Adhesive mechanisms and dynamic performance in smooth adhesive pads of insects

Many insects are capable of running upside down on a smooth surface, carrying loads and withstanding pull-off forces equivalent to more than 100 times their own body weight. The mechanisms of how this impressive performance is achieved and of how they master the conflicting tasks of running and of making stable adhesive contacts, are still largely unknown. We will investigate surface attachment and locomotion in ants and stick insects, which possess smooth adhesive pads at the tips of their legs. Experiments will be conducted to clarify whether attachment forces are due to 'wet' or dry' adhesion, i.e. whether they are mainly based on the properties of a fluid film between pad and surface or on the direct contact between the adhesive pad cuticle and the substrate. We will investigate whether the fluid secretion enhances adhesion and if so, and under what conditions. Attachment forces of single legs (adhesion and friction) will be measured while simultaneously filming their area of contact with the surface. The physical properties of pad cuticle and secretion will be analysed with modern microscopy techniques, and used to compare pad performance with theoretical predictions. To clarify the mechanism of adhesion and the scaling of forces with body size, we will study how adhesive pads detach from the surface. In general, forces are reduced when pads are detached by peeling. We will investigate whether insects are able to avoid peeling and if so, how they do this. To understand how insects manage to combine surface attachment with locomotion, we will study the movements of legs and adhesive pads during attachment and detachment. By analysing high-speed recordings of running insects, we will explore how running is influenced by surface attachment and how insects adjust their legs and feet to cope with different attachment conditions. Our project will clarify how insects adhere to surfaces and how they manage to control attachment forces. Even though numerous potential applications exist, no controllable technical adhesives have yet been developed. Understanding the mechanisms of how insects elegantly control attachment is likely to lead to the discovery of general principles useful for the design of novel 'biomimetic' adhesives.

Insects that possess adhesive pads are capable of withstanding large detachment forces and are yet able to run. The detailed mechanisms of how they master the conflicting tasks of running and of making stable adhesive contacts are still largely unknown. The aim of this project is to clarify the mechanisms of surface attachment in smooth adhesive pads of insects and to understand how adhesive forces are controlled during running. Attachment forces of single legs will be measured under different conditions while simultaneously recording the area of contact. Physical properties of pad secretion and cuticle will be quantified using interference reflection and atomic force microscopy, and used to test theoretical models of adhesion and friction. We will verify whether and under what conditions the fluid secretion enhances adhesion. It will be tested whether attachment forces are based on the properties of a continuous fluid film in the pad contact zone or on direct cuticle-surface contacts. We will test the prediction that the two-phasic nature of the adhesive secretion is essential for static friction forces. To understand the mechanism of adhesion and the physical basis of how insects control adhesion, we will analyse the detailed mode of pad detachment ('peeling' vs. 'sudden separation') and test predictions regarding the scaling of adhesive forces and single pads and whole insects. The movements of legs and adhesive pads will be studied during attachment and detachment. By analysing high-speed recordings of running insects, we will test predictions of how running is influenced by attachment and what leg/tarsus adjustments insects make to cope with different attachment requirements (load, slope, substrate properties, rapid perturbations). Our project aims at dissecting the insects' mechanisms to control attachment forces. Thus, the research has a potential to identify general principles that may prove useful for the development of technical applications.