Hold on tight!
As the above microscope views show, we already knew that the feet of beetles featured flexible structures that enabled forward movement and adherence and that this was achieved, at least in part, by means of a liquid as tiny droplets had been observed. But what were the underlying mechanisms at work here? Could they be represented by an equation? These questions were answered by the researchers from Liege, beginning with a very practical difficulty: “The issue”, remembers Sophie Gernay, “is that the mechanism must be studied in vivo, while the living insect is moving. This is a real challenge because the field of view of the microscope is only a few millimetres, therefore the insect quickly moves away from it. We had to attach the insect to a prop and then bring the insect's foot into the field of vision via an artificial movement. We thus did not observe the natural walk of the insect, which is difficult to obtain under a microscope, but a ‘robotic’ movement (see diagram). We bring its foot in contact with the glass slide of the microscope then we detach it and that gives us images of the hair-like structures as they adhere and detach themselves”. The microscopic images show the tips of the hairs that touch the slide. There is therefore a multiplication of liquid contacts which occur during the walk because the liquid is present on the tip of each ‘hair’.
The interest of the microscopy technique used resides in the production of interference patterns. A luminous ray is reflected on the microscope slide and the other on the hair surface at a certain distance from the slide thus producing interference patterns. These patterns allow understanding the shape of the hair when it is not in contact. “We therefore have not only a 2D image, explains Professor Tristan Gilet, but also information about the third direction and therefore the deformation of the hair just before it comes into contact with the slide. This is what made it possible to reproduce the deformation of the hairs caused by contact with the surface the insect is “walking” on and the capillary forces of the liquid it secretes”.
From there on, the researchers were able to design a model that takes account of the different forces involved in the movement of the hairs. Each of them was considered by the researchers as a deflected beam subjected to capillary forces that are dominant in the liquid meniscus and to the contact forces with the surface. The researchers then studied the balance between these forces. “We were then able to deduce information about the flexibility of the hairs, their deformation by the substrate or the quantity of liquid necessary to ensure adhesion, about one femolitre (the equivalent of a cube with sides of one micrometre) per structure”, explains Sophie Gernay.