Functional Morphology and Design Constraints of Smooth Adhesive Pads

MRS Bulletin ◽  
2007 ◽  
Vol 32 (6) ◽  
pp. 479-485 ◽  
Author(s):  
W. Jon. P. Barnes
Keyword(s):  
Fluid Layer ◽  
Adhesive Force ◽  
Average Thickness ◽  
Force Component ◽  
Design Constraints ◽  
Friction Forces ◽  
Adhesive Pads ◽  
Tree Frogs ◽  
Strong Attachment ◽  
Air Liquid Interface

AbstractSmooth adhesive pads are found among the arthropods, amphibians (particularly tree frogs), and in some mammals. They are used for dynamic adhesion when an animal is climbing steep or overhanging smooth surfaces. There is a need for strong attachment to avoid falling and easy detachment to enable the animal to move. This article describes the morphology and physical properties of smooth adhesive pads, stressing how there is little variation in structure, within tree frogs at least, even among pads that have evolved independently. This is clear evidence of an optimum design; best adhesion occurs when there is a continuous, thin film of fluid between the pad and the surface. Smooth adhesive pads adhere by wet adhesion, the main force component being capillarity, produced by the air/liquid interface (meniscus) around the edge of each pad. Smooth adhesive pads also produce substantial friction forces, probably because of actual contact between the pad surface and substrate (tree frogs) or non-Newtonian properties of the secreted fluid (insects). This is possible because the fluid layer beneath the pad has an average thickness of only a few nanometers. The article also discusses the scaling of adhesive force with size and, finally, implications for biomimetics.

scholarly journals Sticking like sticky tape: tree frogs use friction forces to enhance attachment on overhanging surfaces

2013 ◽  
Vol 10 (80) ◽  
pp. 20120838 ◽  
Author(s):  
Thomas Endlein ◽  
Aihong Ji ◽  
Diana Samuel ◽  
Ning Yao ◽  
Zhongyuan Wang ◽  
...  
Keyword(s):  
Slope Angle ◽  
Three Dimensional ◽  
The Body ◽  
Adhesive Tape ◽  
Large Contribution ◽  
Lateral Direction ◽  
Friction Forces ◽  
Normal Forces ◽  
Adhesive Pads ◽  
Tree Frogs

To live and clamber about in an arboreal habitat, tree frogs have evolved adhesive pads on their toes. In addition, they often have long and slender legs to facilitate not only long jumps, but also to bridge gaps between leaves when climbing. Both adhesive pads and long limbs are used in conjunction, as we will show in this study. Previous research has shown that tree frogs change from a crouched posture (where the limbs are close to the body) to a sprawled posture with extended limbs when clinging on to steeper inclines such as vertical or overhanging slopes. We investigated this change in posture in White's tree frogs ( Litoria caerulea ) by challenging the frogs to cling onto a tiltable platform. The platform consisted of an array of 24 three-dimensional force transducers, which allowed us to measure the ground reaction forces of the frogs during a tilt. Starting from a crouched resting position, the normal forces on the forelimbs changed sign and became increasingly negative with increasing slope angle of the platform. At about 106°±12°, tilt of the platform the frogs reacted by extending one or two of their limbs outwards. At a steeper angle (131°±11°), the frogs spread out all their limbs sideways, with the hindlimbs stretched out to their maximum reach. Although the extension was strongest in the lateral direction, limbs were significantly extended in the fore–aft direction as well. With the extension of the limbs, the lateral forces increased relative to the normal forces. The large contribution of the in-plane forces helped to keep the angle between the force vector and the platform small. The Kendall theory for the peeling of adhesive tape predicts that smaller peel angles lead to higher attachment forces. We compare our data with the predictions of the Kendall model and discuss possible implications of the sliding of the pads on the surface. The forces were indeed much larger for smaller angles and thus can be explained by peeling theory.

scholarly journals Shear-sensitive adhesion enables size-independent adhesive performance in stick insects

2019 ◽  
Vol 286 (1913) ◽  
pp. 20191327 ◽  
Author(s):  
David Labonte ◽  
Marie-Yon Struecker ◽  
Aleksandra V. Birn-Jeffery ◽  
Walter Federle
Keyword(s):  
Body Weight ◽  
Adhesive Force ◽  
Whole Body ◽  
Small Animals ◽  
Shear Forces ◽  
Stick Insects ◽  
Adhesive Pads ◽  
The Difference ◽  
Large Animals ◽  
Adhesive Performance

The ability to climb with adhesive pads conveys significant advantages and is widespread in the animal kingdom. The physics of adhesion predict that attachment is more challenging for large animals, whereas detachment is harder for small animals, due to the difference in surface-to-volume ratios. Here, we use stick insects to show that this problem is solved at both ends of the scale by linking adhesion to the applied shear force. Adhesive forces of individual insect pads, measured with perpendicular pull-offs, increased approximately in proportion to a linear pad dimension across instars. In sharp contrast, whole-body force measurements suggested area scaling of adhesion. This discrepancy is explained by the presence of shear forces during whole-body measurements, as confirmed in experiments with pads sheared prior to detachment. When we applied shear forces proportional to either pad area or body weight, pad adhesion also scaled approximately with area or mass, respectively, providing a mechanism that can compensate for the size-related loss of adhesive performance predicted by isometry. We demonstrate that the adhesion-enhancing effect of shear forces is linked to pad sliding, which increased the maximum adhesive force per area sustainable by the pads. As shear forces in natural conditions are expected to scale with mass, sliding is more frequent and extensive in large animals, thus ensuring that large animals can attach safely, while small animals can still detach their pads effortlessly. Our results therefore help to explain how nature’s climbers maintain a dynamic attachment performance across seven orders of magnitude in body weight.

