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ramé-hart instrument co. December 2015 Newsletter
|A Bug's Life|
I'm going to go out on a limb and say that bugs have to deal with surface tension in a much bigger way than us big humans. In fact, a lot of bugs rely on surface tension for basic and essential functions like propulsion, breathing, and staying dry and afloat.
This month I am going to suggest some ways that bugs rely on surface tension for their survival and provide you with eight examples:
1. The Water Strider depends on the high surface tension of water to walk on it. Their long slender hydrophobic legs distribute the relatively light weight of their body over the film-like surface of water. In addition, the insect's body sports hydrofuge hairs, thousands per square millimeter, which deters wetting when a wave hits the insect's body. If the insect is submerged, tiny pockets of air between the hydrofuge hairs help it pop back up and bring it back to the surface again. Sea Skaters are water striders that can live on the surface of the ocean; most are coastal but a few live on the open ocean, a place that is hostile to almost all other insects.
2. The Rove Beetle shares the same walking-on-water talent that the water strider enjoys. However, in addition, the rove beetle can move itself along the surface of the water by releasing a small amount of pygidial gland secretions which act as a surfactant and propels the insect using what is called the Marangoni effect. This propulsion method works in a fashion similar to the soap boat in this video: https://youtu.be/H311jm2ZBZs.
3. The Water Cricket, like the rove beetle, can propel itself using the Marangoni effect. However, in the case of the water cricket, it releases saliva which acts as a surfactant lowering the surface tension and propelling the insect along at twice its normal speed.
4. The Diving Bell Spider lives almost its entire life underwater. In fact it's the only spider that does so which makes it pretty unique in the spider family. It does breathe air like all spiders but when underwater, an air pocket is trapped by the long and hydrophobic hairs on its legs and abdomen. Underwater life for these spiders begins with a diving bell it constructs from silk and a hydrogel that is spun between plants. The spider inflates it with air that he brings down from the surface. With a fully-inflated diving bell, the diving bell spider can live long periods under water.
5. The Backswimmers (aka, Notonectidea) is a family of insects that have long hairs on its backside and legs. They are able swimmers but do so on their backside like a small rowboat using their long legs as oars. Trapped air in their long hydrofuge hairs on their back and under their wings coupled with their hydrophobic structures provides them with the necessary buoyancy to stay afloat handily. Interestingly, when they do go under water, they rely on an oxygen supply from their hemaglobin. (Most insects that submerge rely on oxygen in the water.)
6. The Mosquito can walk on walls and ceilings using special foot pads covered with setae, stiff bristles that promote adhesion. Geckos and flies also have setae, too, which they use for the same purpose. However, the mosquito can also walk on water. Unlike the water strider, the mosquito does not rely on the setae but rather on pockets of air that are captured in grooves on their legs. This along with the high surface tension of water keeps the legs above the surface. It's been reported that a single mosquito leg can support 23 times its weight. This even surpasses the water strider which boasts 15 times its weight per leg. Surfactants are often used to rid both mosquitoes and their larvea from standing water.
7. The Whirligig Beetle is a most unusual insect in that when they are on the surface of a body of water, their bodies are partly immersed. They rely on surface tension to stay above the waterline. Their bodies minimize water resistance and their eyes are designed so the top half can view above the water and the bottom half below. Similar to the rove beetle, the whirligig is known to secrete a surfactant to help them move along more efficiently.
8. Fire Ants have the amazing ability to build a live raft made up of, well, themselves. It's the ultimate escape vehicle for these creatures who live in flood prone regions of South America. The video below shows how a colony of several hundred thousand fire ants build a raft in under two minutes using themselves as construction material. What appears to be an incredible feat of engineering is aided by adhesive pads on their legs which secretes an oil that helps them stick together. In addition, the ant's shell (called a cuticle) is hydrophobic and permits the ants to trap pockets of air under their body. Due to the high surface tension of water, the pancake-shaped flotilla floats easily. Just don't spray soap in the water - it would act as a surfactant lowering the surface tension and the ants would all go under.
It's amazing how many little creatures rely on the relatively high surface tension of water for their daily grind. Any entomologist worth their salt will have a solid understanding of surface tension and the impact it has on bugs of all variety.
|How to Use your Goniometer as a Digital Optical Comparator|
Our ramé-hart instruments are capable and
versatile surface characterization tools. But occasionally they are also
employed to perform additional tasks beyond what they were originally
designed to do. One such task is taking precise distance measurements of
features on small parts much like you would using an optical comparator.
The video below shows this in action.
In the video above, we are using DROPimage Advanced to take some measurements. Note that the Measure Distance command is also included with DROPimage Standard (but not with DROPimage CA). First, we check our calibration. Since all measurements are referenced back to the calibration, it's important that the calibration be performed properly. We then use the calibration ball to take our first measurement. It measures 4.000mm which is the precise diameter of the calibration ball.
Next we produce a pendant drop and take a few measurements on it: the diameter of the dispensing needle; the drop diameter at the widest point; and the drop height from the bottom of the drop up to the three-phase line (where the drop meets the needle).
In order to show how QC measurements can be taken on a small part, we place a small screw on the specimen stage and then we measure the thread pitch and the diameter of the head of the screw.
Use the Measure Distance command to capture the distance between any two selectable points in your focused image window - no matter what the object is.