Adhesion Basics

8/3/2019 Adhesion Basics

1/12

Apoorva Mahendra Ranjekar

ICNTAA

SMIT


2/12

This interesting insect can freely run along the surface of a water. It's tiny mass and the

geometry of its legs allow it to be supported by the high surface tension of water.


3/12

Water droplet on a

leaf


4/12

Surface tension, is defined as the force

along a line of unit length, where the

force is parallel to the surface butperpendicular to the line.

Surface tension ghas the dimension of

force per unit length, or energy per unit

area.


5/12

In the depths of a volume of liquid,

each molecule is surrounded on all

sides by other molecules; the forces

between them balance out and the

entire mass is in equilibrium. The

situation is different at the surface of a

liquid. At a liquid-air interface forexample, the molecules at the surface

are being attracted by the surrounding

liquid but not by the air. The forces are

imbalanced and consequently the

liquid behaves as if had a stretched

skin.

This creates some internal pressure

and forces liquid surfaces to contract

to the minimal area.


6/12

Molecules liquid state experience strong

intermolecular attractive forces. When those

forces are between like molecules, they are

referred to as cohesive forces.

When the attractive forces are between unlike

molecules, they are said to be adhesive forces.

Adhesion is any attraction process between

dissimilar molecular species that can potentially

bring them in "direct contact.


7/12

The cohesive forces among the liquid molecules are responsible for this

phenomenon of surface tension.

The attractive forces between molecules in a liquid can be viewed as residual

electrostatic forces and are sometimes called van der Waals forces or van der

Waals bonds.

As a general rule, the greater the proportion of polar groups (e.g O-H groups) in a

molecule the stronger the attractive forces between them. Strong attractive forces

give rise to a high surface tension and a tendency to form discreet droplets on a

surface rather than wet it evenly. The large proportion of O-H groups in water are

responsible for its high surface tension. Alcohols, with their smaller proportion of

O-H groups, have lower surface tensions.

surface tension is always parallel to the surface.

The surface tension is the two-dimensional analog to the pressure.

Water has a highest surface tension than most other liquid.

Foreign molecules in water usually lower the surface tension.

Increased temperature lowers surface tension.

Electrical conditions at interfaces can alter surface tension.


8/12


9/12

Surface energy is quantified in terms of the forces

acting on a unit length at the solid-air or the solid-

liquid interface. The units of measurement areexactly the same as for surface tension.

In order to increase the surface area of a mass of

liquid by an amount, A, a quantity of work, gA,

is needed. This work is stored as potential energy. If the surfaces are unequal, the Young-Dupr

equation applies:

W12 = g1 + g2 - g12

where g1 and g2 are the surface energies of thetwo new surfaces, and g12 is the interfacial

tension.


10/12

Where the two surfaces meet, they form a contact angle,

which is the angle the tangent to the surface makes with thesolid surface.

If is greater than 90 the liquid tends to form droplets on

the surface. If is less than 90 the liquid tends to spread out

over the surface and when the liquid forms a thin film,

tends to zero.


11/12

0 indicates vacuum. This is free energy of a unit area of the film


12/12

Documents

Adhesion Basics