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Université de Fribourg

Broad bandwidth optical and mechanical rheometry of wormlike micelle solutions

Willenbacher, N. ; Oelschlaeger, C. ; Schopferer, M. ; Fischer, P. ; Cardinaux, Frédéric ; Scheffold, Frank

In: Physical Review Letters, 2007, vol. 99, no. 6, p. 068302

We characterize the linear viscoelastic shear properties of an aqueous wormlike micellar solution using diffusing wave spectroscopy (DWS) based tracer microrheology as well as various mechanical techniques such as rotational rheometry, oscillatory squeeze flow, and torsional resonance oscillation covering the frequency range from 10⁻¹ to 10⁶ rad/s. Since DWS as well as mechanical oscillatory...

Université de Fribourg

Quantitative modeling of laser speckle imaging

Zakharov, Pavel ; Völker, Andreas ; Buck, Alfred ; Weber, Bruno ; Scheffold, Frank

In: Optics Letters, 2006, vol. 31, no. 23, p. 3465-3467

We have analyzed the image formation and dynamic properties in laser speckle imaging (LSI) both experimentally and with Monte Carlo simulation. We show for the case of a liquid inclusion that the spatial resolution and the signal itself are both significantly affected by scattering from the turbid environment. Multiple scattering leads to blurring of the dynamic inhomogeneity as detected by LSI....

Université de Fribourg

Photocount statistics in mesoscopic optics

Balog, Sandor ; Zakharov, Pavel ; Scheffold, Frank ; Skipetrov, S. E.

In: Physical Review Letters, 2006, vol. 97, p. 103901

We report the first observation of the impact of mesoscopic fluctuations on the photocount statistics of coherent light scattered in a random medium. A Poisson photocount distribution of the incident light widens and gains additional asymmetry upon transmission through a suspension of small dielectric spheres. The effect is only appreciable when the average number of photocounts becomes...

Université de Fribourg

Multiple-scattering suppression in dynamic light scattering based on a digital camera detection scheme

Zakharov, Pavel ; Bhat, Suresh Kumar ; Schurtenberger, Peter ; Scheffold, Frank

In: Applied Optics, 2006, vol. 45(8), p. 1756-1764

We introduce a charge-coupled device (CCD) camera-based detection scheme in dynamic light scattering that provides information on the single-scattered autocorrelation function even for fairly turbid samples. It is based on the single focused laser beam geometry combined with the selective cross-correlation analysis of the scattered light intensity. Using a CCD camera as a multispeckle detector,...

Université de Fribourg

Multispeckle diffusing-wave spectroscopy with a single-mode detection scheme

Zakharov, Pavel ; Cardinaux, Frédéric ; Scheffold, Frank

In: Physical Review E, 2006, vol. 73(1), p. 011413

We present a detection scheme for diffusing-wave spectroscopy (DWS) based on a two-cell geometry that allows efficient ensemble averaging. This is achieved by putting a fast rotating diffuser in the optical path between laser and sample. We show that the recorded (multispeckle) correlation echoes provide an ensemble averaged signal that does not require additional time averaging. Furthermore,...

Université de Fribourg

Laser speckle imaging with an active noise reduction scheme

Völker, Andreas C. ; Zakharov, Pavel ; Weber, B. ; Buck, F. ; Scheffold, Frank

In: Optic Express, 2005, vol. 13(24), p. 9782

We present an optical scheme to actively suppress statistical noise in Laser Speckle Imaging (LSI). This is achieved by illuminating the object surface through a diffuser. Slow rotation of the diffuser leads to statistically independent surface speckles on time scales that can be selected by the rotation speed. Active suppression of statistical noise is achieved by accumulating data over time. We...

Université de Fribourg

Photonic properties of strongly correlated colloidal liquids

Rojas-Ochoa, Luis Fernando ; Mendez-Alcaraz, J. M. ; Sáenz, J. J. ; Schurtenberger, Peter ; Scheffold, Frank

In: Physical Review Letters, 2004, vol. 93, p. 073903-73906

The optical and structural properties of dense colloidal suspensions in the presence of long-range electrostatic repulsion are determined from both light and small-angle neutron scattering experiments. Short-range structural order induces an enhancement of the scattering strength while at the same time the total transmission shows strong wavelength dependence, reminiscent of a photonic crystal....

Université de Fribourg

Optical imaging of the spatiotemporal dynamics of cerebral blood flow and oxidative metabolism in the rat barrel cortex

Weber, B. ; Burger, C. ; Wyss, M. T. ; Schulthess, G. K. von ; Scheffold, Frank ; Buck, Alfred

In: European Journal of Neuroscience, 2004, vol. 20, p. 2664-2670

Oxidative metabolism and cerebral blood flow (CBF) are two of the most important measures in neuroimaging. However, results from concurrent imaging of the two with high spatial and temporal resolution have never been published. We used flavoprotein autofluorescence (AF) and laser speckle imaging (LSI) in the anaesthetized rat to map oxidative metabolism and CBF in response to single vibrissa...

Université de Fribourg

Depolarization of backscattered linear lypolarized ligh

Rojas-Ochoa, Luis Fernando ; Lacoste, David ; Lenke, Ralf ; Schurtenberger, Peter ; Scheffold, Frank

In: Journal of the Optical Society of America: A, 2004, vol. 21, p. 1799-1804

We formulate a quantitative description of backscattered linearly polarized light with an extended photon diffusion formalism taking explicitly into account the scattering anisotropy parameter g of the medium. From diffusing wave spectroscopy measurements, the characteristic depolarization length for linearly polarized light, lp, is deduced. We investigate the dependence of this length on the...

Université de Fribourg

Tracer microrheology of γ-dodecalactone induced gelation of aqueous starch dispersions

Heinemann, Cornelia ; Cardinaux, Frédéric ; Scheffold, Frank ; Schurtenberger, Peter ; Escher, Felix ; Conde-Petit, Béatrice

In: Carbohydrate Polymers, 2004, vol. 55, p. 155-161

We have studied the rheological properties of the natural biopolymer system starch in the presence of the gelation inducing flavor compound γ-dodecalactone. From optical microrheology, based on the multiple scattering of laser light (DWS), we deduce the frequency-dependent moduli G(ω) and G″(ω) of the gelling sample over the full frequency range. Our measurements demonstrate the inherent...