Potatoes Stick to Knife Blade

[lordratner]

Thank you.  This is your personal experience/observation, and I respect it for what it is-- a personal experience/observation.  It is also a viable option to mitigate potato slices sticking to a knife blade, and one used by countless people.

 

I also respect gfweb's observation/experience, even though I wouldn't use it.  But again, it is a viable option, and one that is used by countless people .

 

Going over the 5 (five) pages of this thread, I fail to find any personal observations or experiences from dcarch.  I don't know why this is, but we have a lot of science, climbing up walls with suction cups, sheets of glass, and yet, no personal observations or experience concerning slicing a lowly spud.

 

The "test" is very easy, it is not time consuming, does not require expensive equipment or elaborate set ups, nor does it require extensive training.  It (cutting a simple spud) is also very cheap--I bought potatoes the other day for 69 cents /lb, and after "testing", you can cook and eat the spuds.

Regardless of what Dcarch has personally tested, of the two "scientific" tests performed in this thread to specifically address the topic, both pointed heavily in the direction of air pressure being the primary force acting to keep the potato on the blade, which is exactly what he said from the start. 

 

It doesn't answer the question once and for all, but it's a lot more than "I just don't think air pressure makes sense"

Edited November 13, 2014 by lordratner (log)

[gfweb]

Regardless of what Dcarch has personally tested, of the two "scientific" tests performed in this thread to specifically address the topic, both pointed heavily in the direction of air pressure being the primary force acting to keep the potato on the blade, which is exactly what he said from the start. 

 

It doesn't answer the question once and for all, but it's a lot more than "I just don't think air pressure makes sense"

The oiled knife experiment pretty much kills the air pressure theory.

[dcarch]

The oiled knife experiment pretty much kills the air pressure theory.

 

Not sure what you are trying to say or prove. Oil will have the same effect in the conditions discussed above.

 

Except you just introduced a new factor, i.e. viscosity.

 

Viscosity in physics needs to work with Reynolds numbers.

 

dcarch

[rotuts]

Oil as we all know, decreases the surface tension of water, if not eliminates it.

 

surface tension might be though of as 'sticky-ness :  water sticking to itself at an interface.

 

a tiny tiny amount in pond will eliminate surface tension and mosquitos stoping by for a drink just fall right in and drown

 

the little larvae buggers wiggle up to the surface from time to time for a little O₂ can't 'attach' and die

 

but it true, they are not potatoes.

 

out of Pots. myself.  hope to get some tomorrow for some time in the CSB Ill give the oil exp a try

[lordratner]

The oiled knife experiment pretty much kills the air pressure theory.

This is a potato. Heavily coated in oil and pressed to a knife coated in oil. I did this 2 minutes ago. Olive oil, if it matters.

 

Edit. I redid this a minute ago, rinsing off all the starchy water I could, then dried with paper towels, then coated in oil, an stuck to a dry, then an oiled knife. Same exact result. 

Edited November 13, 2014 by lordratner (log)

[gfweb]

  Worked every time for me when cutting 1/8 " slices out of a Yukon Gold at 328 ft above sea level.

 

You must have a bad potato.

[lordratner]

  Worked every time for me when cutting 1/8 " slices out of a Yukon Gold at 328 ft above sea level.

 

You must have a bad potato.

I can't tell if there's sarcasm in there. 

 

I agree that oil can help keep potatoes from sticking when slicing them, but I submit that it is because it allows air to get between the knife and potato. Perhaps a function of the oil and water repelling each other, I have no clue. But if starchy potato water is the only thing making spuds stick to a knife, then an oiled potato should not stick at all. 

 

But in reality, both oil and water can act as a gasket (if the layer is thin enough) to block air from getting between the blade and potato, creating suction and holding the potato there. The fact I was able to slide the potato up and down the knife while tilting it implies the oil is not acting as a strong adhesive. That leaves air pressure.

[rotuts]

in the Pot. exp above I take this away:

 

the Pot. is in 'pressure' equilibrium or it would move.

 

there is the 'local' ATM of pressure all over the Pot., knife, and Experimentaur.

 

however, the Pot. has its weight ( ie mass ) as an additional force in the downward direction.

 

so the equilibrium of the ATM top of knife and bottom of knife has an additional factor with the vertical 'sticking'

 

it cant be the ATM, it something acting in that cloying interface we have not been able to measure where the Pot. and Knife

 

are enjoying themselves.

 

adhesion its for me, oil or no oil.

