Diffusing Wave Spectroscopy: Microrheology
Diffusing wave spectroscopy (DWS) is an optical rheology technique used to obtain viscoelastic properties without ever touching the sample. Instead of applying shear to it, as is done by mechanical rheometers, the sample is illuminated with laser light. After the incoming light has been scattered many times, the resulting intensity fluctuations are detected and the mean square displacement of the scattering particles is obtained from this. Principles of microrheology are then applied to determine viscous and elastic properties of the sample. What is so exciting about the technique is its ability to measure storage and loss modulus [G’, G’’] within minutes in a huge frequency range.
DWS can use single photon detectors (avalanche photodiodes (APD) or photomultiplier tubes (PMT)) or charge coupled devices (CCD). Response time of the detector limits frequency range which can be measured.
The faster APD and PMT detectors allow storage and loss modulus to be measured from 0.1 to 10,000,000 Hz. Elasticity from 1 to 100,000 Pa can be determined for samples with viscosity as low as 1 mPa•s. Most significantly, only data from APD and PMT detectors are currently comparable to measurements made on mechanical rheometers. The technique is fast enough to allow gel point determination and process monitoring for stability and ageing.
DWS is a microrheology techniuqe and in terms of sample volume it is indeed “micro” since it requires so little sample. Liquid volumes as small as 50 microL can be measured in minutes. Concentrated dispersions (of solids, liquids, or gases in a second liquid) can be measured. DWS can also be used on transparent systems (those which do not scatter light) by adding tracer particles having known properties (such as polystyrene spheres) to the sample.
Mechanical rheometry or “macrorheology” typically requires larger sample volumes than DWS and the sample is difficult to recover for further measurements. Mechanical rheometers can only measure over frequencies from 0.001 to 100 Hz and hours can slip by during a full, frequency sweep measurement. Samples can be degraded by mechanical shear applied at high frequencies and by the resulting frictional heating.
DWS has been known for since the mid 90s, but only recently have clever data treatment algorithms and more powerful computers made it a commercially useful technique. LS Instruments has been selling DWS instruments since 2005. Two models are presently available, the DWS RheoLab and the DWS ResearchLab. Both models make rheological measurements using transmitted light. The ResearchLab also detects backscattering of light to measure particle size from 50 nm -1 micron.