Measuring tiny forces with light

Photons are bizarre: They have no mass, but they do have momentum. And that allows researchers to do counterintuitive things with photons, such as using light to push matter around.

more…

Could this be used to create better telescopes?

For small forces like this, hard vacuum is an absolute necessity.

> It consists of a small cantilever – a miniature diving board – less than 1 cm in length. The bigger the force, the more the cantilever moves. A built-in interferometer acts as a motion sensor.

This cantilever on a chip is now a standard force measuring device, but the version I know uses induction to measure the displacement, not an interferometer. An interferometer could be more accurate.

> can measure forces that are tiny fractions of a newton – from micronewtons all the way down to 15 femtonewtons, 10^-15, at the level of atomic interactions. A piconewton 10^-12 will stretch a DNA molecule out

I wonder how good other methods are?

> The PML team …

Isn’t the correct acronym PMSL?

> Variations on this design could also be used to improve calibrations of atomic force microscopes and even to measure laser power.

True.

> We’re talking nanonewtons for a 1-watt laser beam.

So? That can be measured by a simple propeller mounted on a thin vertical spindle, perhaps.

> Could this be used to create better telescopes?

I was going to say “no” but perhaps the answer is “yes”. Consider adaptive optics. The amount that a telescope mirror has to move to compensate for atmospheric turbulence is small. The normal system is to use piezoelectric devices to apply force to the mirror to move it the small necessary amount. But finer control of the force that shapes the mirror could be applied using lasers to apply smaller forces in more locations – theoretically. But that would run into problems with heating of the mirror – a definite no-no, and perhaps the forces generated would be too small to be of any use.

>> can measure forces that are tiny fractions of a newton – from micronewtons all the way down to 15 femtonewtons, 10^-15, at the level of atomic interactions. A piconewton 10^-12 will stretch a DNA molecule out

> I wonder how good other methods are? … This cantilever on a chip is now a standard force measuring device.

Atomic force microscopes can measure forces down to the sub-piconewton level.

eg. http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3669665/

“This sub-pN level of performance is routinely accessible using a commercial cantilever on a commercial instrument. The two critical results are that force precision and stability were limited by the gold coating on the cantilevers, and smaller yet stiffer cantilevers did not lead to better force precision on time scales longer than 25 ms. … Force is determined by the tip deflection as measured on a quadrant photodiode”.

“Sub-pN force precision in a limited bandwidth … a 5-nm oscillation was applied to the tip at 20 Hz. These improvements allowed the folding pathway of an immunoglobulin to be more carefully examined. The Rief group developed a custom-built, low-drift AFM. The stability of this instrument enabled them to investigate the conformational fluctuations of the calcium-sensing protein calmodulin with high force precision ~2 pN due, in part, to their extraordinarily slow pulling velocity 1 nm/s.”

“Specifically, we achieved a 0.5-pN force precision over a broad bandwidth 0.01 to 10 Hz.”

OK, so the new proposal is aiming for 15 femtonewtons where standard atomic force microscopes are struggling to accurately measure 500 femtonewtons. Nice.

more …

“Magnetic tweezers can measure femtonewton forces” from https://en.wikipedia.org/wiki/Force_spectroscopy

This paper may be describing the same method as in the OP. It claims measurement to an accuracy of +-7 femtonewons
A self-calibrating optomechanical force sensor with femtonewton resolution