View source for CALICE

'''Diffusing-wave spectroscopy''' (DWS) is an optical technique derived from [[dynamic light scattering]] (DLS) that studies the dynamics of scattered light in the limit of strong multiple scattering.<ref>
{{cite journal
 |author=G. Maret, P. E. Wolf
 |year=1987
 |title=Multiple light scattering from disordered media. The effect of brownian motion of scatterers
 |journal=[[Zeitschrift für Physik B]]
 |volume=65 |pages=409
 |doi=10.1007/BF01303762
|bibcode = 1987ZPhyB..65..409M
 |issue=4 }}</ref><ref>
{{cite journal
 |author=D. J. Pine, D. A. Weitz, P. M. Chaikin, and E. Herbolzheimer
 |year=1988
 |title=Diffusing wave spectroscopy
 |journal=[[Physical Review Letters]]
 |volume=60 |pages=1134
 |doi=10.1103/PhysRevLett.60.1134
|bibcode=1988PhRvL..60.1134P
 |issue=12
}}</ref> It has been widely used in the past to study colloidal [[Suspension (chemistry)|suspension]]s, [[emulsions]], [[foams]], gels, biological media and other forms of [[soft matter]]. If carefully calibrated, DWS allows the quantitative measurement of microscopic motion in a soft material, from which the [[rheological]] properties of the complex medium can be extracted ''via'' the so-called [[microrheology]] approach.

==One-speckle diffusing-wave spectroscopy==
Laser light is sent to the sample and the outcoming transmitted or backscattered light is detected by an optoelectric sensor. The light intensity detected is the result of the interference of all the optical waves coming from the different light paths. 

<gallery>
Image:figureDWS.png|Typical setup of diffusing-wave spectroscopy
</gallery>

The signal is analysed by calculating the intensity [[autocorrelation]] function called g<sub>2</sub>. 
<math>g_2(\tau)=\frac{\langle I(t)I(t+\tau)\rangle_t}{\langle I(t)\rangle_t^2}</math>

For the case of non-interacting particles suspended in a (complex) fluid a direct relation between g<sub>2</sub>-1  and the [[mean squared displacement]] of the particles <Δr<sup>2</sup>> can be established. Let's note P(s) the probability density function (PDF) of the photon path length s. The relation can be written as follows:<ref>
{{cite journal
 |author=F. Scheffold ''et al.''
 |year=2004
 |title=New trends in optical microrheology of complex fluids and gels
 |url=http://w3.lcvn.univ-montp2.fr/~lucacip/NewTrendsMicroRheology.pdf
 |journal=[[Progress in Colloid and Polymer Science]]
 |volume=123 |pages=141–146
 |doi=10.1007/b11748
 |isbn=978-3-540-00553-7
}}</ref>

<math>g_2(\tau)-1=[\int {ds P(s) \exp(-(s/l*)k_0^2 \langle\Delta r^2(\tau)\rangle) }]^2</math>

with <math>k_0=\frac{2\pi n}{\lambda}</math> and <math>l*</math> is the transport mean free path of scattered light.

For simple cell geometries, it is thus possible to calculate the mean squared displacement of the particles <Δr<sup>2</sup>> from the measured g<sub>2</sub>-1 values analytically. For example, for the backscattering geometry, an infinitely thick cell, large laser spot illumination and detection of photons coming from the center of the spot, the relation ship between g<sub>2</sub>-1 and <Δr<sup>2</sup>> is:

<math>g_2(\tau)-1=\exp\left(-2 \gamma \sqrt{\langle\Delta r^2(\tau)\rangle k_0^2}\right)</math>, γ value is around 2.

For less thick cells and in transmission, the relationship depends also on l* (the transport length).<ref>
{{cite book
 |author=D. A. Weitz, D. J. Pine
 |year=1993
 |chapter=Diffusing-wave spectroscopy
 |editor=W. Brown
 |title=Dynamic Light scattering
 |pages=652–720
 |publisher=[[Clarendon Press]]
 |isbn=978-0-19-853942-1
}}</ref>

==Multispeckle Diffusing-Wave Spectroscopy (MSDWS)==

This technique either uses a camera to detect many speckle grains (see [[speckle pattern]]) or a ground glass to create a large number of speckle realizations (Echo-DWS <ref>http://spie.org/x8591.xml?highlight=x2404&ArticleID=x8591</ref>). In both cases an average over a large number of statistically independent intensity values is obtained, allowing a much faster data acquisition time.

<gallery>
Image:figureMSDWS.png|Typical setup of Multispeckle Diffusing-wave spectroscopy
</gallery>
<math>g_2(\tau)=\frac{\langle I(t)I(t+\tau)\rangle_p}{\langle I(t)\rangle_p^2}</math>

MSDWS is particularly adapted for the study of slow dynamics and non ergodic media. Echo-DWS allows seamless integration of MSDWS in a traditional DWS-scheme with superior temporal resolution down to 12&nbsp;ns.<ref>
{{cite journal
 |author=P. Zakharov, F. Cardinaux, and F. Scheffold
 |year=2006
 |title=Multispeckle diffusing-wave spectroscopy with a single-mode detection scheme
 |journal=[[Physical Review E]]
 |volume=73 |pages=011413
 |doi=10.1103/PhysRevE.73.011413
|arxiv = cond-mat/0509637 |bibcode = 2006PhRvE..73a1413Z }}</ref> Camera based adaptive image processing allows online measurement of particle dynamics for example during drying.<ref>
{{cite journal
 |author=L. Brunel, A. Brun, P. Snabre, and L. Cipelletti
 |title=Adaptive Speckle Imaging Interferometry: a new technique for the analysis of microstructure dynamics, drying processes and coating formation
 |url=http://www.opticsinfobase.org/oe/abstract.cfm?uri=oe-15-23-15250
 |journal=[[Optics Express]]
 |volume=15 |issue=23 |pages=15250–15259
 |year=2007
 |doi=10.1364/OE.15.015250
|bibcode = 2007OExpr..1515250B
 |pmid=19550809|arxiv = 0711.1219 }}</ref>

==References==
{{reflist}}

==External links==
*[http://www.formulaction.com/technology_dws.html/ Diffusing Wave Spectroscopy Overview with video]
*[http://www.lsinstruments.ch/technology/diffusing_wave_spectroscopy/ Diffusing Wave Spectroscopy Overview]

[[Category:Spectroscopy]]
[[Category:Soft matter]]