Table of contents

The field of complex fluids is a rapidly developing, highly interdisciplinary field that brings together people from a plethora of backgrounds such as mechanical engineering, chemical engineering, materials science, applied mathematics, physics, chemistry and biology. In this melting pot of science, the traditional boundaries of various scientific disciplines have been set aside. It is this very property of the field that has guaranteed its richness and prosperity since the final decade of the 20th century and into the 21st.

The C3 Commission of the International Union of Pure and Applied Physics (IUPAP), which is the commission for statistical physics that organizes the international STATPHYS conferences, encourages various, more focused, satellite meetings to complement the main event. For the STATPHYS22 conference in Bangalore (July 2004), Iran was recognized by the STATPHYS22 organizers as suitable to host such a satellite meeting and the Institute for Advanced Studies in Basic Sciences (IASBS) was chosen to be the site of this meeting. It was decided to organize a meeting in the field of complex fluids, which is a fairly developed field in Iran. This international meeting, and an accompanying summer school, were intended to boost international connections for both the research groups working in Iran, and several other groups working in the Middle East, South Asia and North Africa.

The meeting, entitled `Statistical Physics of Complex Fluids' was held at the Institute for Advanced Studies in Basic Sciences (IASBS) in Zanjan, Iran, from 27 June to 1 July 2004. The main topics discussed at the meeting included: biological statistical physics, wetting and microfluidics, transport in complex media, soft and granular matter, and rheology of complex fluids. At this meeting, 22 invited lectures by eminent scientists were attended by 107 participants from different countries. The poster session consisted of 45 presentations which, in addition to the main topics of the meeting, covered some of the various areas in statistical physics currently active in Iran. About half of the participants came from countries other than Iran, with a relatively broad geographic distribution.

The meeting benefited greatly from the excellent administrative assistance of the conference secretary Ms Ashraf Moosavi and the IASBS staff. We are grateful to Professor Yousef Sobouti, the Director of IASBS, and Professor Reza Mansouri, the Head of the Physical Society of Iran, for their support. We also thank the organizers of STATPHYS22, Professor Rahul Pandit and his colleagues, for their suggestions and support. The conference was supported by donations from the Center for International Research and Collaboration (ISMO) and the Institute for Research and Planning in Higher Education (IRPHE) of the Iranian Ministry of Science, Research and Technology, the Islamic Development Bank, the Abdus Salam International Centre for Theoretical Physics (ICTP), the Tehran Cluster Office of the United Nations Educational, Scientific and Cultural Organization (UNESCO), the Research and Development Directorate of the National Iranian Oil Company, the Physical Society of Iran, the Iranian Meteorological Organization, and the Zanjan City Water and Waste Water Company.

Finally, we would like to express our gratitude to Institute of Physics Publishing, and in particular to Professor Alexei Kornyshev and Dr Richard Palmer for suggesting publishing the proceedings of the meeting and carrying through the editorial processes with the utmost efficiency.

Like-charged polyelectrolytes can attract and condense into compact ordered states via counterion-mediated interactions (Gelbart et al 2000 Phys. Today 53 38–44). Recent examples include DNA toroids and F-actin bundles. We have investigated the structure and dynamics of condensed divalent ions within F-actin polyelectrolyte bundles. Using synchrotron x-ray diffraction, the structural organization of Ba²⁺ ions on F-actin has been directly observed. The Ba²⁺ ions organize into counterion charge density waves (CDWs) parallel to the actin filaments. Moreover, these 1D counterion charge density waves couple to twist deformations of the oppositely charged actin filaments, and mediate attractions by effecting a 'zipper-like' charge alignment between the counterions and the polyelectrolyte charge distribution. We have also examined condensed divalent ²⁵Mg ions within F-actin bundles using solid-state NMR. Preliminary measurements indicate that the longitudinal relaxation time T₁ of Mg²⁺ ions decreases by approximately an order of magnitude as they organize into the CDW state within condensed F-actin bundles. The measured value of T₁ for Mg²⁺ ions in the CDW is intermediate between typical liquid-like and solid-like values.

We review the problem of complex formation between a macroion and an oppositely charged polyelectrolyte on the linear level. We study the effect of changing the ionic strength and the valency of the macroion on the complexed polymer configuration. Distinguishing four different configurational symmetry classes, we are able to map out the global configurational phase diagram. We also study the interaction between two complexes of macroion and polyelectrolyte, which gives rise to configurational changes of two complexes as they approach each other. Most of these calculations are performed on the ground-state level and are thus accurate only for very stiff polymers or highly charged and therefore strongly bound systems. We also give a short outlook into the problem of thermal fluctuations around the polymer ground state configuration as it is wrapped around the macroion. This problem is tackled by a normal-mode analysis. All calculations are performed for parameters corresponding to DNA–histone complexes, which are the basic building blocks of the nucleosomal genetic structure.

