How does ultrasonic oscillation reduce sliding friction?

[foodstarch87]

Hi everyone!

Sorry if I'm not able to work through this problem very much myself... I'm a Food Science student, and I'm trying to read an article about ultrasonic cutting when applied to apple slicing. From the papers they reference, the rapid vibrations on the blade reduce the friction coefficient significantly, but I can't understand why that is the case.

In the end, the friction coefficient is simplified down to μ ≅ 2/π (ζ)
where ζ = (moving velcocity) / (vibrational velocity)

The lower the ratio of the blade velocity to the vibrational velocity, the lower the frictional coefficient. I can't really follow the actual derivation very well, since I've only dealt with friction at the most basic level (like mostly up to F = μN...).

Any help with deconstructing the steps in the article would be highly appreciated! I'll try my best to follow and work through it too! Mostly looking for a basic explanation for what's going on. It just doesn't make intuitive sense to me.

Thanks!

 

[Nik_2213]

Um, Static friction vs Dynamic friction ??

FWIW, I used related technique to move our massive steel lab-benches. Took 'Three Large Guys' or just skinny me. Trick was to use the bench frame's slight flexibility. I'd pull a bench, then kick the nearest leg to start it. The bench would wriggle, its legs became pendulums, and I'd 'walk' it along the aisle without further ado...

 

[foodstarch87]

Sorry if this is a dumb question. Are you suggesting that when it vibrates so fast, it is more of a dynamic friction coefficient in the instantaneous moment? The blade is always moving downward when cutting, but the oscillation seems to somehow affect the friction coefficient. Not super well versed in this haha

 

[ChemAir]

Are you suggesting that when it vibrates so fast, it is more of a dynamic friction coefficient in the instantaneous moment?

I can't speak for @Nik_2213 , but yes, the vibrations do not let the friction go into the static range, which with very few exceptions is higher than the dynamic friction.

It is much harder in most cases to start two surfaces moving against each rather than keeping them moving.

There are other things in play at speed: entrained air/gas, possibly vaporizing lube, lubricant viscosity changes due to pressure/temp, surface heating...others...

I believe though, you are looking at a well documented special case static versus dynamic friction experiment.

 

[Bystander]

https://www.reviewed.com/cooking/best-right-now/the-best-electric-knives

 

[foodstarch87]

Hi everyone,

I was digging around more and saw this explanation on the data sheet of the ultrasonic blade manufacturer. It said that "Because the horn (the blade) is vibrating at such a high frequency, it disrupts the air molecules around it causing a cocoon effect. This cocoon effect virtually eliminates product sticking". Does this sound plausible? How would I describe that as a physics phenomenon?

 

[Baluncore]

The mode of oscillation will determine the principle being employed to cut.

1. Simple vertical oscillation, is chopping. It replaces static friction with dynamic friction, may introduce a cocoon of air.

2. Oscillation along the length of the blade, is slicing. It will convert static to dynamic friction, but introduces less air.

3. Sideways oscillation, opening the kerf and introducing air. It will be messy.

The slots in the cutter look to me like they are there to encourage the vertical oscillation mode
The cutting quality may be because the knife remains clean which reduces sticking.

 

[jbriggs444]

It may or may not be the primary effect, but pulling or pushing a knife longitudinally as it advances into a cut has a useful result. The frictional force of the material on the flat of the knife has a more or less fixed magnitude. That friction acts opposite to the direction of relative motion. If the knife is moving primarily longitudinally, this dynamic friction acts primarily longitudinally. The component of friction that then acts to oppose the knife advancing into the cut is reduced.

Vibration has an advantage over long strokes because it eliminates the need to support the material against the longitudinal force of friction. The material's own inertia does the job.

A vertical vibration into and out of the cut would have much the same effect. The time average of the momentarily upward and momentarily downward forces of friction can average out to something pretty small, even though the net movement averages downward.