Charge induced pattern formation on surfaces: segregation in cylindrical micelles of cationic-anionic peptide-amphiphiles

Micelles, vesicles, and films composed of two species of incompatible heterogeneous molecules exhibit full internal segregation of the component species. This macroscopic segregation can be inhibited by oppositely charging the two different molecular species. The degree of compatibility achieved by the charges leads to either fully homogenous mixtures or to local segregation and the possible formation of regular patterns. We investigate the induction of periodic surface patterns by the presence of opposite charges in flat films and cylindrical micelles. In the strong segregation limit the incompatibility between species can be described by a line tension parameter gamma. The size of the patterns formed is of the order of a characteristic size L approximately (gamma/sigma(2))(1/2), where sigma is the surface charge density. The pattern symmetry on flat surfaces is function only of the fraction of area covered by the components, f: lamellar for 0.34相似文献   

Pseudo-steady-state solutions for solidification in a wedge

Polubarinova-Kochina's analytical differential equation methodis used to determine the pseudo-steady-state solution to problemsinvolving the freezing (solidification) of wedges of liquidwhich are initially at their fusion temperature. In particular,we consider four distinct problems for wedges which are: freezingwith the same constant boundary temperature, freezing with thesame constant boundary heat fluxes, freezing with distinct constantboundary temperatures and freezing with distinct constant fluxesat the boundaries. For the last two problems, a Heun's differentialequation with an unknown singularity is derived, which in bothcases admits a particularly elegant simple solution for thespecial case when the wedge angle is . The moving boundariesobtained are shown pictorially.     

The non-equilibrium charge screening effects in diffusion-driven systems with pattern formation

The effects of non-equilibrium charge screening in mixtures of oppositely charged interacting molecules on surfaces are analyzed in a closed system. The dynamics of charge screening and the strong deviation from the standard Debye-Hückel theory are demonstrated via a new formalism based on computing radial distribution functions suited for analyzing both short-range and long-range spacial ordering effects. At long distances the inhomogeneous molecular distribution is limited by diffusion, whereas at short distances (of the order of several coordination spheres) by a balance of short-range (Lennard-Jones) and long-range (Coulomb) interactions. The non-equilibrium charge screening effects in transient pattern formation are further quantified. It is demonstrated that the use of screened potentials, in the spirit of the Debye-Hückel theory, leads to qualitatively incorrect results.     

Chemically sprayed ZnO:(F,Zr) thin films: Effect of starting solution ageing time and substrate temperature on the physical properties

Conductive and transparent fluorine and zirconium co-doped zinc oxide [ZnO:(F, Zr)] thin films have been deposited onto sodocalcic glass substrates by the chemical spray technique. The effects of starting solution ageing time and the substrate temperature on the transport, structural, and morphological properties of as-deposited ZnO:(F, Zr) thin films are presented in this paper. A decrease in the electrical resistivity values is observed as the starting solution used is more aged, reaching a minimum resistivity of the order of 1.3×10^?2 Ω cm in samples deposited from a 17-day-aged solution, then increasing in samples deposited from solutions aged beyond this ageing time. According to the characterization results, the surface morphology is dependent on the solution ageing time. The X-ray diffraction patterns reveal that the ZnO:(F, Zr) thin films are polycrystalline in nature, fitting well with a hexagonal wurtzite structure, and showing the (0 0 2) planes as preferential growth in all the deposited films. The average optical transmittance, measured in the near UV–visible region, was of the order of 75% in all the cases.     

A computer simulation study of the segregation of amphiphiles in binary immiscible matrices: short asymmetric copolymers in short homopolymers

The bulk and interfacial properties of ternary mixtures with asymmetric amphiphiles (A2B8) in A2 and B2 matrices and in A2 and B10 matrices are investigated by the dissipative particle dynamics type of molecular-dynamics simulations. The monomer concentrations of A2B8(phiA2B8) studied are below the critical micelle concentration (phiA2B8(cmc)) for the formation of micelles in the presence of an adsorbed amphiphilic monolayer at the interface. Macrophase separation from the mixed phase to the segregated state with A-rich and B-rich coexisting phases and the segregation of A2B8 at the interface are thermodynamically gradual but are accompanied by a pronounced stretching and orientation of the constituent chains. The segregation of A2B8 at the interface broadens the interfacial region and reduces the interfacial tension. The chain conformation of the asymmetric amphiphilic molecules and the interfacial properties are dominated by the majority block in the amphiphilic chain and dependent on the composition of the matrix in contact with the majority block. In the A2 and B2 matrices, the B8 blocks in A2B8 chains at the interface resemble a wet brush swollen by short B2 chains. Swelling is responsible for the pronounced stretching and orienting of the amphiphilic chains and the reduced interfacial amphiphile enrichment. At the same interfacial amphiphile excess, however, swollen amphiphiles are more efficient in reducing the interfacial tension than nonswollen amphiphiles.     

Spontaneous chirality via long-range electrostatic forces

We consider a model for periodic patterns of charges constrained over a cylindrical surface. In particular we focus on patterns of chiral helices, achiral rings, or vertical lamellae, with the constraint of global electroneutrality. We study the dependence of the patterns' size and pitch angle on the radius of the cylinder and salt concentration. We obtain a phase diagram by using numerical and analytic techniques. For pure Coulomb interactions, we find a ring phase for small radii and a chiral helical phase for large radii. At a critical salt concentration, the characteristic domain size diverges, resulting in an achiral macroscopic phase-segregated structure. We discuss possible consequences and generalizations of our model.