PROFESSOR: There are four ways that chefs
use to control the viscosity- their reductions,
their starch-based thickener, there's making a emulsion by adding
fat, or using modernist thickeners, where small amounts of material,
small amounts of a polymer, are added to increase
the viscosity by large amounts.
In all cases, what we're doing is adding something to the fluid.
In the case of reduction, we're reducing the amount of liquid.
And what remains are particles, are nonvolatile solids.
This increases the concentration, or the concentration of the solids,
in the liquid.
In the case of starch, we're adding starch granules.
In the case of fat, or making an emulsion, we're adding drops of fat.
And the drops of fat behave just like particles.
In the case of the modernist thickeners, we're adding polymers.
And they also have the effect of adding something to the fluid.
And this is what increases the viscosity of the fluid.
The simplest way to understand this, is to think of particles in the fluid.
When the fluid flows, the particles also flow.
And they have to flow over one another, in order for the whole fluid to flow.
The way they behave depends on the concentration
of the particles in the fluid.
It depends on the volume of the particles, or the volume
that the particles occupy in the fluid.
This is called volume fraction, or the volume fraction of particles,
in the fluid.
We can calculate that easily.
We calculate the volume of each of the particles,
and we take the total number of particles times their individual volume
and divide that by the total volume of the fluid.
When there are very few particles in the fluid,
or their volume fraction is very low, then they have almost no effect at all.
When the fluid flows, the particles just flow with the fluid.
However, when the volume fraction of the particles increases,
then they become crowded.
It's more difficult for the particles to flow over one another.
And they collide with each other, they move on top of each other.
They take much more volume, and the flow is restricted.
In this case, the particles really impact the flow of the fluid
and increase the viscosity of the fluid.
However, for that to happen, the volume fraction of particles
has to be quite high.
It has to be sufficiently high that they can't easily flow over one another.
This doesn't happen unless the volume fraction of the particles
gets to be of the order of 50%, or half the volume of the fluid
is occupied by the particles.
When the volume fraction is so high, then the particles
can't easily flow over one another.
Now we can understand in a very simple way why starch is so
effective at increasing the viscosity.
First, the starch granules expand.
They absorb water, and they get larger.
As we show in this little cartoon, the volume gets larger.
And so, the volume fraction, the effective volume fraction
of the starch granules, increases.
And while you may start out with only 15%
volume fraction, when they've expanded, then they
can occupy a much larger volume fraction.
The second way that starch can increase the viscosity of the fluid
is that the granules stick together.
In that case, they form these more tenuous structures, as we show here.
The volume of these tenuous structures is
the volume of the structures themselves plus all the water that they
enclose, as shown by this circle.
And that's how you can greatly increase the viscosity of a fluid,
even if you don't have, essentially, 50% of the volume.
You have 50% of the volume fraction of particles,
because the particles occupy a much larger effective volume.
The last way to increase the viscosity, is to add these modernist thickeners.
What they are, are large molecules or polymers.
And in that case, only a very, very small amount of material
can lead to a large increase in the viscosity.
The way these work is actually very similar to way the particles
work, except that polymers have a unique feature
that they can occupy a huge volume with very little mass.
To understand that, let's understand the way a polymer actually behaves.
A polymer is a very long, but very flexible, molecule,
and is made up of many, many monomers.
And for a polymer, because it's so flexible,
the bond between each of the monomers can bend in any direction.
It can bend either way.
It can bend one way or the other way.
And it does that, with equal probability.
This has very much the appearance of a random [? walk. ?]
Here's an image of a random walk of an ion.
It can go in any direction.
It goes randomly in all different directions.
This random walk is exactly what describes the behavior of a polymer.
And, in fact, we can lay the polymer shape directly onto this random walk.
So the same way we describe the random walk that describes diffusion of ions
through a fluid, that's the way we describe
the shape of a large polymer molecule.
What this means is that the polymer molecule actually
takes up a huge amount of volume, for a very small amount of mass.
Yet because the polymer spreads and fills the water with this polymer,
it can increase its viscosity by a very large amount.
Because the polymers occupy so little actual volume of the fluid,
they can entangle.
They can come together and intertwine among each other.
That means the effective volume of the polymers
is actually much greater than one.
And the viscosity is even larger.
Remember that the polymers have to get out of the way of one
another, in order for the fluid to flow.
Now they have to disentangle for them to get out of each other's way.
That's called reptation, and that's a very, very slow process.
You can think of it like a bowl of spaghetti.
If you want to take one spaghetti out, you have to pull along its length.
And that takes a long time.
And that's why these thickeners, these polymer thickeners,
are so effective at increasing the viscosity of a fluid.