Saturday, December 02, 2006

Slip, Slide, or Stick: Friction and Lubrication

Your hand slipping over your lover's back, metal pieces sliding past each other in a car engine, and even blood flowing in your blood vessels all involve friction. A little friction is a good thing in the bedroom, but a whole industry has developed to provide products to keep sexual friction under control.

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The amount of friction between two surfaces depends on the roughness of the materials, their chemical composition, and the force pressing the materials together. A lubricant is any substance that reduces friction. Some lubricants are powders, such as graphite, but the lubricants involved in sex are generally liquids, gels, and creams.

Lubricants for sex, which are often called personal lubricants or simply lubes, come in a dizzying array of varieties. There are water-based, silicone-based, and petroleum-based lubes. Some lubes include fragrances; others are edible and offer an array of succulent flavors. Many lubes feature additives such as pigments and dyes, anesthetics, moisturizers, preservatives, and chemicals that warm or cool skin.

How do you know which to choose? The Physics of friction and lubrication can help you figure out the best lube for you and your partner, no matter what activity you have in mind.

Natural Lubricants

The human body is a complex machine, with hundreds of moving parts. Like all machines, it needs lubricants. Elbows, knees, ankles and the rest of your joints would eventually grind themselves to dust if they lacked lubrication. Tears lubricate your eyeballs, saliva lubricates your mouth and throat, and your skin is constantly moistened with sweat and oils to keep it supple as millions of skin cells jostle against each other.

The natural lubricants most important for sex are saliva, vaginal fluids, and male pre-ejaculate.

Saliva consists primarily of water and mucus. Mucus is made of long proteins called mucins, which are coated in compounds related to sugars. The mucins bind to water to make saliva slippery.

The slipperiness of saliva mucus makes it a convenient lubricant for oral sex, hand jobs, and anal sex. Unfortunately the high proportion of water in saliva makes it dry out quickly as the water evaporates.

Vaginal fluids are also packed with mucins to ease penetration and protect vaginal linings from germs. In addition, vaginal fluids include various acids to provide the right chemical environment for sperm, and sugars that nourish sperm swimming through the vaginal fluid toward the ovaries.

Women produce vaginal fluids when they become sexually aroused. The amount varies depending on their age, health, and the timing of their menstrual cycle. Smoking just before sex can reduce fluid production by diminishing blood flow to the vaginal lining. Antihistamines and other drugs can reduce natural lubrication as well. All women need a little lubrication help from time to time, and even the most abundantly lubricating women may need to supplement their vaginal fluid with saliva or artificial lubricants - for extended love sessions.

Men produce some lubricant as well. The Cowper's gland near the prostate secretes a small amount of slippery fluid commonly called pre-ejaculate or pre-cum. Some sex experts believe it helps to lubricate the head of the penis before penetrating a woman's vagina, but most men only produce a few drops and it is often ill timed for insertion. It's more likely that the Cowper's fluid prepares the urethra for the passage of sperm by adjusting acidity, clearing out any traces of urine, and lining the urethra with a slippery, sugar-rich energy source to get the sperm swimming.

There is some lubrication in the anus, but nowhere near enough for most anal sex activities. Generally, you're going to need to bring along some artificial lube for anal play.

Water-Based Lubes

Pure water is an excellent lubricant - sometimes. If you've ever slipped on a wet tile floor, or nearly broken your neck stepping into a tub, you know how slick water can be. Many lubes consist primarily of water.

Unfortunately, water can sometimes dramatically increase friction instead. For example, people may lick a finger to add friction before turning a page in a magazine. Slightly moistening your hands will give you a better grip when trying to take the lid off of a jar, provided you don't make your hands too wet.

When you step into a tub, you usually experience a little of both water's lubrication and its tackiness. After initially sliding over the bottom of a filled tub, your feet will suddenly gain traction, giving you much more grip than you would have had if the tub were totally dry.