The effect of coatings on the cutting process, friction, forces and predictive cutting models in machining operations

2002 ◽  
Vol 216 (3) ◽  
pp. 347-356 ◽  
Author(s):  
E J A Armarego ◽  
S Verezub ◽  
P Samaranayake
Keyword(s):  
High Speed ◽  
Thrust Force ◽  
Orthogonal Cutting ◽  
High Speed Steel ◽  
Cutting Process ◽  
Force Component ◽  
Friction Forces ◽  
Technological Performance ◽  
The Many ◽  
Machining Operations

The effects of the popular TiN and TiCN coatings on the cutting process, friction and predictive mechanics of cutting models for forces and power in machining operations are investigated. Extensive orthogonal cutting and turning tests on steel work materials have shown that the cutting process and predictive force models are not qualitatively affected by these coatings. Quantitatively, both coatings are equally effective in reducing the friction, forces and power in orthogonal cutting when applied to high-speed steel (HSS) tools and equally ineffective when applied to carbide tools. Both TiN and TiCN coatings applied to HSS tools resulted in modest reductions in the power force component and power of 12–30 per cent ‘on average’ with larger reductions in the thrust force component of about 50 per cent ‘on average’. Considerably more research seems necessary to understand better and to predict quantitatively the effects of the many coating-substrate combinations on the technological performance of machining operations, essential for optimizing the economic performance of these operations.

scholarly journals Insect tricks: two-phasic foot pad secretion prevents slipping

2009 ◽  
Vol 7 (45) ◽  
pp. 587-593 ◽  
Author(s):  
Jan-Henning Dirks ◽  
Christofer J. Clemente ◽  
Walter Federle
Keyword(s):  
Adhesive System ◽  
Continuous Phase ◽  
Stick Insect ◽  
Force Measurements ◽  
Friction Forces ◽  
Adhesive Pads ◽  
Foot Pad ◽  
Detailed Function ◽  
Adhesive Secretion

Many insects cling to vertical and inverted surfaces with pads that adhere by nanometre-thin films of liquid secretion. This fluid is an emulsion, consisting of watery droplets in an oily continuous phase. The detailed function of its two-phasic nature has remained unclear. Here we show that the pad emulsion provides a mechanism that prevents insects from slipping on smooth substrates. We discovered that it is possible to manipulate the adhesive secretion in vivo using smooth polyimide substrates that selectively absorb its watery component. While thick layers of polyimide spin-coated onto glass removed all visible hydrophilic droplets, thin coatings left the emulsion in its typical form. Force measurements of stick insect pads sliding on these substrates demonstrated that the reduction of the watery phase resulted in a significant decrease in friction forces. Artificial control pads made of polydimethylsiloxane showed no difference when tested on the same substrates, confirming that the effect is caused by the insects’ fluid-based adhesive system. Our findings suggest that insect adhesive pads use emulsions with non-Newtonian properties, which may have been optimized by natural selection. Emulsions as adhesive secretions combine the benefits of ‘wet’ adhesion and resistance against shear forces.

scholarly journals Dynamic biological adhesion: mechanisms for controlling attachment during locomotion

2019 ◽  
Vol 374 (1784) ◽  
pp. 20190199 ◽  
Author(s):  
Walter Federle ◽  
David Labonte
Keyword(s):  
Elastic Strain Energy ◽  
Neuromuscular Control ◽  
The Body ◽  
Adhesive Systems ◽  
Dynamic Surface ◽  
Surface Attachment ◽  
Adhesive Pads ◽  
Strong Attachment ◽  
Control Principle ◽  
Mechanical Switch

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’.

scholarly journals The use of clamping grips and friction pads by tree frogs for climbing curved surfaces

2017 ◽  
Vol 284 (1849) ◽  
pp. 20162867 ◽  
Author(s):  
Thomas Endlein ◽  
Aihong Ji ◽  
Shanshan Yuan ◽  
Iain Hill ◽  
Huan Wang ◽  
...  
Keyword(s):  
Contact Area ◽  
Shear Stresses ◽  
Curved Surfaces ◽  
Sem Images ◽  
Flat Surfaces ◽  
Normal Forces ◽  
Adhesive Surface ◽  
Reaction Forces ◽  
Adhesive Pads ◽  
Tree Frogs

