13. Class 11th Physics | Surface Tension | Angle of Contact | by Ashish Arora

lets study angle of contact. and to understand
angle of contact we must first analyze cau-hesive and ad-hesive forces. i hope in you early
classes you have studied what do you mean by , cau-hesive forces. Cau-hesive forces
are basically . the attractive forces. between. molecules. of same substance. or it is also
termed as the attractive force between like molecules. and the another one is ad-hesive
forces. which are, termed as, attractive forces. between molecules. of different substances.
or these are also termed as the attractive forces between unlike molecules. and when
we talk about angle of contact it is considered . whenever a liquid surface comes in contact
with the solid surface. like if we just talk about a container. in which a liquid is filled.
then if we talk about the free surface of liquid we can say, all these particles will
experience, cau-hesive forces between the neighboring particles. and those liquid particles
which are at the contact , with the solid surface will experience cau-hesive forces
, inside the volume of liquid and ad-hesive forces. onto the side of , the solid because,
the liquid particles which are in contact with the, solid material of which the container
is, made-up of, will exert ad-hesive forces on the liquid particles. so the resulting
effect of ad-hesive forces will be in horizontal direction which we can write as, f-ad-hesive.
and the effect of net cau-hesive force, as it is acting with in the volume of body it
can be considered. in such a way , that the cau-hesive force. will be acting inside the
volume of body at the point where the liquid surface is in contact with the body. now in
this situation , this cau-hesive force will be having one horizontal component and other
in vertical direction. so this can be written as , f-cauhesive in, horizontal direction
this f-cauhesive in , vertical direction. now the magnitude of f-cau-hesive in horizontal
direction and this f-ad-hesive which are acting in opposite direction will decide , the shape,
or curvature of, the liquid surface in , where it is in contact with the solid. lets consider
two cases . in this situation say, if . f-cau-hesive horizontal is more then, f ad-hesive. then
see what will happen in this situation I will just enlarge try to draw the picture. where
the liquid is in contact. we can see as if f-cau-hesive . horizontal force is more then
the f-ad-hesive force then the liquid will be pulled in slightly inward direction. and
it loose contact with the surface so the, shape of liquid will look like this this will
be the situation, where we can say if f-cau-hesive is acting in. inward direction then f-cau-hesive
horizontal will be acting here. and f-ad-hesive which was acting in, horizontally leftward
direction which is less so it’ll be pulled. and due to . the f-cauhesive vertical force
. it’ll be in contact somewhere. now in this situation if we just draw a tangent to
the surface. then the angle made , between the , tangent and the downward normal of.
this liquid surface at this point or it is along the tangent of the surface, this angle.
theta we term as , angle of contact. and in this situation we can see this angle of contact
is an obtuse angle which is more then ninety degree. and similar situation if we say if
f-cau-hesive horizontal is less then the f-ad-hesive acting on the liquid layer particles. here
we can see the situation will be slightly , different . and i just try to draw the picture
in this situation if f-adhesive is more. it’ll have a tendency to pull the liquid surface
towards the wall of container and shape of liquid will look like this. it’ll be slightly
pulled up . as in this situation f-ad-hesive acting. on the liquid surface is more then
the f-cau-hesive horizontal which is acting , exactly opposite to this. and f-cau-hesive
vertical direction was obviously pulling in downward direction so net f-cau-hesive was
acting here. and in this situation we can state . that. the angle which this tangent
, is making with the vertical direction. theta is the angle of contact and here angle of
contact is less then ninety degree or we can say . it is, an acute angle always if f-ad-hesive
is more then the horizontal component of cau-hesive force. . if we find out the resultant in this
situation . the resultant force would be acting in a direction normal to the free surface,
this is the resultant force acting on it which is in outward direction. and here if we calculate
then f cau-hesive will be relatively large and if we find out the resultant force by
using parallelogram law. it’ll be in inward direction. here we can say . in the situation
the resulting force will be in inward direction which is normal to the free surface of liquid.
this is the way how we can analyze about the angle of contact in one case it is obtuse
and other case it is acute angle. in sequence we can discuss one more concept which is related
to angle of contact that is . for theta less then ninety degree whenever there is, a solid
liquid pair for which the angle of contact is less then ninety degree. we can simply
say that, liquid will. wet the solid surface . the reason is in the
situation when theta is less then ninety degree
the ad-hesive forces are more than the cau-hesive forces . horizontal cau-hesive forces. that
means the shape of . liquid surface would be like this. and in this situation the liquid
will have a tendency to . raise up along the surface of solid. or it’ll be dragged along
with the surface of solid and in this situation the solid surface will be, wet by the liquid
and in this situation this is the angle of contact which we , use as theta which is less
then ninety degree. similar to this if , there is a surface on which. a liquid drop is placed
. in this situation again as soon as it is dropped , as adhesive forces are more . as
soon as it’ll be drop on the surface the shape of, this drop will look like this and
at the edges, or we can say the angle of contact will be theta. on the contrary we can say
if, there are some cases when , theta is more then ninety degree so we can write for theta
more then ninety degree . liquid, do not wet. the solid surface. the reason is , again the
same. if say in a container a liquid is filled. the shape would be like this because here
ad-hesive forces are less. and cau-hesive forces are more. so that’s why this angle
of contact is obtuse or it is more then ninety degree and when liquid will be removed from
the container it’ll be completely remove as liquid is not have any tendency to stick
to the . container material. and in this situation we can say , if on a surface . liquid drop.
is placed. So as soon as it is drop it’ll take a shape like this . as, adhesive forces
are less so , edges will be rounded upward, because cau-hesive forces are more. so in
this situation this angle of contact will be obtuse. the simplest example to understand
as in this situation this is, valid for water glass. pair, and here we can talk about mercury
and glass pair . and this is very common example you should always keep in mind as many applications
we are going to discuss based on this .

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