1,599 research outputs found
Slip flow over structured surfaces with entrapped microbubbles
On hydrophobic surfaces, roughness may lead to a transition to a
superhydrophobic state, where gas bubbles at the surface can have a strong
impact on a detected slip. We present two-phase lattice Boltzmann simulations
of a Couette flow over structured surfaces with attached gas bubbles. Even
though the bubbles add slippery surfaces to the channel, they can cause
negative slip to appear due to the increased roughness. The simulation method
used allows the bubbles to deform due to viscous stresses. We find a decrease
of the detected slip with increasing shear rate which is in contrast to some
recent experimental results implicating that bubble deformation cannot account
for these experiments. Possible applications of bubble surfaces in microfluidic
devices are discussed.Comment: 4 pages, 4 figures. v2: revised version, to appear in Phys. Rev. Let
Slip behavior in liquid films on surfaces of patterned wettability: Comparison between continuum and molecular dynamics simulations
We investigate the behavior of the slip length in Newtonian liquids subject
to planar shear bounded by substrates with mixed boundary conditions. The upper
wall, consisting of a homogenous surface of finite or vanishing slip, moves at
a constant speed parallel to a lower stationary wall, whose surface is
patterned with an array of stripes representing alternating regions of no-shear
and finite or no-slip. Velocity fields and effective slip lengths are computed
both from molecular dynamics (MD) simulations and solution of the Stokes
equation for flow configurations either parallel or perpendicular to the
stripes. Excellent agreement between the hydrodynamic and MD results is
obtained when the normalized width of the slip regions, , where is the (fluid) molecular diameter characterizing the
Lennard-Jones interaction. In this regime, the effective slip length increases
monotonically with to a saturation value. For and transverse flow configurations, the non-uniform interaction
potential at the lower wall constitutes a rough surface whose molecular scale
corrugations strongly reduce the effective slip length below the hydrodynamic
results. The translational symmetry for longitudinal flow eliminates the
influence of molecular scale roughness; however, the reduced molecular ordering
above the wetting regions of finite slip for small values of
increases the value of the effective slip length far above the hydrodynamic
predictions. The strong inverse correlation between the effective slip length
and the liquid structure factor representative of the first fluid layer near
the patterned wall illustrates the influence of molecular ordering effects on
slip in non-inertial flows.Comment: 12 pages, 10 figures Web reference added for animations:
http://www.egr.msu.edu/~priezjev/bubble/bubble.htm
Bouncing or sticky droplets: impalement transitions on superhydrophobic micropatterned surfaces
When a liquid drops impinges a hydrophobic rough surface it can either bounce
off the surface (fakir droplets) or be impaled and strongly stuck on it (Wenzel
droplets). The analysis of drop impact and quasi static ''loading'' experiments
on model microfabricated surfaces allows to clearly identify the forces
hindering the impalement transitions. A simple semi-quantitative model is
proposed to account for the observed relation between the surface topography
and the robustness of fakir non-wetting states. Motivated by potential
applications in microfluidics and in the fabrication of self cleaning surfaces,
we finally propose some guidelines to design robust superhydrophobic surfaces.Comment: 7 pages, 5 figure
Drainage of a nanoconfined simple fluid: rate effects on squeeze-out dynamics
We investigate the effect of loading rate on drainage in molecularly thin
films of a simple fluid made of quasi-spherical molecules
(octamethylcyclotetrasiloxane, OMCTS). We find that (i) rapidly confined OMCTS
retains its tendency to organize into layers parallel to the confining
surfaces, and (ii) flow resistance in such layered films can be described by
bulklike viscous forces if one accounts for the existence of one monolayer
immobilized on each surfaces. The latter result is fully consistent with the
recent work of Becker and Mugele, who reached a similar conclusion by analyzing
the dynamics of squeeze-out fronts in OMCTS [T. Becker and F. Mugele, Phys.
Rev. Lett. {\bf 91} 166104(2003)]. Furthermore, we show that the confinement
rate controls the nature of the thinning transitions: layer-by-layer expulsion
of molecules in metastable, slowly confined films proceeds by a
nucleation/growth mechanism, whereas deeply and rapidly quenched films are
unstable and undergo thinning transitions akin to spinodal decomposition
Survival in pulmonary fibrosis combined with emphysema: likely defined by characteristics of specific patient subpopulations
Authors' Reply to letter from Cottin et al
Disruption of Candida albicans IFF4 gene involves modifications of the cell electrical surface properties.
During the past two decades, the prevalence of candidiasis has increased markedly and Candida albicans has now become one of the most important causes of nosocomial infections, especially after colonization of inert surfaces such as catheters or prostheses. In a previous report, we demonstrated the overexpression of 35 unidentified genes in response to adherence of C. albicans germ tubes to plastic. Therefore, a bioinformatic analysis was performed searching for genes encoding surface proteins potentially involved in adherence. Nineteen genes were thus selected, and one of them, CaIFF4, was further investigated. The deduced protein of this CaIFF4 gene revealed a glycosylphosphatidylinositol (GPI)-anchored site as well as the presence of a N-terminal signal peptide. Disruption of both alleles of CaIFF4 gene from C. albicans parent strain BWP17 was performed by PCR method. Then investigations of properties of null mutant for CaIFF4 gene showed a decrease of adherence of germ tubes to plastic in comparison to the parent strain BWP17. Besides, electrophoretic mobilities of germ tubes of CaIFF4 null mutant and of parental strain BWP17 were measured. Data were then analysed with soft particles analysis theory. Results point out a less important electrophoretic mobility of germ tubes of CaIFF4 null mutant in comparison to germ tubes of BWP17 parental strain
DNA array analysis of Candida albicans gene expression in response to adherence to polystyrene.
Candidiasis is often initiated by the colonization of inert surfaces. In order to elucidate the mechanisms involved in this adherence process, DNA macroarrays were used to analyze the transcriptome of Candida albicans, the main causative agent of this mycoses, in a simple adherence model using germ tubes produced in polystyrene Petri dishes. Non-adherent germ tubes produced on glass surface were used as a control. Analysis of gene expression displayed 77 genes identified as statistically overexpressed in adherent germ tubes. Among these genes, some encoded enzymes participating in metabolism of lipids (such as LIP6), of proteins (such as SAP1) or of carbohydrates (like PGI1, PMI40 and PSA1. Some of these genes have already been reported as playing a role in pathogenesis of C. albicans. However, functions were unknown for a large part (45.5%) of the overexpressed genes which will be analyzed further in order to define their relationship with adherence
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