Case Study Tutorial Wetting and Non-Wetting Basics of Wetting 1.
Lecture 2 Wetting and adhesion - Colloidal Dispersions · ACS© 2007 Lecure 2 - Wetting and...
Transcript of Lecture 2 Wetting and adhesion - Colloidal Dispersions · ACS© 2007 Lecure 2 - Wetting and...
Lecure 2 - Wetting and adhesion 2ACS© 2007
The Molecular Origin of Surface Tension
The molecules at the liquid surface are pulled towards the bulk liquid. To expand the surface requires work. The work is the surface tension times the change in area.
Lecure 2 - Wetting and adhesion 3ACS© 2007
Thermodynamics for surfaces
F AσΔ = Δ
The free energy (work at constant temperature) to compress a gas is:
Where σ is the surface tension.
Δ = ΔF p V
The free energy to stretch a surface is:
When ΔF is negative, the process is spontaneous.
When ΔF is positive, the process reverses.
Lecure 2 - Wetting and adhesion 4ACS© 2007
Coalescence of Droplets
The change in energy is:+
( ) 0
final initial
final initial
F F FA AA
σσ
Δ = −
= −
= Δ<
Therefore the drops coalesce spontaneously.
Lecure 2 - Wetting and adhesion 5ACS© 2007
Coalescence of Droplets with Emulsifier
+
When droplets covered with emulsifier coalesce, some emulsifier must be desorbed. This requires work.
work of desorptionF AσΔ = Δ +
If the emulsifier is strongly adsorbed, the work to remove it is large, and the drops do not coalesce.
Lecure 2 - Wetting and adhesion 6ACS© 2007
Spreading
The energy change per unit area for liquid 2 (top) to spread across the surface 1 (bottom) is:
2 12 1( )F σ σ σΔ = + −
Surfactants reduce the two terms positive terms allowing the drop to spread.
Lecure 2 - Wetting and adhesion 7ACS© 2007
Detergency
Holmberg et al. pp 474-5.
Requires only that the energy of surfactant adsorption is greater than the energy of the new liquid surface created.
Also requires that the surfactant lower the new solid/liquid interface to be less than the previous solid/oil interface.
Requires spontaneous absorption of oil into micelles.
Lecure 2 - Wetting and adhesion 8ACS© 2007
Liquids have different contact angles on different solids
Lecure 2 - Wetting and adhesion 9ACS© 2007
Contact angles: Liquids on solids
The contact angle of 140o is the same for each drop, independent of drop size.
The observation is that the contact angle depends on the materials but not the particular geometry.
Mercury drops on glass.*
Drops vary in size from 4 to 24 grains (1 grain = 64.8 mg)
* Bashforth and Adams, 1883.
Lecure 2 - Wetting and adhesion 10ACS© 2007
The equation for contact angles
σ σ θ σ
σ σ σ θ
= +
− =
cos or
cos
sv lv sl
sv sl lv
The Young-Dupré introduces the idea of a solid surface/vapor surface tension, σsv and a solid/liquid interfacial tension, σsl. The contact angle, q, is independent of the geometry.
σsv
σlv
σslθ
The idea is that the three tensions are balanced:
A sessile liquid drop on a solid:
Lecure 2 - Wetting and adhesion 11ACS© 2007
Works of Cohesion and Adhesion
σsl
σlvσsv
σσ
σ σ σ= + −adhsv lv slW
σ= 2cohW
The work of adhesion is the separation to create two new surfaces from one interface:
The work of cohesion is the separation to create two new surfaces.
Using the Young-Dupré equation: σ θ σ= +cosadhlv lvW
Lecure 2 - Wetting and adhesion 12ACS© 2007
Large surface heterogeneities - contact angle hysteresis
Advancing liquids are held up by low energy spots and show high contact angles.
Receding liquids are held by high energy spots and show low contact angles.
High energy spots –low contact angles.
Low energy spots –high contact angles.
Lecure 2 - Wetting and adhesion 13ACS© 2007
Motion of liquids due to surface energies
Capillary flow –
Motion as a consequence of shape.
Key idea: pressure drop across a curvedsurface
Marangoni flow –
Motion as a consequence of variation insurface tension.
Lecure 2 - Wetting and adhesion 14ACS© 2007
Pressure drops across a curved surface
R1
R2
x x+dx
dz
y
y+dy
1 2
1 1Lp
R Rσ
⎛ ⎞Δ = +⎜ ⎟
⎝ ⎠
The pressure is larger on the concave (inside) of the curved surface.
The Laplace equation:
R1 and R2 are the radii of curvature.
Lecure 2 - Wetting and adhesion 15ACS© 2007
Capillary rise
The final position is determined by 2 principles:(1) The pressure drops across curved interfaces.(2) The pressure in the liquid must be the same at the same depth.
In the final state the pressure drop across the AC interface equals the hydrostatic pressure from C to B.
2 c o sL g hR
σ θ ρ=
Lecure 2 - Wetting and adhesion 19ACS© 2007
Ostwald Ripening
The pressure inside > pressure outside2prσ
Δ =
This equation implies that in an emulsion with a range of drop sizes or a foam with a range of bubble sizes, material diffuses from small drops to large drops.
Also, this equation implies that bubbles are difficult to nucleate.
Lecure 2 - Wetting and adhesion 20ACS© 2007
The Kelvin Equation
2ln m
o
P VP rRT
σ⎛ ⎞=⎜ ⎟
⎝ ⎠
0
2ln mc Vc rRT
σ⎛ ⎞=⎜ ⎟
⎝ ⎠
Similarly, the “curvature” of small particles changes the solubility:
Both these effects cause Ostwald ripening.
The curvature of small drops changes the vapor pressure:
Lecure 2 - Wetting and adhesion 21ACS© 2007
Marangoni Flow
Marangoni flow –
flow resulting from local differences in surface tension.
Causes of Variation in Surface Tension –
Local temperature differences.
Local differences in composition due todifferential evaporation.
Electric charges at surfaces.
Local compression or dilatation ofadsorbed films.