Rheometry Based on a New Optical Detection Scheme:

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Rheometry Based on a New Optical Detection
Scheme Principle and Applications of DWS
Frank Scheffold Soft Condensed Matter
Group Physics Department University of Fribourg -
Switzerland
http//www.unifr.ch/physics/mm
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Thanks !
Pavel Zakharov Echo-DWS
Frederic Cardinaux Microrhelogy, Soft Gels
Peter Schurtenberger
Suresh Bhat Food (Micro)rheology
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Principle
Microrheology ?? Rheometry
Relates the local thermal motion of a probe
particle (typically 1 mm) to the viscoelastic
moduli G(?), G(?)
Advantages

• Fast -gt measurement time as low 1 minute to
  obtain full frequency information
• No external shear -gt measures in linear regime
• Extended frequency range w0.1-105 rad/sec
• Small sample volumes required (0.2-2ml)

Limitations

• only linear rheology
• requires reasonably homogenous systems
• deviations within a factor 1.5-2 possible due to
  imperfect coupling of tracer spheres to medium
• lower precision for naturally opaque systems

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Starting Point
Applications Polymers, Biomaterials, Stomach
mucus, Cytoplasm, Magnetic fluids, Surfactant
Phases (Lamellar, Micellar), Food (milk, starch,
etc.), Emulsions, Dispersions, Ceramics, Soft
Glassy Materials, Clays , Hydrogel scaffolds for
artifical tissue, Film Drying, Cell Rheology,
Actin-Myosin molec. motors,.
Ca. 150 citations
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How it works The Stokes-Einstein-Relation
Brownian particle motion in a viscous fluid

Langevin Eq.
Stokes friction
Measure D for a known particle size Micro -
viscosimeter
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-1
Long time limit Random motion mapped to a
diffusion process
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Generalized ? Microrheology
Elastic solid
Viscous fluid
Particle of Radius R
mmm
Harmonically bound Brownian particle
lt ?r2(t) gt
Optical methods
Viscosity h, spring constant k, G(?), G(?)
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Numerous applications
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How to measure the particle mean square
displacement ?
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Photon correlation spectroscopy (PCS)
Single Photon Counter Correlation
Function Local Brownian motion
Intensity fluctuations
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Probing Nanoscale motion with Diffusing Wave
Spectroscopy
Photon diffusion (transmission)
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Problem Time-averaging in DWS/PCS
Example
w0.1 rad/s ? t 10 seconds time-averaging gt
10000 sec 3h
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New Multi-Speckle DWS with a Single-Mode Fiber !
A new echo-approach to multispeckle DLS/DWS
Motor
Sample
Inspired by K.N. Pham, S.U. Egelhaaf, A.
Moussaid, P.N. Pusey, Ensemble-averaging in
dynamic light scattering by an echo technique,
Rev.Sci.Instt (2004)
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Echo Two-Cell DWS

• At each cycle (multi-speckle) correlation echoes
  appear
• The peak of the echoes is proportional to the
  correlation function of the sample
• The peak area is a more reliable measure of
  g2(t)-1!

Solid Media (Teflon block)
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Echo Two-Cell DWS

• At each cycle (multi-speckle) correlation echoes
  appear
• The peak of the echoes decays as the correlation
  function of the sample

t/sec1/2

• Comparison of
• Duration 12 second Echo experiment (symbols)
• Duration 20 minute time average (solid line)

Two-cell Echo DWS Patent application filed
2005 P. Zakharov, F. Cardinaux and F. Scheffold,
Phys Rev. E (2006) Echo DLS K. Pham, S.
Egelhaaf, A. Moussaid, and P. Pusey, Rev. Sci.
Instrum. 75, 2419 (2004).
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DWS RheoLab The fast and easy-to-use Light
Scattering Rheometer
1 to 5mm cells Automatic Calibration
Temperature Control

• Tracer Motion
• Viscoelastic Properties
• Extremely fast measurements
• due to Two-Cell Echo Tec

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Automated data acquisition
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Example Solutions of giant (polymer like)
micelles
Tracer Sphere

• Cetylpyridinium Chloride (CPy 100mM) with Sodium
  Salicylate (60mM). amphiphilic molecules, 0.005 M
  KCl
• Entangled temporary network
• (x5-10nm correlation length)
• Polystyrene Particles (diameter 720nm) were
  added during preparation (final volume fraction
  ca. 1 ).

Self Assembly -gtPolymer-like micelles
F. Cardinaux, L. Cipelletti, Frank Scheffold and
Peter Schurtenberger, Europhys. Lett. , 57, 738
(2002), F. Cardinaux, Frank Scheffold, Peter
Fischer and N. Willenbacher, to be published
(2006)
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Another Example 2 porc Gelatine after
Temperature Quench 65C ? 25C
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Gelatin after T-Quench
Critical Gelation in a single shot (Gelatine)
Monitor Gelation Down to time resolution 15
sec
Temperature Quench 65C ? 25C
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Critical Gelation Rheology in a single shot
Gel Point Winter/Chambon analysis (same power
law in G,G)
w0.7
Viscoelastic Spectrum G(w), G(w)
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Advantages of DWS Laser Light Rheometry

• Frequency range G(w),G(w) 0.1Hz-100000 Hz,
  range ca. 1- (several) 10 kPa (theoretical DWS
  maximum several 100kPa)
• Very short measurements, typically 1-3 minutes
  to obtain the full frequency spectrum
• Low viscosity samples easily accessible
• No mechnical calibration, No mechanically moving
  parts in sample contact Measures always linear
  rheology
• Possibility of high throughput screening due to
  short measurement time.
• Small sample volumes 0.2-2 ml in standard
  precision glass cells
• Sample is held in a sealed container no
  contact to air (no risk of drying). Further
  storage, treatment and repeated measurements of
  the same sample easily possible.
• Unique possibility to monitor fast processes
• extreme sample conditions e.g. high
  temperature, high pressure, external fields -
  possible but not (yet) implemented

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.thank you for your attention