USA Science and Engineering Festival: The Blog

The Cat’s Meow: A feature by Festival Nifty Fifty Speaker Joe Schwarcz

Have you ever wondered why on some days cats lick themselves more vigorously? I suspect not. But their licking rate is indeed variable. And it just might have to do with the animal’s fear of getting an electric shock. Unfortunately for felines, cat fur loses electrons very readily, and therein lies a problem. Anytime a cat rubs up against something, and they do a lot of rubbing up, electrons are transferred from the cat to the object, leaving the cat positively charged. When the animal now comes close to items that are good electrical conductors and therefore readily give up electrons, it is subjected to an electrifying experience. A spark, which is nothing more than a stream of electrons, can jump from the item to the cat. And then the cat jumps. Unless it has engaged in some prophylactic licking.

The buildup of static electricity is less likely when there is moisture in the air due to a couple of factors. Water in the air makes the air more conductive, making for an easier dissipation of any charge that has built up. Furthermore, water molecules, being polar, also bind to the charged material. “Polar” means that within the molecule electrons are distributed in a fashion so as to make the oxygen atom slightly negative and the hydrogens slightly positive. Cat fur being positively charged attracts the negative end of water molecules which means the positive charge is partially neutralized, making the fur less attractive to any source of electrons. The risk of a spark is diminished. When humidity is low, the cat has to use saliva to moisten its fur to prevent being shocked. Since low humidity is usually associated with good weather, a cat licking itself with increased enthusiasm is a sign that rain is not likely. If you prefer not to use your cat as a barometer, a little spray with water will do the trick. But you may lose some affection.

Let’s move on from licking cats to licking static cling. This too has to do with electron transfer. The tendency for such transfer is known as the “triboelectric effect” and the “triboelectric series” is a list of substances in order of their ability to lose or gain electrons. Substances at the top of the list tend to lose electrons readily, at the bottom, they are more likely to gain electrons.

The triboelectric effect was first described around 600 B.C by the Greek mathematician Thales. Of course there was no reference to electrons which would not be discovered for another two thousand five hundred years by J.J. Thomson. Thales noted that light objects such as feathers were attracted to a chunk of amber that he had been polishing with a piece of fur. As we now understand, the rubbing transfers electrons from the fur to the amber, giving the latter a negative charge. When the negatively charged amber is brought close to a feather, it repels electrons from the feather’s surface, making the surface positive. The attraction between the positive areas of the feather and the negative areas of the amber is an example of “static cling.”

A similar effect occurs when a plastic comb is run through hair. Since hair is above plastic in the triboelectric series, electrons are transferred from the hair to the comb which can then pick up light objects just like the charged amber. Since the hair fibers have lost electrons, they become positively charged. Given that like charges repel each other, the result is the dreaded “fly-away” hair. The solution to this problem, as well as to that of static cling, is the neutralization of any charge that has built up by adding moisture to the surface. But spraying with water is usually not a practical solution.

In the case of hair, we turn to a “conditioner,” composed of molecules that feature both a water-loving, or “hydrophilic” end, and a water hating or “hydrophobic” one. The hydrophobic end sticks to hair and the hydrophilic end attracts water which then dissipates some of the charge on the hair. This same chemistry is used in commercial antistatic agents. A large variety of substances with both hydrophobic and hydrophilic properties are available, ranging from polyethyleneglycol esters to quaternary ammonium salts. The latter are also used in fabric softeners, thereby explaining why these also reduce static cling. Since fabric softeners are also lubricants, they further help to cut static buildup by reducing friction between surfaces.

Materials differ in their susceptibility to the buildup of an electric charge. The determining factor is the conductivity of the material which to a large extent depends on its moisture content. Fibers such as silk, rayon, cotton or wool have a relatively high moisture content and therefore charges are quickly dissipated. But synthetics like polyester, polypropylene and acrylics have a high surface resistance meaning that electrons cannot readily move to neutralize a charge, particularly when humidity is low.

The latest technology to reduce the buildup of static involves the application of coatings that don’t have much of a tendency to lose or gain electrons. An example is a special form of carbon known as a “nanotube” in which carbon atoms are attached to each other to form a cylindrical molecule. These molecules aggregate together to form “nanoparticles,” less than one hundred nanometers in diameter. These nanoparticles form a strong bond to fibers, don’t lose or gain electrons, and are also excellent lubricants. Fabrics coated with nanoparticles also feel soft, resist stains and dry readily.

Had Frank Clewer in Australia been wearing antistatic garments, he would not have caused the stir that he did back in 2005. But he was wearing wool and nylon, both of which are high on the triboelectric series meaning they readily assume a positive charge. When Mr. Clewer walked into a building for a job interview, he set the carpet on fire by causing sparks as electrons jumped from the synthetic material towards his positively charged clothing. The heat generated was enough to ignite the carpet, necessitating the evacuation of the building. There have been no reports of cats sparking such calamities. So when you see your cat licking himself with great gusto, he’s not only protecting himself from electric shocks, he’s protecting your home from a fire. By the way, Mr. Clewer didn’t get the job.

 

Comments

  1. #1 vinylogous
    United States
    December 1, 2012

    So when the feline rubs up against something and loses electrons, it becomes… a cation?

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