Multi Speckle Diffusing Wave Spectroscopy (MS-DWS) monitors the structural properties of a material.
The measurement of the Brownian motion of the particles using the MS-DWS optical method allows determining the 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 the microstructure scale provide information about phase transitions and structure reorganization (crystallization, melting…). This optical method is highly sensitive to the smallest structure change.
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 correlates to particle motion speed.
The 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, the 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.
In drying experiments (using Curinscan), the Fluidity Factor can be monitored over time. It is inversely proportional to characteristic decorrelation time and it provides information about sample fluidity. It allows monitoring of drying and film formation processes. Drying mechanisms (evaporation, packing, particle deformation…) can be detected and a unique signature profile of coating drying can be obtained.