'Gas cushion' model and hydrodynamic boundary conditions for superhydrophobic textures

Superhydrophobic Cassie textures with trapped gas bubbles reduce drag, by generating large effective slip, which is important for a variety of applications that involve a manipulation of liquids at the small scale. Here we discuss how the dissipation in the gas phase of textures modifies their friction properties. We propose an operator method, which allows us the mapping of the flow in the gas subphase to a local slip boundary condition at the liquid/gas interface. The determined uniquely local slip length depends on the viscosity contrast and underlying topography, and can be immediately used to evaluate an effective slip of the texture. Besides superlubricating Cassie surfaces our approach is valid for rough surfaces impregnated by a low-viscosity 'lubricant', and even for Wenzel textures, where a liquid follows the surface relief. These results provide a framework for the rational design of textured surfaces for numerous applications.Comment: 8 pages, 6 figure

Drag force on a sphere moving towards an anisotropic super-hydrophobic plane

We analyze theoretically a high-speed drainage of liquid films squeezed between a hydrophilic sphere and a textured super-hydrophobic plane, that contains trapped gas bubbles. A super-hydrophobic wall is characterized by parameters $L$ (texture characteristic length), $b_1$ and $b_2$ (local slip lengths at solid and gas areas), and $\phi_1$ and $\phi_2$ (fractions of solid and gas areas). Hydrodynamic properties of the plane are fully expressed in terms of the effective slip-length tensor with eigenvalues that depend on texture parameters and $H$ (local separation). The effect of effective slip is predicted to decrease the force as compared with expected for two hydrophilic surfaces and described by the Taylor equation. The presence of additional length scales, $L$, $b_1$ and $b_2$, implies that a film drainage can be much richer than in case of a sphere moving towards a hydrophilic plane. For a large (compared to $L$) gap the reduction of the force is small, and for all textures the force is similar to expected when a sphere is moving towards a smooth hydrophilic plane that is shifted down from the super-hydrophobic wall. The value of this shift is equal to the average of the eigenvalues of the slip-length tensor. By analyzing striped super-hydrophobic surfaces, we then compute the correction to the Taylor equation for an arbitrary gap. We show that at thinner gap the force reduction becomes more pronounced, and that it depends strongly on the fraction of the gas area and local slip lengths. For small separations we derive an exact equation, which relates a correction for effective slip to texture parameters. Our analysis provides a framework for interpreting recent force measurements in the presence of super-hydrophobic surface.Comment: 9 pages, 5 figures, submitted to PRE; EPAPS file include

Young's modulus of polyelectrolyte multilayers from microcapsule swelling

We measure Young's modulus of a free polyelectrolyte multilayer film by studying osmotically induced swelling of polyelectrolyte multilayer microcapsules filled with the polyelectrolyte solution. Different filling techniques and core templates were used for the capsule preparation. Varying the concentration of the polyelectrolyte inside the capsule, its radius and the shell thickness yielded an estimate of an upper limit for Young's modulus of the order of 100 MPa. This corresponds to an elastomer and reflects strong interactions between polyanions and polycations in the multilayer

Flows and mixing in channels with misaligned superhydrophobic walls

Aligned superhydrophobic surfaces with the same texture orientation reduce drag in the channel and generate secondary flows transverse to the direction of the applied pressure gradient. Here we show that a transverse shear can be easily generated by using superhydrophobic channels with misaligned textured surfaces. We propose a general theoretical approach to quantify this transverse flow by introducing the concept of an effective shear tensor. To illustrate its use, we present approximate theoretical solutions and Dissipative Particle Dynamics simulations for striped superhydrophobic channels. Our results demonstrate that the transverse shear leads to complex flow patterns, which provide a new mechanism of a passive vertical mixing at the scale of a texture period. Depending on the value of Reynolds number two different scenarios occur. At relatively low Reynolds number the flow represents a transverse shear superimposed with two co-rotating vortices. For larger Reynolds number these vortices become isolated, by suppressing fluid transport in the transverse direction.Comment: 8 pages, 10 figure

Role of Russia in peaceful settlement of the conflict of North and South Korea: event analysis

This study analyzes the conflict between North Korea and South Korea. The Russian Federation has played an important role in the peaceful settlement of this conflict, which has an impact on the formation of the appearance of Russia in the international arena. The theoretical basis of the study was the theory of images, communication and international conflicts. Using the eventanalysis method, an event-based picture of the conflict was built, comparative assessments were made, conflicting parties, indirect participants and intermediaries were identified. An analysis of the current situation is given, groups of interests and the role of Russia in the peaceful settlement of this conflict are identified.Este estudio analiza el conflicto entre Corea del Norte y Corea del Sur. La Federación de Rusia ha desempeñado un papel importante en la solución pacífica de este conflicto, que tiene un impacto en la formación de la aparición de Rusia en la arena internacional. La base teórica del estudio fue la teoría de las imágenes, la comunicación y los conflictos internacionales. Utilizando el método de análisis de eventos, se construyó una imagen del conflicto basada en eventos, se realizaron evaluaciones comparativas, se identificaron las partes en conflicto, los participantes indirectos y los intermediarios. Se presenta un análisis de la situación actual, se identifican los grupos de intereses y el papel de Rusia en la solución pacífica de este conflicto.Este estudo analisa o conflito entre a Coréia do Norte e a Coréia do Sul. A Federação Russa tem desempenhado um papel importante na solução pacífica deste conflito, que tem um impacto sobre a formação da aparição da Rússia na arena internacional. A base teórica do estudo foi a teoria das imagens, comunicação e conflitos internacionais. Usando o método de análise de eventos, uma imagem baseada em eventos do conflito foi construída, avaliações comparativas foram feitas, partes conflitantes, participantes indiretos e intermediários foram identificados. Uma análise da situação atual é dada, grupos de interesses e o papel da Rússia na solução pacífica deste conflito são identificados