Adhesion on rough surfaces and mechanisms of self-cleaning in insect attachment pads

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Adhesive tapes are made of very soft and tacky materials, allowing them to make contact to moderately rough surfaces. As a result, they are highly susceptible to contamination and can only be used once or twice. By contrast, biological adhesives, as found on the foot pads of insects, allow strong adhesion to rough and contaminated substrates; they can rapidly attach and detach millions of times with no loss of adhesive power. Animal adhesive pads are important models for new 'biomimetic' adhesives, but important aspects of their function are still not fully understood. In this project, we investigate the mechanisms of how insect adhesive pads can cope with surface roughness and contamination. We will use transparent microstructured substrates as a tool to investigate the performance of smooth and hairy adhesive pads of insects. These microstructures consist of micron- and nanometre-sized pillars and ridges and will be fabricated by optical lithography and nanoimprinting. Unlike randomly rough surfaces, these substrates have a well-defined geometry and will allow us to visualize the contact zones of insect pads adhering to them. We will vary the width, height and spacing of the surface structures to explore the detailed parameters of rough surfaces that can be compensated by smooth and hairy adhesive pads of insects. At the same time we will measure what effect roughness has on adhesion and friction forces and we will identify surface conditions that limit insect adhesion. We will study how the pad contact on rough surfaces depends on normal load and shear forces and how it is influenced by time-dependent viscoelastic pad properties. We will also use microstructured substrates to compare two types of foot pads in cockroaches that appear specialized for adhesion and friction (pushing), to test whether they differ in conformability and adhesive strength. We will investigate how natural adhesive pads are able to self-clean by measuring forces after contaminating the pads with standardised particles. We will test three possible mechanisms for self-cleaning, the shedding of particles by alternating pushing and pulling movements, the interlocking of particles when sliding on rough surfaces, and the removal of particles via the secretion of adhesive fluid. Our study of insect adhesion on rough and contaminated surfaces will be important not only for the development of biomimetic adhesives but also for the development of slippery coatings for insect pest control.