Introduction to
DYNAMIC LIGHT SCATTERING (DLS)
or
PHOTON CORRELATION SPECTROSCOPY
(PCS)
Christer Svanberg
Outline
Basic of DLS
– Experimental set-up
– Accessible time- and length-scales
Applications
– Size and shape of sub-micron objects
Research
– Glass transition
– Polymer dynamics
correlation
Experimental set-up
Scattered Electric Field

Es   if (q , t )
  
  
  
        c    T
 c T , 
 T   ,c
  T ,c
DLS probes
• density fluctuations
• concentration fluctuations
Time range of DLS
DLS
QENS
NMR
Raman
-14
Brillouin
-10
Dielectric
-6
-2
LOG(TIME (s))
2
DLS covers a very large time range!
Typically: 10-7 - 103 s! => 10 decades in time!
Experimentally accessible wave vectors
Q-range:
typically 0.6 – 2×10-3 Å-1
DLS is therefore suitable for diffusional studies
of macromolecules, such as polymers and large
biomolecules!
Advantages and Disadvantages
• Wide time range
• Cheap
• Simple experimental set-up
• Only transparent samples
• Very clean samples needed
• Sensitive for mechanical
disturbances
Commerically avaliable products
Scientific instruments
•
•
•
•
More laser power!
Specially designed cryo-furnaces
Polarization options
Vibration isolation table
Scotland, 1827
Brownian Motion
• First observed in 1827 by the botanist Robert Brown. He was looking at pollen
grains under a microscope. The force of life?
• Desaulx in 1877: "In my way of thinking the phenomenon is a result of thermal
molecular motion in the liquid environment (of the particles).”
• The mathematical theory of Brownian motion was developed by Einstein in 1905.
• Jean-Baptiste Perrin verified Einstein's analysis (Nobel Prize 1926).
Brownian Motion
Explanation:
A suspended particle is constantly and
randomly bombarded from all sides by
molecules of the liquid. If the particle is very
small, the number of hits it takes from one
side at a given time will be stronger than the
bumps from other side. This make the particle
jump. These small random jumps are what
make up Brownian motion.
Stoke-Einstein relation:
k BT
D
6r
Applications of DLS
Size:
Using Stoke-Einstein equation DLS can be used to easy, fast and
accurate determination of the hydrodynamic radius of particles.
Typically range: 1 nm – 1μm.
Shape:
Ellipsodial particles results in a small fraction depolarized scattered
light. Can be used for estimation of ellipticity of the particles. Difficult!
Examples
Some examples of sub-micron systems:
–
–
–
–
–
–
–
Micro-emulsions
peptides
micelles
macromolecules
polymers
paint pigments
bacteria, viruses
Estimation of Ellipsodial objects
Lysozome
Hydrodynamic size
Calculated hard sphere
Calculated ellipitic shape
Intensity
Size Distribution of Insulin
Diameter (nm)
Amplitude
The influence of pH on Insulin
Diameter (nm)
Research using DLS
• Determination of size on complex systems:
– “water-in-oil”
– bio-molecules
– cellulose
• Glass transition dynamics
• Polymer dynamics
Glass transition dynamics
log 
-relaxation
–
–
–
–
liquid


cooperative intermolecular motion
stretched exponential decay
non-Arrhenius temp. dep.
freezes at Tg
fast
1/Tg
-relaxation
1/T
1.0
correlation
– local motion
– broad response
– Arrhenius temp. dep.
glass
0.8
0.6
0.4

0.2
0.0
-2
-1
0
log[t/ (s)]
1
2
Poly(propylene glycol)
1
0,8
0,6
192 K
0,4
Temp.
0,2
221 K
0 -6
10
10-4
10-2
100
Time (s)
102
104
Arrhenius Plot
Arrhenius Plot
log[ Relaxation time (s)]
4
Polymer
(n=69)
Dimer
(n=2)
0
Oligomer
(n=7)
Conclusion:
Shorter polymer chains relax
faster than long chains.
Monomer
(n=1)
-4
-8
-12
2 2.5
3 3.5 4 4.5
1000/T (1/K)
5 5.5
6
Glass Transition dynamics in
Free-standing Polymer Films
200 - 500 Å
Polystyrene
Tg=369 K
DLS can be used to probe the
dynamics of thin free-standing
polymer films
Glass-to-glass transitions
copolymer micellar system
SCIENCE 300, 619 (2003)
Multiple Glassy States in a Simple
Model System
Sterically stabilized PMMA-particles
in cis-decalin.
SCIENCE 296 104 ( 2002)
Polymer Dynamics
Dilute solutions
Semi-dilute solutions
f*
2xh
2Rg
Brownian motion
Entangled dynamics
Dynamic Correlation Length ξh
Polymer Gel Electrolytes
Poly(methyl methacrylate)
+
Propylene Carbonate / Ethylene Carbonate
+
Lithium Perchlorate (LiClO4)
Dynamics in a
Polymer Gel Electrolyte
Relaxations and Conductivity in Polymer
Gel Electrolytes
Nernst-Einstein equation
…there is a close connection between the fast
diffusive process and the ionic conductivity!
Outlook: X-ray PCS
Exemplary correlation functions of
colloidal silica suspension measured at
q ~ 7.6 × 10-4 Å-1 using three different
X-ray energies as indicated.
Summary: DLS technique
Probes density and/or concentration fluctuations.
Time scales: ~10-6 - 103 s
Wave vectors: ~10-3 Å-1
Standard characterisation techniques for particles
– Determination of size 1 nm - 1 mm
– Estimation of ellipticity and/or swelling
Research
– Polymer dynamics
– Glass transition dynamics