Unifying disparate rate-dependent rheological regimes in non-Brownian suspensions.

A typical dense non-Brownian particulate suspension exhibits shear thinning (decreasing viscosity) at low shear rate or stress followed by a Newtonian plateau (constant viscosity) at intermediate shear rate or stress values which transitions to shear thickening (increasing viscosity) beyond a critical shear rate or stress value and finally undergoes a second shear thinning transition at extremely high shear rate or stress values. In this study, we unify and quantitatively reproduce all the disparate rate-dependent regimes and the corresponding transitions for a dense non-Brownian suspension with increasing shear rate or stress. We employ discrete particle dynamics simulations based on the proposed mechanism to elucidate its accuracy. We find that a competition between interparticle interactions of hydrodynamic and nonhydrodynamic origins and the switching in the dominant stress scale with increasing the shear rate or stress lead to each of the above transitions. Inclusion of traditional hydrodynamic interactions, attractive or repulsive Derjaguin-Landau-Verwey-Overbeek (DLVO) interactions the interparticle contact interactions, and a constant friction (or other constraint mechanism) reproduces the initial thinning as well as the shear thickening transition. However, to quantitatively capture the intermediate Newtonian plateau and the second shear thinning, an additional nonhydrodynamic interaction of non-DLVO origin and a decreasing coefficient of friction, respectively, are essential, thus providing an explanation for the presence of the intermediate Newtonian plateau along with reproducing the second shear thinning in a single framework. Expressions utilized for various interactions and friction are determined from experimental measurements and hence result in excellent quantitative agreement between the simulations and previous experiments.

Interplay of physical mechanisms and biofilm processes: review of microfluidic methods.

Bacteria in natural and artificial environments often reside in self-organized, integrated communities known as biofilms. Biofilms are highly structured entities consisting of bacterial cells embedded in a matrix of self-produced extracellular polymeric substances (EPS). The EPS matrix acts like a biological 'glue' enabling microbes to adhere to and colonize a wide range of surfaces. Once integrated into biofilms, bacterial cells can withstand various forms of stress such as antibiotics, hydrodynamic shear and other environmental challenges. Because of this, biofilms of pathogenic bacteria can be a significant health hazard often leading to recurrent infections. Biofilms can also lead to clogging and material degradation; on the other hand they are an integral part of various environmental processes such as carbon sequestration and nitrogen cycles. There are several determinants of biofilm morphology and dynamics, including the genotypic and phenotypic states of constituent cells and various environmental conditions. Here, we present an overview of the role of relevant physical processes in biofilm formation, including propulsion mechanisms, hydrodynamic effects, and transport of quorum sensing signals. We also provide a survey of microfluidic techniques utilized to unravel the associated physical mechanisms. Further, we discuss the future research areas for exploring new ways to extend the scope of the microfluidic approach in biofilm studies.

Reorientation of elongated particles at density interfaces.

Density interfaces in the water column are ubiquitously found in oceans and lakes. Interaction of settling particles with pycnoclines plays a pivotal function in nutrient transport between ocean layers and settling rates of marine particles. We perform direct numerical simulations of an elongated particle settling through a density interface and scrutinize the role of stratification on the settling dynamics. It is found that the presence of the density interface tends to turn the long axis of an elongated particle parallel to the settling direction, which is dramatically different from its counterpart in a homogeneous fluid. Although broadside-on settling of the elongated particle is enhanced upon approaching the interface, the long axis rotates toward the settling direction as the particle passes through the interface. We quantify turning couples due to stratification effects, which counteract the pressure-induced torques due to the fluid inertia. A similar behavior is observed for different initial orientations of the particle. It is shown that the reorientation of an elongated particle occurs in both sharp and linear density stratifications.

Effect of solid boundaries on swimming dynamics of microorganisms in a viscoelastic fluid.

