture. The shear-sensitivity of the arolium was present even when corrected for contact area, and was independent of normal preloads covering nearly an order of magnitude. Attachment of both heel and toe pads is thus activated partly by the forces that arise passively in the situations in which they are used by the insects, ensuring safe attachment. Our results suggest that stick insect euplantulae are specialised "friction pads" that produce traction when pressed against the substrate, while arolia are "true" adhesive pads that stick to the substrate when activated by pulling forces

Measuring progess on diet-related NCD's: the need to address the causes of the causes

WHO has developed nine voluntary global monitoring targets and 25 indicators to assess progress in the implementation of national strategies to achieve the global political commitment to reduce the probability of dying from non-communicable diseases (NCDs) for people aged 30–70 years, by 25% by 2025.1,2 Robert Beaglehole and colleagues (Oct 13, p 1283)3 argue that it is better to focus on two of the population-wide targets: tobacco control and dietary salt reduction, and the treatment target, and that the targets should be reported according to socioeconomic status and gender.Department of HE and Training approved lis

Surface contact and design of fibrillar 'friction pads' in stick insects (Carausius morosus): mechanisms for large friction coefficients and negligible adhesion.

Many stick insects and mantophasmids possess tarsal 'heel pads' (euplantulae) covered by arrays of conical, micrometre-sized hairs (acanthae). These pads are used mainly under compression; they respond to load with increasing shear resistance, and show negligible adhesion. Reflected-light microscopy in stick insects (Carausius morosus) revealed that the contact area of 'heel pads' changes with normal load on three hierarchical levels. First, loading brought larger areas of the convex pads into contact. Second, loading increased the density of acanthae in contact. Third, higher loads changed the shape of individual hair contacts gradually from circular (tip contact) to elongated (side contact). The resulting increase in real contact area can explain the load dependence of friction, indicating a constant shear stress between acanthae and substrate. As the euplantula contact area is negligible for small loads (similar to hard materials), but increases sharply with load (resembling soft materials), these pads show high friction coefficients despite little adhesion. This property appears essential for the pads' use in locomotion. Several morphological characteristics of hairy friction pads are in apparent contrast to hairy pads used for adhesion, highlighting key adaptations for both pad types. Our results are relevant for the design of fibrillar structures with high friction coefficients but small adhesion

Making the cut: end effects and the benefits of slicing

Cutting mechanics in soft solids have been a subject of study for several decades, an interest fuelled by the multitude of its applications, including material testing, manufacturing, and biomedical technology. Wire cutting is the simplest model system to analyze the cutting resistance of a soft material. However, even for this simple system, the complex failure mechanisms that underpin cutting are still not completely understood. Several models that connect the critical cutting force to the radius of the wire and the key mechanical properties of the cut material have been proposed. An almost ubiquitous simplifying assumption is a state of plane (and anti-plane) strain in the material. In this paper, we show that this assumption can lead to erroneous conclusions because even such a simple cutting problem is essentially three-dimensional. A planar approximation restricts the analysis to the stress distribution in the mid-plane. However, through finite element modeling, we reveal that the maximal tensile stress - and thus the likely location of cut initiation - is in fact located in the front plane. Friction reduces the magnitude of this stress, but this detrimental effect can be counteracted by large slice-to-push (shear-to-indentation) ratios. The introduction of these end effects helps reconcile a recent controversy around the role of friction in wire cutting, for it implies that slicing can indeed reduce required cutting forces, but only if the slice-push ratio and the friction coefficient are sufficiently large. Material strain-stiffening reduces the critical indentation depth required to initiate the cut further and thus needs to be considered when cutting non-linearly elastic materials

Surface contact and design of fibrillar 'friction pads' in stick insects (Carausius morosus): mechanisms for large friction coefficients and negligible adhesion.

Many stick insects and mantophasmids possess tarsal 'heel pads' (euplantulae) covered by arrays of conical, micrometre-sized hairs (acanthae). These pads are used mainly under compression; they respond to load with increasing shear resistance, and show negligible adhesion. Reflected-light microscopy in stick insects (Carausius morosus) revealed that the contact area of 'heel pads' changes with normal load on three hierarchical levels. First, loading brought larger areas of the convex pads into contact. Second, loading increased the density of acanthae in contact. Third, higher loads changed the shape of individual hair contacts gradually from circular (tip contact) to elongated (side contact). The resulting increase in real contact area can explain the load dependence of friction, indicating a constant shear stress between acanthae and substrate. As the euplantula contact area is negligible for small loads (similar to hard materials), but increases sharply with load (resembling soft materials), these pads show high friction coefficients despite little adhesion. This property appears essential for the pads' use in locomotion. Several morphological characteristics of hairy friction pads are in apparent contrast to hairy pads used for adhesion, highlighting key adaptations for both pad types. Our results are relevant for the design of fibrillar structures with high friction coefficients but small adhesion

Disentangling the role of surface topography and intrinsic wettability in the prey capture mechanism of Nepenthes pitcher plants.

Nepenthes pitcher plants capture prey with leaves specialised as pitfall traps. Insects are trapped when they 'aquaplane' on the pitcher rim (peristome), a surface structured with macroscopic and microscopic radial ridges. What is the functional significance of this hierarchical surface topography? Here, we use insect pad friction measurements, photolithography, wetting experiments and physical modelling to demonstrate that the ridges enhance the trap's efficacy by satisfying two functional demands on prey capture: Macroscopic ridges restrict lateral but enhance radial spreading of water, thereby creating continuous slippery tracks which facilitate prey capture when little water is present. Microscopic ridges, in turn, ensure that the water film between insect pad and peristome remains stable, causing insects to aquaplane. In combination, the hierarchical ridge structure hence renders the peristome wettable, and water films continuous, so avoiding the need for a strongly hydrophilic surface chemistry, which would compromise resistance to desiccation and attract detrimental contamination.BBSRC (BB/I008667/1