We study the dynamics of an isotropic solution of polar filaments coupled by molecular motors acting as active cross-links which generate relative motion of the filaments in two and three dimensions. We investigate the mechanical properties of the homogeneous and isotropic state. We also consider the stability of the homogeneous state for constant motor concentration, taking into account excluded volume and an estimate of entanglement.

The response of a short DNA segment to bending is studied, taking into account the anisotropy in the bending rigidities. It is shown that due to the helical structure of DNA, the anisotropy in the bending rigidities can lead to the formation of kinks and modulations in the curvature, depending on the values of the elastic constants. The typical wavelength for the modulations, or the distance between the neighbouring kinks, is found to be set by half of the DNA pitch.

The microscopic mechanism for thermodiffusion or the Ludwig–Soret effect is investigated for a single charged colloidal particle. For the specific example of a charged permeable membrane, the different forces arising from the interplay between electrostatic interaction and entropy are identified and their magnitudes are calculated in both no-salt and salt-saturated limits. The competition between these forces, which is controlled by the salt density, is shown to decide the direction of motion for the colloids.

We have determined the phase behaviour of dipalmitoyl phosphatidylcholine (DPPC)-cholesterol mixtures from small angle x-ray diffraction studies of oriented multilayers. A cholesterol induced modulated phase, denoted as P_β, is obtained at intermediate cholesterol concentrations, which is distinct from the ripple () phase found in earlier studies on similar systems. We also report some confocal fluorescence microscopy observations on giant unilamellar vesicles (GUVs) made from these mixtures.

The behaviour of a hydrophobic chain near a hydrophobic wall is studied in three dimensions by numerically simulating a modified version of the decorated lattice model of Widom and co-workers. The effective hydrophobic interaction is temperature dependent. The chain stretches and adheres to the hydrophobic wall in a pancake like conformation at intermediate temperatures, whereas it collapses upon itself at slightly higher temperatures. At lower or higher dimensions it is in a random coil state.

The reduction of turbulent energy dissipation by addition of polymers is studied experimentally. We first address the question of where the action of the polymers is taking place. Subsequently, we show that there is a direct correlation of drag reduction with the elongational viscosity of the polymers. For this, the reduction of turbulent energy dissipation by addition of the biopolymer DNA is studied. These results open the way for a direct visualization study of the polymer conformation in a turbulent boundary layer.

We study the propulsion of two model swimmers at low Reynolds number. Inspired by Purcell's model, we propose a very simple one-dimensional swimmer consisting of three spheres that are connected by two arms whose lengths can change between two values. The proposed swimmer can swim with a special type of motion, which breaks the time-reversal symmetry. We also show that an ellipsoidal membrane with tangential travelling wave on it can also propel itself in the direction preferred by the travelling wave. This system resembles the realistic biological animals like Paramecium.

Patterns formed by the flow of an inhomogeneous fluid (suspension) over a smooth inclined surface were studied. It was observed that fractal patterns are formed. There exists a threshold angle for the inclination above which global fractal patterns are formed. This angle depends on the particle size of the suspension. We observed that there are two fractal dimensions for these patterns, depending on the area from which the pattern is extracted. If the pattern is taken from the top which only consists of the beginning steps of the pattern forming, one finds two fractal dimensions, i.e. 1.35–1.45 and 1.6–1.7, in which the first one is dominant. And if the entire pattern is taken, then fractal dimension 1.6–1.7 is observed. The first fractal dimension belongs to the class of flow of water over an inhomogeneous surface, and the second one corresponds to the river network. This may imply that both universality classes are present. However, here disorder is present in the fluid and is transferred to the surface. We have also determined the fractal dimension of the patterns formed below the threshold angle. Testing Horton's laws on different patterns does not lead to a conclusive result. More investigations may give a reliable conclusion.

Pushing a fluid with a less viscous one gives rise to the well known Saffman–Taylor instability. This instability is important in a wide variety of applications involving strongly non-Newtonian fluids that often exhibit a yield stress. Here we investigate the Saffmann–Taylor instability in this type of fluid, in longitudinal flows in Hele–Shaw cells. In particular, we study Darcy's law for yield stress fluids. The dispersion equation for the flow is similar to the equations obtained for ordinary viscous fluids but the viscous terms in the dimensionless numbers conditioning the instability now contain the yield stress. This also has repercussions on the wavelength of the instability as it follows from a linear stability analysis. As a consequence of the presence of yield stress, the wavelength of maximum growth is finite even at vanishing velocities. We study Darcy's law and the fingering patterns experimentally for a yield stress fluid in a linear Hele–Shaw cell. The results are in rather good agreement with the theoretical predictions. In addition we observe different regimes that lead to different morphologies of the fingering patterns, in both rectangular and circular Hele–Shaw cells.

Barchans are crescent-shaped sand dunes forming in arid regions with unidirectional wind and limited sand supply. We report analytical and numerical results for dune shapes under different environmental conditions as obtained from the so-called 'minimal model' of aeolian sand dunes. The profiles of longitudinal vertical slices (i.e. along the wind direction) are analysed as a function of wind speed and sand supply. Shape transitions can be induced by changes of mass, wind speed and sand supply. Within a minimal extension of the model to the transverse direction the scale-invariant profile of transverse vertical cuts can be derived analytically.

