How can flies walk on the window glass upside down? How can geckos climb walls and trees? It looks like a simple question, but it is hard to answer. The secret is that flies and geckos, and many other living species can control the ability to stick to surfaces, ability to adhere. Adhesion has many interpretations, but frequently it refers to ‘van der Waals’ attraction, which adds additional forces in the contact between the surfaces. These additional forces, if controlled, can be used for good. For example, one can think of a gripper of a robot arm which picks up complex 3D shapes to transport in the factory. These robots are used in printing, precision mechanics, semiconductor industry, etc.
Adhesive forces can be also useful in everyday life. Consider a medical plaster for example. Sometimes they detach annoyingly quickly and it was now found why – the geometry of the plaster plays a role here. Recently, a group of researchers from Germany lead by Professor Valentin L. Popov, developed a model to predict the stickiness of flat adhesive surfaces. The model is based on a combination of Boundary Element Method (BEM) and the Griffith criterion for brittle crack propagation. This combination allowed for the calculation of the adhesive stickiness of very peculiar shapes, as shown in the figure below:
From the obtained results, it is clearly seen that the detachment of the adhesives depends on the macroscopic shape of the object and the detachment is initiated at the outstanding parts and sharp corners. This explains why rounded medical plasters will remain intact longer than the squared ones. The model is so robust, that can be applied to virtually any shape. To stress this, an adhesive detachment of the tribonet logo was simulated and the video is shown below:
It is quite clear, that you would not like your medical plaster to be in the shape of the tribonet logo, since it would come off quite fast.
A validation procedure included comparison with various existing analytical solutions and obtained experimental data. From the comparison with experimental data it was found that the model predicts the process of detachment well, even though certain discrepancies attributed to visco-elasticity of materials and half-space approximations were encountered.
Further details can be found in the original article.
. Strength of adhesive contacts: Influence of contact geometry and material gradients, V.L. Popov, R. Pohrt, Q. Li.