In today’s video we are going to talk about contact angle and wettability.
A contact angle gives us an indication of how well (or how poorly) a liquid will spread
over a surface.
While formulating coating, inks, or paints, contact angles provides a useful indication
of how a modification to the ink or paint will affect its spreading over the solid surface.
It also gives a strong indication on the performance of the product as well as successfulness of the process
Basically contact angles express how a liquid interacts with a solid surface.
Suppose we have a solid surface and we put a drop of liquid on it.
We can determine the level of interaction between solid and liquid with this angle theta
which is known as contact angle.
Now there can be several possibilities of how the liquid drop interacts with the solid surface
We might have a drop that looks like a ball where the contact angle is 180°.
In this case we have something that is completely non-wetting.
Wetting is the ability of a liquid to maintain contact with a solid surface.
Wettability is inverse of the contact angle.
Higher the contact angle lower is the wettability.
So Non-wetting means liquid will stay on the solid surface in droplet form; it will not spread
on the solid surface.
As we can see here the liquid droplet can easily roll over the surface due to poor wettability
and high contact angle.
Whereas in another situation we can also have a drop spread out completely and covered the surface
In this case contact angle is 0° and this would be perfect wetting or complete wetting.
So in case of perfect wetting we have theta that is equal to 0° whereas in case of perfect
non-wetting theta is 180°.
But in most cases we have something in between these two, where theta is between 0 and 180°.
So 90° is the angle where two phases show either wetting or non-wetting.
So if the contact angle between the liquid drop and solid surface is less than 90°
it is called wetting and if it's higher than 90° then it is called non wetting.
If taken liquid is water and contact angle is less than 90°, the surface is hydrophilic
and if the contact angle is greater than 90° then surface is hydrophobic.
If the contact angle is close to theoretical limit of 180° then the surface is considered
super hydrophobic.
A contact angle can be large or small, depending on the physical properties of the materials
being investigated such as nature of liquid, type of solid surface etc.
An example of a surface that is non wetting is the lotus leaf which is shown here.
We can see that the contact angle between water droplet and lotus leaf is much higher
than 90°and water droplets forms nearly spherical shape.
On the other hand if we drop water on cotton fabric, water forms zero contact angle with
cotton fabric and spreads out completely showing complete wetting.
So what could be the reason that the water does not spread out on all the solid surfaces
and remain in spherical droplet form?
To understand this first we need to first understand surface tension.
Let’s take example of water droplet.
Due to surface tension water or any other liquid forms spherical shape, it is because
water molecules with in the liquid are attracted to each other.
These water molecules can form H-bonding with other water molecules around them.
So the fact that water and other liquid molecules are attracted to each other is known as cohesion
and this attractive force is known as cohesive force.
Since these water molecules likes to bunch together or form groups, but due to cohesive
forces a molecule is pulled equally in all directions by neighboring molecules
This results in a net force of zero.
However, at the surface there is no water molecule above them.
These are less restricted and are in a state of higher energy compared to the molecules
in the bulk of the liquid.
This allows surface water molecules to group together a little better and form stronger,
tighter bond in such a way that they form a tension that is not present in the water
molecules below the surface.
These bulk water molecules down below will prevent surface molecules from grouping by
pulling the molecules toward the center.
Since the force is only working downward and sidewise, the liquid forms a spherical shape.
So basically surface tension is the tendency of a liquid surfaces to shrink into the minimum
surface area possible because of that liquid molecules are able to make or maintain spherical shape
Now if we have two liquids having high and low surface tension, high surface tension
liquid will form high contact angle or large contact angle and will stay in spherical droplet
form, whereas low surface tension liquid will have low contact angle compared to high surface
tension liquid and will show tendency of wetting that means this will spread out on the solid surface
Now coming back to contact angle, so fundamentally contact angles are a function of the interaction
of the surface tension involved.
There are three boundaries to consider when a droplet is in contact with a solid surface:
the solid, the liquid, and the vapor surrounding them which is usually air.
As we can see that we have surface tension of liquid against vapor; we have surface tension,
or surface energy, which is a accurate term to represent surface tension of solid against
vapor and finally surface tension of liquid against solid.
The figure shown here is a force diagram of the point where all three phases meet.
The three arrows represent the forces exerted by the surface tensions at three interfaces:
liquid-solid, liquid-vapor, and solid-vapor.
Each force is pulling away from the equilibrium point, so if the droplet is in equilibrium,
then the forces are balanced that means leftward force is going to be equal to rightward force,
so surface tension of solid vapor is going to be equal to liquid vapor + liquid solid
cos Ɵ.
So cos Ɵ is equal to the difference in gamma solid vapor – liquid solid/ gamma liquid
vapor.
Now you might have question that are surface energy and surface tension equal?
So numerically in SI system, surface energy and surface tension are equal.
Both have same unit.
Surface tension exists when liquid gas, oil water or gas water phases are present.
Whereas, surface energy is a key property of a solid, which tells about wettability
of that solid surface.
So if we know surface tension and surface energy we can calculate contact angle.
These are the ideal situations where solid surface is smooth and liquid is pure but this
is not the case always.
There are some factors such as surface roughness, impurities, functional group on the surface
etc., that affect contact angle.
In our next video we will discuss factors that affect contact angle.
I hope you enjoyed today’s video.
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Thank you.