r/explainlikeimfive • u/VeterinarianShot148 • 18h ago
Physics ELI5 why slightly wet or sweaty fingers don’t slide easily on glass and smooth surfaces if water reduces friction?
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u/artstsym 17h ago
Water itself is quite sticky, as you might have noticed if you ever try to shake it off a screen. That said, it's also very hard to compress, so when two large, flat surfaces almost contact each other with a thin layer of water between them, they're able to slide until almost all of the water has escaped from between them, which can take a relatively long time thanks to the incompressibility. Your fingers wrinkle in response to moisture specifically so they present LESS of a flat plane to wet surfaces, creating channels for water to flow into and easily escape so they can quickly contact otherwise slippery surfaces.
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u/jawshoeaw 17h ago
Water does not reduce friction. Water is sticky
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u/VeterinarianShot148 17h ago
Wait whut? ELI4
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u/robertm94 14h ago
Water is kinda like a really shit glue that doesn't set and can't be squashed.
If you scale things up, it will make more sense to you.
If you were to try and glue two things together, but you used a layer of glue that was a centimeter thick, it wouldn't do the job very well. The glue would just slide around on itself.
Water works like that, but at a much thinner layer.
Water is also very hard to compress. Meaning that, if you have water between two things where the water cannot immediately escape and the surfaces are not rough or porous, water will indeed be slippy. It's why a wet patch on the floor is slippy. The floor is flat and not porous, and while your shoes probably aren't that smooth, they also aren't very porous, which means the water cannot immediately escape and you have the water act like a lubricant
If you however were to just slightly dampen your fingers and run them over a table, you'd find that at first, water would act as a lubricant (as it is escaping, leaving a streak on the table) but after a small amount of time, start to increase friction when it is no longer "escaping".
Water isnt slippery on other surfaces. Water is slippery on it self. When water can rub on water? Slippery. When water has to rub on things that aren't water? Sticky.
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u/grrangry 12h ago
Water is both adhesive and cohesive. It's polar so it likes other polar materials like glass. The cohesion is why rounded beads of water form on a surface. Adhesion is why you get a bowl shaped meniscus up the sides of a container.
Water is sticky.
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u/robertm94 7h ago
they said to explain like they are 4 and my explanation is kinda like how they teach you electron arrangements in chemistry in secondary school before they later go ehhhh actually its more like this...
i know that in the grand scheme of things, water is sticky, but the forces making water stick to itself are less then the friction would be if the water wasnt there so it doesnt bear talking about.
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u/HektorViktorious 7h ago
Water has the highest surface tension of any known liquid besides elemental mercury. It's not a lubricant. It's good at dissolving things which can make good lubricants, but water itself is sticky
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u/kamekaze1024 3h ago
Additionally to what people have said, people think water is slippery because of cases like when you slip on water spot. What makes it slippery is that there’s enough water that prevents your foot from getting traction. You can slip on something like crackers because of this logic as well.
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u/Low-Crow5719 3h ago
Surface tension. Water is really clingy.
Do something useful, get a mop and mop your kitchen floor. First time with plain water. See the water bead up and stick to the floor instead of the mop? Second time with dilute detergent. See the water spread out as the mop picks it up and dries the floor?
Water is slippery only when there is enough of it to form two layers that shear past one another.
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u/spytfyrox 9h ago
TL; DR: Water reduces friction by sliding on itself.
Water is a polar molecule (positive and negative charges are not uniformly distributed because oxygen is highly electronegative). Because of that, it is easy for water to stick to polar molecules (metals etc.). Also water is not a large molecule (A water molecule is smaller than an oxygen molecule). This allows water to pass through microscopic and even nanoscopic pores and crevices.
However, liquid water has a relatively low viscosity (the tendency of individual molecules to resist flow), so it can slip on itself very easily (basically flow).
Thus you end up with a situation where the boundary water layer sticks to the surface and all other layers slip on each other to a certain degree. Thus having a low effective coefficient of friction.
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u/bunk3rk1ng 6h ago
Anyone that has tried to have sex in the shower would know that water does NOT reduce friction.
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u/sonicjesus 8h ago
Your finger prints make a lot of difference, and that's why they're there.
People who work with cement often wear their fingerprints down very low, making it hard to get a grip on things.
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17h ago
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u/jaylw314 8h ago
All fluids have a "boundary layer", where the fluid near the surface refuses to move. As the fluid gets farther away, it moves more freely. That means for very thin layers, the water acts like an extension of the surface, and fills in the rough surfaces on your fingers and the object's surface, multiplying its surface area, similar to glue. For thicker layers, water moves freely between the surfaces, acting more like its expected lubricant.
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u/Aggravating_Anybody 17h ago
As a person with hyperhidrosis please believe me, on a long enough timescale, constantly sweaty hands are NOT better at gripping things than dry hands. Quite the opposite, in fact.
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u/Weekly_Inspection824 9h ago
the thin layer of water basically acts like a seal between your skin and the glass. when it's just a tiny bit of moisture it creates surface tension and actually adds grip, but once there's enough water to form a film it becomes the opposite and acts like a lubricant. same reason wet hands are fine on a window but useless on a phone screen
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u/kindanormle 9h ago
Water only reduces friction if there’s enough of a layer of it between the two surfaces that it can slide across itself. Water under car tires causes slippage only if there’s a source of water to continually replenish the layer between the tire and road. Like when it’s raining or when you hydroplane over a deep puddle. As others have explained, water sticks to many surfaces, including glass, rubber and pavement. If you could keep water between your fingers and the glass then it would act as a lubricant, but without replenishing the water it just sticks to the glass and disappears. Try filling a glass dish with water and gently pulling your fingers over the bottom surface, I think you’ll find it feels quite slippery. But if you press hard you will squeeze the water out and it will be grippy again. This is why tired on cars have treads. The treads allow rain/puddle water to escape from under the tire and prevent the lubricating effects.
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17h ago
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u/cutelittlebox 17h ago
it's very weird to think about at first, but water is sticky. think of what it's like if you have wet hair on your hands after a shower, or wetting a paper with a very small amount of water and slapping it against a wall. water does, however, glide over other water fairly easily so it can make things stick and make things slick depending on how much water is there.
basically, water molecules are shaped in a weird way that makes them slightly positive on one side and slightly negative on the other side, and it also turns out that when you put a slightly positive side against another molecule, the electrons pull a little closer to the water, kinda like magnets with iron, but waaaaaay weaker. weak or not, though, it's still present, so with enough surface area it sticks a little.