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Selamat sejahtera and good morning.
There is another
learning resource you can use, apart from our
enmodule which is the main
learning platform for
this course. There is another one which I have set up
and I have been using it for a few years
already. This one is called
wiki. Yeah. Wiki.
I am sure you are quite familiar with Wikipedia.
So, Wikipedia the concept is everyone can contribute
the information or the knowledge on any topic.
So people around the world can collaborate to
write on a topic. So that is the concept of
The original concept of Wikipedia is actually based on the wiki
concept, which means that we can build up
the learning resources on any topic
But for this wiki, basically I have
built this up myself.
And I've used it maybe just as a
repository or the place where I can
share some learning resources for this course.
So you can find home is this
page. There is a general instruction on how to use the wiki.
The how to resources for this
course. The lecture
which maybe you have viewed some of the online lectures which I link to enmodule.
And a lecture summary which I just created,
And some other activities and
if you like you can also set up your individual page.
This individual page is just like a web page.
You can put in your you know
your lecture summary. So this are some of examples of your previous
seniors. I keep this pages
as an example of what you can do with wiki page.
And you can also create a group page.
Ok. And if you want to know how to create page and how to use this,
there is an instruction here and even I have
created a tutorial here. These is actually a video tutorial
You can click this and there's a step by step on how to create a page.
So everything is already prepared for you.
So up to you if you want to use it or not.
For example here, if you click the lecture summary
so you will find
a list of lecture by lecture summary.
Ok. Lecture by lecture
summary. So if you have problem to understand certain thing from the
and some other resources and the link for you
to further explore the topic. For example here
lecture 3. So I wrote this the past tense.
yeah and you know as
informal as possible. Yeah
So that you can not understand more from
what we have covered and what we have learned in the class.
So we can go thru this lecture summary and you can print it out.
And in fact actually
you can add to it,t because that is what wiki is about.
You can edit this page. You can click edit
and you can edit it. But please don't delete.
You can add if you like any new information.
That's the spirit to build the knowledge
together. So don't leave everything to me. Okay.
Today we will be discussing about
topic in food rheology
that is visocelasticity.
we have discuss about flow
behavior. And when we talk about the different
form of food materials
or the different forms of food materials.
examples of like liquid
food in the liquid form.
So the easiest example maybe water or fruit juice
or honey. So for this type of liquid
food, we talk about
the flow behavior because they can flow readily.
Okay. Because they appear
and look to us like liquid.
So when you pick up any food materials,
you can look at the appearance, the physical appearance.
And you can tell whether, you can say whether it is a liquid.
You can describe it as a liquid or you can
describe it as a solid. Take something like
cheese. Hard chesse. So maybe you will say that
this is a solid form.
Or something like say ice cube, so it is a
solid, right? You would not say that ice cube is
a liquid before it melts.
Take something like fruit juice, obviously it looks like a liquid.
If you take something like let us say
a very thick tomato sauce,
How would you describe it? Is it a liquid or is it a solid?
Is it a solid or is it a liquid? So sometimes it
is not very obvious. It's not very obvious. Yeah.
But theoretically, theoretically,
any type of material
can be described as
Even water. But
the term viscoelastic is a term to describe
any material would contain some
viscous component and some
Viscous component means it has a liquid like
property. And elastic component
means it has a solid like property.
Okay. So take something like water again.
Obviously you know most people
will say that water is a liquid, right?
Water is a liquid because it appears like a liquid
and it behaves like a liquid. It can flow easily.
Because the viscous component, the liquid like
component in water is the main, the
dominant component in water.
Take something like margarine or butter. It
looks like solid. It appears like solid to our eyes.
Because in in butter we can say that the
elastic component or the solid like
component in butter is the dominant
component. But there is also a liquid like
component in butter. But it is a very small component.
So in the case of butter, because the elastic component or
the solid like
the more dominant one, so it appears to our eyes like a solid
material. And in water,
because the viscous component or the liquid like component is the
more significant one. The more dominant one,
so it appears like a liquid. But
theoretically, if you take any material, it should have
some viscous component or liquid like component and some
solid like component.
Or elastic component. So whether the material would appear
as liquid or would appear as a solid, it
would depend on the properties of the material.
