On the capillary self-focusing in a microfluidic system

A computational framework is developed to address capillary self-focusing in Step Emulsification. The microfluidic system consists of a single shallow and wide microchannel that merges into a deep reservoir. A continuum approach coupled with a volume of fluid method is used to model the capillary self-focusing effect. The original governing equations are reduced using the Hele-Shaw approximation. We show that the interface between the two fluids takes the shape of a neck narrowing in the flow direction just before entering the reservoir, in agreement with our experimental observations. Our computational model relies on the assumption that the pressure at the boundary, where the fluid exits into the reservoir, is the uniform pressure in the reservoir. We investigate this hypothesis by comparing the numerical results with experimental data. We conjecture that the pressure boundary condition becomes important when the width of the neck is comparable to the depth of the microchannel. A correction to the exit pressure boundary condition is then proposed, which is determined by comparison with experimental data. We also present the experimental observations and the numerical results of the transitions of breakup regimes.Comment: To appear in: Fluid Dynamics Researc

Capillary focusing close to a topographic step: Shape and instability of confined liquid filaments

Step-emulsification is a microfluidic technique for droplet generation which relies on the abrupt decrease of confinement of a liquid filament surrounded by a continuous phase. A striking feature of this geometry is the transition between two distinct droplet breakup regimes, the "step-regime" and "jet-regime", at a critical capillary number. In the step-regime, small and monodisperse droplets break off from the filament directly at a topographic step, while in the jet-regime a jet protrudes into the larger channel region and large plug-like droplets are produced. We characterize the breakup behavior as a function of the filament geometry and the capillary number and present experimental results on the shape and evolution of the filament for a wide range of capillary numbers in the jet-regime. We compare the experimental results with numerical simulations. Assumptions based on the smallness of the depth of the microfluidic channel allow to reduce the governing equations to the Hele-Shaw problem with surface tension. The full nonlinear equations are then solved numerically using a volume-of-fluid based algorithm. The computational framework also captures the transition between both regimes, offering a deeper understanding of the underlying breakup mechanism

Spin-orbit induced longitudinal spin-polarized currents in non-magnetic solids

For certain non-magnetic solids with low symmetry the occurrence of spin-polarized longitudinal currents is predicted. These arise due to an interplay of spin-orbit interaction and the particular crystal symmetry. This result is derived using a group-theoretical scheme that allows investigating the symmetry properties of any linear response tensor relevant to the field of spintronics. For the spin conductivity tensor it is shown that only the magnetic Laue group has to be considered in this context. Within the introduced general scheme also the spin Hall- and additional related transverse effects emerge without making reference to the two-current model. Numerical studies confirm these findings and demonstrate for (Au$_{1-x}$Pt$_{\rm x}$)$_4$Sc that the longitudinal spin conductivity may be in the same order of magnitude as the conventional transverse one. The presented formalism only relies on the magnetic space group and therefore is universally applicable to any type of magnetic order.Comment: 5 pages, 1 table, 2 figures (3 & 2 subfigures

Shape of a liquid front upon dewetting

We examine the profile of a liquid front of a film that is dewetting a solid substrate. Since volume is conserved, the material that once covered the substrate is accumulated in a rim close to the three phase contact line. Theoretically, such a profile of a Newtonian liquid resembles an exponentially decaying harmonic oscillation that relaxes into the prepared film thickness. For the first time, we were able to observe this behavior experimentally. A non-Newtonian liquid - a polymer melt - however, behaves differently. Here, viscoelastic properties come into play. We will demonstrate that by analyzing the shape of the rim profile. On a nm scale, we gain access to the rheology of a non-Newtonian liquid.Comment: 4 pages, 4 figure

Thermal noise influences fluid flow in thin films during spinodal dewetting

Experiments on dewetting thin polymer films confirm the theoretical prediction that thermal noise can strongly influence characteristic time-scales of fluid flow and cause coarsening of typical length scales. Comparing the experiments with deterministic simulations, we show that the Navier-Stokes equation has to be extended by a conserved bulk noise term to accomplish the observed spectrum of capillary waves. Due to thermal fluctuations the spectrum changes from an exponential to a power law decay for large wavevectors. Also the time evolution of the typical wavevector of unstable perturbations exhibits noise induced coarsening that is absent in deterministic hydrodynamic flow.Comment: 4 pages, 3 figure

Reanalysis of the FEROS observations of HIP 11952

Aims. We reanalyze FEROS observations of the star HIP 11952 to reassess the existence of the proposed planetary system. Methods. The radial velocity of the spectra were measured by cross-correlating the observed spectrum with a synthetic template. We also analyzed a large dataset of FEROS and HARPS archival data of the calibrator HD 10700 spanning over more than five years. We compared the barycentric velocities computed by the FEROS and HARPS pipelines. Results. The barycentric correction of the FEROS-DRS pipeline was found to be inaccurate and to introduce an artificial one-year period with a semi-amplitude of 62 m/s. Thus the reanalysis of the FEROS data does not support the existence of planets around HIP 11952.Comment: 7 pages, 8 figures, 1 tabl