The two radically different properties of water stem from the attraction that water molecules have for each other and for some other materials. Materials that attract water are called hydrophilic, or water loving, and materials that are not attracted to water are hydrophobic, or water hating. Water beads up on a freshly polished car because the polish is hydrophobic and repels water. Rain droplets spread out on a car that needs waxing because the old oxidized polish is hydrophilic and attracts water.

Water is a liquid because the attraction between the molecules is too weak to turn it into a solid and they slip and slide over each other. It's strong enough, however, to bind water into small droplets when it falls as rain or runs down a window pane.

When your foot slips as you're stepping into a tub, it's because there's a thick layer of water between your foot and the porcelain. The mild attraction between the water molecules makes them act a bit like tiny marbles, with very low friction.

As you put weight on your foot, you squeeze most of the water out of the way, until there is a very thin layer between your skin and the tub. In some places, the layer is only a few molecules thick. Because your skin and the surface of the tub are slightly water-loving, the molecules are attracted to both. The slight attraction the water molecules have for the tub and your foot combine to give you traction.

It's because of the dual lubricating and adhesive properties of water that making love in a pool tub may seem like a good idea, but rarely turns out well. Your skin slides easily over your lover's skin, as long as the contact is light and there is a lot of water between the two of you. Once your skin presses together, you lose water's lubricating properties and the adhesion takes over, which can make vaginal and anal penetration particularly rough experiences.

But it's possible to exploit the forces between molecules to ensure that water stays slippery. That's what's going on in water-based lubes. These types of lubricants work in one of two ways; either by ensuring that the water molecules clump together so that you are less likely to get a thin adhesive layer, or by reducing the water molecules' attraction to each other and other hydrophilic materials. Some lubes have ingredients that do both.

Mixing in glycerin is one way to make water molecules clump together and form a good liquid lubricant. Glycerin is a small molecule that's hydrophilic in two places. As a result, water can attach to each side of a glycerin molecule. Another glycerin then attaches to the water, and so on. Eventually long molecular strings will form. If you could see the mixture through a powerful enough microscope, you would see that the strings tangle up like spaghetti. They slip and slide, like a plate of heavily buttered pasta noodles. To the naked eye, the result is a clear liquid that is much thicker and slicker than water.

The binding between the glycerin and water is weak enough that water molecules can break free of the mixture. They may then evaporate or get absorbed into your skin, which means that lubes relying on glycerin to hold water molecules together will slowly dry out. The glycerin molecules that have lost their water will be mildly attracted to your skin, which makes the lube get sticky as it dries. Adding a little water will restore the glycerin lube's slipperiness.

Glycerin is related to the sugar glucose. If you taste some, you will see that glycerin lubes are sweet. Like sugar, glycerin is a good energy source and sometimes serves as a nutrient for microbes. Women may find that glycerin lubes foster yeast infections. If you or your lover suffers from frequent vaginal infections, look for glycerin-free water-based lubes. There are several other molecules that can hold water together as glycerin does, without feeding populations of vaginal bacteria and fungi.

Another way to make water stay slippery is by mixing it in a gel. Gel lubricants get their jelly-like consistency from long molecules of protein rather than short glycerin molecules. The proteins in gels have many places along their lengths that attract water. Chemical treatments or heat cause the long molecules to crosslink, which means that they connect to each other in some places.

It's like tying pieces of string together at random places to make a loose, three-dimensional web. Water molecules get trapped in the web at the hydrophilic points along the protein molecules. Food gelatins, like Jello, trap water the same way.

The more places that the molecules are connected in a crosslinked gel, the more rigid and jelly-like the gel will be. Like glycerin lubes, water-based gels may dry out in time. Because they're more complex than liquids, with water trapped in a net of crosslinked proteins, you cannot rejuvenate them as well by simply adding water. It's better to add fresh gel if it gets too dry.

Some lubes rely on chemicals called surfactants that reduce the attraction between water molecules. Instead of ensuring that there is a thick layer of water between your skin and your lover's skin, surfactants make thin layers of water less adhesive. They're generally medium length molecules, longer than glycerin and shorter than most proteins.

