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Comparative Study
. 2008 Mar 6;5(20):339-48.
doi: 10.1098/rsif.2007.1077.

Frictional and Elastic Energy in Gecko Adhesive Detachment

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Comparative Study

Frictional and Elastic Energy in Gecko Adhesive Detachment

Nick Gravish et al. J R Soc Interface. .
Free PMC article

Abstract

Geckos use millions of adhesive setae on their toes to climb vertical surfaces at speeds of over 1 m s(-1). Climbing presents a significant challenge for an adhesive since it requires both strong attachment and easy, rapid removal. Conventional pressure-sensitive adhesives are either strong and difficult to remove (e.g. duct tape) or weak and easy to remove (e.g. sticky notes). We discovered that the energy required to detach adhering tokay gecko setae (W(d)) is modulated by the angle (theta) of a linear path of detachment. Gecko setae resist detachment when dragged towards the animal during detachment (theta = 30 degrees ) requiring W(d) = 5.0+/-0.86(s.e.) J m(-2) to detach, largely due to frictional losses. This external frictional loss is analogous to viscous internal frictional losses during detachment of pressure-sensitive adhesives. We found that, remarkably, setae possess a built-in release mechanism. Setae acted as springs when loaded in tension during attachment and returned elastic energy when detached along the optimal path (theta=130 degrees ), resulting in W(d) = -0.8+/-0.12 J m(-2). The release of elastic energy from the setal shaft probably causes spontaneous release, suggesting that curved shafts may enable easy detachment in natural, and synthetic, gecko adhesives.