We numerically study the effect of solid boundaries on the swimming behavior of a motile microorganism in viscoelastic media. Understanding the swimmer-wall hydrodynamic interactions is crucial to elucidate the adhesion of bacterial cells to nearby substrates which is precursor to the formation of the microbial biofilms. The microorganism is simulated using a squirmer model that captures the major swimming mechanisms of potential, extensile, and contractile types of swimmers, while neglecting the biological complexities. A Giesekus constitutive equation is utilized to describe both viscoelasticity and shear-thinning behavior of the background fluid. We found that the viscoelasticity strongly affects the near-wall motion of a squirmer by generating an opposing polymeric torque which impedes the rotation of the swimmer away from the wall. In particular, the time a neutral squirmer spends at the close proximity of the wall is shown to increase with polymer relaxation time and reaches a maximum at Weissenberg number of unity. The shear-thinning effect is found to weaken the solvent stress and therefore, increases the swimmer-wall contact time. For a puller swimmer, the polymer stretching mainly occurs around its lateral sides, leading to reduced elastic resistance against its locomotion. The neutral and puller swimmers eventually escape the wall attraction effect due to a releasing force generated by the Newtonian viscous stress. In contrast, the pusher is found to be perpetually trapped near the wall as a result of the formation of a highly stretched region behind its body. It is shown that the shear-thinning property of the fluid weakens the wall-trapping effect for the pusher squirmer.

Interaction between a pair of particles settling in a stratified fluid.

The anisotropic structure of fluidized suspensions is governed by their microstructures which are in turn determined by the dynamics of particle pair interactions. Here, we present a numerical simulation of particle interaction in linearly stratified fluids. It is shown that unlike homogeneous fluids, stratification results in the attraction of particles settling abreast. The attraction between the particles is characterized by the combined effects of buoyancy, inertia, and diffusion. The interaction of the particles settling in tandem can be fundamentally altered due to the presence of the background density gradients and the drafting-kissing-tumbling behavior in a homogeneous fluid can be replaced by drafting-kissing-separation or drafting-separation phenomenon depending on the strength of the stratification. In the case of weak stratification, drafting-kissing-tumbling occurs, however, a prolonged kissing time is observed and the rate of change of the orientation of particles is reduced. It is shown that the formation of the buoyancy-induced vortical structures and the generation of stratified jets behind the particles are the fundamental mechanisms in governing the dynamics of the particle pair interaction in stratified fluids.

Swimming of a model ciliate near an air-liquid interface.

In this work, the role of the hydrodynamic forces on a swimming microorganism near an air-liquid interface is studied. The lubrication theory is utilized to analyze hydrodynamic effects within the narrow gap between a flat interface and a small swimmer. By using an archetypal low-Reynolds-number swimming model called "squirmer," we find that the magnitude of the vertical swimming velocity is on the order of O(εlnε), where ε is the ratio of the gap width to the swimmer's body size. The reduced swimming velocity near an interface can explain experimental observations of the aggregation of microorganisms near a liquid interface.

Hydrodynamic mechanisms of cell and particle trapping in microfluidics.

Focusing and sorting cells and particles utilizing microfluidic phenomena have been flourishing areas of development in recent years. These processes are largely beneficial in biomedical applications and fundamental studies of cell biology as they provide cost-effective and point-of-care miniaturized diagnostic devices and rare cell enrichment techniques. Due to inherent problems of isolation methods based on the biomarkers and antigens, separation approaches exploiting physical characteristics of cells of interest, such as size, deformability, and electric and magnetic properties, have gained currency in many medical assays. Here, we present an overview of the cell/particle sorting techniques by harnessing intrinsic hydrodynamic effects in microchannels. Our emphasis is on the underlying fluid dynamical mechanisms causing cross stream migration of objects in shear and vortical flows. We also highlight the advantages and drawbacks of each method in terms of throughput, separation efficiency, and cell viability. Finally, we discuss the future research areas for extending the scope of hydrodynamic mechanisms and exploring new physical directions for microfluidic applications.

