2. A BRIEF INTRODUCTION
Particles of fluids are constantly in randomised motion. Due to this molecules of liquids have high
energies.
Particles of fluids can be divided into two types:
Bulk particles
Surface particles
There is a presence of high amount of attractive and repulsive forces between the particles of the
liquid.
These forces create potential energy across particles in the liquid. The potential energy of each
particle depends on the number of molecules in its vicinity with the energy decreasing with an increase
of molecules around it.
As the bulk molecules are surrounded in all directions as compared to surface molecules which only
have molecules beside and underneath them, the total potential energy of the bulk molecules
is much lower than that of the surface molecules
Due to this liquids tend to try to reduce the amount of surface area to as little as possible.
This extra energy at the surface and the tendency to try to remain in the minimum possible surface area
results in the occurrence of Surface Tension.
3. SURFACE TENSION
Imagine a line drawn on the surface of a liquid, the two sides of the line pull each other with a force
proportional to the length of the line. Surface Tension is defined as the Force per Unit Length or
𝑆 =
𝐹
𝐿
(𝑁𝑚−1
)
As discussed earlier, liquids tend to try to keep their surface area to a minimum, this can be seen in
water droplets which take the shape of a sphere as a sphere has the smallest surface area for a given
volume.
Surface Tension is a property of the liquid and is dependent on temperature.
It is inversely proportional to the temperature i.e. it decreases on increasing temperature.
This is because the particles move more and more rapidly as temperature increases and thus the
intermolecular forces become more unstable.
Surface Tension can be seen in our daily lives, for example, a needle can be seen floating on the surface
of water if kept carefully but falls down if it pierces through the surface even slightly, or when you fill a
glass to the brim the water level is slightly over the surface without falling down the sides. Both of these
scenarios can be explained thanks to Surface Tension.
Water has a relatively high surface tension and can allow denser objects to float on it, allowing some
organisms such as water striders to “walk on water”
4. SURFACE ENERGY
Molecules in the surface layer remain in slight turbulence compared to the molecules in the bulk.
Molecules from the surface are constantly pulled into the bulk and replaced by other molecules from the
bulk.
The surface of a liquid can be compared to a stretched elastic membrane. This stretched elastic
membrane contains some potential energy, similarly the energy possessed by the surface of a liquid is
called surface energy.
When a molecule is pulled from the bulk to the surface, work is done against the downward
resultant force, this results in an increase in potential energy and a difference in the potential
energy between the surface and bulk, This extra energy possessed by the surface is known as Surface
Energy.
Surface energy can be defined as the energy required per unit area to increase the size of the
surface. It has the units of N/m.
Surface energy is defined as E = S ∗ (change in area) and its units are Jm⁻²
Due to this surface energy, if the interaction energy between a liquid and a solid is lower than this
energy, it will stick to it to reduce its surface area and thus its energy.
The attraction force can be measured using weights. A balance beam is kept with one surface on the
liquid with some weight on it. Weights are gradually added to the other side until the surface of water is
right above the surface of liquid. The extra weight added will be equal to the surface tension and the
interaction force between the liquid and solid
(A Tensiometer, Measures Surface Tension)
5. EXCESS PRESSURE
Imagine a drop of water, as the size of the drop is fall external agents (excluding atmospheric pressure)
such as gravity can be neglected. The surface tension on the surface of the drop is constantly pushing in
trying to reduce the size of the drop and reduce it’s surface area.
The pressure increases due to this force.
This difference in pressure between the interior of the drop and the exterior of the drop is
termed as excess pressure.
In case of a water drop, the pressure inside the drop is greater than the outside by 2S/R where S is
the surface tension of the water while R is the radius of the drop
In case of a soap bubble, the pressure inside the bubble is greater than the outside by 4S/R where S
is the surface tension of the water and R is the radius of the bubble
The excess pressure in case of a bubble is double that in case of drop due to the fact that a bubble is a
film which has 2 sides. The film may appear to be thin but is still over hundreds of thousands of
atoms thick.
Excess pressure is responsible for some properties of liquids, for example, smaller bubbles are harder to
blow as the force due to pressure will be greater than the force of attraction between the molecules of
the gas.
6. CONTACT ANGLES, COHESIVE, AND ADHESIVE FORCES
The surface of liquid in the vicinity of another medium is generally curved. For example, when liquids
are kept in a tube exposed to the atmosphere they develop a curved surface called a “meniscus”.
The angle between the tangent to the liquid surface at the point of contact and solid surface inside the
liquid is termed as angle of contact.
Angle of contact depends on the level attraction among the liquid molecules and between the liquid and
solid
Forces between molecules of the same material are known as cohesive forces whereas forces between
molecules of different materials are known as adhesive forces.
If the cohesive forces are stronger than the adhesive forces, the liquid will maintain its shape and if the
adhesive forces are stronger it will spread out.
The meniscus formed between liquids and containers can be explained by cohesive. A concave meniscus
(Water in a glass tube) can be seen when the adhesive forces between the liquid and container are
stronger than the cohesive forces among the molecules and the convex meniscus (Mercury in a glass
tube) occurs when the cohesive forces are stronger.
Angle of contact can be used to determine whether water will form droplets or spread, if it is obtuse,
water will from droplets (Water on leaves) else it will spread on the surface (Water on a plastic sheet).
7. CAPILLARY RISE
When one end of a thin tube is dipped into a liquid the liquid may rise up despite the presence of
gravity. When angle of contact is acute, the liquid rises and when the angle of angle of contact is
obtuse, the liquid is depressed.
Capillary rise occurs when the adhesive forces are stronger than the cohesive forces and depends on the
radius of the capillary tube, the smaller the radius the higher the rise. If a capillary rise appears, the
meniscus is observed to be concave and the pressure right above the liquid is less than atmospheric
pressure.
The liquid keeps rising until the pressure above the liquid is equal to atmospheric pressure.
The height in capillary rise is given by ℎ = 2𝑆/(𝜌𝑔𝑎) where S is the surface tension, ρ is the density, g is
acceleration due to gravity, a is the radius.
Capillary rise is necessary for multiple reasons in our environment. For example transport of water in
plants through xylem occurs through the phenomenon of capillary rise.
(Experiment showing Capillary Rise in plants
notice the color of the leaves)
8. DETERGENTS
Water alone cannot clean dirty clothes and remove grease as water does not wet greasy dirt. There is
little to no area of contact between them
If water could wet grease, it can be cleaned. This is where detergents come in.
Detergents act as surfactants or wetting agents by reducing the surface tension of water and allowing
it to spread and increase it’s surface area.
Detergent molecules have two ends, a hydrophilic or water loving head and a hydrophobic tail.
The hydrophilic head interacts with the water and the hydrophobic tail interacts with the grease. This
helps the grease combine with the water.
The molecules surround the blob of grease and break it down into pieces that can flow easily. Thus
detergents clean the grease present and help it flow away.
Detergents reduce the surface tension of the water surface and help increase the formation of the
grease blobs surrounded by water.
