Raft Instability of Biopolymer Gels

Following recent X-ray diffraction experiments by Wong, Li, and Safinya on biopolymer gels, we apply Onsager excluded volume theory to a nematic mixture of rigid rods and strong ``$\pi/2$'' cross-linkers obtaining a long-ranged, highly anisotropic depletion attraction between the linkers. This attraction leads to breakdown of the percolation theory for this class of gels, to breakdown of Onsager's second-order virial method, and to formation of heterogeneities in the form of raft-like ribbons.Comment: 5 pages, 4 figure

Nomadic firms in a globalizing economy: A comparative study

The location patterns of modern firms appear to exhibit floating patternswith a tendency towards footlooseness. The spatial-economic dynamics - sometimes across teh border - of firms is encapsulated in the term 'nomadic firms'. This paper adresses the issue of nomadic behaviour of firms against the background of globalisation trends. After a critical discussion of globalisation phenomena and a review of the literature on nomadic entrepreneurial behaviour, the paper sets out t formaulate a series of relevant hypotheses of spatial relocation behaviour of international firms in a globalizing network economy. The analytical framework is tested by means of interviews among actual or potential nomadic firms, in both the Netherlands and abroad. Infrastructure quality and geographical accessibility appear to play an important role, but also opportunity seeking behaviour has a prominent place in nomadic behaviour. The comparitive study among the various firms located in various countries, which aims to identify the critical relocation factors, is based on principles of modern meta-analysis.

Effective adhesion strength of specifically bound vesicles

A theoretical approach has been undertaken in order to model the thermodynamic equilibrium of a vesicle adhering to a flat substrate. The vesicle is treated in a canonical description with a fixed number of sites. A finite number of these sites are occupied by mobile ligands that are capable of interacting with a discrete number of receptors immobilized on the substrate. Explicit consideration of the bending energy of the vesicle shape has shown that the problem of the vesicle shape can be decoupled from the determination of the optimum allocation of ligands over the vesicle. The allocation of bound and free ligands in the vesicle could be determined as a function of the size of the contact zone, the ligand-receptor binding strength and the concentration of the system constituents. Several approximate solutions for different regions of system parameters are determined and in particular, the distinction between receptor-dominated equilibria and ligand-dominated equilibria is found to be important. The crossover between these two types of solutions is found to occur at a critical size of the contact zone. The presented approach enables the calculation of the effective adhesion strength of the vesicle and thus permits meaningful comparisons with relevant experiments as well as connecting the presented model with the proven success of the continuum approach for modeling the shapes of adhering vesicles. The behavior of the effective adhesion strength is analyzed in detail and several approximate expressions for it are given.Comment: 19 pages, 6 figures. To appear in Phys. Rev.

Adhesion-induced phase separation of multiple species of membrane junctions

A theory is presented for the membrane junction separation induced by the adhesion between two biomimetic membranes that contain two different types of anchored junctions (receptor/ligand complexes). The analysis shows that several mechanisms contribute to the membrane junction separation. These mechanisms include (i) the height difference between type-1 and type-2 junctions is the main factor which drives the junction separation, (ii) when type-1 and type-2 junctions have different rigidities against stretch and compression, the ``softer'' junctions are the ``favored'' species, and the aggregation of the softer junction can occur, (iii) the elasticity of the membranes mediates a non-local interaction between the junctions, (iv) the thermally activated shape fluctuations of the membranes also contribute to the junction separation by inducing another non-local interaction between the junctions and renormalizing the binding energy of the junctions. The combined effect of these mechanisms is that when junction separation occurs, the system separates into two domains with different relative and total junction densities.Comment: 23 pages, 6 figure

Phase diagram of the random field Ising model on the Bethe lattice

The phase diagram of the random field Ising model on the Bethe lattice with a symmetric dichotomous random field is closely investigated with respect to the transition between the ferromagnetic and paramagnetic regime. Refining arguments of Bleher, Ruiz and Zagrebnov [J. Stat. Phys. 93, 33 (1998)] an exact upper bound for the existence of a unique paramagnetic phase is found which considerably improves the earlier results. Several numerical estimates of transition lines between a ferromagnetic and a paramagnetic regime are presented. The obtained results do not coincide with a lower bound for the onset of ferromagnetism proposed by Bruinsma [Phys. Rev. B 30, 289 (1984)]. If the latter one proves correct this would hint to a region of coexistence of stable ferromagnetic phases and a stable paramagnetic phase.Comment: Article has been condensed and reorganized; Figs 3,5,6 merged; Fig 4 omitted; Some discussion added at end of Sec. III; 9 pages, 5 figs, RevTeX4, AMSTe