Do you know what Oreos are? It works like this: you spin them and
bam, almost every time, the cream ends up on either one of the cookies.
Except that with my favorite cookies, Prince cookies... uh-uh, I don't think
I've ever managed to separate them perfectly even once,
without having some chocolate leftover on both cookies...
I would like it if, just like Oreos,
all of the cream, the chocolate, would stay on one cookie only
so I can enjoy it to the max.
Because as it is, my pleasure is spoiled.
So how come this works for one cookie
and not for the other?
And I hope I have your undivided attention
because I'm not sure there is a single topic on Earth right now
that is more important than this... /sarcasm
{I got lost on the Internet (again)}
Between the fact that an article was published this year
on the topic of Oreo separation (for real!), and the fact that I was going to be able to eat
Prince cookies "for work", I must admit that the admit that the video was pretty inevitable.
The article is very serious (as is my desire to eat Prince cookies).
It has been published by 2 researchers from MIT in the journal Physics of Fluids.
But the article only talks about Oreos, not Princes. It is quite common
in the scientific literature to come across gaping holes in the literature.
There's a hole in the Prince domain, that's the way it is. So I contacted the researcher
to see if she could extrapolate her knowledge about Oreos to Princes.
"I think this type of failure is due to two things.
The most important one is probably the softness of the filling.
I don't know what kind of filling you have in your cookies
but if it's very soft, that will probably distribute itself between the two sides
because that will be the weakest link in the mechanical structure."
What she's talking about is the cohesion of the filling.
If we take liquid cream for example, it's super easy to separate.
On the other hand, Oreo filling, which is more viscous, it's harder to separate because
it's held together. So when you separate the two cookies, on the few occasions that it's
the filling that breaks, it could be because the filling is too soft.
So the break could be due to a lack of cohesion.
"The second aspect that can affect how it fails is how it was manufactured originally.
So if you can form a better adhesive bond between things,
then even if you have a harder cream, if it's just stuck very well,
maybe the sides of the cookie are more porous or something,
so the cream can actually get inside.
Then maybe that makes the cream stronger.
This time, she describes a form
of rupture due to lack of adhesion; what we usually get in our Oreos. This is
quite intuitive, it's the fact that the cream doesn't cling well to the cookie.
This may seem like a superficial a topic (and it is), but actually,
we're gonna have to do some rheology! I can hear your enthusiasm
because rheology is so cool: it allows you to study how a fluid
reacts to a deformation and to a rate of deformation. Let's say you try
to make yourself a little bit of a special sandwich. First you spread some cheese, then
you add some concrete, then lava, toothpaste, blood, paint...
why not. You will quickly realize that it is not all that easy
to spread out all the ingredients, or even to keep them clinging to the toast.
And that's because these are fluids that have different behaviors depending on
how we deform them with our knife, or under the simple influence of gravity.
And the small specificities of fluids, can be measured... with a rheometer.
In our case, we'd just need to place a cookie between the two plates, then rotate each
biscuit in relation to each other and that'll give us a measure like the viscosity,
or the torque needed to reach a certain shear rate... in short,
fluid characteristics. So we we don't have a rheometer, but we have an...
Oreo-meter.
Get it?
It's not my pun, the researcher came up with it.
This is our Oreometer.
This is the same principle as the rheometer but
in vertical form. We will wedge each of the biscuits in the machine, then we'll apply a force
that is far from the center on only one of the two biscuits by adding coins.
This creates a rotational movement on one of the biscuits which will get the two biscuits
to separate. It is exactly what happens when we do it by hand, but in a slightly
more controlled manner. The Oreometer was designed by Crystal, and printed by
technicians of the chemical engineering department of Polytechnique Montreal -
where I went, in particular to meet with Bruno. We ran the first tests together.
Bruno: I'm not an expert in Oreo detachment...
Viviane: Oh yeah, is that not your title?
B: Professor of Oreology, that would be a great title.
Let's start, first with 5...
Up to 10...
25...
Ah...!
If I open it...
That was a really good separation. There's still a kind of
deformation or flow above the surface of the cream, you can really see
a rotation pattern.
The flow we're talking about happened because
the cream, remember, we consider to be a fluid... which is taken along
by this rotational movement of the Oreometer. The rotation generates a stress that we
call shearing. And if you don't know what shearing is, it's a type of
mechanical stress. You already know about compression which comes from pressing,
and traction where we pull on something, and then there's shearing, when it... shears. That's just
the word. And so when we do this movement there, the cream undergoes a shearing
which is a bit special because it's rotating.
V: The shear, because it's rotating, and it's only because it's rotating
that it's true, we won't have the same shear at the center of the cookie compared to the
on the outside of the cookie. B: Absolutely, because at the bottom, if
you look at your cookie, in the center, the center of the cookie as such, when I spin it...
You see here. If you look between the letters R and E, this point is not
moving. So if there is no movement, there is no shearing, there is no
deformation. So it's really towards the outside of the cookie that the
deformation happens, where we impose the highest shear rate within the cream.
So the movement applied to the biscuit is not homogeneous.
V: But then, it's particularly impressive to have an effort that is
not homogeneous, but to have a result that is homogeneous.
