The molecular structure of liquids

define fluidity

the ability to be moved in a certain direction by small forces

What do fluids not support at rest?

shear stress

What is it called when a molecule oscillates about an equilibrium node for a short period of time, and then jumos to another node?

relaxation time

What does relaxation time depend on?

• the liquid’s nature

• temperature

How long is the relaxation time?

around 10^-13 secs

What is the quasicrystalline structure of liquids?

the crystal-like way the molecules are arranged at the eqm. node but the arrangement is slowly lost as it moves away from the node

Which two properties do ordinary liquids have?

• isotropic

• amorphous

define isotropic

having a physical property which has the same value when measured in different directions

define amorphous

lacking a clear structure or not crystalline

Give two properties of VdW forces.

• repulsive at small distances

• attractive at larger distances and decreases away from molecule

What is the radius of action?

the smallest distance between two molecules where the VdW forces become negligible

What is the sphere of molecular action?

a sphere around a molecule with the radius equal to the radius of action of the molecule

What happens when a force is exerted on a molecule deeper than r?

the molecule is pulled equally in all directions by other liquid molecules so there’s no net force

What happens when this force is exerted?

net force f_k is normal to the free surface and not zero and directed inwards as less bonds are formed, which is cohesion force

define cohesion pressure (p_k)

the sum of the cohesion forces acting on all molecules in the unit area of the surface layer of the liquid

what does cohesion pressure do?

keeps liquids compressed

what is p_k for water?

10⁹ Pa

do molecules of the surface layer have higher potential energy or kinetic energy than bulk liquid molecules and why?

they have higher potential energy due to the cohesion pressure

What happens to the energy of the molecule when it moves to the bulk from the surface layer?

the kinetic energy increases

What happens to the energy of the molecule when it moves to the surface layer from the bulk?

the kinetic energy decreases and the potential energy increases

what are surface tension forces?

the forces parallel to the free surface

what shape does the free liquid take?

the shape with minimum potential energy of the surface tension forces, called the spherical shape

define the coefficient of surface tension (alpha)

the sum F of surface tension forces per unit length along an arbitrary line L in the free surface of the liquid 

alpha = F/L

what is alpha measured in?

N/m

How can you also define the coefficient of surface tension?

the work needed to increase the area of the surface by one unit of area           alpha = A/S

what does alpha depend on?

• the liquid’s nature

• temperature

• concentration and nature of dissolved substances

what are surfactants?

additives that change the alpha of a liquid

what do positive surfactants do and give examples

reduce alpha eg. soap, polymers

what do negative surfactants do and give examples

increase alpha eg. sugars, inorganic salts

define wetting

the curving of the free surface of a liquid at the boundary line between a liquid, gas, and solid

what is wetting caused by?

different coefficients of the three states of matter

What must the net surface tension force be at points along the boundary line?

zero, which is possible if the free surface is curved

what is the contact angle?

the angle between the solid surface under the liquid and a tangent the the free surface, passing through a point on the boundary line

When is the liquid absolutely wetting?

when the contact angle is 0 degrees

When is the liquid wetting?

when the contact angle is from 0 to 90 degrees

When is the liquid non-wetting?

when the contact angle is between 90 and 180 degrees

When is the liquid absolutely non-wetting?

when the contact angle is 180 degrees

Why is wetting a relative property? and give examples

a liquid may wet some solid bodies but not others eg. water wets glass and not paraffin wax

What is the meniscus?

the curved free surface close to the walls of the vessel it’s in

When is the meniscus concave?

when the liquid wets the walls

When is the meniscus convex?

when the liquid doesn’t wet the walls

What is the Laplace pressure?

the pressure due to the unbalanced surface tension forces at the curved free surface

What does the Laplace pressure depend on? (Δp)

• the radius of the curvature R of the meniscus

• the alpha of the liquid

• Δp = 2alpha/R

Where is the Laplace pressure directed?

towards the centre of the meniscus

Define capillary action.

rise or fall of the free surface of a liquid inside a small-diameter tube

What is a gas embolism?

when a gas bubble enters a capillary, it may stop the flow of blood

What are the possible complications of a gas embolism?

severe functional disorders and even death

What happens to the blood in the capillary when there’s a gas embolism?

• the front meniscus has a smaller radius

• the Laplace pressure at the front meniscus is larger

• this causes a net force opposing blood flow

Give examples of gas bubbles appearing in the bloodstream.

• injecting liquids

• surrounding pressure drops quickly

What is a similar phenomenon?

fat embolism caused by fat droplets in the bloodstream