Alexander Z. Patashinski

The Mosaic of Surface Charge in Contact Electrification

Electrification caused by rubbing two objects creates patches of positive and negative charge on both surfaces. When dielectric materials are brought into contact and then separated, they develop static electricity. For centuries, it has been assumed that such contact charging derives from the spatially homogeneous material properties (along the material’s surface) and that within a given pair of materials, one charges uniformly positively and the other negatively. We demonstrate that this picture of contact charging is incorrect. Whereas each contact-electrified piece develops a net charge of either positive or negative polarity, each surface supports a random “mosaic” of oppositely charged regions of nanoscopic dimensions. These mosaics of surface charge have the same topological characteristics for different types of electrified dielectrics and accommodate significantly more charge per unit area than previously thought.

Ultrasensitive detection of toxic cations through changes in the tunnelling current across films of striped nanoparticles

Although multiple methods have been developed to detect metal cations, only a few offer sensitivities below 1 pM, and many require complicated procedures and sophisticated equipment. Here, we describe a class of simple solid-state sensors for the ultrasensitive detection of heavy-metal cations (notably, an unprecedented attomolar limit for the detection of CH(3)Hg(+) in both standardized solutions and environmental samples) through changes in the tunnelling current across films of nanoparticles (NPs) protected with striped monolayers of organic ligands. The sensors are also highly selective because of the ligand-shell organization of the NPs. On binding of metal cations, the electronic structure of the molecular bridges between proximal NPs changes, the tunnelling current increases and highly conductive paths ultimately percolate the entire film. The nanoscale heterogeneity of the structure of the film broadens the range of the cation-binding constants, which leads to wide sensitivity ranges (remarkably, over 18 orders of magnitude in CH(3)Hg(+) concentration).

TOWARDS UNDERSTANDING THE LOCAL STRUCTURE OF LIQUIDS

In this article we discuss the problem of well-defined crystalline patterns of local atomic arrangements in equilibrium liquids, and their statistical mechanics modelling. We present arguments in favor of the existence of local crystalline structures in liquids (local crystal order hypothesis) and discuss a generalized energy landscape picture in the theory of the liquid state. This picture allows a quantification of the hypothesis of local order and offers basic concepts for the statistical mechanics modelling of the melting phase transition. We review recent results of probabilistic-based searches for local structures in various two- and three-dimensional computer-simulated liquids. Next, some statistical-mechanics models of melting and amorphization in terms of structural states of small clusters are proposed. The models, which have only two characteristic energies, that of the orientationally disordered locally crystalline state, and that of completely amorphous state, are studied in a mean-probability approximation. If the amorphization energy is high, the material retains local crystallinity even in the melt; at higher temperatures a crossover to the locally amorphous state occurs. A material that has a low energy non-crystalline local packing exhibits an amorphization melting; the phase transition is from orientationally ordered crystal state to a locally amorphous melt.

The unstable and expanding interface between reacting liquids: theoretical interpretation of negative surface tension

When a chemical reaction between two immiscible liquids creates surfactant molecules at the interface between them, the interfacial surface tension decreases with increasing amount of surfactant. In particular, an interfacial reaction that is faster than the time scale of system’s equilibration can cause a marked increase in the interfacial area due to the surface tension becoming effectively negative. Under these highly nonequilibrium conditions, the interface roughens and develops a variety of interfacial structures ranging from ‘‘ripples’’ to micelle-like formations; in systems of droplets, this process can lead to cycles of droplet elongation and self-division into smaller progenies. In the present work, the emergence and implications of negative surface tension over a ‘‘reactive’’ interface are studied theoretically and using computer simulations. The onset of interfacial instabilities can be described analytically using the methods of linear stability analysis of the continuum theory. For longer times, Molecular Dynamics simulations are implemented which reproduce the formation and increase of interfacial ‘‘ripples’’ at the initial stage, when the interface is a monolayer of surfactant, and widening of the reactive/mixing layer at later times.

Heterogeneous Structure, Heterogeneous Dynamics, and Complex Behavior in Two-Dimensional Liquids.

