When singer Bryan Ferry said that we should stick together, I wondered two things: Did he really like me that much? And how exactly were we going to stick to each other?
Many scientists have been pondering questions related to adhesion for a long time. Why can a gecko run up a wall? How do bacteria stay attached to a surface? And why can flies attach themselves to pretty much any surface?
But they've also been slowly uncovering various approaches to adhesion. One structure seems to be fairly ubiquitous: a sort of mushroom-shaped foot. The mechanism of adhesion between the face of the mushroom and a surface is electrostatic, so you might think it would be better to use a flat surface, which is structurally simpler. Early studies had revealed that despite their similarities, the failure of adhesion is different for flat and mushroom-shaped feet. But although there was some speculation about the differences, no one really knew why the two failed in different ways. Until now, that is.
To investigate the way that adhesion fails, scientists created artificial mushroom feet and filmed the unsticking process. Initially, they filmed the feet unsticking at 5,400 frames per second, but the feet went from stuck to unstuck in the 185 microseconds between frames. Very difficult to learn anything from that. So the researchers ramped up the frame rate to 180,000 frames per second, and even then they had some difficulties.
At this speed, the amount of noise in each frame was dramatically increased, so the frame-by-frame analysis of detachment was a little more complicated. Despite this, it was shown that detachment followed a consistent pattern. First, as the foot is pulled up, the contact area between the foot and the surfaces reduces slowly. Then, at some location, a small crack between the foot and the surface appears. This crack slowly expands until a critical point is reached, after which the edge of the crack expands rapidly until the entire foot pops free of the surface.
Critically, even when the foot is not stuck properly—there is a crack between the surface and the foot before pulling the foot off—detachment doesn't necessarily begin at this weak point, which indicates that imperfect attachment does not always change the strength of the adhesion for mushroom-shaped feet. According to the researchers, this means that the strength of adhesion is determined more by the type and shape of the surface rather than by how perfectly the foot is placed on the surface.
Most importantly, the experimental results support a model where the mushroom shape acts to redistribute stress evenly over the entire attachment surface. The flat shape, however, maximizes the stress near the edges. By redistributing the stress, there are no weak points where detachment can begin. This lack of an initial point to begin separation delays the separation and results in stronger attachment. More importantly, the attachment is robust, even in the face of imperfect placement. Which is really important because, in real life, a fly doesn't often have time to ensure that its feet are perfectly seated against a surface.
Physical Review Letters, DOI: 10.1103/PhysRevLett.111.104301