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#### BillJates

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##### How thin can a needle be?
« on: December 28, 2016, 09:25:22 AM »

Do microscopically thin needles exist/are they possible? Extremely thin, but also extremely rigid.

What are the thinnest needles that exist? How thin could a needle possibly get, while still being rigid (an actual needle)?

What would it have to be made of?
« Last Edit: December 28, 2016, 12:06:53 PM by BillJates »
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#### Arkcon

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##### Re: How thin could a needle be?
« Reply #1 on: December 28, 2016, 10:01:33 AM »

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That all depends on how reasonable we're all willing to be.  I just want my friends back, except for Cartman, you can keep him.

#### Enthalpy

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##### Re: How thin can a needle be?
« Reply #2 on: December 29, 2016, 05:42:56 AM »

Atoms are the only limit, as Arkcon pointed out. And if a thin tip is more interesting than a thin cylinder, you have a single atom at the sensing point of a tunnel effect microscope or an atomic force microscope.

You have some intermediate diameters between drawn steel wire and nanowires. For instance the usual carbon fibres used in composites have commonly 4µm to 7µm diameter. Gold wires used (less and less) to bond silicon chips are but thicker. Beams of silicon can be made narrower by semiconductor processes. Asbestos fibres split down to some 20nm diameter.

One limit is stiffness, yes. No material is magic here: steel is already excellent, a few ceramics are just 2× stiffer, nanotubes 5×
https://en.wikipedia.org/wiki/Young's_modulus
So the diameter makes all, and the stiffness of a cylinder (or of any beam) varies as the thickness high 4. If comparing at identical length, the limit is close. I didn't use steel wires, but steel sheets of 0.1mm are already quite flexible, so you couldn't push with a good human force on a 0.1m long sample.

On the other hand, if the needle's length scales down like the diameter, then there is no such limit, because a beam's stiffness varies as the length high -3 too. You may also refer to beam buckling by compression
which tells that, if the length and diameter vary by the same amount, then the buckling force varies as much as the cross-section area does, just like the limit by the material strength does.
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#### Irlanur

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##### Re: How thin can a needle be?
« Reply #3 on: January 08, 2017, 11:22:06 PM »

Just to widen the scope a bit here: The bacterial Type-VI secretion system uses "needles" of about 20 nm diameter, but they are also filled with effector proteins.

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#### P

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##### Re: How thin can a needle be?
« Reply #4 on: January 10, 2017, 11:54:12 PM »

Enthalpy beat me too it, but I too was going to comment on the stylus of an ATM. 1 atom thick at the point.
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Tonight I’m going to party like it’s on sale for $19.99! - Apu Nahasapeemapetilon #### Irlanur • Chemist • Full Member • Mole Snacks: +31/-4 • Offline • Posts: 415 ##### Re: How thin can a needle be? « Reply #5 on: January 11, 2017, 12:06:53 AM » Also: I guess the thickness of a needle is not only defined by the thickness of its tip Logged #### P • Full Member • Mole Snacks: +53/-15 • Offline • Gender: • Posts: 556 • I am what I am ##### Re: How thin can a needle be? « Reply #6 on: January 11, 2017, 05:08:49 AM » Good point. What about a nano tube? If that was used to deliver something through it's tube then would it be considered a needle? When does a pipe or a tube become a needle by definition? When it's purpose is also to penetrate through a membrane? Logged Tonight I’m going to party like it’s on sale for$19.99!

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