Emergence of a limit cycle for swimming microorganisms in a vortical flow of a viscoelastic fluid.

We propose that the rheological properties of background fluid play an important role in the interaction of microorganisms with the flow field. The viscoelastic-induced migration of microorganisms in a vortical flow leads to the emergence of a limit cycle. The shape and formation rate of patterns depend on motility, vorticity strength, and rheological properties of the background fluid. Given the inherent viscoelasticity of exopolysaccharides secreted by microorganisms, our results can suggest new mechanisms leading to the vital behavior of microorganisms such as bacterial aggregation and biofilm formation.

Stratlets: low Reynolds number point-force solutions in a stratified fluid.

We present fundamental solutions of low Reynolds number flows in a stratified fluid, including the case of a point force (Stokeslet) and a doublet. Stratification dramatically alters the flow by creating toroidal eddies, and velocity decays much faster than in a homogeneous fluid. The fundamental length scale is set by the competition of buoyancy, diffusion and viscosity, and is O(100 µm-1 mm) in aquatic environments. Stratification can therefore affect the swimming of small organisms and the sinking of marine snow particles, and diminish the effectiveness of mechanosensing in the ocean.

Instability of stationary liquid sheets.

The rupture of a 3D stationary free liquid film under the competing effects of surface tension and van der Waals forces is studied as a linearized stability problem in a purely irrotational analysis utilizing the dissipation method. The results of the foregoing analysis are compared with a 2D long-wave approximation that has given rise to an extensive literature on the rupture problem. The irrotational and long-wave approximations are here compared with the exact 2D solution. The exact solution and the two approximate theories give the same results for infinitely long waves. The problem considered depends on two dimensionless parameters, the Hamaker number and the Ohnesorge number. The Hamaker number is a dimensionless number defined as a measure of the ratio of van der Waals forces to surface tension. The exact solution and the two approximate solutions differ by < 1% when the Hamaker number is small for all values of the Ohnesorge number. When the Ohhnesorge number is close to one, as in the case of water films separated by distance 100 A, the long-wave approximation overestimates and the potential flow approximation underestimates the exact solution by similar small amounts. The high accuracy of the dissipation method shows that the effects of vorticity are small for small to moderate Hamaker numbers.

Two repressor elements inhibit expression of the von Willebrand factor gene promoter in vitro.

A fragment of the human von Willebrand factor (VWF) gene promoter corresponding to sequences -487 to +247 bp functions as an endothelial specific promoter in cell culture. We have previously reported that a GATA transcription factor functions as an activator and an NF1 like protein functions as a repressor of this promoter fragment. We have now identified a second negative regulatory element in the VWF promoter that interacts with nuclear factor(s) (designated R) in both bovine aortic endothelial and smooth muscle cells. Inhibition of either the NF1 or the R repressor alone is not sufficient to activate the VWF promoter in smooth muscle cells. The present studies reveal that simultaneous inhibition of both repressors activates the VWF promoter in smooth muscle cells. The data support a model of selective derepression to explain the endothelial cell specific activity of the 487 to +247 fragment of the VWF promoter in vitro.

Structure of the rat inhibin and activin betaA-subunit gene and regulation in an ovarian granulosa cell line.

