Switchability induced by mechanical instability in bioinspired adhesives

Non-covalent adhesion produced by the gecko is attributed to the structured surface of its toes. The synthetic adhesives mimicking this principle have now been around for a decade. However, the characteristic features of reversibility and self-cleaning ability of the gecko adhesive system have not yet been successfully integrated. The present work focuses on developing a switchable adhesive system responsive to an external stimulus. Elastomeric polydimethylsiloxane surfaces are structured with fibrillar arrays. Mechanical instability of the fibrils is recognized and utilized to produce a reversible switch between adhesion and non-adhesion. Normal compression caused the fibrils to buckle inducing a contact transition from their tips to the sides. When the contact transition occurred under moderate compressive loads, tip contact re-formed upon reversal of buckling and adhesion was reversible. However, when reversible buckling occurred under large compressive loads or when fibril side peeled without unbuckling, contact re-formation was impaired. Drastic change incontact area in the re-formed state resulted in a low adhesion state. The role of fibril contact shape, radius, aspect ratio, orientation and the applied compressive load in the adhesion switchability was examined. In situ visualization was employed to study the contact mechanisms. Contact shape, fibril orientation and preload were identified as the key parameters for controlling switchable adhesion.Die nicht-kovalente Adhäsion von Geckos beruht auf der Oberflächenstruktur ihrer Zehen. Während der letzten zehn Jahre wurden künstliche Haftsysteme hergestellt, die auf diesem Prinzip beruhen. Die charakteristischen Eigenschaften des Gecko-Haftsystems, Reversibilität und Selbstreinigung, konnten jedoch bisher nicht integriert werden. Ziel dieser Arbeit ist die Entwicklung und Untersuchung eines reversiblen Haftsystems, das durch einen externen Stimulus geschaltet werden kann. Dazu wurden fibilläre Oberflächen aus Polydimethylsiloxan hergestellt. Die mechanische Instabilität der Fibrillen wurde zur reversiblen Schaltung zwischen haftendem und nicht-haftendem Zustand verwendet. Senkrechter Druck auf die Fibrillen führte zu Knickung, wobei ein Übergang von Spitzen- zu Seiten-Kontakt der Strukturen beobachtet wurde. Bei moderatem Druck konnte sich nach Entlasten und dem Wiederaufrichten der Fibrillen der Spitzen-Kontakt wiederherstellen, was zu hoher Adhäsion führte. Bei starkem Druck wurde der Spitzenkontakt nach der Knickung nicht wieder hergestellt. Die starke Änderung der Kontaktfläche führte dann zu einer niedrigen Adhäsion. Der Einfluss von Druck, Kontaktform, Radius und Aspektverhältnis sowie Ausrichtung der Fibrillen auf die Schaltbarkeit der Adhäsion wurde untersucht. Die Kontaktbildungsmechanismen wurden mittels in situ Visualisierung beobachtet. Kontaktform, Fibrillenausrichtung und Druck wurden als Schlüsselparameter zur Kontrolle der schaltbaren Adhäsion identifiziert

Switchability induced by mechanical instability in bioinspired adhesives

Non-covalent adhesion produced by the gecko is attributed to the structured surface of its toes. The synthetic adhesives mimicking this principle have now been around for a decade. However, the characteristic features of reversibility and self-cleaning ability of the gecko adhesive system have not yet been successfully integrated. The present work focuses on developing a switchable adhesive system responsive to an external stimulus. Elastomeric polydimethylsiloxane surfaces are structured with fibrillar arrays. Mechanical instability of the fibrils is recognized and utilized to produce a reversible switch between adhesion and non-adhesion. Normal compression caused the fibrils to buckle inducing a contact transition from their tips to the sides. When the contact transition occurred under moderate compressive loads, tip contact re-formed upon reversal of buckling and adhesion was reversible. However, when reversible buckling occurred under large compressive loads or when fibril side peeled without unbuckling, contact re-formation was impaired. Drastic change incontact area in the re-formed state resulted in a low adhesion state. The role of fibril contact shape, radius, aspect ratio, orientation and the applied compressive load in the adhesion switchability was examined. In situ visualization was employed to study the contact mechanisms. Contact shape, fibril orientation and preload were identified as the key parameters for controlling switchable adhesion.Die nicht-kovalente Adhäsion von Geckos beruht auf der Oberflächenstruktur ihrer Zehen. Während der letzten zehn Jahre wurden künstliche Haftsysteme hergestellt, die auf diesem Prinzip beruhen. Die charakteristischen Eigenschaften des Gecko-Haftsystems, Reversibilität und Selbstreinigung, konnten jedoch bisher nicht integriert werden. Ziel dieser Arbeit ist die Entwicklung und Untersuchung eines reversiblen Haftsystems, das durch einen externen Stimulus geschaltet werden kann. Dazu wurden fibilläre Oberflächen aus Polydimethylsiloxan hergestellt. Die mechanische Instabilität der Fibrillen wurde zur reversiblen Schaltung zwischen haftendem und nicht-haftendem Zustand verwendet. Senkrechter Druck auf die Fibrillen führte zu Knickung, wobei ein Übergang von Spitzen- zu Seiten-Kontakt der Strukturen beobachtet wurde. Bei moderatem Druck konnte sich nach Entlasten und dem Wiederaufrichten der Fibrillen der Spitzen-Kontakt wiederherstellen, was zu hoher Adhäsion führte. Bei starkem Druck wurde der Spitzenkontakt nach der Knickung nicht wieder hergestellt. Die starke Änderung der Kontaktfläche führte dann zu einer niedrigen Adhäsion. Der Einfluss von Druck, Kontaktform, Radius und Aspektverhältnis sowie Ausrichtung der Fibrillen auf die Schaltbarkeit der Adhäsion wurde untersucht. Die Kontaktbildungsmechanismen wurden mittels in situ Visualisierung beobachtet. Kontaktform, Fibrillenausrichtung und Druck wurden als Schlüsselparameter zur Kontrolle der schaltbaren Adhäsion identifiziert