Figures

Figure 1
Figure 1
An overview of the gecko adhesive from macroscale to microscale. (a) A tokay gecko's toes are covered in adhesive hairs (setae). (b) Setae are grouped into arrays along the underside of the toe. Pulling setal arrays in the proximal direction produces adhesion and high friction. Pushing distally causes no adhesion to occur and low friction. (c) Setal arrays along the underside of the toe. (d) Setal arrays are densely packed but achieve high contact areas owing to setal branching and flexibility. (e) A single seta is approximately 110 μm in length and 2.1 μm in radius. Setae have a slight curvature and branch at their tips ending in 200 nm wide spatula.
Figure 2
Figure 2
Geometry of isolated setal array detachment tests. (a) Side view of a tokay gecko toe showing the proximal (0°; towards animal) and distal (180°; away from animal) directions. (b) Isolated setal arrays were loaded vertically and then dragged proximally to attach. Once adhering, setal arrays were detached along a linear path of angles ranging from 30° to 150° in increments of 10°. Setal array reaction force and displacement was measured during detachment, yielding work of detachment (Wd).
Figure 3
Figure 3
The mechanics of detaching isolated tokay gecko setal arrays along a path of angle θ from depths producing maximum (LDPmax) and average (LDPavg) adhesion. θ=0° is proximal shearing, θ=90° is vertical detachment, and θ=180° is distal shearing. (a) The detachment energy (Wd) for both loading conditions was maximum at θ=30° and decreased with increasing θ becoming negative at θ∼110° and reaching a minimum at θ=130°. Maximum Wd was 5.0±0.86 J m−2 for LDPmax and 3.0±0.81 J m−2 for LDPavg. Minimum Wd was −0.8±0.12 J m−2 for LDPmax and −0.55±0.09 J m−2 for LDPavg. (b) The average detachment stress (σ) had identical extrema as Wd for both loading conditions. However, σ stayed relatively constant until θ∼110°, whereas Wd decreased steadily. Maximum σ was 53±7.6 kPa for LDPmax and 35±9.2 kPa for LDPavg. Minimum σ was 15±1.5 kPa for LDPmax arrays and 12±2.5 kPa for LDPavg.
Figure 4
Figure 4
After a vertical preload, a proximal drag is required to engage setae in adhesion. (a) In a separate experiment, a single seta was observed during loading to measure the kinematics of attachment. The amount of proximal drag distance is noted for each frame showing that after approximately 20 μm the seta is fully straightened. Subsequent dragging of the seta resulted in sliding while shaft tension was maintained. (b) Average friction stresses measured during setal array attachment are shown for both loading methods as a function of engagement drag distance, illustrating that setae rapidly approach maximum stress during the first approximately 20 μm. The initial constant slopes indicate elastic loading for the first approximately 10 μm with setal elastic coefficients of kmax=1.11 N m−1 for LDPmax and kavg=0.61 N m−1 for LDPavg. After the initial elastic loading, setal stresses began to level off as the seta straightens and tip sliding eventually occurs. Within 20 μm of dragging friction stresses are very near the stresses present at detachment.
Figure 5
Figure 5
Detachment energy (Wd) components for distal angles (θ>90°) illustrate the elastic energy return during engagement. (a) Normal Wd shows that setae adhere (Wd>0) up to θ=120°, after which energy is returned for greater θ. As the detachment angle becomes increasingly distal, setal tension relaxes earlier along the detachment trajectory allowing for large normal energy return during detachment. (b) Shear Wd becomes negative as soon as the displacement becomes distal (θ>90°), indicating that shaft tension is being relaxed elastically. Detachment at angles above 120° relaxes shafts and then begins to compress them axially resulting in an increasing Wd.
Figure 6
Figure 6
Average unloading friction and adhesion curves from setal array detachment tests (LDPavg) are shown for three detachment angles. For each detachment angle, a sequence of still images from a video of a single seta illustrates the kinematics of detachment at that angle. Positive shear force indicates that setae are pulled in tension, and a negative normal force indicates that setae are adhering. The white line in the single seta image sequences indicates the initial setal tip position prior to detachment, and the grey bar shows the total distance of tip sliding. The last frame of each sequence has a 50 μm scale bar in the upper left corner. Left tip displacement is distal and right tip displacement is proximal. (a) Detaching along a linear path oriented at 30° causes shafts to remain in tension during detachment. The shear force slowly decreases during the largely proximal detachment path until removed from the surface. The setae adhered during the whole detachment motion requiring a large energy input necessary to detach. The single seta images show that the tip slid approximately 30–50 μm prior to detachment. (b) Detachment along a 130° linear path resulted in a rapid decrease of both shear and normal force signifying that shaft tension was released rapidly. The single seta images show no tip displacement during the entire unloading cycle. This suggests that 130° is the optimum detachment angle because it allows elastic shaft energy to return without dissipating any energy frictionally. (c) A linear detachment path oriented at 150° resulted in a rapid decrease of shear force and normal force. However instead of remaining small, the shear and normal forces became substantially large in the opposite direction indicating that the setae was compressing during detachment. The image sequence shows that, at such a high detachment angle, the shaft is compressing axially and sliding during detachment, which resulted in energy dissipation and inefficient detachment.
Figure 7
Figure 7
Frictional (Wd>0) and spring-loaded (Wd<0) detachment mechanics for an individual seta detaching over a retraction angle θ. Grey arrows illustrate the net energy input (towards the system) and output (away form the system) that occur. (a) During detachment along a 130° linear path, the setal tip remains stationary while the shaft unloads elastically, resulting in spontaneous debonding (Wd<0). (b) A largely proximal linear detachment path causes setae to remain in tension, inhibiting elastic energy return to the system. The setal tip slides along the substrate during separation until the setal shaft angle reaches the critical angle of detachment α*=26–30°, at which point debonding occurs. The setal tip sliding that occurs prior to debonding results in frictional energy loss during detachment and thus a large Wd.
Figure 8
Figure 8
Frictional tip sliding results in detachment energy dissipation (Wd) that varies as a function of θ. Modelling a seta in tension as a rigid rotatable beam Wd can be approximated as West(θ)=F·s(θ) from the geometric tip displacement s(θ) of the detaching beam. A seta 110 μm in length is loaded 35 μm from its original height of 70 μm making an initial 18° shaft-substrate angle that increases until detachment occurs at α=26°. Assuming an average setal stress of 161 kPa produces the fit for Wd. The illustration inset in the bottom left corner highlights the frictional sliding of the setal tips during detachment (θ<110°). The illustration inset on the right side shows how negative energy results from the spring-loaded setae unloading during high angle detachment (θ>110°).

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