Molecular-level relation between intra-particle glass transition temperature and stability of colloidal suspensions

In many colloidal suspensions, the dispersed colloidal particles are amorphous solids resulting from vitrification. A crucial open problem is understanding how colloidal stability is affected by the intra-particle glass transition. By dealing with the latter process from a solid-state perspective, we estabilish a proportionality relation between the intra-particle glass transition temperature, $T_{\textrm{g}},$ and the Hamaker constant, $A_\textrm{H},$ of a generic suspension of nanoparticles. It follows that $T_\textrm{g}$ can be used as a convenient parameter (alternative to $A_\textrm{H}$) for controlling the stability of colloidal systems. Within DLVO theory, we show that the novel relationship, connecting $T_\textrm{g}$ to $A_\textrm{H},$ implies the critical coagulation ionic strength (CCIS) to be a monotonically decreasing function of $T_{\textrm{g}}.$ We connect our predictions to recent experimental findings

Approximate analytical description of the nonaffine response of amorphous solids

An approximation scheme for model disordered solids is proposed that leads to the fully analytical evaluation of the elastic constants under explicit account of the inhomogeneity (nonaffinity) of the atomic displacements. The theory is in quantitative agreement with simulations for central-force systems and predicts the vanishing of the shear modulus at the isostatic point with the linear law {\mu} ~ (z - 2d), where z is the coordination number. The vanishing of rigidity at the isostatic point is shown to be a consequence of the canceling out of positive affine and negative nonaffine terms

Shear flow of non-Brownian rod-sphere mixtures near jamming

Varying particle shape provides a route to rationally design the viscosity and jamming of complex fluids. The underlying physical mechanisms, however, remain unexplored. Here we use the discrete element method, taking particle contact and hydrodynamic lubrication into account, to unveil the shear rheology of mixtures of frictionless non-Brownian spheres and rod-like (spherocylinders) particles in the dense regime of packing fraction. By increasing the aspect ratio of the rods, while keeping constant either the total (rods plus spheres) particle packing fraction and the rod-sphere mixing ratio, the viscosity $\eta$ of the mixture is observed to vary in a non-monotonical fashion. An initial decrease of viscosity with increasing aspect ratio is followed by a subsequent increase after that a minimum is reached when the rods have an aspect ratio of approximately $1.5.$ This minimum represents the absolute one, in the particular case of no spheres present in the system. A mechanistic interpretation of this discovery is provided in terms of packing and excluded-volume arguments

Density-based fractionation of soil organic matter: effects of heavy liquid and heavy fraction washing

Physical fractionation methods used in soil organic matter (SOM) research commonly include density-based procedures with heavy liquids to separate SOM pools with varying turnover rates and functions. Once separated, the heavy SOM pools are often thoroughly rinsed with water to wash off any residues of the heavy liquids. Using four soils with contrasting properties, we investigated the effects of using either sodium polytungstate (SPT) or sodium iodide (NaI), two of the most commonly used heavy liquids, on the distribution of organic carbon (C) and total nitrogen (N) in free light, intra-aggregate light, and mineral-associated heavy SOM pools isolated by a common fractionation scheme. We also determined the effects of washing the mineral-associated heavy SOM fractions on the recovery of organic C and total N after separation. Because of its smaller viscosity compared to that of NaI, SPT consistently yielded greater intra-aggregate and smaller mineral-associated soil organic C contents. We also confirm that some commercial SPT products, such as the one used here, can contaminate organo-mineral heavy pools with N during density-based fractionation procedures. We do not recommend the repeated washing of heavy fractions separated with Na-based heavy liquids, as this can mobilize SOM

Validating the regional estimates of changes in soil organic carbon by using the data from paired-sites: the case study of Mediterranean arable lands

