An order parameter equation for the dynamic yield stress in dense colloidal suspensions

We study the dynamic yield stress in dense colloidal suspensions by analyzing the time evolution of the pair distribution function for colloidal particles interacting through a Lennard-Jones potential. We find that the equilibrium pair distribution function is unstable with respect to a certain anisotropic perturbation in the regime of low temperature and high density. By applying a bifurcation analysis to a system near the critical state at which the stability changes, we derive an amplitude equation for the critical mode. This equation is analogous to order parameter equations used to describe phase transitions. It is found that this amplitude equation describes the appearance of the dynamic yield stress, and it gives a value of 2/3 for the shear thinning exponent. This value is related to the mean field value of the critical exponent $\delta$ in the Ising model.Comment: 8 pages, 2 figure

Interaction of tigliane and daphnane diterpenoid esters with protein kinase C isozymes in vitro

Different biological effects elicited by different phorbol esters in mammalian cells are thought to be due mainly to their differential interaction with, and modification of, the cellular pool of PKC isozymes. However, in most investigations only TPA, as the most potent tumor-promoting phorbol ester, has been used. Inspite of observations that other phorbol esters exert more specific biological effects in vivo, relatively few studies have been reported so far on their interaction with individual PKC isozymes in vitro or in vivo. In vivo interaction of different phorbol esters with PKC involves penetration of the phorbol ester molecule into the inner layer of the cellular membrane and binding to the regulatory domain of PKC followed by redistribution of the PKC pool from the cytosolic to the membrane bound fraction. It is thought that PKC redistribution ("translocation") is followed by an activation step and phosphorylation of substrate(s). Tigliane (TPA, PdBu, DOPP, DOPPA and Sap A) and daphnane (Thy A and Rx) diterpenoids of the phorbol ester group, were investigated for their ability to interact with purified recombinant protein kinase C (PKC) isozymes. Representative compounds of distinct biological activity were chosen in an attempt to establish a correlation between their in vivo effects and their ability to interact with individual PKC isozymes. Utilising PKC isozymes α, β1, β2, ỿ, δ, ε, and ζ purified from a baculovirus/Sf9 insect cell expression system and a phosphatidylserine / Triton X-100 mixed micellar system as an in vitro cell membrane model, binding and activation of PKC isozymes by seven different phorbol esters was studied. Binding affinity and activation potency of individual compounds were found to correlate well with high tumor promoting activity of TPA and PdBu on one side and with the non-promoting action of DOPPA and Rx. However, the non-promoters DOPP and Sap A and a second stage tumor promoter Thy A were effective agonists of PKC isozymes. To study the ability of some PEs to induce association of PKC isozymes with cellular membranes (i.e. "translocation"), a membrane fraction obtained from HL-60 cells was used, in order to approach in vivo conditions. Although the ability of the investigated PEs to induce "translocation" of PKC isozymes corresponded to their ability to induce PKC activation, the ability of micromolar Ca2+ concentrations to induce membrane association of n-PKCs, ε and δ, was not in agreement with our activation results and current theory of Ca2+ independency of the n-PKC isozymes. These results suggested that specific biological effects of different phorbol esters could not solely be explained through differences in their interaction with PKC isozymes in vitro. It is possible that an intracellular component, absent in an artificial system, is responsible for modulation of phorbol ester effects in vivo. Additionally a daphnane diterpene and a second stage tumor promoter mezerein, was isolated from previously uninvestigated Daphne blagayana, a plant indegenous to the Balkan. For the first time, detailed one and two dimensional NMR (1H, 13C, COSY and NOESY) experiments were conducted to confirm the previously determined structure of mezerein. Computer assisted molecular modelling and structure analysis enabled determination of molecular minimum free energy and interatomic distances of the pharmacophore's functional groups. These values were similar to those obtained for a highly potent tumor promoter TPA. As an activator of individual PKC isozymes in vitro, mezerein appeared to be different from TPA. Mezerein was relatively less potent (when compared with TPA) as an activator of the novel PKC isozymes δ and ε. This suggested that differences in biological activity of mezerein and TPA could be, in part, due to differences in their ability to activate the PKC isozymes δ and ε

Non-ergodic transitions in many-body Langevin systems: a method of dynamical system reduction

We study a non-ergodic transition in a many-body Langevin system. We first derive an equation for the two-point time correlation function of density fluctuations, ignoring the contributions of the third- and fourth-order cumulants. For this equation, with the average density fixed, we find that there is a critical temperature at which the qualitative nature of the trajectories around the trivial solution changes. Using a method of dynamical system reduction around the critical temperature, we simplify the equation for the time correlation function into a two-dimensional ordinary differential equation. Analyzing this differential equation, we demonstrate that a non-ergodic transition occurs at some temperature slightly higher than the critical temperature.Comment: 8 pages, 1 figure; ver.3: Calculation errors have been fixe

Introduction to Programming Using dBASE

An introduction to information technology and information systems is normally a component in business school curricula. Students are often required to write a few simple programs, mostly using programming language BASIC. However, dBASE is a good alternative to BASIC in fulfilling this function; indeed, in many respects it is a superior option. The choice of the language is based on many factors. Reasons such as dBASE\u27s excellent debugging facility, students\u27 previous (and possibly frustrating) experience with programming in BASIC, and a perception that dBASE will more likely be used after graduation than BASIC may support the substitution of BASIC with dBASE. The problem with using dBASE at this elementary level is that it offers much too extensive a set of commands and functions, which could easily overwhelm beginning students. However, a small subset of dBASE programming language, provides sufficient functionality to reach the above goals. This minimal subset should support variables, data input and output, database file creation, loading, listing, updating and sequential processing, program editing, saving, execution and debugging, and structured program constructs sequence, iteration (or repetition) and decision (or selection). This paper presents a functional subset of dBASE commands and the order of coverage which have been successfully used on many occasions. The paper can also be useful to prepare handout materials for students

NπN\pi scattering in the Roper channel

We present results from our recent lattice QCD study of $N\pi$ scattering in the positive-parity nucleon channel, where the puzzling Roper resonance $N^*(1440)$ resides in experiment. Using a variety of hadron operators, that include $qqq$-like, $N\pi$ in $p$-wave and $N\sigma$ in $s$-wave, we systematically extract the excited lattice spectrum in the nucleon channel up to 1.65 GeV. Our lattice results indicate that N$\pi$ scattering in the elastic approximation alone does not describe a low-lying Roper. Coupled channel effects between $N\pi$ and $N\pi\pi$ seem to be crucial to render a low-lying Roper in experiment, reinforcing the notion that this state could be a dynamically generated resonance. After giving a brief motivation for studying the Roper channel and the relevant technical details to this study, we will discuss the results and the conclusions based on our lattice investigation and in comparison with other lattice calculations.Comment: 8 pages, 5 figures, presented at the 35th International Symposium on Lattice Field Theory, 18-24 June 2017, Granada, Spai

Dynamics of k-core percolation in a random graph

We study the edge deletion process of random graphs near a k-core percolation point. We find that the time-dependent number of edges in the process exhibits critically divergent fluctuations. We first show theoretically that the k-core percolation point is exactly given as the saddle-node bifurcation point in a dynamical system. We then determine all the exponents for the divergence based on a universal description of fluctuations near the saddle-node bifurcation.Comment: 16 pages, 4 figure