And later we will define one parameter call
Deborah number which actually
you can see the relationship between
the viscous or the elastic
property of the material and the rate of deformation.
Okay. So before we go further
I want to show a video which
perhaps some of you would never imagine.
This video that you're going to watch
In this video, water is used to cut the metal.
So this machine is called water jet cutter.
So that is a water jet actually.
So, have you ever imagined
that water, a liquid
that we drink, can be used to cut metal. Do you know about this?
So it is amazing isn't it?
Perhaps we have experience. Sometimes when there is
a heavy rain, you know and you walk in the rain
the raindrop on your
face and you can feel sometimes can be
quite painful right. Or when
we play with the water jet or that
water pistol or water jet,
and you try to point to someone,
it can be quite painful. Because
what happens here? What is the principal here?
Ok. So the water is at a very high pressure
The water is
actually at a very high pressure. Coming out from the nozzle at a very, very high
What was the pressure just now mentioned in the video? To cut that metal
This is still early in the morning. I thought you would listen to everything,
you catch everything in the video.
What was the pressure, the water jet pleasure just now? 80
PSI. What is the atmospheric pressure,
now? The atmospheric pressure?
1 atm. The atmospheric pressure. 1 atm, equals to
how much, in PSI?
1 atm equals to
equal to 15
PSI and this is 80,000 PSI
divide by 15. So can you imagine ,the pressure
the water jet just now at 80,000 PSI
can cut the metal precisely
to whatever shape that we want. When we
shoot the water at a very high pressure. Here,
think about the deformation of the water.
Imagine. Let us say that
we pour the water now, from this bottle.
The water deforms
and flow, right? Rheology is the science of
deformation and flow. But
now let's take the factor of time.
The time factor here. So when the water flows
when I just pour like this, the water flows slowly due to the gravity,
right? And the water deforms
slowly. If we measure the shear rate, the shear rate is perhaps at a low
shear rate. But now, the water is
pumped thru a small jet at a very high pressure.
80,000 PSI. Think about the rate
of deformation here. The rate of deformation is very, very, very
very, very fast. So
now the point that I want to stress here. Even water, a
liquid water, would behave like
a solid depending on the
rate of deformation.
Another scenario. Let's go to the swimming pool
but do not jump into the swimming pool. Maybe just
sit at the
outside of the swimming pool and just play
with the water with your hand. And
try to just move your hand slowly
in the water. You can feel the water, right?
You can feel the water. Now
make it faster. Move your hand faster and faster
and faster and faster. The faster you move your hand in the water
the more resistant you can feet, right?
It becomes harder and harder and harder. Now,
again it is about the rate of deformation of the water.
The harder you move your hand, the faster is the deformation of
And the water now feels more and more and more like a solid.
The same thing
with the water that is flowing in the
sea. You know you imagine that,
you know if we just
jump into the sea or into the swimming pool.
Nothing happens. You know, you can swim nicely,
if you can swim. Otherwise you will drown.
But now imagine, you go to the Penang Bridge
and climb to the top of the Penang bridge
Just image. Don't do it. I would not be responsible.
It is being recorded now. But just imagine, you
climb to the top of the Penang Bridge and jump
into the Penang Bridge. You'll die.
The moment we hit the surface of the water from a very high
point, the water feels now like rock-solid.
Because the moment you hit the water,
the rate of deformation
at that point
is very, very fast. And now
at a very high deformation rate the, water would behave like a solid.
Remember the water
should have also the viscous or the liquid like component and
the sold like component or the elastic component.
Which component now would become more dominant?
That is another factor. The rate of deformation.
Imagine also, when now you can watch it on YouTube,
the plane crashes on the sea
and the moment it hits the water, it will
disintegrate into pieces.
I like to watch this program on Discovery Channel
on the plane crash investigation. But I always fly.
Sometimes when I
imagine those accidents it can be very scary. But then the
the point that I'm trying to make here, when the plane crash
into the sea surface, why does it break into pieces?
Because now, the plane is at a very, very, high speed.
Maybe 100 kilometer per hour or more
It hits the surface of the water at a very high speed.
So, the water actually deforms at a very, very fast
rate. And will behave like a solid.
So I think that's enough examples before we
learn more about concept of visco elasticity.
