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Name: Paul
Status: other
Grade: other
Location: N/A 
Country: N/A
Date: 2/18/2005


Question:
Hi. This has been bugging me for a while...

How come a little bit of water is "sticky" or a lot acts as a lubricant?

For example, sand sticks to wet feet, but not to dry feet. And, wet sand does not stick to wet feet. Also, when your feet are wet they stick to a dry floor, but a wet floor is usually slippy.


Replies:
My two cents. Not really based on knowledge of the molecules etc. just a guess.

I would think the properties of surface tension and capillary action are what is perceived as "sticky". The difference between a little and a lot, is the comparison between individual drops and a multitude of drops. A certain amount of the individual drops exhibit the quality of surface tension, but there is a tipping point where they merge and become a surface. As a surface the behavior is different. Whereas drops can deal with the individual grains of sand, the surface becomes a barrier, hence the behavior you noticed.

James Przewoznik


Paul-

Wet stickiness and lubrication are different things, independent from each other. The stickiness you have noticed is not in any conflict with lubrication. In fact it can help make a good lubricant. Lots of thick, gooey greases are great lubricants, but hold sand like crazy. The stickiness is due to surface tension trying to pull the liquid into contact with solid surfaces. Lubrication is something that happens when a space between two solid surfaces is filled with liquid, and then is difficult to expel when abrupt forces push the solids towards each other.

Tacky, half-melted rubber is a case of higher friction due to liquid-like surface adhesion. But in that case, there is a semi-solid network of molecules connecting various wetted spots, so the rubber cannot flow like a liquid. A sticky solid will have high friction. A sticky liquid will lubricate.

If you imagine there is a thin liquid layer on the surface of solid rubber, if that layer is broke up into millions of tiny spots, wetted spots on a solid surface tend to be stuck to one spot; a certain level of force is required for them to flow laterally like a crawling snail. Substantial friction can result if there are millions of boundaries of tiny wetted-spots. So, once you fill the entire space between surfaces, no boundaries remain, and friction vanishes. High viscosity in an extremely thin layer of liquid might still provide some drag, but it would allow continuous creeping slippage.

A different phenomenon is water between two glass slides. If there is plenty of water, the glass surfaces are forced apart and the slides can slip over each other very freely. If there is not quite enough water to fill the whole contact area, then the edges of the wet spots have a high concave surface curvature, and surface tension will pull the two glass faces towards each other. The thinner the liquid-layer, the tighter the curvature, the higher the force. The highest spots on the solid surfaces are pulled into hard, direct contact with each other, and in that circumstance friction can abruptly get extremely high. Something like cold-welding between the faces can even occur.

just some examples to think about-

Jim Swenson

PS- viscosity of the liquid is a third independent thing.


That is a tricky question to answer, as there are a couple of forces acting there. (but I will do my best)

The first force to understand is Surface tension. Simply put, water likes to have a barrier along its surface, like a balloon. Fill up your sink, and drop some very small items onto the surface, such as pieces of thread. If you look closely, you will see a slight depression on the water's surface around these particles. If you did this with heavier objects, they would be able to force thier way right through the surface and sink. (or float, but either way, they would not sit on top of the water like thread or water bugs.)

Now take half a glass of water, and look where the water touches the sides. The water appears to be a little sticky, not rising to level in some places, and climbing the sides in others. This is another instance of surface tension in action.

The dry sand on wet feet sticks because it is caught in the same effect as the water clinging to the sides of the glass. Wet sand is just as stuck to more wet sand, and is thus perfectly content to remain stuck to the beach.

As for slippery floors, that is another effect altogether. That is called hydroplaning. You step in a puddle, but not all of the water can be easily shoved aside by the weight of your foot. Now you have a very thin layer of water between your foot and the floor you were hoping to gain traction on. This thin layer of water is already in contact with both your foot and the floor, so there are no surface tension effects to keep you from sliding right along. even if your foot slips to the edge of the puddle, your weight is more than enough to push right past what surface tension there is.

I hope these answers help. They are not the most precise explanation, but they should give you a good idea.

Ryan Belscamper


Although it might not seem like it, water is sticky in a peculiar way, even though it is a liquid. Water has surface tension that tries to pull isolated small drops of water (like rain or fog) into the shape of a ball. Sort of like a balloon that you fill with water. Surface tension is like the balloon itself. Most liquids have surface tension.

If the water "wets" a surface then the water sticks to that material too. A tiny amount of water wets a sand grain and also wets your skin, and the surface tension of that small amount of water keeps the sand stuck to you. The surface tension is not large enough to have large pebbles stick to you. Sometimes scientists call this effect capillary attraction.

Surface tension only present when there is a water-air interface (actually, there other kind of surface energies, but they arent as important). So sand under the water does not stick to you because there is no air/water interface to allow the surface tension to exist.

Off course, if the amount of water is large, the surface tension effects are proportionately small. With a slippery floor, water separates your foot from the flooring material. Water is a liquid and your foot may slide on a layer liquid. Oil would be even slipperier.

If your feet are just a little bit wet, then the capillary effects can be noticed and your foot might have high friction.

Wet clothing is very difficult to remove, especially socks. Socks need to slide off your feet and wet fabric makes thousands of small capillary bridges to your skin and doesnt slide well.

Bob Erck



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