Mechanics of Insect Adhesion

Many insects secrete liquids in order to adhere to surfaces, allowing them to climb vertically and upside-down even on smooth surfaces. However, the details of how this is achieved remain unclear. There are open questions about the nature of the adhesive, as well as the dynamics of operation of the adhesive. This project will focus on developing models of different proposed mechanisms that can be compared with existing data in the biological literature, as well as new experiments from the laboratory of Walter Federle (Zoology, Cambridge). An understanding of the operation of such adhesives is important both from a biological perspective, and also as a first step in exploiting similar mechanisms in man-made adhesives.

There are two key strands of the project. The first strand is motivated by the observation that the liquid secretion used by many insects is an emulsion of water droplets in oil. However, the purpose of this emulsion is unclear: does the insect rely on the formation of many capillary bridges that then provide a large adhesive force or is the emulsion instead a way to modify the dynamic properties of the adhesive (for example by giving it a finite yield stress)? I will develop models of such emulsions focussed on understanding the force-displacement response that would be expected if capillary bridges were important. I will also begin to understand how such emulsions age, for example through Ostwald ripening.
The second strand concerns the manner in which the fluid is secreted through the sac of the insect's adhesive apparatus. Previously, this has been modelled using simple box-models that neglect some of the important mechanics of the problem. I will develop physically-based models to better understand both this secretion process and how it may be reversed when the insect wishes to detach from a surface: previous experimental observations have suggested that depending on how the insect detaches from the surface, the secretion is either left behind on the surface or removed (presumably being reabsorbed into the insect's adhesive apparatus). This is one example of suspected passive mechanisms in insect adhesion that are not at all understood.

The approach that I will adopt will involve the development of a hierarchy of mathematical models to understand how such adhesives operate. Here I will benefit from the existing expertise in the group concerning surface tension-dominated phenomena, particularly as it applies to the deformation of soft objects. I will also ensure that I work closely with experimentalists both in Walter Federle's group and with Robert Style (Materials Science, ETH-Zurich): this will allow me to develop models that are biologically relevant (Federle group), whilst also being able to perform quantitative tests in idealised scenarios (Rob Style). This combination of mathematical techniques (asymptotic analysis and some numerical techniques) tested against real experimental data from biological and idealised systems makes this research both timely and novel.

This project falls within the EPSRC Fluid Dynamics research area