Luis F. Rojas-Ochoa

Study of translational and rotational dynamics of birefringent colloidal particles by depolarized light scattering in the far- and near-field regimes.

We characterize the translational and rotational dynamics of birefringent spherical colloidal particles by depolarized light scattering in the far- and near-field regimes. For this purpose, we use depolarized dynamic light scattering and propose an extension of dynamic heterodyne near-field light scattering that takes into account the polarization state of the light. Such a combination of methods allows to access colloidal dynamics in an extended q-range and permits to evaluate different modes of particles motion in suspension. Furthermore, we obtain a good agreement between results from the far- and near-field approaches thus validating our proposal and opening the possibility to investigate simultaneously the subtle interplay between translational and rotational motions of anisotropic colloidal particles in length-scales from the order of the particle size to several interparticle distances.

Time-resolved study of optical properties and microscopic dynamics during the drying of TiO 2 films by spectral diffusing wave spectroscopy

We present a combined experimental, theoretical, and numerical study of photon transport and microscopic dynamics in rigid and drying turbid thin films. Our setup is based in multispeckle diffusing wave spectroscopy and is adapted for frequency sweep of the illuminating source. We apply our approach to simultaneously monitor the changes in optical properties and microscopic dynamics of turbid thin films of rutile TiO2 powder dispersed in ethanol during the full drying process. Accordingly, we introduce an extension of the photon diffusion model for spectral speckle intensity correlations to account for system microscopic dynamics. We find that our results are well described by the model, where parameters required as the time-dependent sample thickness and transport mean free path are obtained from experiments. Furthermore, our findings are validated by numerical simulations of speckle dynamics based on the copula scheme. We consider that our scheme could be useful in time-resolved physical characterization of time-evolving turbid thin systems.

Characterization of slow dynamics in turbid colloidal systems by a cross-correlation scheme based on echo dynamic light scattering.

We describe the implementation of echo dynamic light scattering in a cross-correlation detection scheme, which enables the study of slow dynamics in moderately turbid colloidal systems by adapting a commercial light scattering device. Our setup combines a 3D cross-correlation detection scheme (3DDLS), which allows for suppression of multiple scattering, with the speckle echo technique for dynamic light scattering. The recorded cross-correlation echoes provide precise ensemble-averaged results that appropriately describe sample dynamics of ergodic and non-ergodic colloidal systems of different turbidities. Additionally, the high mechanical stability achieved in our setup makes possible an absolute estimation of the scattering intensity correlation function (ICF) directly from the height of echoes, thus making unnecessary any correction for imperfect rotation of the sample or of any ad hoc assumption regarding the correspondence between the absolute values of echo height and ICF. Furthermore, we find that zeroth-order echo height represents the coherence factor of the 3DDLS experiment.

Investigating the time dynamics of photon sequences scattered by tracer particles immersed in a polymeric gel

By dynamic light scattering, we study the effects of spatial confinement of tracer colloidal particles immersed in polyacrylamide networks on the time dynamics of sequences of elapsed times of scattered photons. We vary the degree of particles spatial confinement by changing the concentration of crosslinker in solution, from 0 to 2% in monomer mass fraction, which allows us to change the elasticity of the polymer gel. By calculating the Allan factor of the point processes of scattered photons arrival times, we are able to characterize the statistical properties of the scattering process. For all the samples we find that: i) the statistics of the photon arrival times is not Poissonian but clusterized in a certain counting time interval; ii) the time dynamics of the photon sequences is fractal in an ample range of counting times, from ~10−3 to ~10−1 s, with scaling exponents ranging between ~1.7 and ~1.0; iii) the crosslinking density of the polymer network, and thus, the system elasticity is correlated with the value of the fractal exponent. The value of the fractal exponent decreases as the system elasticity, or tracers spatial confinement, increases.

Effect of ionic strength on the aggregation kinetics of the amidated amyloid beta peptide Aβ (1-40) in aqueous solutions

In this work we study the effect of solution ionic strength on the structural evolution of amidated amyloid beta peptide Aβ (1-40) oligomers at the early stages of fibril formation. By light scattering, we follow the time evolution of the structure and short-time dynamics of peptide structures at low ionic strengths. Our results allow identifying initial oligomer structures as the effective building blocks in the amyloid fibrils formation and indicate that the oligomers growth pathway, from compact structures to flexible chain-like structures, becomes faster as the solution ionic strength is increased. Furthermore, we find no evidence of structural branching what suggests that elongation of amyloid fibrils is dominated by linear association. To describe our results we adapt a phenomenological model based on population balance equations and linear polymer growth, where the parameters required are obtained from the experiments. Model calculations are in good agreement with experimentally-obtained estimates for the radius of gyration of Aβ (1-40) oligomers, thus further supporting our findings. Additionally, we introduce a model for the effective interaction among initial Aβ structures that captures the dependence of the effective association rates on solution ionic strength.