1,292 research outputs found
'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 (texture characteristic length), and (local slip
lengths at solid and gas areas), and and (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 (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, , and , 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 ) 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 pacÃfica 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 teorÃa 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 pacÃfica 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 pacÃfica 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 construÃda, 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 pacÃfica deste conflito são identificados
- …