A dynamical light scattering technique and its application in viscoelastic networks in soft matter

In this paper, we present a dynamic light scattering technique using diffusing wave spectroscopy to track the dynamics of colloidal particles embedded in a complex fluid which allows us to obtain structural and dynamical information of a transparent viscoelastic material. Scattered light of a single speckle is detected by a photomultiplier tube and the time correlation function of light intensity is calculated using a temporal average. If the particles can not explore the entire phase space, temporal average and ensemble average are not the same. This is a necessary condition to relate ensemble average from the scattering by many particles to intensity temporal fluctuations. To overcome non-ergodicity for large lag times, a CCD camera is used for the acquisition of the scattered light were pixels form an array of detectors which enables us to perform thousands of simultaneous experiments. In this manner, the time correlation function is obtained directly by taking the ensamble average instead of using a temporal average. For short lag times, the non-ergodicity problem can be avoided by remixing the scattered light coming from the sample by the use of a slowly rotating diffuser disk placed before the collection optics of the photomultiplier tube. This procedure provides a true ensemble-averaged time correlation function over ~ 7-8 decades of time. As an example of the application of this technique, the dynamics of microspheres embedded in cross-linked polymer matrix, namely, an acrylamide-bisacrylamide gel is studied. This polymer network is known to swell or shrink by changing the solvent composition. The description of the arrested dynamics of the microspheres can be obtained, as well as the viscoelastic properties of the polymer network at different cage sizes.

[1]  D. Weitz,et al.  Microrheology of cross-linked polyacrylamide networks. , 2005, Physical review. E, Statistical, nonlinear, and soft matter physics.

[2]  B. Berne,et al.  Dynamic Light Scattering: With Applications to Chemistry, Biology, and Physics , 1976 .

[3]  F Scheffold,et al.  Diffusing-wave spectroscopy of nonergodic media. , 2001, Physical review. E, Statistical, nonlinear, and soft matter physics.

[4]  Wu,et al.  Nonergodicity and light scattering from polymer gels. , 1992, Physical review. A, Atomic, molecular, and optical physics.

[5]  S. Middleman Rheology: Principles, measurements, and applications by C. Macosko, VCH Publishers, 1994, 550pp., $95.00 , 1995 .

[6]  V. Viasnoff,et al.  How are colloidal suspensions that age rejuvenated by strain application? , 2002, Faraday discussions.

[7]  R. Pecora Dynamic Light Scattering , 1985 .

[8]  A. Gadomski,et al.  Thermokinetic approach of single particles and clusters involving anomalous diffusion under viscoelastic response. , 2007, The journal of physical chemistry. B.

[9]  Mason,et al.  Optical measurements of frequency-dependent linear viscoelastic moduli of complex fluids. , 1995, Physical review letters.

[10]  T. Waigh Microrheology of complex fluids , 2005 .

[11]  Thomas G. Mason,et al.  Estimating the viscoelastic moduli of complex fluids using the generalized Stokes–Einstein equation , 2000 .

[12]  Virgile.Viasnoff,et al.  Multispeckle diffusing-wave spectroscopy: A tool to study slow relaxation and time-dependent dynamics , 2002 .

[13]  R. Castillo,et al.  Absolute values of transport mean free path of light in non-absorbing media using transmission and reflectance measurements , 2008 .

[14]  P. Schurtenberger,et al.  Diffusing wave spectroscopy and small-angle neutron scattering from concentrated colloidal suspensions. , 2002, Physical review. E, Statistical, nonlinear, and soft matter physics.

[15]  D. López‐Diaz,et al.  Microrheology and characteristic lengths in wormlike micelles made of a zwitterionic surfactant and SDS in brine. , 2010, The journal of physical chemistry. B.

[16]  J. Harden,et al.  Recent advances in DWS-based micro-rheology , 2001 .

[17]  D. Weitz,et al.  Diffusing wave spectroscopy. , 1988, Physical review letters.

[18]  R. Castillo,et al.  Diffusing wave spectroscopy in Maxwellian fluids , 2008, The European Physical Journal E : Soft matter.

[19]  P. Zakharov,et al.  Multispeckle diffusing-wave spectroscopy with a single-mode detection scheme. , 2005, Physical review. E, Statistical, nonlinear, and soft matter physics.

[20]  Shao-Tang Sun,et al.  Phase transitions in ionic gels , 1980 .