Measuring Very Small Forces in the Lab

[zoobyshoe]

I'm curious about how very small forces are actually measured in the lab. I read somewhere that a dyne, for instance, exerts about the same force as a mosquito landing. What kind of set ups and instruments are needed to accurately measure a force that small, and smaller?

 

[berkeman]

Maybe laser interferometry on a weak deflector? I'd be interested in hearing about actual lab setups too.

 

[zoobyshoe]

Maybe laser interferometry on a weak deflector?

What's the principle behind this?

 

[berkeman]

Laser interferometry is just a way to measure very small displacements, on the order of the wavelength of the laser light. You combine a reflected version of the light (off of whatever is deflecting from the force) with some of the original beam, and count the number of fringes that the combination passes through as the deflecting element moves.

Laser interferometry is relative, though, so you need to start with the deflecting element stationary in a rest position first. Then apply the mosquito force, and count the number of fringes that the combined beam goes through until the deflection is stationary again. You'd need to calibrate for the force versus displacement of the deflecting element, of course, but hopefully that would just me a mechanics/material science exercise.

I'm not sure what the best deflecting element would be -- depends on the force magnitude, direction, stability, etc. You could even use a micromachined cantilever board on silicon if the thing you want to measure could be interfaced to the end of the lever...

 

[zoobyshoe]

That method would take care of the very tiny forces for sure. Pretty fascinating.

I wonder about in- between situations for which that would be too sensitive.

 

[krab]

One could use an electrostatic balance. This is a balance that uses electrostatic forces rather than counter weights.

 

[zoobyshoe]

One could use an electrostatic balance. This is a balance that uses electrostatic forces rather than counter weights.

It looks like someone has made such a thing:

http://adsabs.harvard.edu/cgi-bin/n...amp;db_key=PHY&data_type=HTML&format=

i.e. not just for measuring electrostatic forces, but there's no details about it.