Energy dissipation in sheared wet granular assemblies

Energy dissipation in sheared dry and wet granulates is considered in the presence of an externally applied confining pressure. Discrete element simulations reveal that for sufficiently small confining pressures, the energy dissipation is dominated by the effects related to the presence of cohesive forces between the particles. The residual resistance against shear can be quantitatively explained by a combination of two effects arising in a wet granulate: (i) enhanced friction at particle contacts in the presence of attractive capillary forces and (ii) energy dissipation due to the rupture and reformation of liquid bridges. Coulomb friction at grain contacts gives rise to an energy dissipation which grows linearly with increasing confining pressure for both dry and wet granulates. Because of a lower Coulomb friction coefficient in the case of wet grains, as the confining pressure increases the energy dissipation for dry systems is faster than for wet ones

Band structure of helimagnons in MnSi resolved by inelastic neutron scattering

A magnetic helix realizes a one-dimensional magnetic crystal with a period given by the pitch length $\lambda_h$. Its spin-wave excitations -- the helimagnons -- experience Bragg scattering off this periodicity leading to gaps in the spectrum that inhibit their propagation along the pitch direction. Using high-resolution inelastic neutron scattering the resulting band structure of helimagnons was resolved by preparing a single crystal of MnSi in a single magnetic-helix domain. At least five helimagnon bands could be identified that cover the crossover from flat bands at low energies with helimagnons basically localized along the pitch direction to dispersing bands at higher energies. In the low-energy limit, we find the helimagnon spectrum to be determined by a universal, parameter-free theory. Taking into account corrections to this low-energy theory, quantitative agreement is obtained in the entire energy range studied with the help of a single fitting parameter.Comment: 5 pages, 3 figures; (v2) slight modifications, published versio

pZMO7-Derived shuttle vectors for heterologous protein expression and proteomic applications in the ethanol-producing bacterium Zymomonas mobilis

Background The ethanol-producing bacterium Zymomonas mobilis has attracted considerable scientific and commercial interest due to its exceptional physiological properties. Shuttle vectors derived from native plasmids have previously been successfully used for heterologous gene expression in this bacterium for a variety of purposes, most notably for metabolic engineering applications. Results A quantitative PCR (qPCR) approach was used to determine the copy numbers of two endogenous double stranded DNA plasmids: pZMO1A (1,647 bp) and pZMO7 (pZA1003; 4,551 bp) within the NCIMB 11163 strain of Z. mobilis. Data indicated pZMO1A and pZMO7 were present at ca. 3-5 and ca. 1-2 copies per cell, respectively. A ca. 1,900 bp fragment from plasmid pZMO7 was used to construct two Escherichia coli - Z. mobilis shuttle vectors (pZ7C and pZ7-184). The intracellular stabilities and copy numbers of pZ7C and pZ7-184 were characterized within the NCIMB 11163, ATCC 29191 and (ATCC 10988-derived) CU1 Rif2 strains of Z. mobilis. Both shuttle vectors could be stably maintained within the ATCC 29191 strain (ca. 20-40 copies per cell), and the CU1 Rif2 strain (ca. 2-3 copies per cell), for more than 50 generations in the absence of an antibiotic selectable marker. A selectable marker was required for shuttle vector maintenance in the parental NCIMB 11163 strain; most probably due to competition for replication with the endogenous pZMO7 plasmid molecules. N-terminal glutathione S-transferase (GST)-fusions of four endogenous proteins, namely the acyl-carrier protein (AcpP); 2-dehydro-3-deoxyphosphooctonate aldolase (KdsA); DNA polymerase III chi subunit (HolC); and the RNA chaperone protein Hfq; were successfully expressed from pZ7C-derived shuttle vectors, and their protein-protein binding interactions were analyzed in Z. mobilis ATCC 29191. Using this approach, proteins that co-purified with AcpP and KdsA were identified. Conclusions We show that a shuttle vector-based protein affinity 'pull-down' approach can be used to probe protein interaction networks in Z. mobilis cells. Our results demonstrate that protein expression plasmids derived from pZMO7 have significant potential for use in future biological or biotechnological applications within Z. mobilis.published_or_final_versio

Thickness-dependent spontaneous dewetting morphology of ultrathin Ag films

We show here that the morphological pathway of spontaneous dewetting of ultrathin Ag films on SiO2 under nanosecond laser melting is found to be film thickness dependent. For films with thickness h between 2 <= h <= 9.5 nm, the morphology during the intermediate stages of dewetting consisted of bicontinuous structures. For films 11.5 <= h <= 20 nm, the intermediate stages consisted of regularly-sized holes. Measurement of the characteristic length scales for different stages of dewetting as a function of film thickness showed a systematic increase, which is consistent with the spinodal dewetting instability over the entire thickness range investigated. This change in morphology with thickness is consistent with observations made previously for polymer films [A. Sharma et al, Phys. Rev. Lett., v81, pp3463 (1998); R. Seemann et al, J. Phys. Cond. Matt., v13, pp4925, (2001)]. Based on the behavior of free energy curvature that incorporates intermolecular forces, we have estimated the morphological transition thickness for the intermolecular forces for Ag on SiO2 . The theory predictions agree well with observations for Ag. These results show that it is possible to form a variety of complex Ag nanomorphologies in a consistent manner, which could be useful in optical applications of Ag surfaces, such as in surface enhanced Raman sensing.Comment: 20 pages, 5 figure