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

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.

Biological adhesives, as the foot pads of insects, are important models for novel synthetic adhesives, but important aspects of their function are still not understood. They allow strong adhesion to rough and contaminated substrates, where other adhesives fail, and can repeatedly attach and detach without losing adhesive power. In this project, we investigate the mechanisms of how insect adhesive pads can cope with surface roughness and contamination. We will use transparent micro- and nanostructured substrates to study the performance of smooth and hairy adhesive pads of insects. These substrates consist of pillars and ridges fabricated from polymers or inorganic ceramic materials by optical lithography and nanoimprinting. They have a well-defined geometry and will allow us to visualize the contact zones of insect pads adhering to them. By varying width, height and spacing of the structures we will explore the detailed roughness parameters that can be compensated by insects. At the same time we will quantify the effect of roughness on adhesion and friction, thereby identifying conditions that limit insect adhesion. We will study how pad contact on rough surfaces depends on normal and shear forces as well as time-dependent viscoelastic properties. Microstructured substrates will be used to compare proximal vs. distal foot pads, specialised for adhesion and friction, to test whether they differ in conformability and adhesive strength. We will investigate how insect adhesive pads are able to self-clean by measuring forces after contaminating pads with standardised particles. We will test three possible mechanisms for self-cleaning, the shedding of particles by alternating pushing/pulling movements, the interlocking of particles when sliding on rough surfaces, and the removal of particles via adhesive fluid. Our study will be important for the development of biomimetic adhesives and the production of slippery coatings for insect pest control.

Biomimetic adhesives, mimicking animal adhesive organs, are currently being developed by many research groups worldwide. They may be used for a wide range of applications in production processes, climbing robots and medical applications, and therefore have a high potential economic impact. Efforts towards the development of biomimetic adhesives are limited by the fact that the biological models are still not sufficiently understood. Research on the function of natural adhesive structures is therefore essential. We anticipate that our findings will be important for the manufacture of synthetic, biomimetic adhesives. Given the importance of adhesives in technology, the economic impact of such novel adhesives will be substantial. A second area in which our research on the insect adhesion on rough surfaces will have impact is the development of insect-repellent coatings for pest control. Crawling insects cause a considerable economic loss. A detailed understanding of how surface roughness limits insect adhesion will be essential for the development of such coatings. Durable, non-toxic insect-repellent coatings that take advantage of surface roughness are feasible but still haven't been developed. We will disseminate our results via publications in international scientific journals, talks at conferences and by presenting them to colleagues in our fields, as well as in engineering and materials science. We will communicate our results to potential industrial partners for the development of insect-repellent coatings and biomimetic adhesives. To promote this activity, we are going to prepare information leaflets summarising the main findings of our research for industrial partners. To reach a wider audience outside academia, we will present findings to the general public and in the media. One industrial partner has already agreed to support our research on insect-repellent coatings and to supply us with materials for testing. Both PI and Co-PI have worked with Cambridge Enterprise, a wholly owned subsidiary of the University of Cambridge, which provides services to the research groups to file patents, and provide aid in the commercialisation of the registered IP. We therefore have the necessary experience and contact to quickly and effectively secure IP that will emerge from this project.