Rotational and translational microrheology from shape-anisotropic particles.

In this work, we report measurements of the mean squared angular and translational displacements of a colloidal dumbbell immersed in a viscoelastic fluid using digital microscopy. From the mean squared displacements, we obtain the mechanical properties of the media. Both angular and translational motions provide the same viscoelastic complex modulus and agree with that obtained from the translational motion of a spherical probe particle.

Massive replica exchange Monte Carlo algorithm: a tool to access high pressure thermodynamics of hard systems.

We have explored the idea of producing the equilibrium equation of state, i.e. the pressure as a function of packing fraction, ßP(φ), of a confined system up to very high pressures to yield the configuration that leads to the maximum packing fraction. For this purpose we have massively implemented the replica exchange Monte Carlo algorithm in graphics processing units (GPUs), in such a way that each GPU core handles a single simulation cell. This yields a very easy scheme to implement parallelization for a very large amount of replicas (thousands), which densely sample configuration space. We have tested this idea with a very well studied system, i.e. discs confined in a circular cavity, for a number of particles N ≤ 125. In all cases, our outcomes for configurations having maximum packing fractions are in perfect agreement with those already reported and conjectured optimal in the literature, for which there is no formal mathematical proof, strongly suggesting that they are indeed optimal configurations. Furthermore, in most cases, we have obtained the same function ßP(φ), by compressing loose random configurations and by decompressing copies of the configuration having the largest packing fraction. This reveals numerically that the so obtained maximum packing configurations are the correct answer.

Exposure to silver nanoparticles produces oxidative stress and affects macromolecular and metabolic biomarkers in the goodeid fish Chapalichthys pardalis.

Silver nanoparticles (AgNPs) are the most commercialized nanomaterial worldwide, mainly due to their microbicidal activity. Although, AgNPs have been shown to be toxic to aquatic species, their effect on endemic fish, like Goodeidae, has not been demonstrated. Endemic species are under strong pressures by anthropogenic contamination and destruction of their habitat; therefore, we studied adult Chapalichthys pardalis, an endemic fish of Mexico. We evaluated the toxic effect of AgNPs through oxidative stress, macromolecular and metabolic biomarkers. We determined the LC50 (96h) and performed subchronic tests (21days) using sublethal AgNPs concentrations (equivalent to CL1 and CL10). At the end of the bioassay, we quantified 10 stress biomarkers in the liver, gills, and muscle, including the antioxidant enzymes (superoxide dismutase [SOD], catalase [CAT], and glutathione [GPx]), thiobarbituric acid reactive species (TBARS), protein oxidation (CO), macromolecules (proteins, lipids, and carbohydrates), and metabolites (glucose and lactate). In addition, we determined the integrated biomarkers response (IBR). LC50 was of 10.32mgL-1. Results of subchronic exposure (21days) revealed that AgNPs produce oxidative stress in C. pardalis adults, as evidenced by a diminution in antioxidant enzymes activity and an increase in TBARS and oxidized proteins. AgNPs also diminished levels of macromolecules and generated a high-energy consumption, reflected in the reduction of glucose levels, although lactate levels were not altered. The IBR analysis evidenced that the largest effect was produced in organisms exposed to LC10, being the liver and gills the organs with the greatest damage. Results demonstrated that exposure to AgNPs induces acute and chronic toxic effects on C. pardalis and forewarns about the impact that these nanomaterials can exert on these ecologically relevant aquatic organisms.

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.

Simultaneous characterization of rotational and translational diffusion of optically anisotropic particles by optical microscopy.

We probe the roto-translational Brownian motion of optically anisotropic particles suspended in water with a simple and straightforward optical microscopy experiment that does not require positional or rotational particle tracking. We acquire a movie of the suspension placed between two polarizing elements and we extract the translational diffusion coefficient D T and the rotational diffusion coefficient D R from the analysis of the temporal correlation properties of the spatial Fourier modes of the intensity fluctuations in the movie. Our method is successfully tested with a dilute suspension of birefringent spherical colloidal particles obtained by polymerizing an emulsion of droplets of liquid crystal in a nematic phase, whose roto-translational dynamics is found to be well described by theory. The simplicity of our approach makes our method a viable alternative to particle tracking and depolarized dynamic light scattering.

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.

Confinement-induced fluid-gel transition in polymeric solutions.

The viscoelastic properties of confined polymer solutions are probed by particle-tracking microrheology. The mean squared displacement of spherical probe particles embedded in the solution and the storage and loss moduli of the system are measured as the level of confinement is increased. It is found that those quantities change continuously as the confinement increases, and, at severe conditions, when the constrain reaches the size of the polymer molecule, the system undergoes a transition from a viscoelastic fluid to a gel.

Microrheology of viscoelastic fluids containing light-scattering inclusions.

The microrheology of viscoelastic fluids containing light-scattering inclusions is measured by depolarized dynamic light scattering (DDLS) from optically anisotropic spherical colloidal probes. The anisotropy of the probes allows us to measure both their translational and the rotational mean squared displacements simultaneously, and DDLS allows us to suppress the light scattered from the inclusions. The storage and loss moduli are determined from both mean squared displacements and the results compared with mechanical measurements.