Slippery Surface!

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Freiburg research team develops artificial surfaces insects cannot stick to

Beetles, cockroaches, and ants will have a harder time walking on facades or air conditioners in the future – thanks to the bio-inspired, anti-adhesive surfaces Prof. Dr. Thomas Speck, Dr. Bettina Prüm, and Dr. Holger Bohn are developing together with the Plant Biomechanics Group of the University of Freiburg. The team studied plant surfaces in order to determine what influence cell form and microstructure as well as surface chemistry exert on the adhesion behavior of insects.

The researchers conducted adhesion experiments in which Colorado potato beetles walked across differently structured plant surfaces as well as replicas made of synthetic resins. The team used a highly sensitive sensor to measure the traction forces of the beetles on various surfaces. They discovered that wavy or strongly curved cells can increase the adhesive powers of beetles, whereas microstructures composed of wax crystals or cuticular folds reduce them. The latter are tiny folds in the cuticle, a protective layer on the surface of the leaf resembling polyester. The beetles had the hardest time walking on surfaces with cuticular folds with a height and width of approximately 0.5 micrometers and a spacing of between 0.5 and 1.5 micrometers. “That is the perfect anti-adhesion surface. The insects slip off of it much easier than off glass,” says project director Thomas Speck. The cuticular folds reduce the contact area between the adhesive hairs on the beetles’ legs and the plant surface. Unlike on more coarsely structured surfaces, the beetle can’t dig its feet firmly into the cuticular folds. Thus, the microstructure of the surface has a stronger effect on the adhesion of the beetle than the cell form.

The team also took contact angle measurements to investigate the wettability of the various surfaces. The researchers used hydrophobic and hydrophilic artificial moldings of the microstructured plant surfaces in order to study the influence of the surface chemistry on surface wettability and the beetles’ walking behavior. Much like wax crystals, cuticular folds are very good at repelling water. In contrast to the wettability, which depends on both the microstructure and the surface chemistry, the walking behavior of the beetles is not influenced by the surface chemistry. This means that the beetle’s adhesive power depends solely on the physical microstructure of the surface.

Speck and his team published their findings in the current issue of the journal Acta Biomaterialia. In the future, the anti-adhesion surfaces could be used to line the ventilation pipes of air conditioners, which are often teeming with cockroaches and other insects. In addition, they could also be applied to facades and window frames to prevent insects that move predominantly by walking from entering the house and invading the cupboard and medicine cabinet. “This aspect is particularly important in the tropics,” says Speck.

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Can the tree whisperer save our forests?

Driven by a warming climate and human intervention, hordes of tiny beetles have devastated North America’s great forests over the past 20 years.

In this edited excerpt from his new book, Calgary author Andrew Nikiforuk examines how one quirky homemade invention provided some insight into ways of controlling a seemingly unstoppable beetle outbreak

During 2005, David Dunn often wandered the hilly outskirts of Santa Fe looking like a medieval plague doctor. Armed with headphones and a tape recorder, the avantgarde music composer and violin player poked the thin bark of pinyon trees with a special homemade device.

The odd contraption consisted of a meat thermometer and a piezoelectric transducer from a Hallmark greeting card. After inserting the modified thermometer-cum-microphone into the tree’s inner bark, Dunn patiently listened to the voices inside the tree. The bespectacled artist made an ungainly apparition in the desert forest as he perched against trees for hours on end.

Dunn became a tree whisperer after New Mexico started to lose half of its famed pinyon trees to an unprecedented beetle outbreak. Anxious landowners wanted a clear diagnosis on their trees before they pulled out their chainsaws.

Because Dunn had the listening tools, he got recruited for the job. Whenever the sound engineer heard noises that resembled running water or creaking winds in a pinyon, he’d give the tree an all-clear for beetles. Such a diagnosis inevitably invited two possible prescriptions: the landowner could water the tree more often, to build resin resistance, or he or she could spray the pinyon with the pesticide carbaryl. If Dunn heard squirrel-like pops and clicks, that meant the beetle had taken up residence and was now building its own magical sound universe.

Such a diagnosis invariably resulted in someone pulling out a saw. When people offered to pay for his unique service, Dunn gracefully accepted a donation on behalf of his non-profit Art and Science Laboratory. Dunn, after all, was collecting data on one of the world’s most remarkable animals for one of the strangest and most unlikely of science experiments.

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Misty Aqua – Water Collection in the Desert

Namib desert beetle (possibly of genus Onymacr...

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A simple and inexpensive way to produce drinking water

IN THE dry desert on the west coast of Namibia, where the annual average rainfall is a meagre 40mm, the Namib beetle (Stenocara gracilipes) has evolved a unique mechanism to drink. It collects moisture from the early-morning fog that is produced when ocean breezes from the Atlantic collide with the hot desert air. Drawing inspiration from the beetle’s fog-harvesting trick, Shreerang Chhatre, a graduate student at the Massachusetts Institute of Technology, and his colleagues have developed a simple and inexpensive way to produce drinking water.

The Namibia mist rapidly dissipates once the sun rises, so the beetle has just a brief opportunity to collect water. The insect typically finds a ridge of sand and faces the breeze, angling its lower body upwards with its specially adapted wings outstretched. The wings have bumps made of a hydrophilic substance that attracts minute water droplets. As they accumulate, the droplets grow larger until their weight causes them to run off into troughs in the beetle’s wings. These troughs are covered with a waxy water-repelling substance which has the effect of rolling the droplets down the beetle’s inclined body towards its mouth. The insect then promptly drinks them.

Fog harvesting is not a new idea. FogQuest, a Canadian charity, has been installing devices using a plastic mesh to catch water droplets in developing countries for more than a decade. Mr Chhatre says what he and his colleagues have done is to increase the efficiency of water collection by using a variety of surface coatings.

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