# The smallest particle we can feel

1. Feb 19, 2015

### tonyxon22

How large is the smallest particle a human being can detect or feel with his fingertips? If it was, let’s say, a pure gold particle, how many atoms would that be?

2. Feb 19, 2015

### Suraj M

3. Feb 19, 2015

### Staff: Mentor

Sensitivity is different for everyone. It is dependent on the density of receptors on the skin, for example Merkel cells. Merkel cells are the most sensitive of the different receptor types - there are 4 types as I remember.

But.
Your question seems to have a bad assumption. Your fingertip skin, like or not, is covered with microbial cells, bacteria. Generally they are good guys unless you just finished mucking out a horse stall. They are millions of atoms in size. Humans cannot feel one.

To get an answer for yourself - I would go to a hardware store and buy several sheets silicon dioxide (black) sandpaper. The grit size on the back(like 1000) is the particle size - 1000 per inch. Get sheets ranging from about 400 .... 5000 grit. Using fine tweezers (or good forceps) tease off some loose grit from each paper type. As your progress through the grit sizes, you will find that at some point it still feels slidy, like talcum powder, but you cannot feel the grit. That is the limit of perception. Save the paper for refinishing furniture or cars. It is not a waste.

If you are supersentive try some chromium oxide (green) buffing powder - 8000 grit.

PS: Here is a very approximate calculation, I used UNIX bc, so the precision is large but the input parms are not that accurate, so this is approximate number of atoms in a piece of 5000 grit:
Code (Text):

Volume (1/2.54)^3 = .06102374409473228394 cc
Volume * density of Silicon dioxide =  2.65 * .06102374409473228394 = .16171292185104055244g

So the mass of one piece of grit is approximately:
.16171292185104055244 / (5000/2.54)^3
.00000000002119999999g

The number of molecules is Avogadro's number time the mass of one grit
.00000000002119999999 * (6.022141^23)
18222460 molecules
atoms = molcules * 3 since silicon dioxide has three atoms, let's just round the number
atoms ~ 60000000

Last edited: Feb 20, 2015