[lordratner]

in the Pot. exp above I take this away:

 

the Pot. is in 'pressure' equilibrium or it would move.

 

there is the 'local' ATM of pressure all over the Pot., knife, and Experimentaur.

 

however, the Pot. has its weight ( ie mass ) as an additional force in the downward direction.

 

so the equilibrium of the ATM top of knife and bottom of knife has an additional factor with the vertical 'sticking'

 

it cant be the ATM, it something acting in that cloying interface we have not been able to measure where the Pot. and Knife

 

are enjoying themselves.

 

adhesion its for me, oil or no oil.

If it was adhesion, the potato wouldn't slide around on the knife without falling off. Think of any adhesive. Bumper sticker, glue stick, scotch tape, crazy glue. You can never slide the two adhered objects around once the adhesive is set. Adhesives work by grabbing on to micro imperfections in the surface of the objects. That's why smooth surfaces sometimes need to be sanded for an adhesive to stick. But once stuck, they don't slide on each other. 

 

A suction cup, however, works better when lightly wet. It's not the water making it stick, though, it's the suction. The water just acts as a better gasket than the cheap rubber. And you can slide suction cups around on smooth surfaces without the suction breaking and falling off, because it's air pressure, not adhesive holding the suction cup in place. 

 

Once again, the only tests done in this thread point to suction. I wonder exactly what proof would help change your mind. 

 

By the way, this is fun.

Edited November 13, 2014 by lordratner (log)

[rotuts]

""  adhesive is set ""

 

true.  surface tension or adhesion of water is fluid.  when water sets, its ice

 

""  it's the suction ""

 

indeed.  an inner area of less than one, or your current ATM

 

so  why does the vertical potato have 1 ATM of pressure on the top and 1 ATM of pressure on the bottom + its mass effect

 

and the mass effect that cant be related to air not fall off ?

[Anna N]

I just sliced almost 3 pounds of potatoes on my Benriner. Not a single slice stuck to the blade. Just saying......

[rotuts]

Excellent   AnnaN

 

I also use that slicer  for  you know

 

casserollish dishes  etc

 

and my experience is the same

 

thank you.

[dcarch]

Oil as we all know, decreases the surface tension of water, if not eliminates it.

 

surface tension might be though of as 'sticky-ness :  water sticking to itself at an interface.

 

 

You are correct that oil decreases surface tension of water. That's why I suggest to try all the experiments with soapy water, You will find out the surface tension of water has nothing to do with why two closely positioned together are very difficult to be pulled apart.

 

It is indeed strange that so many other experiments involving no water and no potatoes are not good enough to proof that there is atmospheric pressure.

 

I mentioned about viscosity, and Rynold's Number. It all has to do with viscosity. Air has very little viscosity. It flows quickly. The purpose of that one small drop of water in between two sheets of glass is because water has much higher viscosity to prevent air from leaking in between to two sheets of glass. Please, please, please try to understand the theory of hydraulics, read it thoroughly, if you don't do this homework, you will forever believe that water adhesive is stronger than 14.7 lbs/sq. in. The moment air leaks into the space between the two sheets of glass, instantly, based on the law of hydraulics, atmospheric pressure is neutralized, and the glass sheet can be separated easily. Oil, depends on water kind of oil, is higher in viscosity than water and will make it even more difficult to separate the glass sheets.

 

dcarch

[dcarch]

I just sliced almost 3 pounds of potatoes on my Benriner. Not a single slice stuck to the blade. Just saying......

 

And that is exactly to be expected, as i have explained many many times. The condition obeys exactly the law of physics about no work is needed against a vertical force. It is in every General Physics 101 book.

 

I see a lot of misunderstanding here to distinguish the vector of force and work. Horizontal sliding and vertical pulling proves very different theories.

 

dcarch

Edited November 13, 2014 by dcarch (log)

[gfweb]

So if I take a plate of potatoes dauphinoise and invert it,  why doesn't the air pressure keep the potatoes on the plate and off of the floor?

 

What about the few potatoes that remain on the plate?

[dcarch]

So if I take a plate of potatoes dauphinoise and invert it,  why doesn't the air pressure keep the potatoes on the plate and off of the floor?

 

What about the few potatoes that remain on the plate?

 

Excellent questions! Because you are a lousy potatoes dauphinoise maker. ( Haha kidding!)

 

The  dauphinoise  is not rigid and is not evenly weighted. Part of it will sag creating the peeling action to allow air to leak in between potato slices and atmospheric force is equalized to allow gravity to pull the slices down. 