In situ observation of contact mechanisms in bioinspired adhesives at high magnification

We analyzed the contact mechanisms of bioinspired microfibrillar adhesives using in situ scanning electron microscopy. During adhesion tests we observed that (i) the superior adhesion of mushroom-shaped fibrils is assisted by the stochastic nature of detachment, (ii) the aspect ratio of microfibrils influences the bending/buckling behavior and the contact reformation, and (iii) the backing layer deformation causes the microfibrils to elastically interact with each other. These studies give new insights into the mechanisms responsible for adhesion of bioinspired fibrillar adhesives

Neuronal diversity of the amygdala and the bed nucleus of the stria terminalis

The amygdala complex is a diverse group of more than 13 nuclei, segregated in five major groups: the basolateral (BLA), central (CeA), medial (MeA), cortical (CoA), and basomedial (BMA) amygdala nuclei. These nuclei can be distinguished depending on their cytoarchitectonic properties, connectivity, genetic, and molecular identity, and most importantly, on their functional role in animal behavior. The extended amygdala includes the CeA and the bed nucleus of the stria terminalis (BNST). Both CeA and the BNST share similar cellular organization, including common neuron types, reciprocal connectivity, and many overlapping downstream targets. In this section, we describe the advances of our knowledge on neuronal diversity in the amygdala complex and the BNST, based on recent functional studies, performed at genetic, molecular, physiological, and anatomical levels in rodent models, especially rats and mice. Molecular and connection property can be used separately, or in combinations, to define neuronal populations, leading to a multiplexed neuronal diversity-supporting different functional roles. © 2020 Elsevier B.V

Construction of constant fatigue life diagram for a carbon fiber composite

Using the constant amplitude fatigue data at various stress ratios, the constant fatigue life (CFL) diagram was constructed for the CFC material which is useful in prediction of fatigue life under variable amplitude fatigue loads. A quasi-isotropic lay-up sequenced carbon fiber-epoxy composite (CFC) laminate was fabricated by resin infusion technique. The tensile and compression tests were carried out to determine the static strength of the material. About 175 mm long, constant rectangular cross-sectioned fatigue test specimens were cut and prepared from the laminate. The stress-controlled, constant-amplitude fatigue tests were conducted in a 100 kN servo-hydraulic test machine, at room temperature and in lab air atmosphere. All the fatigue tests were performed with a sinusoidal waveform and a frequency of 1-3 Hz. The fatigue tests were conducted at four different stress ratio, R = �min/�max, Viz., 0.7,0.5 (tension-tension), -1.0 (tension-compression), and 4.0 (compression-compression). Anti-buckling guide was employed during the fatigue tests which involved compressive load cycles . The CFL Diagrams are material specific with predictive capability for any designed levels from selective experimental data

Optimizing Cold Rolling and Mode of Annealing of Interstitial-Free High-Strength Steel Sheets

Percentage of cold rolling and mode of annealing significantly influence the formation of structures and textures of steel sheets. The objective is to co-relate the micro-structures and textures at each step of processing and their effect on mechanical properties. In addition, an attempt has been made to distinguish the batch and continuous mode of annealing. In the present investigation, interstitial free high strength (IF-HS) steel was first hot rolled to 85-90%, followed by cold rolling (CR) and then annealed. Annealing was carried out in the batch as well as the continuous mode. It was found that a fine pancake type and strong hot band texture was developed after hot rolling. Its very strong orientations were (332)[(11) over bar3] and (112)[1 (1) over bar0]. An almost uniform alpha-fibre and gamma-fibre were developed at 80% cold reduction. After annealing, the new gamma-fibre was found to be developed replacing the old one for both modes of annealing i.e. batch and continuous. Further, it was observed that the intensity of the gamma-fibre was higher at 80% CR after continuous annealing than after batch annealing. The strain energy introduced during cold rolling could not be removed completely in batch annealing. Therefore, the drawability obtained through continuous annealing was higher than that after batch annealing. The process route of 80% CR and annealing, either in batch or in continuous mode, has developed optimum strength and drawability in steel sheets. Either batch or continuous mode can be used for annealing to cold rolled IF-HS steel sheets, but the latter one is found to be superior to the former one

