Preventing Catastrophic Failure of Microfibrillar Adhesives in Compliant Systems Based on Statistical Analysis of Adhesive Strength

Abstract

Adhesives based on fibrillar surface microstructures have shown great potential for handling applications requiring strong, reversible, and switchable adhesion. Recently, the importance of the statistical distribution of adhesive strength of individual fibrils in controlling the overall performance was revealed. Strength variations physically correspond to different interfacial defect sizes, which, among other factors, are related to surface roughness. For analysis of the strength distribution, Weibull's statistical theory of fracture was introduced. In this study, the importance of the statistical properties in controlling the stability of attachment is explored. Considering the compliance of the loading system, we develop a stability criterion based on the Weibull statistical parameters. It is shown that when the distribution in fibril adhesive strength is narrow, the global strength is higher but unstable detachment is more likely. Experimental variation of the loading system compliance for a specimen of differing statistical properties shows a transition to unstable detachment at low system stiffness, in good agreement with the theoretical stability map. This map serves to inform the design of gripper compliance, when coupled with statistical analysis of strength on the target surface of interest. Such a treatment could prevent catastrophic failure by spontaneous detachment of an object from an adhesive gripper.

Attachment of bioinspired microfibrils in fluids: transition from a hydrodynamic to hydrostatic mechanism

“…The curves are in fairly good agreement up to the maximum force of about 5.8 mN. However, detachment happens earlier in the experiment than in theory; this is possibly caused by unstable growth of a pre-existing crack in the compliant test set-up [34], which is neglected in the theory. In view of the agreement between experimental tests and theoretical calculations, we note that the initial gap, h0 remains a fitting parameter.…”

Application of Machine Learning to Object Manipulation with Bio-Inspired Microstructures

“…Also, the reliability of handling asymmetric objects with various geometries and forms can profit. More specifically, the data collected directly using the robotic arm bridge the gap between laboratory adhesion experiments in displacement-control and the actual pick and place application under force-controlled loading [31,33]. Certain limitations of the present approach should be noted.…”

“…Videos of the tests were recorded and correlated with the force-displacement measurement. To characterize the spread of the distribution of individual fibril pull-off forces, a cumulative Weibull distribution was fitted (Equation 1 in the supporting information, see also [31,32]. The Weibull moduli, m; and the reference elongations, u 0 ; of the arrays are reported in Table 1.…”

“…As the glass surface was free of contaminants, the likely reason for this distribution lies in the limitations of the replica molding process: surface defects and deviations in fibril radius induced a spread in pull-off strength across each array. These variations were quantified by fitting Weibull distributions (equation 1 in SI) and extracting characteristic values as explained in previous reports [27,31,32], and presented in Table 1. divulging the variation of the number of attached fibrils N a for similar misalignment angles (a $ 0:6 and b $ 0:02 ).…”

Soft Tactile Coil‐Based Sensor for Misalignment Detection of Adhesive Fibrillary Gripping Systems

“…For aligning the pad an optical setup leveraging the principal of FTIR inspired by previous publications by Tinnemann et al was integrated in the setup. [2,34,35] Mechanical properties of the foam in terms of hysteresis and deformation behavior and the influence of the measuring layer material on the adhesive force were investigated with this setup. A set of tests was carried out on the samples to characterize them.…”