arXiv subjects

Christof M. Aegerter

Publications and source records attributed to Christof M. Aegerter.

11 recordsLinked to original sources

arXiv2025

An integrated vertex model of the mesoderm invagination during the embryonic development of Drosophila

The mesoderm invagination of the Drosophila embryo is known as an archetypal morphogenic process. To explore the roles of the active cellular forces and the regulation of these forces, we developed an integrated vertex model that combines the regulation of morphogen expression with cell movements and tissue mechanics. Our results suggest that a successful furrow formation requires an apical tension gradient, decreased basal tension, and increased lateral tension, which corresponds to apical constriction, basal expansion, and apicobasal shortening respectively. Our model also considers the mechanical feedback which leads to an ectopic twist expression with external compression as observed in experiments. Our model predicts that ectopic invagination could happen if an external compressive gradient is applied.

q-bio.TO↗

arXiv2021

How shape and flapping rate affect the distribution of fluid forces on flexible hydrofoils

We address the fluid-structure interaction of flexible fin models oscillated in a water flow. Here, we investigate in particular the dependence of hydrodynamic force distributions on fin geometry and flapping frequency. For this purpose, we employ state-of-the-art techniques in pressure evaluation to describe fluid force maps with high temporal and spatial resolution on the deforming surfaces of the hydrofoils. Particle tracking velocimetry (PTV) is used to measure the 3D fluid velocity field, and the hydrodynamic stress tensor is subsequently calculated based on the Navier-Stokes equation. The shape and kinematics of the fin-like foils are linked to their ability to generate propulsive thrust efficiently, as well as the accumulation of external contact forces and the resulting internal tension throughout a flapping cycle.

physics.flu-dyn↗

arXiv2019

Hydrodynamic stress maps on the surface of a flexible fin-like foil

We determine the time dependence of pressure and shear stress distributions on the surface of a pitching and deforming hydrofoil from measurements of the three dimensional flow field. Period-averaged stress maps are obtained both in the presence and absence of steady flow around the foil. The velocity vector field is determined via volumetric three-component particle tracking velocimetry and subsequently inserted into the Navier-Stokes equation to calculate the total hydrodynamic stress tensor. In addition, we also present a careful error analysis of such measurements, showing that local evaluations of stress distributions are possible. The flapping foil used in the experiments is designed to allow comparison with a small trapezoidal fish fin, in terms of the scaling laws that govern the oscillatory flow regime. Unsteady Euler-Bernoulli beam theory is employed to derive instantaneous transversal force distributions on the deflecting hydrofoil from its deflection and thereby validate the spatial distributions of hydrodynamic stresses obtained from the fluid velocity field. The consistency of the force time-dependence is verified using a control volume analysis.

cond-mat.soft↗

arXiv2019

Magnetic field effects on one-dimensional Anderson localization of light

Transport of coherent waves in multiple-scattering media may exhibit fundamental, non intuitive phenomena such as halt of diffusion by disorder called Anderson localization. For electromagnetic waves, this phenomenon was observed only in one and two dimensions so far. However, none of these experiments studied the contribution of reciprocal paths nor their manipulation by external fields. In order to weaken the effect of reciprocity of coherent wave transport on Anderson localization in one dimension (1D), we studied light propagation through stacks of parallel Faraday-active glass slides exposed to magnetic fields up to 18 Tesla. Measurements of light transmission statistics are presented and compared to 1D transfer-matrix simulations. The latter reveals a self-organization of the polarization states in this system leading to a saturation of the Faraday rotation-induced reciprocity breaking, an increase of the localization length, and a decrease of transmission fluctuations when reciprocity is broken. This is confirmed experimentally for samples containing small numbers of slides while for larger samples a crossover from a 1D to a quasi-1D transport regime is found.

physics.optics↗

arXiv2019

Tunable high-index photonic glasses

Materials with extreme photonic properties such as maximum diffuse reflectance, high albedo, or tunable band gaps are essential in many current and future photonic devices and coatings. While photonic crystals, periodic anisotropic structures, are well established, their disordered counterparts, photonic glasses (PGs), are less understood despite their most interesting isotropic photonic properties. Here, we introduce a controlled high index model PG system. It is made of monodisperse spherical TiO$_2$ colloids to exploit strongly resonant Mie scattering for optimal turbidity. We report spectrally resolved combined measurements of turbidity and light energy velocity from large monolithic crack-free samples. This material class reveals pronounced resonances enabled by the possibility to tune both the refractive index of the extremely low polydisperse constituents and their radius. All our results are rationalized by a model based on the energy coherent potential approximation, which is free of any fitting parameter. Surprisingly good quantitative agreement is found even at high index and elevated packing fraction. This class of PGs may be the key to optimized tunable photonic materials and also central to understand fundamental questions such as isotropic structural colors, random lasing or strong light localization in 3D.

physics.optics↗

arXiv2017

Resonant transport and near-field effects in photonic glasses

A fundamental quantity in multiple scattering is the transport mean free path the inverse of which describes the scattering strength of a sample. In this paper, we emphasize the importance of an appropriate description of the effective refractive index $n_{\mathrm{eff}}$ in multiple light scattering to accurately describe the light transport in dense photonic glasses. Using $n_{\mathrm{eff}}$ as calculated by the energy-density coherent potential approximation we are able to predict the transport mean free path of monodisperse photonic glasses. This model without any fit parameter is in qualitative agreement with numerical simulations and in fair quantitative agreement with spectrally resolved coherent backscattering measurements on new specially synthesized polystyrene photonic glasses. These materials exhibit resonant light scattering perturbed by strong near-field coupling, all captured within the model. Our model might be used to maximize the scattering strength of high index photonic glasses, which are a key in the search for Anderson localization of light in three dimensions.

cond-mat.dis-nn↗

arXiv2016

Can 3D light localization be reached in "white paint"?