B: That's a good point. I don't know if we can really say that this is
a homogeneous result or if it is not the result of a cascade of events.
What probably happened is that there was a disbonding or a lack of adhesion
towards the periphery of the cookie, and then automatically after that, the rest follows as well.
It's a bit like starting a tear, and then the rest comes free with it.
Ok, so what we understand is that the cream is only detached from the cookie only
around the edges. But since it is quite viscous, it is pretty cohesive,
the edges 'lead' the rest of the cream to continue to come off. But in principle, tht is not
at all what happens to the Prince cookies because we never get a
clean surface where the cream comes off properly. So the only thing we can do is
to run some tests on the Prince cookies with our machine, which is
unfortunately only adapted to the size of Oreos.
So my idea is to trim a Prince down to the size of an Oreo
so that it fits in the machine.
Ok... it works.
Here we go: 1 2 3 4 5.
60... I can't fit any more!
What can I do?
Gotta start again.
Well, it's going to be a little bit biased now but...
c'est la vie.
80...
85...
100.
I can't add any more coins.
The system is not designed for that...
I'll do it by hand.
Ah, there's nothing to do, it's not budging!
The grip is crazy on these cookies, it's amazing.
I didn't expect that :(
Ok so, since we didn't manage to unbind the cookie, what we're going to do is
try to affect the fluidity of the filling that's inside the Prince. For that, I'm going to put it
in the microwave for a few seconds and hopefully that will be enough to soften the cream so that
we can finally twist it open.
20... boop!
Now we've had a separation...
Ooh! *laughs in defeat* ...yes.
10 seconds was too much microwaving, BUT.
There's something interesting here. You can see very clearly that it was melted,
but where it has come off here, it came off in one area... But it didn't
come off everywhere. There are some little dots, and I think that they
play a role in Prince cookies having such a strong grip: the biscuit itself
is quite deep. There are clinging points for the cream to get it to really stay
inside the cookie.
And that's why it's going to be more difficult
to get the cream to slide off the cookie rather than have it break off like it did here.
So it's going to be interesting to compare
our tests with Crystal's findings.
Here are the curves she got.
This curve is called a stress-strain curve.
It shows that when I increase the number of coins (which corresponds to
increasing the stress); when I increase the number of coins here, I actually get
very little deformation. So it increases here without changing much on the deformation axis.
And then I reach a moment when suddenly, for a very small increase in stress
(a very small addition of coins), it starts to move a lot,
to move across this axis, meaning it deforms a lot. This curve is obtained
with a rheometer. Our Oreometer doesn't give us that info,
we only get the maximum point (the so-called elastic limit, if you were curious).
Let me digress for a moment on the importance
of this curve. It is unmissable in the study of any material in existence.
Let me give you an example with simulations that we can do for surgery.
You're looking at a spine that has a scoliosis and needs to be
corrected surgically. When you want to optimize the treatment, the choice of materials,
the number of screws, the positioning etc., it's best to simulate these factors on
digital patients rather than living patients. Obviously.
In order to run these simulations, you have to tell the computer that
this ligament, this muscle, this bone, this rod, this screw, etc. -
when a force is applied to it, it will deform in such and such a way.
And this "in such a way" is the curve that we just saw.
That's where the information comes from.
Hence the importance of these curves... and this is true in
all areas. We need this fundamental data as starting points from which
to simulate surgeries, airplanes, lava flows...
Or cookies.
Now, if we get back to our results, what Crystal finds is that
cohesive failure occurs with much less stress (far fewer coins),
than adhesion failure does, which is when the Oreo separates perfectly.
~It's for science.~
And for Prince cookies, it was the other way around.
The only time they separated was when adhesion was poor. Because
Prince biscuits have a really good grip
and I think it's thanks to these holes in the biscuits.
If you look closely, you'll see;
you can see it on the microscope. There are almost cracks in the cookie for the cream
to really get inside and cling to it.
The other way that the cookie separated
was when we changed the cohesive properties of the cream by
liquefying it. So the Prince's filling also has a much better consistency
than that of the Oreos.
Clearly, Princes were not designed to be separated.
All in all, the article does not explain how all cookies with filling can be opened up...
An opportunity for those of you who might like to initiate a new field of
research... The only question is who would agree to fund it.
Oh and by the way, this would make for a great hands-on mechanics project,
for the the teachers among you.
Have you noticed that in the past years, the quality of easy-open packages has really improved?
That's a big change, because in general, easy opens are never easy.
it's never easy. And if you don't already know why they're so hard to open,
you're in luck! I've already made a video on this topic.
How lucky... :)
I hope this video has got you excited to subscribe, because I would really like to
reach 500K subscribers before the end of the year while continuing to work
on these kinds of topics... aka a little quirky. I can't do it without you
so please check that you *are* subscribed, and I'll see you next time.
Bye!
Bruno: I prefer to eat the biscuit
with the cream on it last, and start with the one without.
Viviane: Oh you start with the one without cream?
B: Yeah, saving the best for last. That's my perspective.
V: I eat the cream first. And then I eat the biscuits.
B: Both biscuits? V: Yeah, both biscuits.