Diffusing Wave Spectroscopy
Diffusing Wave Spectroscopy (or DWS in short) is a technique which allows
one to do light scattering studies on multiply scattering samples. This
tool is helpful to extract useful information on particle motion
on very short length scales and particle sizing.
Basically there are two aprroximations in DWS: The theory is good in the
limit of very high multiple scattering, where each photon is scattered a
large number of times and the second is that the individual
scattering events are approximated by the contribution of an average
scattering event.
It neglects any interference effects of the light as it propagates through
the medium and assumes that the light is completely diffused by the time
it reaches the detector. A schematic diagram of the essential nature of
the experiment is shown below. For more information on DWS and
microrheology you can check the following references:
D.J. Pine, D.A. Weitz, P.M. Chaikin and E. Herbolzheimer, Diffusion
wave spectroscopy, Phys. Rev. Lett., 60, 1134-1137
(1988)
David A. Weitz and David Pine, Diffusiing-wave spectroscopy,
Dynamic Light Scattering, Ed. Wyn Brown , O.U.P. (1992)
D.A. Weitz, J. X. Zhu, D.J. Durian, H. Gang and D.J. Pine, Diffusing
wave spectroscopy: the technique and some applications, Phys.
Scr., T49, 610-621 (1993)
D.J. Pine, D.A. Weitz, J. X. Zhu and E. Herbolzheimer, Diffusiong
Wave Spectroscopy : dynamic light scattering in the multiple scattering
limit, J. Phys. France, 51, 2101-2127 (1990)
We are presently using DWS to study the microrheological properties of
(Poly)-ethylene oxide (PEO) , a flexible polymer. We introduce
polystyrene beads of various sizes in the polymer and extract the mean
square displacements of these colloidal particles using DWS. This response
of the beads to the polymer determines the rheology of the system. Using
the more conventional single scattering we can extend the temporal range
of the response function by a few more decades. Thus by employing these
two different light scattering techniques we have not only been able to
achieve an excellent comparison but also a larger frequency range of when
compared to the bulk measurements. The other advantages of using this
technique over the conventional measurements are that it requires very
little sample volume (<2ml) and there are no macroscopic stresses that are
exerted on the sample.
Correlation function from DWS for 2.2% 900K PEO with different bead sizes.
Mean square displacement for the above correlation function. Notice the
small displacements ~10nm that can be ascertained using DWS.
Comparison between single scattering and DWS. The single scattering data
overlaps very nicely with the DWS measurements
Comparision of microrheology with macro rheology. The microrheological
data spans almost seven decades and it has an excellent overlap with the
bulk measurements.
Click here for a recent talk that I gave at
the APS March meeting (PDF format).
Bivash Dasgupta
9 & 15 Oxford Street
Gordon McKay Labs
Harvard University
Cambridge MA 02138
Ph. 617 496 9562
bivash@deas.harvard.edu