Why are your hands slippery when dry, get “grippy” when they get a little bit wet, then slippery again if very wet?
In: Physics
van der waals forces, and probably water displaces microscopic air pockets creating a slight suction effect
“There’s a fine line between between ideal friction and ideal lubrication.” – Dr. Ruth Westheimer
Water has a high surface tension. Think of this as water molecules being so attracted to themselves that they minimize the surface area at any water/air interface. This is why water forms into beads or droplets. If you add a little bit of water to your finger and apply it to a plastic grocery bag or book page, when you pull away the top layer of the bag or book page will move with your finger. Separating from your wet finger would require that the water/air surface area increases against the surface tension force which is trying to minimise the area.
If there is enough water to produce a thin film between two sliding surfaces lubrication will occur. Some good examples of this are bearings, air hockey tables, or walking on wet brick. You would only notice this briefly while walking, but the thin layer of water would cause you to slip before your shoes contact the ground.
The surface tension of water is greater than the surface tension of your dry skin, which is mostly just tiny dead flaky bumps. Once you have too much water, no, you’re no longer dealing with service tension and now you’re dealing with fluid viscosity, which counteracts the tension.
Source: science and 25 years of hyperhidrosis presenting primarily in the hands and feet
Old ELI5…
Actually, when the ridge on your fingertips and hand get hydrated with water, combined with the crease, will act as a ventouse. Kinda like if you had a thousand of little ventouse on the inside of your palm hand so it stick to slipery surface.
I’ve always said that certain things are slippery when they’re too dry, but also slippery when they’re too wet. A lot of systems on earth balance on the correct amount of water. Slipperiness is no exclusion.
No ones hands are “slippery when dry.”
What are you talking about?
We literally chalk our hands before lifting heavy weights or doing bar work to get them as dry and abrasive as possible to maximize friction so we don’t slip.
People this thread are weird
Water sticks really well to lots of things, so a little bit of it makes things grippy / sticky, BUT it doesn’t stick to itself nearly so well so too much of it makes things slippy.
your hands get grippy when they’re a little bit wet?
This is a question of friction. There are two main types of forces contributing to friction.
First we have literal roughness/unevenness in the surface (termed [Asperity](https://en.wikipedia.org/wiki/Asperity_(materials_science))) which means two surfaces can hook into each other, sort of like velcro, and your hands has a lot of these (e.g. finger prints).
The second is a bit less intuitive and is called the Van der Waals force. This can cause friction between smooth surfaces because the force comes from the atoms and molecules themselves. This is the reason you get a better grip on your cellphone case that is made of soft rubber than you get from the glass and hard plastic of the phone itself.
Beyond this it can often be hard to specifically determine what forces will dominate for a given scenario.^([1]) Climbers generally want to chaulk their hands completely dry to get a better grip on the rough surfaces they climb on. If the surface is really smooth then you might want to rely on the Van der Waals force instead, for instance by using indoor running shoes with soft rubber soles when in a gym with equally soft rubber flooring. However, be careful of any lubricant that can come between the smooth surfaces, as this will remove the Van der Waals force completely and make it extremely slippery.
For your specific example I would conjecture that somewhat wet hands strikes a good balance between using the Van der Waals force and the roughness between surfaces to obtain relatively strong friction for a variety of scenarios. Too wet hands could for example act as lubricant on the Van der Waals force. (Dry hands I am less sure of but I have a feeling it could have something to do with reduced surface contact)
>[1] The relationship between frictional interactions and asperity geometry is complex and poorly understood. It has been reported that an increased roughness may under certain circumstances result in weaker frictional interactions while smoother surfaces may in fact exhibit high levels of friction owing to high levels of true contact.” [wikipedia.org/wiki/Asperity](https://en.wikipedia.org/wiki/Asperity_(materials_science))
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