On viscoelastic drop impact onto thin films: axisymmetric simulations and experimental analysis.

This study investigates the effect of fluid elasticity on axisymmetric droplets colliding with pre-existing liquid films, using both numerical and experimental approaches. The numerical simulations involve solving the incompressible flow momentum equations with viscoelastic constitutive laws using the finite volume method and the volume of fluid (VOF) technique to track the liquid's free surface. Here, the Oldroyd-B model is used as the constitutive equation for the viscoelastic phase. Experiments are also performed for dilute viscoelastic solutions with 0.005% and 0.01% (w/w) polyacrylamide in 80:20 glycerin/water solutions, in order to ensure the validity of the numerical solution and to investigate the elasticity effect. The formation and temporal evolution of the crown parameters are quantified by considering the flow parameters, including the fluid's elasticity. The results indicate that the axisymmetric numerical solutions reasonably agree with the experimental observations. Generally, the fluid's elasticity can enlarge the crown dimension at different thicknesses of the fluid film. Moreover, at intermediate values of the Weissenberg number, the extensional force in the crown wall can control the crown propagation. Furthermore, the results reveal that the effects of the Weber number and the viscosity ratio on this problem are more significant at higher values of the Weissenberg number.

Viscoplastic fingering in rectangular channels.

We experimentally study the viscous fingering problem of viscoplastic fluids in channels of rectangular cross section. We find that a yield stress-dependent capillary number (Ca^{*}) and an aspect ratio-dependent Bond number (Bo^{*}) can classify the finger shape into ramified and unified fingering patterns, and the finger flow regime into yield stress, viscosity, and aspect ratio-buoyancy-dominated regimes. For these regimes, we provide the transition boundaries using Ca^{*} and Bo^{*} and propose simple relations to predict the finger width, for a wide range of flow parameters, versus the capillary number, the channel aspect ratio, and the rheology of the viscoplastic fluid.

A microfluidic method and custom model for continuous, non-intrusive biofilm viscosity measurements under different nutrient conditions.

Straight, low-aspect ratio micro flow cells are used to support biofilm attachment and preferential accumulation at the short side-wall, which progressively reduces the effective channel width. The biofilm shifts downstream at measurable velocities under the imposed force from the constant laminar co-flowing nutrient stream. The dynamic behaviour of the biofilm viscosity is modeled semi-analytically, based on experimental measurements of biofilm dimensions and velocity as inputs. The technique advances the study of biofilm mechanical properties by strongly limiting biases related to non-Newtonian biofilm properties (e.g., shear dependent viscosity) with excellent time resolution. To demonstrate the proof of principle, young Pseudomonas sp. biofilms were analyzed under different nutrient concentrations and constant micro-flow conditions. The striking results show that large initial differences in biofilm viscosities grown under different nutrient concentrations become nearly identical in less than one day, followed by a continuous thickening process. The technique verifies that in 50 h from inoculation to early maturation stages, biofilm viscosity could grow by over 2 orders of magnitude. The approach opens the way for detailed studies of mechanical properties under a wide variety of physiochemical conditions, such as ionic strength, temperature, and shear stress.

Through thick and thin: a microfluidic approach for continuous measurements of biofilm viscosity and the effect of ionic strength.

Continuous, non-intrusive measurements of time-varying viscosity of Pseudomonas sp. biofilms are made using a microfluidic method that combines video tracking with a semi-empirical viscous flow model. The approach uses measured velocity and height of tracked biofilm segments, which move under the constant laminar flow of a nutrient solution. Following a low viscosity growth stage, rapid thickening was observed. During this stage, viscosity increased by over an order of magnitude in less than ten hours. The technique was also demonstrated as a promising platform for parallel experiments by subjecting multiple biofilm-laden microchannels to nutrient solutions containing NaCl in the range of 0 to 34 mM. Preliminary data suggest a strong relationship between ionic strength and biofilm properties, such as average viscosity and rapid thickening onset time. The technique opens the way for a combinatorial approach to study the response of biofilm viscosity under well-controlled physical, chemical and biological growth conditions.

Regularized thin-fiber model for nanofiber formation by centrifugal spinning.

