Angle of contact - Mechanical Properties of Fluids Video Lecture - Class 11

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now steady angle of contact
what is angle of contact it is the angle
at which a liquid interface meets a
solid surface so here will basically
study the interaction between a liquid
interface and a solid surface so there
is always an angle which the soil with
the liquid interface makes with a solid
surface let's suppose generally this
denoted by theta now different the angle
of contact is different at interfaces of
different pairs of liquids and solids so
what will be the angle of contact or the
magnitude of angle of contact depends
upon the material which constitutes the
interface that is it is different for
every different pair of solid and liquid
now if you take a real-life example you
would have seen a droplet of water on a
look to see if you see this this water
droplet has a surface which is in
contact with this solid surface with the
deep surface of the leaf is the solid
surface and this is the liquid surface
so this is the surface of contact so at
this surface of contact the liquid
surface needs some angle with the soil
in seconds so that angle is known as the
angle of contact which is generally
denoted by theta now what is the
significance of angle of contact and why
do we need angle of contact angle of
contact
determines whether a liquid will spray
on the surface of a solid or it will
form droplets so I hope you understand
the difference between spread and
droplets if you see when you pour water
on say a lotus leaf how does the water
look like or how what shape does the
water
takes when it falls on the lunar sleep
it takes the form of an droplet but
there are many substances where when you
pour water it is suppose you pour water
on a piece of float it will not form
droplets it will just go and spread over
the clothes and clothes then get wet
so what is the factor on which it
depends whether the water will spread on
the solid surface or it will form a
droplet that is basically determined by
the angle of contact how if the angle of
contact is obtuse then a droplet is form
of choice means the angle which is
greater than 90 degree on the contrary
if the angle of contact is a cubed then
the water will form a spread so this is
the point which you need to remember if
the angle of contact is obtuse it will
form a droplet and if it is acute it
will spread on the surface of the solid
now let us understand it with the help
of this diagram here we have both the
scenarios in the first scenario it forms
a droplet in the second scenario we see
that it spreads over the circles now how
does this angle determine which is this
angle that we exactly look at this is my
solid surface this is the liquid droplet
anguses if right so when I say solid
liquid interface which is the solid
liquid interface this line this is the
line which joins the solid and the
liquid surface so this is basically s
that is solid liquid interface so when I
pop up solid am interface it is in this
direction as you can see this is a solid
this is air so this is solid air
interface now the third one is liquid
air interface so this is liquid this is
air so obviously this would be the
liquid air interface so here we have
chosen this point you can
anyway if you want to reduce this point
of contact also so even there also you
would get the same result so in this
case if you see that the angle which the
liquid air interface makes with the
solid liquid interface that is theta
this theta is greater than ninety degree
that is an obtuse angle so in this case
a droplet is formed that is where a
droplet is formed when a liquid hits the
surface of a solid and it forms a draw
then the angle of contact that is the
angle which the liquid mates with the
solid surface that angle is obtuse
similarly in case of a spread here you
see a spread is formed in this case
there is no droplet the water just
spreads over the solid surface in this
case you see that this angle here also
this is solid liquid interface this is
solid air interface and this is liquid
clear interprets the angle in which it
forms with the solid surface is acute
and that is lesser than ninety degrees
so the point which you need to remember
here is whenever an angle of contact is
less than ninety degree the water or the
fluid spreads on the soil surface when
the angle of contact is greater than
ninety degree the liquid or the fluid
forms a drop or a droplet over the solid
surface now having given this thing a
thought that why is it that all the
drops and bubbles are all these
spherical in shape as we discussed thank
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