Enhanced Triboelectric Charge Stability by Air-Stable Radicals.

This paper demonstrates that air-stable radicals enhance the stability of triboelectric charge on surfaces. While charge on surfaces is often undesirable (e.g.

ab initio study of Mn-based systems for oxidative degradation.

Organic contaminants can be removed from water/wastewater by oxidative degradation using oxidants such as manganese oxides and/or aqueous manganese ions. The Mn species show a wide range of activity, which is related to the oxidation state of Mn. Here, we use ab initio molecular dynamics simulations to address Mn oxidation states in these systems.

Microscale Bipolar Charge Distributions on Surfaces after Liquid Wetting and Evaporation in an Electric Field.

Studies have shown that when insulator surfaces become electrostatically charged, complex spatial distributions of charge are produced, which are made up of micrometer-scale regions of both charge polarities. The origin of these charge patterns, often called "charge mosaics", is not understood. Here, we carried out controlled Kelvin force microscopy experiments on microfabricated interdigitated electrode systems to show that the process of wetting a surface by a liquid followed by evaporation of the liquid in an electric field can lead to neighboring micrometer-scale regions of positive and negative charge, which remain stable long after the electric field is removed.

Relative importance of electrostatic and van der Waals forces in particle adhesion to rough conducting surfaces.

It is commonly assumed that van der Waals forces dominate adhesion in dry systems and electrostatic forces are of second order importance and can be safely neglected. This is unambiguously the case for particles interacting with flat surfaces. However, all surfaces have some degree of roughness.

Particle adhesion to rough surfaces.

While particle adhesion to smooth surfaces is well understood, real surfaces are not perfectly smooth, and the effects of surface roughness on adhesion are not easily characterized. We develop a theory for the effects of surface roughness on the strength of particle adhesion due to van der Waals forces, in the Derjaguin-Muller-Toporov (DMT)-type adhesion regime. We first address a well-defined rough surface created by embedding spheres in a smooth substrate, which had been previously examined experimentally.

Humidity transforms immobile surface charges into mobile charges during triboelectric charging.

Triboelectric charging - which children see when they rub balloons on their hair - has important consequences in many industries and natural phenomena. Despite its importance, the identity of the charge carriers that lead to triboelectric charging is uncertain. For polymers, previous X-ray photoelectron spectroscopy studies definitively show that bonds break during triboelectric charging.

Extraction of bioactive compounds from mango ( L. . ) seed kernel with ethanol-water binary solvent systems.

Mango seed kernel, a by-product of the processing industry, can be valorized as a potential source of bioactive compounds. Binary mixtures of ethanol and water, used in solid-liquid extraction (SLE), have drawn interest as an effective means of recovering phytochemicals from plant materials because these solvents can be used in food applications and their synergistic effect makes them a superior solvent over their pure counterparts. Total phenolic content (TPC) and HPLC chromatograms of each ethanolic extract revealed that ethanol concentration had a significant effect on phenolic compound recovery, wherein, TPC of mango kernel varied from 18.

The interplay of structure and dynamics at grain boundaries.

Molecular simulations are carried out to address the structure and atomic diffusion at grain boundaries. We use an inherent structure approach, which maps each configuration in a molecular dynamics trajectory to the potential energy minimum ("inherent structure") it would reach by a steepest descent quench. Dynamics are then decomposed into a combination of displacements within an inherent structure and transitions between inherent structures.

Single-stranded DNA oligomer brush structure is dominated by intramolecular interactions mediated by the ion environment.

Single-stranded DNA (ssDNA) brushes, in which ssDNA oligomers are tethered to surfaces in dense monolayers, are being investigated for potential biosensing applications. The structure of the brush can affect the selectivity and the hybridization efficiency of the device. The structure is commonly thought to result from the balance of intramolecular interactions, intermolecular interactions within the monolayer, and molecule-surface interactions.

Oxazine Ring-Related Vibrational Modes of Benzoxazine Monomers Using Fully Aromatically Substituted, Deuterated, N Isotope Exchanged, and Oxazine-Ring-Substituted Compounds and Theoretical Calculations.

Polymerization of benzoxazine resins is indicated by the disappearance of a 960-900 cm band in infrared spectroscopy (IR). Historically, this band was assigned to the C-H out-of-plane bending of the benzene to which the oxazine ring is attached. This study shows that this band is a mixture of the O-C stretching of the oxazine ring and the phenolic ring vibrational modes.

Effect of surfactant shape on solvophobicity and surface activity in alcohol-water systems.

Here we study the relationship between a surfactant's molecular shape and its tendency to partition to the interface in ethanol-water mixtures. In general, finding surfactants that are effective in alcohol-water mixtures is more challenging than finding ones that are effective in pure water. This is because the solvophobic effect that partitions surfactants from bulk solution to the interface becomes weaker as ethanol concentration increases.

Surface activity of octanoic acid in ethanol-water solutions from molecular simulation and experiment.

The surface activity of a typical surfactant, octanoic acid (OA), in ethanol-water solutions is investigated with a combined experimental and molecular simulation approach. The experiments show that OA reduces the surface tension of ethanol-water solutions at low ethanol concentration, but the effectiveness decreases with increasing ethanol concentration and vanishes for ethanol concentrations above 60%. Molecular dynamics simulations are used to obtain free energy landscapes for OA as a function of the distance from the surface.

