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Diffusing Wave Spectroscopy (MS-DWS)

Multi Speckle Diffusing Wave Spectroscopy (MS-DWS) to monitor structural properties of a material.

The measurement of Brownian motion of the particles using MS-DWS optical method allows to determine structure-related properties of the sample. MS-DWS can be used with different types of sample forms. In a bulk sample, viscoelastic properties related to gel formation and network structure evolution can be studied with time and temperature. When a thin film layer is applied, coating drying and film formation without any contact with the sample can be characterized. In relation to fast temperature ramps or cycles, the dynamics at microstructure scale provide information about phase transitions and structure reorganization (crystallization, melting...). This optical method is highly sensitive to the smallest structure change.

Main advantages

  • Works on opaque & concentrated dispersions (emulsions, suspensions, foams)
  • Sensitive to short particles displacement (nanometer range)
  • Average signal over a large number of particles at microstructure scale

FROM MS-DWS MEASUREMENTS, IT IS POSSIBLE TO:

  • Determine rheological properties in bulk
  • Monitor evolution (gelation, stability...)
  • Monitor drying of coatings
  • Monitor phase transitions

How does it work?

A laser beam is scattered by moving particles in a specific pattern. A Camera monitors the image of the resulting interfering wave (speckle). Depending on the mobility of the particles the pattern fluctuates. Pixel by pixel analysis of the speckle allows to plot decorrelation curve as a function of characteristic time and directly correlate to particle motion speed.
Decorrelation curve can be analyzed using different physical models allowing to determine multiple structure properties.

-          In a passive microrheology approach (Rheolaser MASTER), Mean Square Displacement (MSD) can be determined to quantify the displacement of the particles (in nm).
If MSD is a linear function of decorrelation time, followed particle displacement can be considered “free”. This indicates that material structure is purely viscous. In most cases however, the MSD curve encounters a plateau, particle is partially retained by encountered network structure and is no longer free. This means that analyzed material is characterized by both viscous and elastic behavior

-          Fluidity and Elasticity factors can be calculated to quantify viscoelastic properties. Fluidity Index, inversely proportional to characteristic decorrelation time, provides information about sample fluidity. Monitored over time (with Rheolaser COATING), it allows to follow drying and film formation. Drying mechanisms (evaporation, packing, particle deformation…) can be detected and a unique signature of coating drying can be obtained.

-          In thermally activated analysis (Rheolaser CRYSTAL), micro-dynamics of the structure (in Hz) can be monitored to study its evolution during thermal stress (temperature ramps and cycles). Any change to the structure: reorganization, crystallization, polymorphic transition, will impact the dynamics and result in a characteristic peak. The integration of the peak (as a function of time or temperature) allows to determine characteristic transition parameters.