Texture and Formability of One-Step and Two-Step Cold-Rolled and Annealed Interstitial Free High-Strength Steel Sheets

In the current experimental investigation, a comparative study has been carried out to understand the development of texture under different conditions, viz. controlled hot-rolled, one-step cold-rolled (CR) and annealed, and two-step cold-rolled and annealed conditions. Mechanical properties were determined and experimental formability limit diagrams (FLDs) were plotted after both one-step and two-step cold-rolled and annealed conditions. From the tabulated data, it was found that hot band texture of 85 to 90 pct deformation was strong and the main orientations were (112) [1 (1) over bar0] and (332)[(1) over bar(1) over bar3] One-step cold rolling developed the strong and uniform alpha-fiber and gamma-fiber at 80 pct cold reduction. The strong and uniform new gamma-fiber was obtained at the one-step 80 pct CR annealed condition. In addition, the highest drawability was found at the one-step 80 pct CR annealed condition. In two-step cold rolling, orientation (223)[1 (1) over bar0] was the main texture component along with extremely strong gamma-fiber. Moreover, batch annealing of two-step cold-rolled steel sheets developed exceptionally strong and uniform gamma-fiber, and all mechanical properties were enhanced significantly except yield strength. From FLDs, it is observed that the formability properties of interstitial free (IF) high-strength (HS) steel sheets were excellent at both one-step and two-step cold-rolled and annealed conditions. However, the two-step cold-rolling and annealing process was found to be superior to the one-step process. The data of this investigation may be used at the industrial level to design the entire processing of IF-HS steel sheets

Effect of ply-drop on fatigue life of a carbon fiber composite under a fighter aircraft spectrum load sequence

Carbon fiber composite laminate containing symmetric internal ply-drop simulating thickness variation was fabricated and tested to determine fatigue life under a standard mini-FALSTAFF spectrum load sequence. The fatigue life of ply-drop composite was significantly lower than that of plain composite due mainly to initiation and growth of delamination near the ply-drop location. The spectrum fatigue life was also predicted by empirical method. For this purpose, static and constant amplitude fatigue data was generated to construct constant life diagrams (CLDs). The spectrum fatigue life predicted using CLDs was in good agreement with experimental results for both plain and ply-drop composites

Bioinspired pressure actuated adhesive system

We developed a dry snythetic adhesive system inspired by gecko feet that can switch reversibly from adhesion to non-adhesion with applied pressure as external stimulus. Micropatterned polydimethylsiloxane (PDMS) surfaces with pillars of 30 µm length and 10 µm diameter were fabricated using photolithography and moulding. Adhesion properties were determined with a flat probe as a function of preload. For low and moderate applied compressive preloads, measured adhesion was 7.5 times higher on the patterned surfaces than on flat controls whereas for high preloads adhesion dropped to very low values. In situ imaging showed that the increased preload caused the pillars to deform by bending and/or buckling and to lose their adhesive contact. The elasticity of PDMS aids the pillar recovery to the upright position upon removal of preload enabling repeatability of the switch. Such systems have promising properties e.g. for industrial pick-and-carry operations

Effect of two step cold rolling — continuous annealing on micro-structures, textures and mechanical properties in IF and IF-HS steel sheets

In the present work, the objectives were to investigate the type of microstructure and texture that developed after two step cold rolling — continuous annealing. Further, an attempt has been made to correlate the microstructure and texture with mechanical properties. Interstitial free (IF) and interstitial free — high strength (IF-HS) steel sheets which contain almost γ-fiber textures, were subjected to two step cold rolling for different percentage of reduction and further continuously annealed. It was observed that cold rolling strengthened the region of (223)[10] and 335[10] orientations of α-fiber as well as γ-fiber. After continuous annealing, cold rolled γ-fiber as well as (223)[10] and 335[10] orientations transformed to extremely strong new γ-fiber. It was found that at two steps 80% cold rolling -continuous annealing, drawability of IF steel sheet was exceptionally high i.e. around 3.15, whereas for IF-HS steel sheets, it was around 2.7. Although, two steps cold rolling -continuous annealing enhanced percentage elongation and stretchability exceptionally, but it reduced the yield strength substantially. However, ultimate tensile strength was hardly affected. In addition, it is also observed that two steps cold rolling — continuous annealing developed uniform grain size in both the grades of steel sheets