Today’s New Reason To Believe

By Katie Galloway

[Katie Galloway is an RTB volunteer apologist. She is completing her PhD at Caltech in Chemical Engineering with an emphasis in biological systems.]

Imagine putting tons of super glue on your hands and feet and trying to climb the Empire State Building like Spider-man. Your dynamic display of agility doesn’t get you too far. You’re stuck! That’s because the chemical bonds between the sticky glue and the building surface are stronger than the force of your weight. If the bonds on your feet and hands hold your weight, you will have to pull off with at least a fourth the force of your weight for each new grip-not a very satisfying solution if you have to climb 102 stories to the top. And by the way, you will probably have to reapply glue for each step since the glue will not stay sticky. Clearly this represents a poor design strategy for scaling sheer surfaces-so how do geckos do it?

Geckos have the amazing ability to adhere to sheer surfaces, but it’s their capacity to control their adhesion and not stay stuck that allows them to run straight up walls. Their feet aren’t sticky or else they would get stuck just like you and your super glue. Instead, geckos adhere to surfaces using spatulae composed of billions of setae, hard bristle structures that look like split-ends gone crazy. Adhesion is promoted via Van der Waals forces, the sum of the attractive intermolecular interactions between the surface and the setae.

To put Van der Waals forces into effect, the gecko employs shear force (the force applied parallel to the surface) to get its setae in close contact with the surface. By regulating the shear force, the gecko controls the area of the setae in contact with the surface of a tree or wall. The more force applied, the more the otherwise perpendicular setae bend, creating a larger surface area for the Van der Waals interactions that create adhesion. Fortunately, this process is reversible. To take another step, the gecko reduces (unloads) the shear force on the foot so the setae are no longer induced to bend. The contact area between the gecko’s foot and the wall decreases until the gecko can easily lift and replace the foot.

To see this principle in action, slide the tips of your fingers down a window. They slide pretty easily, but when you put your entire hand on the glass and pull down you encounter greater resistance. This is due to friction created by the interaction of atoms in the glass with atoms in your hand. With only your fingertips, the small area of contact results in a small frictional force. More Van der Waals forces are generated with the larger surface area of your hand, requiring more sliding force to overcome. Additionally, you can observe that even more force is required to slide your hand as you increase the pressure. This is because increasing the pressure puts more of your hand in contact with the glass, creating more intermolecular bonds. So why can’t you scale the Empire State Building like a gecko?

From your perspective, it appears that your hand is in complete contact with the glass, but in actuality, very little of your hand is touching the surface. Due to the roughness of the glass and your hand, the contact area is still relatively small, and the atoms close enough for Van der Waals interactions number very few. Putting more pressure on the surface achieves greater contact, but it also causes you to push away from the wall, which is the opposite of the direction you want to go. Without flexible setae, humans do not have much control over their degree of contact with surfaces like glass or the sides of buildings. Our ability to enhance or reduce this contact area without pushing off the surface is very limited, unlike the wieldy gecko.

While humans lack natural setae, wings, sonar, and a host of other biologically inspired devices, we are endowed with a set of powerful tools to unlock the mysteries of Earth’s creatures and harvest knowledge of the natural realm to benefit mankind. As reported in the January 30, 2008, issue of Science Daily, researchers at UC Berkeley put their curiosity and brains to work in designing a gecko-mimetic tape that adheres when loaded with shear force, yet can be easily removed from a surface without leaving a residue¹. Synthetic stiff polymer microfiber arrays were fabricated from polypropylene. Forty-two million stiff plastic fibres 20um in length and only 0.6 um in diameter were arranged in 1 cm². Two cm² of this tape were able to hold up 400 g, or nearly 1 pound. The team from Berkeley reported that:

“In the absence of shear forces, these fibres show minimal normal adhesion. However, sliding parallel to the substrate with a spherical probe produces a frictional adhesion effect which is not seen in the flat control.”²

By harnessing the geckos’ mechanical strategy to induce Van der Waal interactions via shear force, the researchers demonstrated that stiff polymers can be used to attain sliding-induced adhesion. While this tape marks a step forward for gecko-inspired synthetic adhesives (GSA)gecko-inspired adhesives, more research into a system for nonfouling adhesives is necessary before anyone “Spider-mans” up a skyscraper.