Most studies on the adhesive mechanisms of climbing animals have addressed attachment against flat surfaces, yet many animals can climb highly curved surfaces, like twigs and small branches. Here we investigated whether tree frogs use a clamping grip by recording the ground reaction forces on a cylindrical object with either a smooth or anti-adhesive, rough surface. Furthermore, we measured the contact area of fore and hindlimbs against differently sized transparent cylinders and the forces of individual pads and subarticular tubercles in restrained animals. Our study revealed that frogs use friction and normal forces of roughly a similar magnitude for holding on to cylindrical objects. When challenged with climbing a non-adhesive surface, the compressive forces between opposite legs nearly doubled, indicating a stronger clamping grip. In contrast to climbing flat surfaces, frogs increased the contact area on all limbs by engaging not just adhesive pads but also subarticular tubercles on curved surfaces. Our force measurements showed that tubercles can withstand larger shear stresses than pads. SEM images of tubercles revealed a similar structure to that of toe pads including the presence of nanopillars, though channels surrounding epithelial cells were less pronounced. The tubercles' smaller size, proximal location on the toes and shallow cells make them probably less prone to buckling and thus ideal for gripping curved surfaces.

A Theory of Hydrodynamic Friction Forces in Starved Point Contact Considering Cavitation

1974 ◽  
Vol 96 (2) ◽  
pp. 237-245 ◽  
Author(s):  
Y. P. Chiu
Keyword(s):  
Fluid Layer ◽  
Point Contact ◽  
Rolling Friction ◽  
Rigid Bodies ◽  
Variable Degree ◽  
Hydrodynamic Friction ◽  
Friction Forces ◽  
Viscosity Parameter

Expressions for the hydrodynamic friction forces between two rigid bodies each having two principal radii of curvature have been found using an exponential pressure-viscosity assumptions. The rolling friction forces are presented graphically in terms of a dimensionless meniscus distance. These formulas are applicable to the computation of the rolling friction in a ball-race contact and in a ball-cage contact with a variable degree of starvation. In particular, the rolling friction forces can be predicted as a function of the usual speed-viscosity parameter and the thickness of the inlet fluid layer far upstream of the inlet.

scholarly journals Sliding-induced adhesion of stiff polymer microfibre arrays. II. Microscale behaviour

2008 ◽  
Vol 5 (25) ◽  
pp. 845-853 ◽  
Author(s):  
Bryan Schubert ◽  
Jongho Lee ◽  
Carmel Majidi ◽  
Ronald S Fearing
Keyword(s):  
Shear Force ◽  
Strong Dependence ◽  
Adhesive Force ◽  
Shear Forces ◽  
Spherical Probe ◽  
Adhesive Pads ◽  
Normal Adhesion ◽  
Fibre Angle ◽  
First Time ◽  
Adhesion Effect

The adhesive pads of geckos provide control of normal adhesive force by controlling the applied shear force. This frictional adhesion effect is one of the key principles used for rapid detachment in animals running up vertical surfaces. We developed polypropylene microfibre arrays composed of vertical, 0.3 μm radius fibres with elastic modulus of 1 GPa which show this effect for the first time using a stiff polymer. In the absence of shear forces, these fibres show minimal normal adhesion. However, sliding parallel to the substrate with a spherical probe produces a frictional adhesion effect which is not seen in the flat control. A cantilever model for the fibres and the spherical probe indicates a strong dependence on the initial fibre angle. A novel feature of the microfibre arrays is that adhesion improves with use. Repeated shearing of fibres temporarily increases maximum shear and pull-off forces.

scholarly journals Biomechanics of shear-sensitive adhesion in climbing animals: peeling, pre-tension and sliding-induced changes in interface strength

2016 ◽  
Vol 13 (122) ◽  
pp. 20160373 ◽  
Author(s):  
David Labonte ◽  
Walter Federle
Keyword(s):  
Fluid Layer ◽  
Stick Insect ◽  
Linear Increase ◽  
Shear Forces ◽  
Conflicting Demands ◽  
Adhesive Pads ◽  
Direction Dependence ◽  
Peel Force ◽  
Induced Changes ◽  
Adhesive Forces

Many arthropods and small vertebrates use adhesive pads for climbing. These biological adhesives have to meet conflicting demands: attachment must be strong and reliable, yet detachment should be fast and effortless. Climbing animals can rapidly and reversibly control their pads' adhesive strength by shear forces, but the mechanisms underlying this coupling have remained unclear. Here, we show that adhesive forces of stick insect pads closely followed the predictions from tape peeling models when shear forces were small, but strongly exceeded them when shear forces were large, resulting in an approximately linear increase of adhesion with friction. Adhesion sharply increased at peel angles less than ca 30°, allowing a rapid switch between attachment and detachment. The departure from classic peeling theory coincided with the appearance of pad sliding, which dramatically increased the peel force via a combination of two mechanisms. First, partial sliding pre-stretched the pads, so that they were effectively stiffer upon detachment and peeled increasingly like inextensible tape. Second, pad sliding reduces the thickness of the fluid layer in the contact zone, thereby increasing the stress levels required for peeling. In combination, these effects can explain the coupling between adhesion and friction that is fundamental to adhesion control across all climbing animals. Our results highlight that control of adhesion is not solely achieved by direction-dependence and morphological anisotropy, suggesting promising new routes for the development of controllable bio-inspired adhesives.

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