BACKGROUND: Legacy data are unique occasions for estimating soil organic carbon (SOC) concentration changes and spatial variability, but their use showed limitations due to the sampling schemes adopted and improvements may be needed in the analysis methodologies. When SOC changes is estimated with legacy data, the use of soil samples collected in different plots (i.e., non-paired data) may lead to biased results. In the present work, N = 302 georeferenced soil samples were selected from a regional (Sicily, south of Italy) soil database. An operational sampling approach was developed to spot SOC concentration changes from 1994 to 2017 in the same plots at the 0-30 cm soil depth and tested. RESULTS: The measurements were conducted after computing the minimum number of samples needed to have a reliable estimate of SOC variation after 23 years. By applying an effect size based methodology, 30 out of 302 sites were resampled in 2017 to achieve a power of 80%, and an α = 0.05. A Wilcoxon test applied to the variation of SOC from 1994 to 2017 suggested that there was not a statistical difference in SOC concentration after 23 years (Z = - 0.556; 2-tailed asymptotic significance = 0.578). In particular, only 40% of resampled sites showed a higher SOC concentration than in 2017. CONCLUSIONS: This finding contrasts with a previous SOC concentration increase that was found in 2008 (75.8% increase when estimated as differences of 2 models built with non-paired data), when compared to 1994 observed data (Z = - 9.119; 2-tailed asymptotic significance < 0.001). This suggests that the use of legacy data to estimate SOC concentration dynamics requires soil resampling in the same locations to overcome the stochastic model errors. Further experiment is needed to identify the percentage of the sites to resample in order to align two legacy datasets in the same area

Effect of Hydrodynamic Interactions on the Lifetime of Colloidal Bonds

We use analytical theory and numerical simulation to study the role of short-range hydrodynamics (lubrication forces) in determining the lifetime of colloidal bonds. Such insight is useful in understanding many aspects of colloidal systems, such as gelation, nucleation, yielding, and rejuvenation, and as a paradigm for diffusion-controlled dissociation reactions in liquids. Our model system consists of spherical particles with an attractive square-well potential of variable width δ. We find that the predicted colloidal bond lifetimes can be substantially increased upon the inclusion of lubrication forces, to an extent that depends on the attraction range. An analytical law is derived that predicts this enhancement as a function of the well width, in quantitative agreement with simulation data. For sufficiently short-ranged attraction, lubrication forces dramatically enhance the drag on two bonded particles, leading to reduced effective diffusion coefficients and, hence, longer bond lifetimes. This effect disappears upon an increase in the width of the attractive wells beyond a length scale comparable to the particle diameter. The simulation further suggests that the role of lubrication forces becomes less important as confinement is increased, i.e., upon approaching the supersaturation limit, ϕ ≈ 0.5, where caging effects become important. Our findings complement recent studies of the role of long-range hydrodynamic interactions, contributing to a comprehensive description of the subtle link between hydrodynamics and bonding in attractive colloids.CN is supported by the Maudslay-Butler Research Fellowship at Pembroke College, Cambridge

Nonmonotonic dependence of polymer glass mechanical response on chain bending stiffness

We investigate the mechanical properties of amorphous polymers by means of coarse-grained simulations and nona ne lattice dynamics theory. A small increase of polymer chain bending sti ness leads rst to softening of the material, while hardening happens only upon further strengthening of the backbones. This nonmonotonic variation of the storage modulus G0 with bending sti ness is caused by a competition between additional resistance to deformation o ered by sti er backbones and decreased density of the material due to a necessary decrease in monomer-monomer coordination. This counter-intuitive nding suggests that the strength of polymer glasses may in some circumstances be enhanced by softening the bending of constituent chains.CN acknowledges the Maudslay-Butler Research Fellowship at Pembroke College, Cambridge for financial support; VVP and AZ acknowledge financial support from the US Army Research Laboratory under grant nr. W911NF 16-2-0091

Chain-assisted charge transport in semicrystalline conjugated polymers

Charge-carrier transport in a paradigmatic semicrystalline polymer semiconductor (P3HT) is important for both fundamental understanding and applications. In samples with enhanced structural disorder due to ad-hoc point defects, the mobility displays rich behavior as a function of electric field (F) and temperature (T). At low T, the mobility increases with the applied field, but upon further increasing T, the field-dependence becomes shallower. Eventually, at the highest T considered, the slope changes sign and the mobility then decreases with the field. This phenomenon can be interpreted with our model as a result of the competition between intrachain conductive-like transport (which slows on increasing F) and interchain activated transport (which is faster at higher F). The former is controlling at high T where interchain hops are strictly limited to nearest-neighbor monomers on adjacent chains. At low T, instead, interchain hops to distant sites are allowed and control the positive correlation of the mobility with the field.This study was supported by the Winton Programme for the Physics of Sustainability (B.O.C.).This is the author accepted manuscript. The final version is available from ACS at http://dx.doi.org/10.1021/acs.jpcc.6b04714