Effect of Patterned Slip on Micro and Nanofluidic Flows

We consider the flow of a Newtonian fluid in a nano or microchannel with walls that have patterned variations in slip length. We formulate a set of equations to describe the effects on an incompressible Newtonian flow of small variations in slip, and solve these equations for slow flows. We test these equations using molecular dynamics simulations of flow between two walls which have patterned variations in wettability. Good qualitative agreement and a reasonable degree of quantitative agreement is found between the theory and the molecular dynamics simulations. The results of both analyses show that patterned wettability can be used to induce complex variations in flow. Finally we discuss the implications of our results for the design of microfluidic mixers using slip.Comment: 13 pages, 12 figures, final version for publicatio

Effective slip boundary conditions for flows over nanoscale chemical heterogeneities

We study slip boundary conditions for simple fluids at surfaces with nanoscale chemical heterogeneities. Using a perturbative approach, we examine the flow of a Newtonian fluid far from a surface described by a heterogeneous Navier slip boundary condition. In the far-field, we obtain expressions for an effective slip boundary condition in certain limiting cases. These expressions are compared to numerical solutions which show they work well when applied in the appropriate limits. The implications for experimental measurements and for the design of surfaces that exhibit large slip lengths are discussed.Comment: 14 pages, 3 figure

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, $a/\sigma \gtrsim {\cal O}(10)$, where $\sigma$ is the (fluid) molecular diameter characterizing the Lennard-Jones interaction. In this regime, the effective slip length increases monotonically with $a/\sigma$ to a saturation value. For $a/\sigma \lesssim {\cal O}(10)$ 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 $a/\sigma$ 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

Nanoscale fluid flows in the vicinity of patterned surfaces

Molecular dynamics simulations of dense and rarefied fluids comprising small chain molecules in chemically patterned nano-channels predict a novel switching from Poiseuille to plug flow along the channel. We also demonstrate behavior akin to the lotus effect for a nanodrop on a chemically patterned substrate. Our results show that one can control and exploit the behavior of fluids at the nanoscale using chemical patterning.Comment: Phys. Rev. Lett. in pres

Two-dimensional conical dispersion in ZrTe5 evidenced by optical spectroscopy

Zirconium pentatelluride was recently reported to be a 3D Dirac semimetal, with a single conical band, located at the center of the Brillouin zone. The cone's lack of protection by the lattice symmetry immediately sparked vast discussions about the size and topological/trivial nature of a possible gap opening. Here we report on a combined optical and transport study of ZrTe5, which reveals an alternative view of electronic bands in this material. We conclude that the dispersion is approximately linear only in the a-c plane, while remaining relatively flat and parabolic in the third direction (along the b axis). Therefore, the electronic states in ZrTe5 cannot be described using the model of 3D Dirac massless electrons, even when staying at energies well above the band gap 6 meV found in our experiments at low temperatures.Comment: Physical Review Letters 122, 217402 (2019). Corrected acknowledgment

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

M. ulcerans infection diagnosis by fine needle aspiration

Date du colloque&nbsp;: 04/2009</p

Report of the laboratory confirmation of cases from the CDTUB Pobé, Benin

In 2009, 420 diagnostic samples (corresponding to 301 patients suspected of M. ulcerans infection) from the Centre de Dépistage et de Traitement de l’Ulcère de Buruli of Benin were subjected to IS2404 PCR at the Centre Hospitalier Universitaire of Angers. 180 samples (corresponding to 141 patients) were confirmed positive to M. ulcerans infection by PCR

Assessment of the usefulness of performing bacterial identification and antimicrobial susceptibility testing 24 h a day in a clinical microbiology laboratory

The impact of inoculating agar media with positive blood cultures and of performing bacterial identification and antimicrobial susceptibility testing (AST) for positive urine cultures, blood cultures and certain fluid cultures after day hours (night service (NS)) was evaluated in a clinical microbiology laboratory. The impact of the NS was assessed in terms of decreases in the delays from the time of sampling to the time at which results became available and of the consequences for patient management and antimicrobial treatment. Two major benefits were obtained: initiation of earlier appropriate treatment, and change to a reduced-spectrum but still efficient regimen. The hours of laboratory testing and the availability and transmission of results to the clinical staff were recorded. Concurrently, these hours were estimated as though laboratory tests had been performed in the absence of NS. Reductions in delay were defined as the differences between the hours actually spent and the estimated hours. Economic concerns were also considered. Overall, 430 samples for which an identification and/or AST were performed during the NS were included in the study. The NS led to the implementation of earlier appropriate therapy in 97 cases (22.6%), and to the change to reduced-spectrum but still efficient regimens in 23 additional cases (5.3%). In conclusion, there appeared to be benefits from a system providing bacterial identification and AST overnight, but a study of the cost-effectiveness of the NS would be useful to back up this observation

Equilibrium Simulation of the Slip Coefficient in Nanoscale Pores

Accurate prediction of interfacial slip in nanoscale channels is required by many microfluidic applications. Existing hydrodynamic solutions based on Maxwellian boundary conditions include an empirical parameter that depends on material properties and pore dimensions. This paper presents a derivation of a new expression for the slip coefficient that is not based on the assumptions concerning the details of solid-fluid collisions and whose parameters are obtainable from \textit{equilibrium} simulation. The results for the slip coefficient and flow rates are in good agreement with non-equilibrium molecular dynamics simulation.Comment: 11 pages, 4 figures, submitted to Phys Rev Let