DWS Microrheology

 
 

 

The MSD of the tracer particles is extracted using Diffusing Wave Spectroscopy (DWS). This technique probes much larger sample volumes than DLS- and microscopy-based microrheology, which yields enhanced statistics at measuring times on the order of one minute. In addition, DWS has a greatly improved spatial resolution with respect to DLS and microscopy.

 

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Image depicts the newest version of DWS RheoLab.

DWS Rheolab

 
 

Therefore, it allows for measurements on highly viscous and even arrested (non-ergodic) samples where motion of tracer particles is strongly restricted. Finally, the accessibility to frequencies as high as 107 rad/s is one of the most important features of DWS microrheology. For comparison, mechanical rheometers are typically limited to frequencies up to 102 rad/s.

 

The DWS RheoLab from LS Instruments is a compact and versatile instrument, which takes advantage of modern light scattering technology. In contrast to traditional mechanical rheology, samples are sealed in glass cuvettes and, therefore, can be studied over extended time ranges to assess their stability or aging. Moreover, typical mechanical rheometers are limited to frequencies up to ~100 rad/s, and may take several hours to complete a frequency sweep. By harnessing our patented DWS Echo-technique, the DWS RheoLab can perform accurate and reliable measurements of \(G'(\omega)\) and \(G''(\omega)\) over an extended frequency range from 10-1 to 106 rad/s in a matter of minutes.

 
 

Using suitable tracer particles DWS microrheology can measure loss and storage moduli in a wider frequency range than oscillatory mechanical rheology.

 

Fig. 3: Storage \(G'(\omega)\) and loss \(G''(\omega)\) moduli of an aqueous solution with 0.55% wt/v xanthan, measured by a mechanical rheometer and DWS RheoLab. For microrheology,  polystyrene particles (980 nm diameter) were added at 1%wt/v. Note that DWS extends the measurements of \(G'(\omega)\) and \(G''(\omega)\) to considerably higher frequencies.

 
 

[1]  T.G. Mason and D. A. Weitz:
Optical Measurements of Frequency-Dependent Linear Viscoelastic Moduli of Complex Fluids,
Physical Review Letters 74, 1250-1253 (1995).

 

[2] B.W. Mansel, S. Keen, P.J. Patty, Y. Hemar and M.A.K. Williams:
A Practical Review of Microrheological Techniques, Rheology - New Concepts, Applications and Methods, Associate Prof. Rajkumar Durairaj (Ed.), ISBN: 978-953-51-0953-2, InTech, DOI: 10.5772/53639 (2013)