We propose a regularized thin-fiber (string) model that overcomes past numerical limitations and allows determination of the steady fiber velocity and diameter of a semi-infinite Newtonian viscous fiber emerging from a nozzle rotating about an axis in the presence of centrifugal, inertial, and viscous forces of arbitrary magnitudes. The results are controlled by two dimensionless groups, namely, the Rossby number Rb expressing the ratio of inertial to centrifugal forces and the Reynolds number Re, the ratio of inertial to viscous forces. We find that for Rb < 0.5 and Re < 1, regularization of the string-model equations is required to provide numerical stability, which we achieve. Solutions are thereby obtained in which viscosity reduces curvature in the fiber trajectory; these solutions asymptotically approach the inviscid solution at large distances along the spin line. Thus, long spin lines reach a diameter that is independent of viscosity, as long as the fiber is sufficiently long, a criterion that is made clear in the paper. At Rb > 0.5, regularization is not required, the curvature in fiber trajectory is increased by viscosity, and the solution at large distances along the spin line does not converge to the inviscid result. Regimes of behavior in the plane formed by Re and Rb are mapped out and example behavior is given for each regime.

Isolation and in silico functional analysis of MtATP6, a 6-kDa subunit of mitochondrial F1F0-ATP synthase, in response to abiotic stress.

Mitochondrial F(1)F(0)-ATP synthase is a key enzymatic complex of energy metabolism that provides ATP for the cell. Subunits of this enzyme over-express under stress conditions. Little is known about the structure and regulatory mechanism of the F(0) portion of this enzyme. We isolated the full-length coding sequence of the RMtATP6 gene from rice and wheat, and partial sequences from Aegilops crassa and Triticum monococcum (Poaceae). We found that the sequence of rice RMtATP6 is 1965 bp long and contains two exons and one intron in 3'-UTR. Then, we analyzed the 2000-bp upstream region of the initiation codon ATG of the RMtATP6 and AtMtATP6, as promoter. The RMtATP6 coding sequence was found to be much conserved in the different plant species, possibly because of its key role under stress conditions. Promoter analysis demonstrated that RMtATP6 and AtMtATP6 include cis-acting elements such as ABRE, MYC/MYB, GT element in the upstream region, which respond to abscisic acid stress hormone and might show vital its roles in biotic and abiotic tolerance as an early-stress responsive gene. A mitochondrial signal peptide of 30 amino acids in length and an N-terminal cleavage site between amino acids 20 and 21 were discovered in RMtATP6. In addition, we found a transmembrane domain with an alpha helix structure that possibly passed through the mitochondrial inner membrane and established the 6-kDa subunit in the F(0) portion of the enzyme complex. Apparently, under stress conditions, with increasing ATP consumption by the cell, the 6-kDa subunit accumulates; by switching on F(1)F(0)-ATP synthase it provides additional energy needed for cell homeostasis.

Occurrence of shallow bark canker of walnut (Juglans regia) in southern provinces of Iran.

From April 2001 to November 2002, samples of walnut branches and trunks with symptoms of shallow bark canker were collected from Fars and Kohgiluyeh-va-Boyerahmad provinces. Symptoms of the disease were small cracks in the bark of the trunk and scaffold branches of mature trees with dark watery exudates which stained the affected trunk or limb. By removal of phelloderm, extensive necrosis of the underlying tissues was observed. In some cases, necrosis extended to cambium and outer xylem. Sixty-one strains of a bacterium were isolated from infected tissues using EMB and YDC media. On the basis of standard biochemical and physiological tests the bacterium was identified as Brenneria nigrifluens. The pathogen was found to be wide-spread in the provinces. Isolates were compared by physiological and biochemical characters, antibiotic sensitivity and protein electrophoretic pattern. Most of the strains were fairly similar in phenotypic features and electrophoretic profiles ofwhole-cell proteins were similar to each other and to reference strain (B. nigrifluens 5D313). Inoculation of 1-2 years-old walnut seedlings in May and June produced blackening symptoms and the bacterium survived for long period in infected tissues. This is the first report of the shallow bark canker of walnut in southern Iran.