Measurement of Surface Forces

Surface Forces & Colloid Stability

Copyright © 2014 R. Sedev. All rights reserved.

by Rossen Sedev

A Liquid Film – Thick or Thin?

g

g

h

Independent interfaces = thick film Interacting interfaces = thin film

g

g

h

2f g int2f f hg

Disjoining Pressure (Surface Force)

The interaction between the two interfaces is given by the disjoining pressure, Π:

P h P

, , iT V

h

f

h

f dh

The interaction between the two interfaces can be discussed in terms of disjoining

pressure Π (force per unit area, N/m²) or, alternatively, free energy f (energy per

unit area, J/m²):

The Measurement Principle

h

Δx

Repulsion

Attraction

f k x

Hooke’s law

• Force • Distance

Derjaguin Approximation

1 2

1 2force between spheres energy between planes

2R R

f h F hR R

h h R1

R2

Validity:

1,2h R

Measurement Methods

• Surface Force Apparatus (SFA)

• Atomic Force Microscopy (AFM)

• Thin Film Balance (TFB)

• Optical Tweezers (OT)

Surface Force Apparatus

The distance (film thickness):

• Interferometry (FECO)

• Resolution is about 0.1 nm

The force:

• Elastic deformation of a

cantilever spring

• Sensitivity is about 10 nN

FECO Interferometry

Heuberger (2001)

mica

mica

Ag layer

Ag layer

Wavelength [Å]

Lat

eral

Posi

tio

n [

µm

]

Example

Mica – Aqueous NaCl – Mica

Horn (1990)

Adhesion Force

Israelachvili (2004)

Atomic Force Microscope

Nanotechnology Encyclopedia

Force Measurement

NPL

Colloid Probe Microscopy

6 μm silica particle 1.5 μm silica particle 20 μm cellulose bead

Ralston et al (2005)

Example

Silica Particle – Aqueous NaCl – Silica Particle

Toikka & Hayes (1997)

Spheres

• R1 = 1.03 µm

• R2 = 1.15 µm

NaCl (pH = 5.8):

• 0.2 mM (circles)

• 2.0 mM (squares)

PC

Π

Capillary Tube (Scheludko Cell)

Manev & Nguyen (2005)

Thin Film Balance

The disjoining pressure in the film is set

by the gas pressure in the cell.

Manev & Nguyen (2005)

Example

Air – Aqueous F108 – Air

hW

Uniform versus real structure:

Sedev & Exerowa (1999)

7 µM F108 in 0.1 mM NaCl (23 °C)

Optical Tweezers

Molloy & Padgett (2002)

Setup

Force measurement:

Optical trap displacement

Distance measurement:

Image analysis

Elmahdy et al (2009)

Example

CaCl2 (pH = 7):

• 0.01 mM (pentagon)

• 0.04 mM (squares)

• 0.1 mM (circles)

• 0.2 mM (triangles)

• 0.1 mM (rhombus)

• 1.0 mM (hexagon)

Silica Particle – Aqueous CaCl2 – Silica Particle

Elmahdy et al (2009)