5) Moleculer Structure Of Surface Tension Flashcards
Molecular structure of liquids
The molecules are close to each other
Large molecular forces of attraction exist
A characteristic property is fluidity
-the ability to be moved in a particular direction by relative small forces
The quasicrystalline structure of liquids
The molecules oscillate about an equilibrium node for a short period of time (relaxation time) and then jump to another node
Relaxation time:
- Nature of the liquid
- temperature
- is usually around 10^-13 seconds
The quasicrystalline structure of liquids 2
At the equilibrium node the molecules are arranged in a crystal like way
Away from the node the arrangement is usually lost
Ordinary liquids are isotopic and amorphous
Van der waals forces
The molecules of liquids interact with VDWF
- repulsive at small distances
- attractive at larger distances and decreasing away from molecule
Radius of action r- the smallest distance between 2 molecules at which the force is weak so it’s neglected
Sphere of molecular action- sphere centred at molecule with radius= to radius of action
Van der Waals forces 2
Radius r
The smallest distance between to molecules at which the force becomes small enough to be neglected
Intermolecular forces
DRAWING
Force on a molecule submerged deeper than r
The molecule is pulled equally in all directions by neighbouring liquid molecules. net force is 0
Intermolecular forces 2
The net force F_k on the molecule is not 0
Drawing
Cohesion pressure
P_k is the sum of cohesion forces acting on the unit area of all molecules
I’m water
P_k= 10^9 pa
Energy of molecules
Due to cohesion pressure the molecules on the surface have higher potential energy than other molecules
When molecule passes from surface to a layer in the bulk its kinetic energy increases and vice versa
Surface tension
The molecules on the surface interact with forces parallel to the surface =surface tension forces
Coefficient of surface tension
Alpha is the coefficient
F is the sum of surface tension
L is length along line
Alpha=f/L
Alpha is measured in N/m
Surfactant is additive that modified surface tension
Positive= reduced alpha( soaps)
Negative= increases alpha (salt)
The coefficient of surface tension 2
It also also be defined as the work needed to increase the area of the surface by one unit
Alpha = A/s
Alpha depends on:
The nature of the liquid
Temperature
Wetting
The curving of the free surface of the liquid at the boundary between liquid gas and solid
A liquid may wet a sold body but not wet another
Eg: water wets glass but not paraffin wax
Contact angle
The angle between the solid surface under the liquid and tangent to the free surface of the liquid
DRAWINF
Contact angle 2
The liquid is
Absolutely wetting
0 = theata
Wetting
0
Contact angle 3
The liquid is
Non wetting
90
Meniscus
Meniscus is the curved free surface close to the walls of the vessels
Meniscus is concave when liquid wets walls
Meniscus is convex when it doesn’t wet the walls
DRAWING
Laplace pressure
Due to the unbalanced surface tension of the curved free surface
DRAWINGS
Capillary action
Fall or rise of the free surface inside a small tube.
Phg= 🔺p = 2 alpha/ R
Height of the liquid column:
h= 2 alpha/ pgR
Gas embolism
When a gas bubble enters a narrow tube it stops the blood flow through vessels. Can cause death
.front meniscus has smaller radius
.laplace pressure at front is larger
.net force that opposes the blood flow
DRAWING