When waves scatter multiple times in 3D random media, a disorder driven phase transition from diffusion to localization may occur [Anderson 1958 Phys. Rev. 109 1492-505; Abrahams et al 1979 Phys. Rev. Lett. 42 673-6]. In "The question of classical localization: A theory of white paint?" P.W. Anderson suggested the possibility to observe light localization in TiO$_2$ samples [Anderson 1985 Phil. Mag. B 52 505-9]. We recently claimed the observation of localization effects measuring photon time of flight (ToF) distributions [Störzer et al 2006 Phys. Rev. Lett. 96 063904] and evaluating transmission profiles (TP) [Sperling et al 2013 Nat. Photonics 7 48-52] in such TiO$_2$ samples. Here we present a careful study of the long time tail of ToF distributions and the long time behavior of the TP width for very thin samples and different turbidities that questions the localization interpretation. We further show new data that allow an alternative consistent explanation of these previous data by a fluorescence process. An adapted diffusion model including an appropriate exponential fluorescence decay accounts for the shape of the ToF distributions and the TP width. These observations question whether the strong localization regime can be reached with visible light scattering in polydisperse TiO$_2$ samples, since the disorder parameter can hardly be increased any further in such a "white paint" material.

cond-mat.dis-nn↗

arXiv2014

Inelastic scattering puts in question recent claims of Anderson localization of light

The interplay between nonlinear effects and Anderson localization in disordered optical fibres1 has recently attracted great interest, and it is important in the action of random lasers in which closed multiple scattering loops have enhanced intensity. As optical nonlinearities in TiO2 can give valuable information on the nature of light transport in strongly scattering powders, we studied these effects in an extended experimental and theoretical investigation (to be published). Now, Scheffold and Wiersma have ...

cond-mat.dis-nn↗

arXiv2005

Observation of the critical regime near Anderson localization of light

Diffusive transport is among the most common phenomena in nature [1]. However, as predicted by Anderson [2], diffusion may break down due to interference. This transition from diffusive transport to localization of waves should occur for any type of classical or quantum wave in any media as long as the wavelength becomes comparable to the transport mean free path $\ell^*$ [3]. The signatures of localization and those of absorption, or bound states, can however be similar, such that an unequivocal proof of the existence of wave localization in disordered bulk materials is still lacking. Here we present measurements of time resolved non-classical diffusion of visible light in strongly scattering samples, which cannot be explained by absorption, sample geometry or reduction in transport velocity. Deviations from classical diffusion increase strongly with decreasing $\ell^*$ as expected for a phase transition. This constitutes an experimental realization of the critical regime in the approach to Anderson localization.

cond-mat.dis-nn↗

arXiv2005

Avalanches and Self-Organized Criticality in Superconductors

We review the use of superconductors as a playground for the experimental study of front roughening and avalanches. Using the magneto-optical technique, the spatial distribution of the vortex density in the sample is monitored as a function of time. The roughness and growth exponents corresponding to the vortex landscape are determined and compared to the exponents that characterize the avalanches in the framework of Self-Organized Criticality. For those situations where a thermo-magnetic instability arises, an analytical non-linear and non-local model is discussed, which is found to be consistent to great detail with the experimental results. On anisotropic substrates, the anisotropy regularizes the avalanches.

cond-mat.stat-mech↗

arXiv2004

Model for the robust establishment of precise proportions in the early Drosophila embryo

During embryonic development, a spatial pattern is formed in which proportions are established precisely. As an early pattern formation step in Drosophila embryos, an anterior-posterior gradient of Bicoid (Bcd) induces hunchback (hb) expression (Driever et al. 1989; Tautz et al. 1988). In contrast to the Bcd gradient, the Hb profile includes information about the scale of the embryo. Furthermore, the resulting hb expression pattern shows a much lower embryo-to-embryo variability than the Bcd gradient (Houchmandzadeh et al. 2002). An additional graded posterior repressing activity could theoretically account for the observed scaling. However, we show that such a model cannot produce the observed precision in the Hb boundary, such that a fundamentally different mechanism must be at work. We describe and simulate a model that can account for the observed precise generation of the scaled Hb profile in a highly robust manner. The proposed mechanism includes Staufen (Stau), an RNA binding protein that appears essential to precision scaling (Houchmandzadeh et al. 2002). In the model, Stau is released from both ends of the embryo and relocalises hb RNA by increasing its mobility. This leads to an effective transport of hb away from the respective Stau sources. The balance between these opposing effects then gives rise to scaling and precision. Considering the biological importance of robust precision scaling and the simplicity of the model, the same principle may be employed more often during development.

q-bio.CB↗