Scientists have been studying gecko adhesion for decades, but even before formal science picked up interest, the Bible was cluing in readers to the gecko’s unique abilities. In Proverbs 30:24-28, the Bible says, “A lizard can be caught with the hand, yet it is found in kings’ palaces.” Today we have greater insight into how these little guys can be so stealthy!

References:

¹ J. Lee, C. Majidi, B. Schubert, and R. Fearing, “ Sliding induced adhesion of stiff polymer microfiber arrays: 1. Macroscale behaviour,” Journal of the Royal Society, Interface

² B. Schubert, J. Lee, C. Majidi, and R. Fearing, “Sliding induced adhesion of stiff polymer microfiber arrays: 2. Microscale behaviour,” Journal of the Royal Society, Interface

Bee Foraging Provides Insight on Optimization of Server Allocation

By Katie Galloway

[Katie Galloway is an RTB volunteer apologist. She is completing her PhD at Caltech in Chemical Engineering with an emphasis in biological systems.]

Perhaps you don’t instantly recognize the connection between the Internet and honeybees. But researchers from the Georgia Institute of Technology (where the school mascot is, coincidentally, the Yellow Jacket) do. A Georgia Tech group has developed an algorithm based on honeybee foraging strategies to optimize server allocation to keep websites running smoothly.

Honeybees and servers face similar optimization problems: efficiently allocating limited resources (e.g. worker bees or computing power) in the face of variable demand. Through swarm intelligence, honeybees operate entirely devoid of central command. The hive’s survival depends on worker bees finding and retrieving nectar. So how do the honeybees ensure that they work productively? They dance, of course.

Honeybees scout for nectar until they find it and then return to the hive. The scout bee crawls up on a vertical honeycomb and starts dancing. This jig is called the “waggle dance.” Every turn has meaning. The direction of the dance indicates the direction (relative to the sun) of the nectar, the number of waggles tells how far to fly, and the length of the dance connotes how sweet the source. The forager bees learn the directions by following the dance, turning the hive into a Broadway show-stopper.

When the source is very sweet, the waggle dance can go on for quite a while. This allows more forager bees to see the dance so greater numbers will be recruited to that nectar site. Yet during this time, as the sun continues to move, the position of the nectar source relative to the Sun constantly changes. Amazingly, the dancing bees adjust their angle to account for the Sun’s movement.

Returning bees keep the dance going as long as the source is still abundant. As the bees deplete the source, the dancing slowly tapers off. Meanwhile, scout bees from new sources enter the dance floor, allowing the hive to deftly shift from a dwindling supply to a fresh one. This competition ensures that available nectar sites are optimally staffed with workers at all times as the conditions change.

The researchers at Georgia Tech recognized that server optimization problems resulting from “abnormal” demand could be solved by developing an algorithm based on the bee foraging strategies. Servers allocate computing power to websites as the demand on those sites varies. Under previous strategies, sites were optimized for “normal” demand, but when strained these sites would crash. All the while, servers for other sites remained idle.

The honeybee algorithm guarantees less busy servers don’t sit by idly during times of high demand. When an overloaded server receives a request for access to a website, it places an ad to recruit other servers to the in-demand site. The higher the demand, the longer the ad remains posted, and the more computing power is contributed to the run the site (examples, like Ticketmaster or YouTube). “Computational results indicate that the new algorithm is highly adaptive to widely varying external environments and quite competitive against benchmark assessment algorithms.” ¹ In laymen’s terms, this new algorithm is the bee’s knees!

The research from the field of biomimetics (biologically inspired engineering) continues to demonstrate that natural systems are endowed with specialized knowledge from which even the most intelligent human designers can benefit. Engineered biomimicry has brought society a wide array of technologies from robust adhesives to optimized server strategies.

Wisely, researchers heed the biblical imperative to “ask the animals and they will teach you.” In Proverbs 30:24-28, the Bible says, “Four things on earth are small, yet they are extremely wise: Ants are creatures of little strength, yet they store up their food in the summer; coneys are creatures of little power, yet they make their home in the crags; locusts have no king, yet they advance together in ranks; a lizard can be caught with the hand, yet it is found in kings’ palaces.”

What can we learn from the animals today?