A laser facility based on a linear image sensor with a sampling period of 100 µs allows the investigation of the dissipative dynamics of a vibrated granular matter under gravity. The laser reveals the vertical movement of an individual zirconia–yttria stabilized 2 mm ball at the surface of a weakly excited 3D granular matter bed. The stochastic realizations are measured from the top of the container. Then, power spectra measurements reveal the different cooperative dynamics of the fluidized gap. We also carried out measurements for one steel ball and many balls in 1D and 3D systems. We fit the measured different regimes with generalized Langevin pictures. We introduce a fractional temporal operator to characterize the ensemble of dissipative particles which cannot be represented by a single Langevin particle in a complex fluid.

Embryonic development is the generation of a multicellular organism from a single cell. During this process, tissues and organs are differentiated and positioned at different parts of the organism. In this paper, we deal with the precision (embryo to embryo variations) of this process by looking at the early development of the fruit fly embryo. We show that precision of the antero-posterior gene expression pattern is hard coded at the very first stages of development. The noise reduction mechanism is extremely robust and resistant to mutations, in contrast to the actual position of the tissues themselves.

We propose an agent-based model of a single-asset financial market, described in terms of a small number of parameters, which generates price returns with statistical properties similar to the stylized facts observed in financial time series. Our agent-based model generically leads to the absence of autocorrelation in returns, self-sustaining excess volatility, mean-reverting volatility, volatility clustering and endogenous bursts of market activity non-attributable to external noise. The parsimonious structure of the model allows the identification of feedback and heterogeneity as the key mechanisms leading to these effects.

A two-parameter family of discrete-time asymmetric exclusion processes on a one-dimensional lattice is considered. Among these processes are the asymmetric simple exclusion model and the drop–push model. Using the Bethe ansatz, the exact solution to the master equation and from that the drift rates of the two-particle sector are obtained.

A general identification of the positional specific heat as the thermodynamic response function associated with the static relaxation length is proposed, and a phenomenological description for the thermal dependence of the static relaxation length in supercooled liquids is presented. Accordingly, through a phenomenological determination of the positional specific heat of supercooled liquids, we arrive at the thermal variation of the static relaxation length ξ, which is found to vary in accordance with in the quasi-equilibrium supercooled temperature regime, where T₀ is the Vogel–Fulcher temperature and exponent ν equals unity. This result to a certain degree agrees with that obtained from mean field theory of the random-first-order transition, which suggests a power law temperature variation for ξ with an apparent divergence at T₀. However, the phenomenological exponent ν = 1 is higher than the corresponding mean field estimate (becoming exact in infinite dimensions), and in perfect agreement with the relaxation length exponent as obtained from numerical simulations of the same models of structural glass in three spatial dimensions.

Liquid is represented as a complicated system of disclinations according to a defect description of liquids and glasses. The expressions for the linear disclination field of arbitrary form and the energy of the inter-disclination interaction are derived in the framework of the gauge theory of defects. This allows us to describe liquid as a disordered system of topological moments and reduce this model to the Edwards–Anderson model with long-range interaction. Within the framework of this approach, vitrification is represented as a 'hierarchical' phase transition. The suggested model allows us to explain the process of Fischer cluster formation and the slow dynamics in supercooled liquids close to the liquid–glass transition point.

The application of cumulant expansion is generalized to a ferrimagnetic system of large spins. An effective Hamiltonian is obtained in terms of classical variables for a quantum spin chain to describe the thermodynamic properties at moderate and high temperature regimes. We have considered quantum corrections up to second order in the cumulant expansion where fairly good results are observed in the convergent regime, i.e T>Js. (J is the exchange coupling between spins, s is the smaller spin (S₁,s₂) and T is temperature.) Our results show that the correlation of more than two neighbouring sites is negligible at moderate and high temperature. Thus the results of a single ferrimagnet molecule can be applied to the chain of interacting molecules for temperature greater than the classical energy scale, i.e. T>JS₁s₂.

We study the ground state properties of the t–J model on two and three leg ladders. The phase diagram of the anisotropic couplings along the rungs and legs is investigated to find the opening of a charge transfer gap which appears as a charge density plateau. In a perturbative approach we start from isolated rungs at zero leg couplings. Then the stability of the charge density plateau is examined by first order perturbation theory in terms of an effective Hamiltonian using the numerical Lanczos method on finite size ladders. The results of this approach are compared with the exact diagonalization of finite systems. We have found that an improvement of our results depends on the initial clusters for zeroth order perturbation which cannot be recovered by higher order perturbation in rung clusters. For the two leg ladder case the application of 4-site (2 × 2) plaquettes helped us to overcome earlier flaws when the charge density is greater than one-half. We have also addressed the generalization of our method to trace phase separation in this model, where a linear dependence of ground state energy versus charge density gives evidence of a phase separated state.