But the picture here is a nice
another manifestation of
viscoelastic property. Okay.
If you stir water, water is a Newtonian fluid.
You stir water slowly,
and faster and faster. Have you ever
seen the water climb up the rod?
The water will not climb up the rod. When you stir water
very, very fast, what you get is a vortex.
Right? What you get is a vortex. Not the water will climb up the rod.
If you can show that, then perhaps that is something new.
For a Newtonian fluid like water of fruit juice
or honey even, when you stir very, very fast,
like this, using a stirring rod, you will never see the water climbing up the rod,
but you will get a vortex.
But something like this, dough in this case,
when you stir very fast, actually it will
climb up the rod, like this.
This is a good
evidence of visco-elastic property.
Meaning in this sample, in this material,
it has a higher
elastic component then the viscous component.
Higher solid like component compared to the
liquid like component.
when we look at the different type of
material, food material it can range from
sold like, example like; margarine, butter
chocolate bar anything that looks like a solid
to a liquid like. Looks like a liquid.
Feel like a liquid. Flow like a liquid.
But most material would behave
somewhere in between.
Most material would behave somewhere between the
ideal solid or ideal fluid. So,
this material, we call it visco-
the viscoelastic material, the range you can have
is actually somewhere between something that has a purely elastic
or purely viscous.
Something like water, maybe you want to
think of it as almost purely viscous.
Newtonian behavior. Purely elastic.
Can you think of one example? Usually we use
maybe in physics especially
we use spring, as an example of ideal
elastic body or elastic material.
Sometimes we called it as as a Hookean
So meaning that, something like a spring when you stretch it
and when you release, it will
go back to its original shape. But if we stretch too much,
maybe it would not go back to its original shape. You stretched it over the elastic limit.
So most material is
viscoestatic. Meaning that it has some visco
or viscous component. Which is the liquid like component and
elastic component which is the solid like component. Which one is
more significant? Which one is higher?
So that would define,
how the material would look like, if there is more solid like
or elastic component, maybe it would appear like a solid. If there is more liquid
like component, maybe it would appear more like a liquid.
So viscoelasticity means
having both visocus
or liquid and elastic or solid like properties.
this is the so called just now, the rod climbing
effect. So this is a good demonstration.
This is a viscoelastic material here.
this is the mixing rod, the stirring rod . So you can see the liquid
climb up the rod, due to the normal force actually.
So these rod climbing phenomenon is also known as
the Weissenberg effect. So when we
see something like this,this phenomenon; the Weissenberg
effect, so we know that the material is a viscoelastic
component and it has a significant
elastic or solid like component. If you do this,
repeat this experiment using Newtonian fluid, water
fruit juice, honey, you will not see these effect. So that is one simple way
to differentiate between
a simple Newtonian fluid and viscoelastic material.Okay.
So the unique manifestation of these effects will depend on the ratio
of elastic to viscous components in the visco
elastic material. If you have more
or higher elastic component, meaning the ratio of elastic to viscous is
high, then you can see maybe this material would climb up
even higher. Okay.
So rod climbing phenomenon or Weissenberg effect is a manifestation
of the elastic component, or the solid like
component in the sample. And another
viscoelastic component is so called snapback
property. Snapback property,
probably you have experience it also. When you pour something from the bottle,
maybe the tooth paste or maybe the sauce.
You'll notice that when
you poor then after that you
stop. Whatever left at the end of the bottle will
sort of go back. So just like elastic material, you stretch
then you release. It will go back.So
that is a snap back property.
Which is also the evidence of
the elastic component in
For example the sauce retreating back to the bottle after a portion is squeezed
in the previous lecture, I have given you an example of chocolate processing.
The chocolate melt dispensed from the
nozzle from the dispenser. Then
you want the liquid chocolate to stop
flowing. So this is
called the die swelling effect. Depending on
how much is the elastic component in the material,
the higher the elastic component in the material
the more it will expand.
So that will depend on the elastic component or the
solid like component in the
So now to represent this,
we can build up a mechanical
model which consist of a spring which
represents a purely elastic component
or we can also describe it as a
Hookean solid. And
we can represent the purely viscous component by using a dashpot.
A dashpot is like a syringe.
