INSTRUCTOR: So this screencast is part two
of screencasts discussing the properties of fluids.
In the first screencast,
we discussed density and the bulk modulus.
In this screencast,
what we're going to talk about
is the vapor pressure and surface tension.
So let's start with the vapor pressure.
So the vapor pressure when you have a liquid and a vapor,
and they're in equilibrium,
the vapor is said to be saturated,
and the pressure that the vapor exerts
on the liquid surface is known as the vapor pressure.
And when the absolute pressure in the fluid
equals the vapor pressure, boiling occurs.
So it's importance to fluids lies in the fact
that very low pressures can develop
in a flow due to fluid motion.
So when this pressure is reached, boiling occurs.
And these bubbles can then flow
into a movement of higher pressure,
which then the bubbles collapse,
and it has enough intensity to cause structural damage.
And this process is known as cavitation.
So in other words,
when these bubbles burst and cause this kind of damage,
that's cavitation.
So many engineering problems involve
finding the point of lowest pressure in a system
to ensure that this is above the vapor pressure.
Generally this pressure will occur
at the highest point of the system,
since pressure increases as you go down in height.
So the higher the cavitation number,
the bigger the difference between
the pressure of the system and the vapor pressure.
Therefore, it's less likely that cavitation
will occur with a higher cavitation number.
So now let's talk about surface tension.
So surface tension, and we abbreviate this as sigma,
occurs due to an imbalance
of the cohesive forces at the interface.
And this is usually between a liquid,
so we have liquid here,
and we have some kind of gas on the surface.
But it can also occur between two immiscible liquids.
So on the interior, right in here,
the molecules are equally attracted to each other.
However, here the molecules on the surface
are subjected to a net force towards the interior.
This tensile force is known as surface tension,
and it has units of newtons per meter.
So let's consider the forces that are acting on, let's say,
half of a liquid drop here that has a radius of R.
So we have forces that are due to surface tension
that oppose the forces due to a change in pressure
between the interior and the exterior.
So a force balance says that two-pi-R,
which is the radius of the drop times the surface tension,
has to equal that change in pressure
times its area, pi-R-squared.
So from that we can determine
what that change in pressure is.
Two times sigma divided by R.
So one of the phenomena associated with
surface tension is capillary rise.
So if we have a small tube,
and it's inserted into water,
what will happen is the water in the tube
will rise above the level of the outside water.
So here's your water,
and what will happen here
is you'll get what's known as capillary rise.
And so a force balance is going to give us that h,
which is the height above the water level,
is equal to two times the surface tension
times the cosine of theta.
And this is divided by the specific weight times the radius.
And this theta is equal to
what's known as the contact angle.
And the way we calculate the contact angle
is if you draw a line coming from that dip
due to the capillary rise,
the angle between that and the tube itself
is what's known as theta.
So surface tension is a measure
of a fluid's attraction with itself.
So a drop size in this scenario depends on
how long a drop will stay and accumulate on the pin.
So if the fluid is more attracted to itself,
there'll be less surface area
between the forming drop and the pin.
And the consequence of less surface area,
the attraction between the pin and the liquid will be less,
requiring less liquid mass to break the mass attraction.
So with this,
we're gonna finish up the four
of the properties that determine fluids.
Our vapor pressure and surface tension here,
and our density and bulk modulus on the previous screencast.
In addition, take a look at our viscosity screencast
that describes an extremely important fluid property,
viscosity, and its relationship
to an extremely important fluids concept, shear stress.