Longterm Influence of Inertia on the Diffusion of a Brownian Particle

We demonstrate experimentally that a Brownian particle is subject to inertial effects at long time scales. By using a blinking optical tweezers, we extend the range of previous experiments by several orders of magnitude up to a few seconds. The measured mean square displacement of a freely diffusing Brownian particle in a liquid shows a deviation from the Einstein-Smoluchowsky theory that diverges with time. These results are consistent with a generalized theory that takes into account not only the particle inertia but also the inertia of the fluid surrounding the particle. This can lead to a bias in the estimation of the diffusion coefficient from finite-time measurements. We show that the decay of the relative error is polynomial and not exponential and, therefore, can have significant effects at time scales relevant for experiments.Comment: 5 pages, 4 figure

Dietary interventions to contrast the onset and progression of diabetic nephropathy. a critical survey of new data

This article is a critical overview of recent contributions on the dietary corrections and the foods that have been claimed to delay or hinder the onset of diabetic nephropathy (DN) and its progression to end-stage renal disease. Innovative dietary and behavioral approaches to the prevention and therapy of DN appear the more captivating in consideration of the rather well established protocols for glucose and blood pressure control in use. In addition to restricted caloric intake to contrast obesity and the metabolic syndrome, adjustments in the patient's macronutrients intake, and in particular some degree of reduction in protein, have been long considered in the prevention of DN progression. More recently, the focus has shifted to the source of proteins and the content of glycotoxins in the diet as well as to the role of specific micronutrients. Few clinical trials have specifically addressed the role of those micronutrients associated with diet proteins that show the most protective effect against DN. Research on clinical outcome and mechanisms of action of such micronutrients appears the most promising in order to develop both effective intervention on nutritional education of the patient and selection of functional foods capable of contrasting the onset and progression of DN

Simultaneous measurements of electrophoretic and dielectrophoretic forces using optical tweezers

Herein, charged microbeads handled with optical tweezers are used as a sensitive probe for simultaneous measurements of electrophoretic and dielectrophoretic forces. We first determine the electric charge carried by a single bead by keeping it in a predictable uniform electric field produced by two parallel planar electrodes, then, we examine same bead's response in proximity to a tip electrode. In this case, besides electric forces, the bead simultaneously experiences non-negligible dielectrophoretic forces produced by the strong electric field gradient. The stochastic and deterministic motions of the trapped bead are theoretically and experimentally analysed in terms of the autocorrelation function. By fitting the experimental data, we are able to extract simultaneously the spatial distribution of electrophoretic and dielectrophoretic forces around the tip. Our approach can be used for determining actual, total force components in the presence of high-curvature electrodes or metal scanning probe tips.Comment: 9 pages, 3 figure

Cage Size and Jump Precursors in Glass-Forming Liquids: Experiment and Simulations

Glassy dynamics is intermittent, as particles suddenly jump out of the cage formed by their neighbours, and heterogeneous, as these jumps are not uniformly distributed across the system. Relating these features of the dynamics to the diverse local environments explored by the particles is essential to rationalize the relaxation process. Here we investigate this issue characterizing the local environment of a particle with the amplitude of its short time vibrational motion, as determined by segmenting in cages and jumps the particle trajectories. Both simulations of supercooled liquids and experiments on colloidal suspensions show that particles in large cages are likely to jump after a small time-lag, and that, on average, the cage enlarges shortly before the particle jumps. At large time-lags, the cage has essentially a constant value, which is smaller for longer-lasting cages. Finally, we clarify how this coupling between cage size and duration controls the average behaviour and opens the way to a better understanding of the relaxation process in glass--forming liquids.Comment: Letter, 4 figure

Surface charge and hydrodynamic coefficient measurements of {\it Bacillus subtilis} spore by Optical Tweezers

In this work we report on the simultaneous measurement of the hydrodynamic coefficient and the electric charge of single {\it Bacillus subtilis} spores. The latter has great importance in protein binding to spores and in the adhesion of spores onto surfaces. The charge and the hydrodynamic coefficient were measured by an accurate procedure based on the analysis of the motion of single spores confined by an optical trap. The technique has been validated using charged spherical polystyrene beads. The excellent agreement of our results with the expected values demonstrates the quality of our procedure. We measured the charge of spores of {\it B. subtilis} purified from a wild type strain and from two isogenic mutants characterized by an altered spore surface. Our technique is able to discriminate the three spore types used, by their charge and by their hydrodynamic coefficient which is related to the hydrophobic properties of the spore surface.Comment: 21 pages 5 figure

Differential Variance Analysis: A direct method to quantify and visualize dynamic heterogeneities

Many amorphous materials show spatially heterogenous dynamics, as different regions of the same system relax at different rates. Such a signature, known as Dynamic Heterogeneity, has been crucial to understand the nature of the jamming transition in simple model systems and is currently considered very promising to characterize more complex fluids of industrial and biological relevance. Unfortunately, measurements of dynamic heterogeneities typically require sophisticated experimental set-ups and are performed by few specialized groups. It is now possible to quantitatively characterize the relaxation process and the emergence of dynamic heterogeneities using a straightforward method, here validated on video microscopy data of hard-sphere colloidal glasses. We call this method Differential Variance Analysis (DVA), since it focuses on the variance of the differential frames, obtained subtracting images at different time-lags. Moreover, direct visualization of dynamic heterogeneities naturally appears in the differential frames, when the time-lag is set to the one corresponding to the maximum dynamic susceptibility. This approach opens the way to effectively characterize and tailor a wide variety of soft materials, from complex formulated products to biological tissues