Particle size effects in particle-particle triboelectric charging studied with an integrated fluidized bed and electrostatic separator system.

Fundamental studies of triboelectric charging of granular materials via particle-particle contact are challenging to control and interpret because of foreign material surfaces that are difficult to avoid during contacting and measurement. The measurement of particle charge itself can also induce charging, altering results. Here, we introduce a completely integrated fluidized bed and electrostatic separator system that charges particles solely by interparticle interactions and characterizes their charge on line.

Insertion of liquid crystal molecules into hydrocarbon monolayers.

Atomistic molecular dynamics simulations were carried out to investigate the molecular mechanisms of vertical surface alignment of liquid crystals. We study the insertion of nCB (4-Cyano-4'-n-biphenyl) molecules with n = 0,…,6 into a bent-core liquid crystal monolayer that was recently found to provide good vertical alignment for liquid crystals. The results suggest a complex-free energy landscape for the liquid crystal within the layer.

Atomic mobility in a polymer glass after shear and thermal cycles.

Molecular dynamics simulations and energy landscape analyses are carried out to study the atomic mobility of a polymer glass during the physical aging process that follows shear and thermal cycles. The mobility is characterized by the fraction of atoms moving more than their diameter in a given time interval. The mobility is enhanced after a shear or thermal cycle, and this enhancement decays with time.

The unpredictability of electrostatic charging.

Capricious charges: the electrostatic charging that occurs when two surfaces come into contact is familiar to everyone, and has been known for millennia. Nonetheless, the scientific understanding of the phenomenon is poor, and it is not possible to reliably predict which surface will charge positively and which will charge negatively. Recent work shows why electrostatic charging may never be predictable.

Sheared polymer glass and the question of mechanical rejuvenation.

There has been much recent debate as to whether mechanical deformation reverses the aging of a material, and returns it to a structure characteristic of the system at a higher temperature. We use molecular dynamics simulation to address this problem by carrying out shear and temperature increase simulation on atactic glassy polystyrene. Our results show explicitly that the structure (as quantified by the torsion population) changes associated with shear and temperature increase are quantitatively--and in some cases qualitatively--different.

Isotope fractionation by thermal diffusion in silicate melts.

Isotopes fractionate in thermal gradients, but there is little quantitative understanding of this effect in complex fluids. Here we present results of experiments and molecular dynamics simulations on silicate melts. We show that isotope fractionation arises from classical mechanical effects, and that a scaling relation based on Chapman-Enskog theory predicts the behavior seen in complex fluids without arbitrary fitting parameters.

Strain-induced reversal of charge transfer in contact electrification.

We have contact! Material strain can have a dominating effect on contact electrification. When a deflated (relaxed) balloon is rubbed against teflon, the teflon surface charges positively, but when the same balloon is inflated (strained), the teflon surface charges negatively. This result illustrates that material strain can control contact electrification and alter the driving force of some (yet unknown) charge-transfer species.

Transport coefficients in silicate melts from structural data via a structure-thermodynamics-dynamics relationship.

The viscosity and diffusivities of silicate melts under high-pressure, high-temperature conditions are difficult to obtain experimentally. Estimation and extrapolation of transport coefficients are further complicated by their extreme sensitivity to melt composition. Our molecular-dynamics simulations show that, over a broad range of melt composition, temperature, and pressure, the diffusivities correlate with the excess entropy; approximations to the latter can be obtained from the knowledge of the radial distribution function.

Effects of tether length on the behavior of amphiphilic bent-core molecules at water surfaces.

Alignment layers for bulk liquid crystalline phases can be created with monolayers formed by Langmuir-Schaefer techniques. Monolayer stability is a function of the propensity of the component molecule to effectively pack at a water interface; this propensity is enhanced when the molecule has an appropriate balance of hydrophilicity and hydrophobicity and the desired liquid crystalline order, as well as other structural factors. Our experiments show that molecules based on a bent-core with one hydrophilic and one hydrophobic end can form stable monolayers that act as effective alignment layers.

Obtaining charge distributions on geometrically generic nanostructures using scanning force microscopy.

We develop the self-consistent sum of dipoles (SCSD) theory for the purpose of recovering charge densities present on nanostructures using scanning force microscope (SFM) force-separation experiments. The dielectric probe is discretized into volume elements characterized by their atomic polarizabilities. Magnitudes of the induced dipole in each element are calculated based on discrete charges placed on the surfaces, dipole-dipole interactions, and dielectric and ionic properties of the surrounding medium.

Fold catastrophes and the dependence of free-energy barriers to conformational transitions on applied force.

Applied mechanical force (f) can activate conformational change in molecules by reducing the height of a free-energy barrier (DeltaG(b)). In this paper, molecular dynamics simulations are carried out with umbrella sampling and self-consistent histogram methods to determine free-energy profiles for a coarse-grained model of a protein under an applied force. Applied force is shown to cause fold catastrophes, where free-energy minima are destabilized until they disappear.

Electrification of granular systems of identical insulators.

Insulating particles can become highly electrified during powder handling, volcanic eruptions, and the wind-blown transport of dust, sand, and snow. Measurements in these granular systems have found that smaller particles generally charge negatively, while larger particles charge positively. These observations are puzzling since particles in these systems are generally chemically identical and thus have no contact potential difference.