The “Rehbinder Effect”

[mengwong]

I came across a viral video of a nail going through a ceramic mug in a water bath


The alleged explanation involves the “Rehbinder effect”

The Rehbinder effect in physics is the reduction in the hardness and ductility of a material, particularly metals, by a surfactant film.

A proposed explanation for this effect is the disruption of surface oxide films, and the reduction of surface energy by surfactants.

The effect is of particular importance in machining, as lubricants reduce cutting forces.

I found this explanation more convincing:

There is always some ductile deformation on a microscopic level, but it is negligible for this kind of ceramic. The deformation will be almost entirely brittle. For example, if I did a tensile test on a ceramic under water it's not suddenly going to get 10% strain to failure.

The answer to why the cup doesn't crack really is to do with changing the energetic favourability of creating new surfaces, as the post above says, rather than increasing the material's ductility. It's a fracture mechanics phenomenon, not a solid mechanics one, if that helps.

By putting the cup in a high surface energy fluid (water) there is a much higher energetic cost of creating new ceramic/water interfacial surfaces. Consequently, crack propagation is greatly reduced and less new surface is created.

A couple of interesting things to think about. If you add a little soap to the water, it's much less effective, as the soap reduces the energetic cost of creating new water/soap surface. Alternatively, if you did it in mercury, you'd get an even stronger crack propagation suppression (I think!).

I don't think rock is a useful analogy in this case. There's lots of different kind of rock, and it's not my area of expertise, but in many cases they're closer to a sort of inorganic composite. I suspect the effect of water in that case is to soften some sort of matrix material, which increases its ductility. So the effect is a solid mechanics one and not a fracture mechanics one.

(From Reddit - Dive into anything )

Both theories – around the breakdown of ceramics under dry vs wet conditions, and surfactant-dependent ductility in metals – should have interesting application to the behaviour of stones and steels in sharpening (and sandpaper and SiC powder).

I expect there are a few KKFers whose day jobs involve knowing a lot about this.

 

[lesslemming]

Is the effect related to the ceramic being wet, or is it because the medium the mug is immersed in (water) conducts the shockwave differently?

 

[mengwong]

Is the effect related to the ceramic being wet, or is it because the medium the mug is immersed in (water) conducts the shockwave differently?

I’d guess having a relatively incompressible, yet shock-absorbing medium around the mug is a lot like sandwiching your metal workpiece between a couple blocks of wood before drilling/sawing into it… maybe that’s what the energy analysis is saying?

 

[MarcelNL]

relatively incompressable? I think water is not relatively incompressable but pretty much incompressable. I also suspect that is the key reason for this to work...similar to why you'd put a piece of sheet metal on a wooden surface if you want to drive a nail through it.
I have a hard time believing the water film has anything to do with this...perhaps try with the bottom surface of the mug not being wet but the inside under water

 

[mengwong]

Of course, you are right about the incompressibilty… I just meant that with the mug being open and the tub being open, the water does have the option to flow out of the way, so there is some give – though at the timescale of a hammer tap, not much give. Maybe the mug seals to the tub so it isn’t all that open?

Now I want to reproduce the experiment using oobleck.

 

[mengwong]

This made me laugh

https://www.sciencedirect.com/science/article/abs/pii/S0890695518301214

 

[Naftoor]

Man, a whole mat sci degree and I never heard of this. Life is freaking awesome! Lanyard holes in some jnats?

 

[WildBoar]

Yeah, this thread gives me the shivers. Failure of Materials was one of the most math-intensive courses of my engineering education. It's the class that made me be okay with getting a minor in ESM instead of maybe pushing for a double-major. I remember formulas for calculating the energy needed for crack propagation going on for something like three lines of paper, and that was with my very small handwriting. The only other class that came close was Deep Foundations. Even matrix structural analysis was a walk in the park compared to that Failure of Materials class.

 

[Mr.Wizard]

Thanks for introducing me to this concept. I am quite skeptical of this "viral video" however. It is shot in a way that you really cannot see what's going on, and in the second attempt it looks to me like there is something over the bottom of the cup with a slight blue hue. I would bet he filled the bottom with resin to inhibit crack propagation, and maybe other tricks unrelated to "Rehbinder Effect."

 

[Mr.Wizard]

I think water is not relatively incompressable but pretty much incompressable.

We can argue about degree and terminology but water does in fact decrease in volume under pressure, so calling it incompressible is at best a Lie-to-Children. Water is an order of magnitude more compressible than quartz crystal, yet we are happy to describe compression of quartz in the context of the piezoelectric effect. It would be far better to simply introduce the concept of bulk modulus when appropriate than to teach falsehoods for the sake of simplicity.

 

[mengwong]

I am quite skeptical of this "viral video" however.

Indeed—
https://www.reddit.com/r/physicsgifs/s/SIJn6UEP7F

 

[mengwong]

Anyway, coming back from the distraction of metal-on-ceramic, this article gives a more comprehensive survey relevant to ceramic-on-metal action

https://www.sciencedirect.com/science/article/pii/S0264127520302227

 

[Mr.Wizard]

https://www.sciencedirect.com/science/article/pii/S0264127520302227

Very interesting stuff!

Therefore, the Rehbinder effect can be understood to manifest in two
phases: (1) enhanced mobility of dislocations at the surface due to the
intermolecular attraction between the surfactant and the workpiece
that promotes the dislodging of workpiece atoms by slip, which leads
to (2) the pile-up of intragranular dislocations that result in strain local-
ization and the eventual embrittlement of the material. In ultraprecision
microcutting, the stress zone can be explained to initiate from two ends
of the shear plane [73] under the influence of the surface-active medium
as compared to the presumptuous stress zone initiated from the tool-tip
[74,75]. Fig. 9 demonstrates a micro-crack that has evolved on the free
surface of the chip during deformation along the shear plane and
under the influence of a surfactant. As the micro-crack propagates, the
enhanced emission of dislocations in the plastic zone of the crack tip
moves along the slip plane of the grain and begins to pile up at pinning
centers and entangles with dislocations emitted from the tool-tip. This
results in a quicker strain localization within the chip and produces
thinner deformed chips with serration-like microfeatures on the free
surface of the chip. Conversely, the conventional chip formation process
without the surface-active medium would only comprise of a plastic
zone that initiates from the tool-tip and eventually forms bulges on
the free surface of the chip.

Fig. 9. Theorized sequence of events of the Rehbinder effect during chip formation.