The remarkable abilities of flies and other insects to walk with ease on vertical smooth surfaces, and even upside down, has aroused human curiosity for hundreds of years. How do they do it? Furthermore, are there ways for us to do it? If we can master the technology, is there money to be made via innovation? In a helpful review article, Jan-Henning Dirks looks at the history of research into his phenomenon, and draws some fascinating conclusions.
"Even back [in the 17th Century], scientists knew that there is more to insects than meets the eye. While the most basic system of mechanical interlocking found in arthropods is the claw, insects do not merely have a miniature version of this. Many surfaces in the natural world are simply not soft enough to allow claws to be inserted, or are too smooth to provide a safe grip. The question of how insects stick, crawl and run on vertical surfaces and even upside down remains as hotly debated between scientists now as it was in the 17th century."
In 1665 Robert Hooke was one of the first to publish hand-drawn images of a fly's foot in his well-known book Micrographia (left). He speculated that flies adhere by using small hairs that interlock with the roughness of the substrate. Today, we can use more elaborate methods to study the adhesive pads of insects. This electron-microscope image shows a close-up of the smooth pad of a living weaver ant depositing adhesive fluid on a substrate (right). (Image courtesy: Walter Federle, source here)
These insects have tarsal attachment pads that can secrete a nanometre-thin film of a special fluid. The fluid has to provide an adhesive bond that can (at least) support the weight of the insect, but the bond must also be easily broken so that the insect can detach its legs easily for walking or taking flight. On smooth vertical surfaces, the secretion must provide not only adhesion, but also friction, so that the insect does not slide down, out of control. A first milestone in research has been to determine the chemistry of the fluids. The findings reveal several layers of complexity.
"Chemical analyses of this fluid in beetles showed that it contains saturated and unsaturated linear hydrocarbons of C20-C28 chain length, fatty acids and alcohols, as well as true waxes. A recent study on locusts detected not only shorter chain fatty acids (C16-C20) but also significant amounts of polar and amphiphilic components such as saccharides, amino acids and cholesterol. From the low contact angles of footprints on glass, it was concluded that the secretion consists of lipid nano-droplets in an aqueous fluid. However, in vivo observations using interference reflection microscopy (IRM) in ants and stick insects showed that the emulsion's continuous phase is oily and contains volatile, hydrophilic droplets."
The discovery that the adhesive is an emulsion stimulated hypotheses about the breaking of the bond when walking and also the generation of static friction to avoid sliding. It is known that many emulsions have a higher viscosity than their components and some display marked non-Newtonian properties. In a research paper, Dirks et al. report on experimental work designed to test these hypotheses.
"In this study, we test the effect of the pad secretion's two-phasic nature on friction forces by selectively reducing the hydrophilic phase of a stick insect's emulsion in vivo, using a polymeric, water-absorbing substrate."
The authors confirmed that the adhesive secretion is a water-in-oil emulsion and that the two phases were essential to avoid sliding. They found that, on the nano-scale of operation, non-Newtonian effects were even more marked than anticipated.
"When confined to thin films, emulsions can show even more complex non-Newtonian behaviour, which may no longer be dominated by bulk-continuum properties but by processes at the interface. Many emulsions are shear-thinning and exhibit Bingham flow, where shear stress is linearly dependent on shear rate, with a positive intercept. Thus, Bingham fluids require an initial minimum 'yield stress' before they start to flow. The stick insects' relationship between shear stress and sliding velocity is consistent with Bingham flow. A yield point of the two-phasic pad secretion provides an explanation for the significant static friction observed in insects."
Although emulsions are widely used in cosmetics and food technology, each emulsion should be regarded as an engineered fluid - fine-tuned for purpose. This principle is even more relevant to the adhesive secretions of insects.
"The insects' use of an emulsion as an adhesive fluid conveys the benefits of 'wet' adhesion (better adhesion to rough surfaces, wear resistance) without sacrificing the essential ability to withstand shear forces. Our results suggest that insect adhesive organs take advantage of non-Newtonian properties of emulsions and these properties may have been optimized by natural selection. Thus far, engineers have concentrated on biomimetic adhesives inspired by the dry, fibrillar system of geckos. However, the insects' secretion-aided attachment systems also provide an as yet unexplored source of inspiration for novel biomimetic adhesives. Moreover, understanding the detailed function of the insect adhesive system may lead to the development of a new type of non-toxic, wear-resistant insect-repellent coating."
The more we understand these adhesive fluids, the more sophisticated they appear to be. There are still many unexplored aspects - for example, we do not know how the fluids are produced, nor do we understand the self-cleaning mechanisms. Attributing the fine-tuning of the fluid compositions to natural selection is not because the authors have gathered evidence to show that natural selection is responsible. What they have done is show that fine-tuning characterises the materials. Their working theory is one that drives a specific deduction about the cause of this fine-tuning. This is not evidence against design, but the promotion of an alternative theoretical model. The evidence for design lies in the "insect pad secretion" system being characterised as 'exquisitely assembled' rather than 'cobbled together'. It is the Darwinists who have championed evolutionary transformation as a blind, cobbling together, process - then let them demonstrate the cobbled-together features before they appeal to the system being "optimized by natural selection".
Insect tricks: two-phasic foot pad secretion prevents slipping
Jan-Henning Dirks, Christofer J. Clemente and Walter Federle
Journal of the Royal Society Interface, 6 April 2010, Vol. 7, no. 45, 587-593 | doi: 10.1098/rsif.2009.0308
Abstract: Many insects cling to vertical and inverted surfaces with pads that adhere by nanometre-thin films of liquid secretion. This fluid is an emulsion, consisting of watery droplets in an oily continuous phase. The detailed function of its two-phasic nature has remained unclear. Here we show that the pad emulsion provides a mechanism that prevents insects from slipping on smooth substrates. We discovered that it is possible to manipulate the adhesive secretion in vivo using smooth polyimide substrates that selectively absorb its watery component. While thick layers of polyimide spin-coated onto glass removed all visible hydrophilic droplets, thin coatings left the emulsion in its typical form. Force measurements of stick insect pads sliding on these substrates demonstrated that the reduction of the watery phase resulted in a significant decrease in friction forces. Artificial control pads made of polydimethylsiloxane showed no difference when tested on the same substrates, confirming that the effect is caused by the insects' fluid-based adhesive system. Our findings suggest that insect adhesive pads use emulsions with non-Newtonian properties, which may have been optimized by natural selection. Emulsions as adhesive secretions combine the benefits of 'wet' adhesion and resistance against shear forces.
Dirks, J-H., Not slippery when wet, (physicsworld.com, Dec 1, 2010)
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