Surfactant molecules each have a hydrophilic connection at one end. Surfactant molecules link up with water, effectively making the molecules larger and more bulky. This keeps them farther apart. The attraction that water molecules feel for each other gets much weaker if they are even slightly separated. The reduced attraction also reduces the adhesion of the surfactant-water mix.

Many gel and liquid lubes include surfactants to make the water in them ultra slippery.

Water-based lubes are safe for use with latex condoms and diaphragms, as well as all sex toys. They wash off easily with nothing more than warm water. Of course, that means they rinse away too readily for making love in the bath.

Oil-based Lubes

Some lubes don't include any water at all. Lubricants based on vegetable oils and petroleum products are often very slick and long lasting. Vegetable oils common in lubes include olive, sesame, and palm oils, to name just a few of the many possible varieties. Most petroleum-based lubes are varying grades of petroleum jelly, with Vaseline being the best known brand.

Both plant oils and petroleum lubricants are made of hydrocarbon chains, long strings of carbon atoms with hydrogen atoms attached to the sides. The texture of petroleum-based lubes is determined primarily by the lengths of the carbon chains.

Molecules made of chains ten to fifteen carbon atoms long form mineral oils and light watery lubricants. Longer chains are heavier and clump together to form jellies like Vaseline. Still longer chains result in paraffin wax.

Vegetable oils are a little more complicated. They're also made of hydrocarbon chains. However, they typically consist of multiple chains linked together by a glycerin molecule. Like petroleum products, heavier oils usually are made of longer chains. In addition, light, watery oils can be made to solidify by changing the number of hydrogen atoms attached to the chains, through a process known as hydrogenation. Margarine and shortening are made from light vegetable oils that have been hydrogenated.

Hydrocarbon chains that make up oils are highly hydrophobic. If you've ever made salad dressing with oil and vinegar (which is mostly water) you've seen how hard it is to mix the two.

The molecules in oils and petroleum products don't attract each other or your skin very strongly, which is why they're slippery.

You might think oils and petroleum jellies would be easier to clean off of your body if they're not as strongly attracted to your skin as is water, but that's clearly not the case. One reason it's harder to remove hydrocarbon lubes is that their large molecules don't evaporate very well. If you get water on your skin, just wait a while and it'll dry all by itself. Oils and petroleum products will stick around for ages with little or no sign of evaporation, which is good for long lovemaking sessions, but not so great for the post-coital clean up.

The fact that the lubes are hydrophobic also means that you cannot simply rinse them off with water. To remove the lubes you'll need to wash with soap.

Many men prefer oils and petroleum products over water-based lubes for masturbation. Some people feel that they are better suited for anal sex because the petroleum jellies in particular are heavier and last longer. Hydrophobic vegetable oils and petroleum lubes work well in the tub or pool because they won't rinse away.

Unfortunately, all oils and petroleum products dissolve latex, and should never be used in combination with latex condoms, diaphragms, and latex sex toys. It's also generally a bad idea to use them for vaginal sex because the soap necessary to clean the lubes away removes the protective vaginal mucous as well. This leaves the delicate membranes open to infection.

Silicone-based Lubes

Some of the newest lubes on the market are silicones. Silicone molecules have essentially the same structure as petroleum except that the long carbon chains are replaced by chains of alternating silicon and oxygen atoms. Like oils and petroleum products they are hydrophobic, long lasting lubes. They're just as slippery as oils but will not dissolve latex. They wash off with soapy water, making them less than ideal as vaginal lubricants but much better alternatives for anal sex when latex condoms are involved.

There are also water-based lubes that replace glycerin with dimethicone, a silicone molecule that can link water molecules into long slippery just as glycerin does. They're good lubrication alternatives for vaginal intercourse if you want to avoid glycerin. Like all water-based lubes, those that include dimethicone wash off easily with water and are no good in the tub or pool. They're safe to use in conjunction with latex products, but will still damage silicone sex toys.

