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Finger Friction and Hydration

08/18/2021 10:09 PM

I think I've found an answer to something that's been puzzling me for a while, and I thought I would share it.

To open a plastic bag, your fingers have to be damp but not wet. (These days, wearing a mask in the grocery store, it's hard to lick your fingers.)

But why does this work?

Here is the answer I found in Quora:

Fingers have hills and valleys (finger ridges) and when damp there is both water and air trapped between the finger and plastic surface. Water, because of its surface tension, is attracted to the hydrophilic skin and plastic. Its least energy is when the water/air surface is minimum and the water/skin and water/plastic surface are maximum. Moving the finger stretches the water/air surface and increases this energy, so there is a force opposing this motion. This force is transferred to the plastic so the bag can be opened.

How does moisture sometimes generate more friction (aiding better grips then dry hands for example) but other times generate less friction (slippery floor for example)? - Quora

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#1

Re: Finger friction and hydration

08/18/2021 11:19 PM

Wet fingers tend to wrinkle, and dry fingers are smooth....the wrinkly surface gives better traction because of the roughness...

https://www.nytimes.com/2013/01/15/science/wrinkled-fingers-get-better-grip-on-wet-objects.html

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#2
In reply to #1

Re: Finger friction and hydration

08/19/2021 1:06 AM

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#3
In reply to #1

Re: Finger friction and hydration

08/19/2021 8:12 AM

Wrinkled skin probably does grip better, but dampening your fingers works immediately, long before the skin has absorbed enough water to wrinkle.

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#6
In reply to #3

Re: Finger friction and hydration

08/19/2021 1:57 PM

Moisture softens the skin and promotes elasticity, which enables wrinkling when dragging along a surface...

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#4

Re: Finger Friction and Hydration

08/19/2021 8:47 AM

I rub the bag vertically,at the entry end, between my hands,and the static electricity

will separate the layers,and also create a wrinkled surface for better grip.

Also,I avoid the clumsy effort to put large odd-shaped items in the flimsy bag by

simply inserting both hands into the bag,and inverting the bag over the

object,grasping the object through the bag.

Works great with bananas,etc.

Especially good when picking up poultry so as to avoid contact; even though they are

wrapped,they are still wet with XXX ??.

The checkout clerks also appreciate this too,(they have said so) and it keeps the

checkout conveyor cleaner.

Shop smarter,not harder!

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#5
In reply to #4

Re: Finger Friction and Hydration

08/19/2021 10:24 AM

Good tips, thanks! I'll try that next time I go to the grocery store.

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#7

Re: Finger Friction and Hydration

08/21/2021 7:22 PM

Einstein explained that the reason wet sand will support more weight than dry sand is because of the surface tension of the water binding the particles together.

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#8

Re: Finger Friction and Hydration

08/23/2021 9:55 AM

(These days, wearing a mask in the grocery store, it's hard to lick your fingers.)

In a similar vein, I've found myself several times, trying to blow the swarf away from a drilled hole whilst wearing a mask. There's a split second whilst you realise what happened then you feel really stupid.

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#9
In reply to #8

Re: Finger Friction and Hydration

08/30/2021 1:51 AM

Even worse - blow your nose.

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