 

Those few slices remain on the plate is because they are much lighter in weigh to create the peeling action.

 

dcarch

[dcarch]

As I promised, I am posting a video of a knife I designed and made which prevents potatoes from sticking to the blade. The design is based on my understanding of the physics and mechanics of slicing a potato or similar vegetables.

 

The knife needs some sharpening. I have not done that for a long time.

 

Perhaps this will answer part of OP's question?

 

dcarch

 

 

[gfweb]

So the knife negates air pressure?

[lordratner]

So the knife negates air pressure?

Hard to tell from the video, but it looks like the blade has an uneven surface, unlike the first knife. It's also a lot narrower.

 

The narrower blade provides less surface area, so if the potato does get a good seal, the force of air pressure holding it to the knife is lower (since it is a function of surface area).

 

More likely is that the knife has an uneven surface. Similar to the knifes with scallops in the side, the uneven surface allows air to get between the potato and the blade, which balances the force of air pressure on the spud, leaving gravity to pull it where it will.

 

If the answer was sticky starch water, why wouldn't the potato stick to all metal knifes, regardless of shape? Does duct tape only stick to flat surfaces?

[dcarch]

So the knife negates air pressure?

 

Depends on what you mean by "negate".

 

The design of the knife takes advantage of all the factors discussed in this thread to prevent "vacuum" from developing between the blade and the potato slice. The only time you have to counter act atmospheric pressure is when your action creates a vacuum. Vacuum by definition (on earth) = -14.7 lbs/ sq. in.

 

The video demonstrates "peeling" allows air to leak in between surfaces, in the video, frame 1.13 minute, , even the santoku knife can prevent potato from sticking if the potato slice is thin enough to curve away from the blade.

 

The video also shows the difference of vertical v.s. horizontal force to remove the slice from the blade.

 

The potato knife's thin blade with single bevel edge grind "peels" the slices away and never allow vacuum to develop.

 

 

dcarch

[Norm Matthews]

I think the main reason it didn't stick is that it is sharpened on one side which allows the potato to peel away.  The thin blade also helps but I think such a knife would make it difficult to get even slices on the potatoes because it is so thin that the blade is easier to twist during the cut.

 

edit: oops. Didn't see Dcarch post until after I posted.

Edited November 15, 2014 by Norm Matthews (log)

[Edward J]

What role--if any--does the fact that the white knife is a good deal narrower, I'd say 25% of the surface area compared to the black knife, have? 

 

More knife surface area= more contact with the spud=more adhesion, or if you will, "stickiness".

 

Russets are an o.k. "general purpose" spud, but Yukon golds and reds have a much higher starch content

[lordratner]

What role--if any--does the fact that the white knife is a good deal narrower, I'd say 25% of the surface area compared to the black knife, have?

More knife surface area= more contact with the spud=more adhesion, or if you will, "stickiness".

Russets are an o.k. "general purpose" spud, but Yukon golds and reds have a much higher starch content

Correct. But the argument being made (and supported by the vacuum tests) is that the adhesion force is much less than the suction force. So yes, there is less adhesion, but the adhesion from the starch was never enough to keep it on the blade (which is why it falls off under vacuum). But because the knife prevents suction by allowing air between the potato and blade, no sticking spud.

[Norm Matthews]

What is also is a factor is that the knife is sharpened at a  considerable angle on the side that faces away from the potato. That helps the potato slice peel away from the knife by pushing it away as the knife passes through.

Edited November 15, 2014 by Norm Matthews (log)

[Edward J]

Spending summer holidays at my uncle's farm gave me an appreciation for many things, including axes, since I had to split firewood while I was there.  I never knew there was a difference between a felling axe and a splitting axe, a side hatchet (single bevel) and a double bevel .  As I soon found out, a splitting axe need not be very sharp, it just needs to be very "fat" or have a wide bevel.  Of course, at lot of "exertion" needs to be spent in getting the axe wedged in the wood in order to split the wood apart.  Oh, and elm doesn't split, no matter how much you hack and chop.  A nugget of info I gleaned after a practical joke from my cousins..... 

 

What's all that got to do with spuds?  If you have a "fat" or wide bevel on your knife, you will need to exert a lot more force to slice a spud, since you are basically using a wedge to split apart the spud.  Multiple this force by 8-10 spuds and you'll want to go to a knife with a thinner bevel.....