Emulsions and Creams

Although water and oil don't mix as a rule, there's a way to almost make them get together, which leads to another type of lubricant - emulsions. Surfactants do the trick. You've already seen that surfactant molecules that are hydrophilic on one end can make water more slippery. If the other end of the surfactant molecule is hydrophobic, then it can help get water and oil molecules close together, even if they don't actually mix.

Creams and many creamy lotions are emulsions. If you mix a surfactant in water and then add oil, the surfactant molecules will surround oil droplets with their water-hating ends pointed in, toward the oil, and their water-loving ends pointed outward. The surfactants create tiny balloons of oil in the water called micelles. The oil and water still don't actually mix, but the micelles act like large dissolved particles in the water. Mayonnaise is a common emulsion of vegetable oil and water, with a bit of egg white mixed in to act as an emulsifying surfactant.

All emulsions, including most lubes advertised as creams or lotions, have either oil or silicone mixed with water. Be sure to check the ingredients before using an emulsion in combination with latex or silicone prophylactics and toys; all the same precautions apply for emulsions as do for simple oils, petroleum jellies, and silicones.

To summarize. . .

- Water-based lubes are usually safe with latex condoms as well as silicone and rubber, provided they don't have any oils or silicone surfactants. (Check the label to be certain.) They're also easy to clean up.

- Oil and petroleum lubes are super slick and long lasting, but destroy latex condoms and toys, and are hard to wash off.

- And finally, silicone lubes are also very slick and long lasting, but don't harm latex products. Although they can be as tough to clean as oils, and will damage silicone toys

The options for sexual lubes are vast. So which is best for you? It's hard to say, but I can't think of a better way to answer the question than buying several types and spending a night trying them out. It'll be a slippery, sloppy session of sexual fun, and in the end I hope you'll have a new appreciation for the physics of friction and lubrication.

Next time . . . Pumped Up and Ready for Love: Fluid Physics and Sex


Anonymous said...

Ewwww, gross!

Mind Releaver said...

Very usefull information! would look forward for more of such information

inef said...


--One of my favorite posts; not gonna lie.

But some of us might appreciate a little credit where credit is due: this is a chemistry post! :P

Anonymous said...

Oil/petroleum based lubricants also create a favorable environment for bacteria to thrive in, thereby, they are often the culprits of UTI's.

smelly said...

you mentioned gels being used in, i assume commercial products. any idea if aloe could be used as a home substitute? great, thorough article. i never thought about underwater sex that way but it explained a lot. much obliged. :)

Anonymous said...

It might be worthwhile to note that the long-term health effects of silicone, dimethicone and parabens, which are also often added to lube are largely unknown.

Subcutaneous injection of silicone is known to cause horrible tumors (see the book _Travesti_ by Don Kulick in particular). While breast implants with silicone gel were re-approved by the FDA in 2006, this was due to there being little evidence of increased cancer risk from intact silicone implants. If a silicone implant ruptures however, it is very serious, very carcinogenic and may be linked to connective tissue disorders. Silicone is absolutely not approved for human consumption.

Any of the more traditional places you would want to put lube are particularly good at absorbing chemicals, which is why various types of suppositories are being increasingly suggested by doctors (if not prescribed to the squeamish). The potential for absorption of fairly toxic chemicals makes me pretty shy about using silicone based or pure silicone lube.

I have noticed that it makes a great skin conditioner. If I had problems with glycerin and the sex was good enough I would definitely consider it.

I got a sample of ForPlay Succulent Passion Fruit, which is not something I would have tried on my own, mostly because of the lugubrious name and the fact that it's flavored. I *highly* recommend it. Looking at the label, I have no idea what the flavoring content is, but glycerin and potassium sorbate have been around for a long time.

Anonymous said...

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rabbit vibrator said...

Oh, those stuffs may cause some infection I think. Its not really good.