§8.2 Surface phenomenon of liquid

8.2.3 wetting and spreading

g

S

l

g-l + g-s s-l

G = s-l – (g-l + g-s) = -Wa

Work of Adhesion

(1) adhesion

1. Some definitions

Wa = g-l + g-s – s-l

Wa > 0

The solid can be wetted by the liquid.

8.2.3 wetting and spreading

(2) immersion

g-s s-l

G = s-l - g-s = -Wi Wi = g-s - s-l > 0

Work of immersion

(3) spreading

g-s s-l + l-g

G = s-l + l-g - s-g = -S

S = s-g - s-l - l-g > 0

The liquid spreads over the solid spontaneously.

spreading coefficient

2. Contact angle ()

Hydrophobicity of conversion layer on Mg alloy goniometer

The contact angle () is the angle measured through the liquid, where a liquid/vapor interface meets a solid surface

g-l

g-s

s-l

Under equilibrium:

g-l cos + s-l = g-s

Young equationlg

slgs

cos

When :g-s - s-l = g-l , cos =1, = 0 o, Complete wettable.

When :g-s-s-l< g-l , 0<cos <1, <90 o, wettable.

When :g-s < s-l , cos < 0, > 90 o, nonwettable.

The direction of surface tension

3. Lyophobic and lyophilic solids

The greater the specific energy, the easier the spreading of liquid over solid.

g-s – g-l – s-l > 0

g-s > g-l + s-l

g-s > g-l

g-l

g-s

s-l

g-s > 100 mN m-1, high-energy surface : Metals, oxides,

chlorides, inorganic salts. g-s 500 ~ 5000 mN m-1

g-s < 100 mN m-1, low-energy surface: organic solids,

polymers. PTFE: g-s 18 mN m-1

Nonstick cooker

4. Spreading over liquid

SO/W = - G = W - O - W/O

SO/W > 0,

oil can spread over water

SO/W < 0,

oil floats in shape of lens.

Liquids Iso-C5H12O C6H6 C6H12 CS2 CH2I2

SO/W 44.0 8.8 3.4 -8.2 -26.5

Floating oil drop on chicken soup

Floating oil on sea surface

Clapham Common (2000 m2)

1774 Benjamin Franklin (2.4 nm)

The film formed over water is of one molecule thick. (proved

by Pockels and Rayleigh):

Unimolecular film, monolayer, Insolvable film

wreck of a tanker

Spreading of oil over seawater

A environmental disaster

2010年 5 月 5 日，美国墨西哥湾原油泄漏事件

2.3.4. Capillarity Capillary rise / depression

1 2

2( )

lp p

h gr

grh

)(

2

21

Rr cos

gRh

)(

cos2

21

Discussion

This relation can be used to

determine the surface tension

of liquids – capillary rise

method

gRh

)(

cos2

21

Measurement of porosity distribution:

Mercury methodp