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Université de Fribourg

Self-diffusion and structural properties of confined fluids in dynamic coexistence

Sousa de, N. ; Sáenz, Juan José ; Scheffold, Frank ; García-Martín, A. ; Froufe-Pérez, L. S.

In: Journal of Physics: Condensed Matter, 2016, vol. 28, no. 13, p. 135101

Self-diffusion and radial distribution functions are studied in a strongly confined Lennard-Jones fluid. Surprisingly, in the solid–liquid phase transition region, where the system exhibits dynamic coexistence, the self-diffusion constants are shown to present up to three-fold variations from solid to liquid phases at fixed temperature, while the radial distribution function corresponding...

Université de Fribourg

High-quality photonic crystals with a nearly complete band gap obtained by direct inversion of woodpile templates with titanium dioxide

Marichy, Catherine ; Muller, Nicolas ; Froufe-Pérez, Luis S. ; Scheffold, Frank

In: Scientific Reports, 2016, vol. 6, p. 21818

Photonic crystal materials are based on a periodic modulation of the dielectric constant on length scales comparable to the wavelength of light. These materials can exhibit photonic band gaps; frequency regions for which the propagation of electromagnetic radiation is forbidden due to the depletion of the density of states. In order to exhibit a full band gap, 3D PCs must present a threshold...

Université de Fribourg

Lifetime of fluorescent dye molecules in dense aqueous suspensions of polystyrene nanoparticles

Scalia, Giuseppe ; Scheffold, Frank

In: Optics Express, 2015, vol. 23, no. 23, p. 29342

We study the lifetime of two common fluorescent dye molecules from the Alexa Fluor NHS Ester family dissolved in water in an opaque aqueous dispersion of dielectric polystyrene nanoparticles. We investigate the role of the dispersion composition by varying the particle concentration and adding SDS (sodium dodecyl sulfate) surfactant molecules. The observed strong changes in lifetime of Alexa 430...

Université de Fribourg

Laser speckle analysis synchronised with cardiac cycle

Zakharov, Pavel ; Scheffold, Frank ; Weber, Bruno

In: Proceedings SPIE, 2015, vol. 9540, p. 954008–954008–9

We present an improved Laser speckle imaging approach to investigate the cerebral blood flow response following function stimulation of a single vibrissa. By synchronising speckle analysis with the cardiac cycle we are able to obtain robust averaging of the correlation signals while at the same time removing the contributions due to the pulsation of blood flow and associated tissue adaptation....

Université de Fribourg

Controlling dispersion forces between small particles with artificially created random light fields

Brügger, Georges ; Froufe-Pérez, Luis S. ; Scheffold, Frank ; Sáenz, Juan José

In: Nature Communications, 2015, vol. 6, p. 7460

Appropriate combinations of laser beams can be used to trap and manipulate small particles with optical tweezers as well as to induce significant optical binding forces between particles. These interaction forces are usually strongly anisotropic depending on the interference landscape of the external fields. This is in contrast with the familiar isotropic, translationally invariant, van der Waals...

Université de Fribourg

Structure of marginally jammed polydisperse packings of frictionless spheres

Zhang, Chi ; O'Donovan, Cathal B. ; Corwin, Eric I. ; Cardinaux, Frédéric ; Mason, Thomas G. ; Möbius, Matthias E. ; Scheffold, Frank

In: Physical Review E, 2015, vol. 91, no. 3, p. 032302

We model the packing structure of a marginally jammed bulk ensemble of polydisperse spheres. To this end we expand on the granocentric model [Clusel et al., Nature (London) 460, 611 (2009)], explicitly taking into account rattlers. This leads to a relationship between the characteristic parameters of the packing, such as the mean number of neighbors and the fraction of rattlers, and the radial...

Université de Fribourg

Accounting for inertia effects to access the high-frequency microrheology of viscoelastic fluids

Domínguez-García, P. ; Cardinaux, Frédéric ; Bertseva, Elena ; Forró, László ; Scheffold, Frank ; Jeney, Sylvia

In: Physical Review E, 2014, vol. 90, no. 6, p. 060301

We study the Brownian motion of microbeads immersed in water and in a viscoelastic wormlike micelles solution by optical trapping interferometry and diffusing wave spectroscopy. Through the mean-square displacement obtained from both techniques, we deduce the mechanical properties of the fluids at high frequencies by explicitly accounting for inertia effects of the particle and the surrounding...

Université de Fribourg

Static versus dynamic analysis of the influence of gravity on concentration non-equilibrium fluctuations

Croccolo, Fabrizio ; Bataller, Henri ; Scheffold, Frank

In: The European Physical Journal E, 2014, vol. 37, no. 11, p. 1–5

In a binary fluid mixture subject to gravity and a stabilizing concentration gradient, concentration non-equilibrium fluctuations are long-ranged. While the gradient leads to an enhancement of the respective equilibrium fluctuations, the effect of gravity is a damping of fluctuations larger than a “characteristic” size. This damping is visible both in the fluctuation power spectrum probed by...

Université de Fribourg

Inclined layer convection in a colloidal suspension with negative Soret coefficient at large solutal Rayleigh numbers

Italia, Matteo ; Croccolo, Fabrizio ; Scheffold, Frank ; Vailati, Alberto

In: The European Physical Journal E, 2014, vol. 37, no. 10, p. 1–6

Convection in an inclined layer of fluid is affected by the presence of a component of the acceleration of gravity perpendicular to the density gradient that drives the convective motion. In this work we investigate the solutal convection of a colloidal suspension characterized by a negative Soret coefficient. Convection is induced by heating the suspension from above, and at large solutal...

Université de Fribourg

Principles and Fundamentals of Optical Imaging

Scheffold, Frank

In: Optical Imaging of Neocortical Dynamics, 2014, vol. 85, p. 19–32

In this chapter I will give a brief general introduction to optical imaging and then discuss in more detail some of the methods specifically used for imaging cortical dynamics today. Absorption and fluorescence microscopy can be used to form direct, diffraction-limited images but standard methods are often only applicable to superficial layers of cortical tissue. Two-photon microscopy takes an...