Syringe. You fill the syringe with liquid
and you can push or pull the plunger.
Right? So we have. And
the liquid inside this dashpot is a
Newtonian liquid. So for the viscous component,
we can use a dashpot to represent it.
For the elastic component in the viscoelastic material, we can use
spring to represent it. Now,
if we build up a model by,
A simplest one will be one spring and one dashpot. Then,
we can make it more and more complicated. We can have
a few springs, a few dashpots. We can arrange it in series.We can arrange it
you know, in many different ways.
Just like in the in the electrical circuit. We can
arrange the register everything in different ways.
So therefore, by having a different number
of springs and different number of dashpots. And arrange them in different
arrangement, we can get actually
a different degree all viscoelastic material with different degree of
elastic material and different degree of viscous component. So don't worry
about this. I do not want to go further
into this. But I just have to say something about this, because when you
any book on rheology or viscoelastic,
probably you will see this. So next time when you see this, it would not be very
you know, very strange to you. The idea is just to
sort of have a model to describe the different degree of
elastic component and different degree of viscous component
in any viscoelastic material. So we can combine several units of spring and
dashpot with different arrangements to represent material with different
viscoelasticity. This particular lecture
is also available as an online lecture. And I
shared the link in enmodule. And I actually expect you to watch
that before you come for the lecture. How many of you
have not watched? Don't worry.
You can raise your hand. You have not watched
the lecture. You haven't watch?
This lecture. The link given in enmodule.
Its okay. But it is there.
And also in the wiki. Just now.
So please register for the wiki. I'll give you the address shortly.
And if you're not very clear, you can listen to that
online lecture again. Now
I want to introduce this important parameter.
When we discuss about viscoelastic material,
this one parameter that we have to really
understand. This is call
This parameter is called the... How do you pronounce that?
Deborah number. This number is to describe
behavior. Now recall
what we learned in the previous lecture about the non
Newtonian behavior. We have two types. The non Newtonian
time independent and the non Newtonian
time dependent. And
similarly for viscoelastic material, we have also
time dependent viscoelastic behavior.
But it should't be
very difficult for you to understand now. When we say time dependent
it means that behavior or how the material would behave
and appear to our eyes is
actually dependent on time
of deformation or deformation time.
How fast the material deforms
and reforms. Again
we use an elastic rubber band as an example here.
We stretch the rubber band. We can stretch it slowly.
We can stretch it very fast. When we release
it will recover the original shape. How fast it will recover?
Just like when we pour the sauce on the plate, how fast it will recover its original
It would depend on the time dependency of
how far it will recover the original structure. So that it can go back to the
Right? So it's all about the rate
of deformation. How fast it will deform
and how fast it will reform
the structure. Rebuilt the structure.
So that is a time dependent.
Okay. To characterize viscoelastic properties
we must consider the time frame required
to record a measurement. A measurement. What we measure depends on how
rapidly we measure.
That's in the red text.
Red colour. What we measure depends on
how rapidly we measure. Just like when you move your
hand in the water. You more slowly.
Basically you are measuring, you are feeling the water, right?
You are measuring or you are feeling the water. So you move your hand
slowly, you feel what the water.
Okay. You know its a liquid. You move faster,
you feel the resistant more and more.
And in the end, if you move very, very fast. Just like the water jet is
pumped at a very, very high a high-pressure. The rate of deformation is very high
and it feels like a solid now. Do this simple experiment. Move your hand through
the water. So we have done this
experiment, right? In our mind.
you know the more the faster you move,
the faster you move your hand means you are applying a fast, higher shear rate.
Higher shear rate, means higher
and faster deformation.
The more resistance you feel and the water feels like a solid
rather than a fluid. Or rather than a liquid.
Remember go back to our first lecture,
everything flows if you wait long enough.
If we wait long enough, the glass is flowing but we cannot see.
But if we wait long enough, we will see that actually
it flows. Long enough means a time frame is long.
The water jet just now
flow at very, very fast shear rate.