Analysis of the metrical and topological features of the local structure in a freezing two-dimensional Lennard-Jones system found that in a narrow strip [Formula: see text] of thermodynamic states close to the melting line, the liquid becomes a complex liquid characterized by a super-Arrhenius increase of relaxation times, stretched-exponential decay of correlations in time, and a power-law distribution of waiting times for changes in the local order. In [Formula: see text], the structure of the liquid and its dynamics are spatially heterogeneous; the sizes of ordered clusters are power-law distributed. Those features are governed by local structure evolution between solid-like and liquid-like (disordered) patterns. The liquid inside the strip [Formula: see text] gives a unique opportunity to study how heterogeneous structure, dynamics and complexity are intertwined with each other on a microscopic level.

Tomography and Static-Mechanical Properties of Adherent Cells

A tomography approach is used to reconstruct 3D cell shapes and, simultaneously, the shapes/positions of the nuclei within these cells. Subjecting the cells to well-defined microconfinements of various diameters allow for relating the steady-state shapes of cells to their static-mechanical properties. The observed shapes show striking regularities between different cell types and all fit to a model that takes into account the cell membrane, cortical actin, and the nucleus.

Inherent amorphous structures and statistical mechanics of melting

The statistical mechanics of local and global order in a condensed system is studied in a coarsened model in which the atomic arrangements in small volumes may be crystalline or amorphous. The melting behavior of the material is determined by two characteristic energies, the energy of an orientationally disordered locally crystalline state and the energy of a completely amorphous state. If the amorphization energy is high, the material retains local crystallinity even in the melt; then, at higher temperatures there is a crossover to a locally amorphous state. A material with a low-energy noncrystalline local packing exhibits amorphization melting; the phase transition is from an orientationally ordered crystal state to a fully amorphous melt. Strong interactions that are not of a two-body type are suggested to favor the first behavior, and to lead to structural liquids just above the melting point.

Modeling melting in binary systems

A coarsened model for a binary system with limited miscibility of components is proposed; the system is described in terms of structural states in small parts of the material. The material is assumed to have two alternative types of crystalline local arrangements associated with two components of the alloy. Fluctuating characteristics of a cluster are the type and the space orientation of its crystalline arrangement. There are two different phase transitions in the model system, an orientation order-disorder transition representing melting, and a phase transition between phases differing in concentration of components. Depending on the parameters characterizing the interaction in the system, this last transition may take place both in the crystalline and in the amorphous (molten) phase. A special approximation is used to study the thermodynamics of the system. The calculated phase diagram describes, at least qualitatively, the most important features of a binary system.

Fluctuations of conformational states in biological molecules: theory for anomalous spectral diffusion dynamics

Abstract An anomalous power law behavior of spectral diffusion broadening of persistent holes is found in large biological molecules dissolved in a glassy host at very low temperature. We argue that this is caused by the internal degrees of freedom of the biomolecule itself rather than by excitations of the glassy host. To explain the observed universal time dependence of the hole width w ∼ t 1/4 , we propose a stochastic model of protein dynamics close to the equilibrium, which describes this process in terms of the quasi-one-dimensional diffusion of proteins in conformation space. Assuming that each step of diffusive motion changes the electronic excitation energy randomly, we derive the observed time behavior of the spectral hole. The physical mechanisms involved are discussed.

ORIENTATION RELAXATION IN GLASSY POLYMERS. II. DIPOLE-SIZE SPECTROSCOPY AND SHORT-TIME KINETICS

The orientational diffusion of a rodlike particle embedded in a glassy polymeric matrix is considered; the underlying kinetics is that of local rearrangements. A defining parameter of the theory is the length of the particle. The timing of steps of the random walk in orientation space is determined by rearrangements. We discuss the physical properties of the glass state in connection with the rearrangement kinetics. The orientational diffusion is influenced by the local disorder; this influence is different for dipoles of different length. For a short dipole, the resulting diffusion is of generalized Debye type. Nonexponential relaxation of physical quantities may then be caused by the distribution of rearrangement barriers. For longer dipoles and if the orientation is uniquely determined by the configuration of the embedding cluster, the motion is a random walk on a given random map on a sphere. An ensemble of random mappings is considered. For even longer dipoles, hierarchical (multiscale) relaxation is...