The time frame of the deformation is very, very
short. So that's the meaning of
the time frame. Now we define the Deborah number now,
which is the ratio. Deborah number is a ratio.
so it is a ratio. So there is no dimension.
dimensionless. It is a
ratio of characteristic relaxation
time of a material to a characteristic time
of the relevant deformation
process. Well does this
definition make sense to you? The ratio of characteristic
relaxation time of a material
to. So we give a symbol t
to a characteristic time of the relevant deformation
characteristic relaxation time of a material. Any material
will have their own unique
characteristic relaxation time.
We stretch a rubber band rubber, the band will
go back to its original shape because it has
its own relaxation time.
To recover. Meaning to recover its original
structure. That is the relaxation time.
chewing gum. Stretch a chewing gum after you
chew and you take all the sweets.
You are just left with that part.
And you try to stretch the chewing gum.
Will it go back to its original structure?
In this case, maybe no. Because the chewing gum has its
own unique characteristic relaxation time.
The rubber band has is own unique
relaxation time. It is about the recovery of the structure.
Okay. The characteristic time of the relevant deformation process. How fast we deform the
material. We pour slowly
or we pump at 80000psi just like the water jet.
That is the characteristic time of the relevant
deformation process. How fast it deforms.
So we define Deborah number as this
now, with this in mind
For an ideal hookean elastic solid,
the t which is the
characteristic relaxation time,
For a Newtonian viscous liquid the t is 0.
So now, Newtonian,
the Deborah number is zero. For
elastic hookean solid,
it is infinite. And for viscoelastic
we have somewhere between zero and infinity.
So we can define now, a
high Deborah number
much larger than value much larger than 1
So any material with a very high Deborah number
would appear or
would show a solid like behavior.
any material with a low De. number Deborah number
lower than one, would display
liquid like behavior.
So anything between this
will give you a measure or a degree of the visco
elastic property of the material.
So the implication of this, the material can appear
like a solid like when
it has a very long characteristic relaxation
time. Or the relevant deformation
process is very fast.
A material can appear solid like, like water.
It can appear like solid like. It can cut the metal.
Either it has a very long characters relaxation time.
In this case no. Water does not have a
very long characteristic relaxation
time. It has a very very small
relaxation time. But in this case
when water flows at 80,000 PSI
the relevant the deformation process is very
The relevant the deformation process is very fast. We go back to this
equation. When water flows
at 80000PSI as the water jet cutter,
the deformation time is very,
very fast. Meaning this value is very small.
Maybe a fraction of a second. When this value is very small
and this is constant for water
so the De number now will be very big.
Much higher than 1. So just now we say
any when Deborah number is much higher that much higher than one,
the water will behave like a solid.
When we pour
the water slowly from the bottle.
the rate of deformation
or the deformation time is long.
So we have and this is still constant.
So we have now bigger t value
And the De will be much smaller.
the De number is
much smaller than one. And now the water will behave
more like a liquid. I hope it is too confusing.
Any question? Any part
that is not clear about the concept viscoelasticty?
It is Okay.
The first time you feel like you understand, but maybe not
And may be the first time you
want to ask question also you don't know what to ask. That is this the reason why
I want to watch the lecture
if its available, before you come to the lecture.
So when you watch the lecture before you come
to the lecture. And maybe you want to
read more and explore more then when you come to the lecture,
whatever I say, whatever I explain in the class makes more sense.
Then only you can start ask good question.
But now maybe this is the first time you hear about viscoelastic,
The first time that you know that water can cut a metal. So you just
too much. Maybe you cannot digest it. You cannot ask a good question.
Okay. So I'm trying actually
The reason why the record this lecture so you know, in
the future, I mean you're junior. We will have all the lectures already
for the whole semester.
And they can watch even before they register for the course, even.
During the long holidays.They can watch all the lectures,
if they are hardworking enough.
And when they come to the lecture,
it should be easier for them to understand
especially if they have done some reading
you know on their own. And it makes more sense. So this in with the new model that we
want to use.
Currently for my just maybe a few lectures available online especially for
this topic on rheology. Emulsion maybe a few,
And you can actually start
to the online lectures on emulsion. It is already there.
Yeah. On crystallization.
If you want. So when you come to the lecture, at least this is not something new
because you have
you have listened to the lecture before. Yeah
okay I'm going to stop here. But before that,
I want you to register in the wiki
space. So the address is simple.
The address is here.
The address is imk209.
So see you next week.