We have isolated the rat inhibin and activin betaA-subunit gene, which is composed of three exons, and have characterized a 571-bp region upstream from the transcriptional start site that functions as a promoter in transient transfection studies in an ovarian granulosa cell line, GRMO2. Deletion analysis of the 571-bp promoter region has identified DNA sequences between -362 bp and -110 bp to be essential in mediating basal promoter activity and activation by forskolin (FSK) and/or 12-O-tetradecanoylphorbol-13-acetate (TPA). Within this region, a variant CRE (cAMP response element) has been identified at -120 bp. Point mutations in the variant CRE substantially reduce the ability of FSK and/or TPA to induce promoter activity in GRMO2 cells. A single nucleotide change in the variant CRE, which converts it to a consensus CRE, does not enhance promoter activity in response to FSK and/or TPA, but rather reduces promoter activity to the same extent as the other inactivating mutation in the variant CRE, suggesting that this element does not act as a classical CRE. Consistent with this, electrophoretic mobility shift assays performed using antibodies to a variety of cAMP and phorbol ester-responsive transcription factors indicate that the AP-1 family proteins jun-B and fos-B are present in the protein complex binding to the variant CRE. Overexpression of jun-B and fos-B in GRMO2 cells resulted in a robust activation of the betaA-subunit promoter. Our results suggest that this novel variant CRE sequence mediates both cAMP and phorbol ester regulation through its interactions with AP-1family proteins.

Ionizing irradiation increases transcription of the von Willebrand factor gene in endothelial cells.

Ionizing irradiation damage to the vasculature results in an increase in procoagulant activity of endothelial cells, including elevated von Willebrand factor (vWf) secretion. We investigated the mechanism of irradiation induction of vWf release and demonstrated that vWf mRNA levels were increased when either human or bovine endothelial cells were exposed to 20 Gy irradiation. This response to irradiation was independent to de novo protein synthesis, but required new transcription. Nuclear run-on experiments indicated that increased vWf transcriptional activity was partly responsible for the higher levels of the mRNA accumulation. Transfection analyses with plasmids in which a human growth hormone structural gene was under the control of the endothelial-cell-specific vWf promoter demonstrated that irradiation increased promoter activity. Deletion analyses demonstrated that sequences necessary for irradiation induction of the promoter activity were located within the 112-bp sequences (-90 to +22) that constitute the non-endothelial-cell-specific core promoter region of the vWf gene. Results of gel mobility assays and deletion analyses demonstrated that a site in the vWf promoter other than the putative NF-kB binding site is involved in the mechanism of irradiation induction of the vWf.

An NF1-like protein functions as a repressor of the von Willebrand factor promoter.

The expression of the von Willebrand factor (vWf) gene is restricted to endothelial cells and megakaryocytes. We have previously reported the identification of a region of the vWf gene that regulates its cell-type-specific expression in cell culture. This region (spanning nucleotides -487 to +247) consists of a core promoter (spanning nucleotides -90 to +22), a positive regulatory region (spanning nucleotides +155 to +247), and a negative regulatory region spanning nucleotides -312 to -487. To identify the trans-acting factor(s) that interacts with the negative regulatory region, we carried out gel mobility and DNase1 footprint analyses of sequences -312 to -487. These analyses demonstrated that an NF1-like protein interacts with DNA sequences spanning -440 to -470 nucleotides in the negative regulatory region of the vWf promoter. Base substitution mutations of the NF1 binding site abolished the NF1-DNA interaction. Furthermore, mutation of the NF1 binding site in the promoter fragment (-487 to +155) that contained the core and the negative regulatory region resulted in activation of the mutant promoter in both endothelial and nonendothelial cells. The wild type promoter fragment (-487 to +155) was not activated in either cell type. These results demonstrate that an NF1-like protein functions as a repressor of vWf promoter activity. In contrast, the mutation of the same NF1 binding site, but now in the context of the larger 734-base pair endothelial cell-specific promoter fragment (-487 to +247), did not result in promoter activation in nonendothelial cells. The data indicate that there are additional repressor elements within the vWf promoter region suppressing its activity specifically, in nonendothelial cells, and suggest that there is a secondary repressor element(s) that is located in the terminal region of the first exon of this gene.

Nuevo medio solido para el crecimiento de Borrelia persica y Borrelia microti / New solid means for the growing of Borrelia persica and Borrelia microti

Se describe un nuevo medio solido para la rapida deteccion de Borrelia persica y Borrelia microti. Corrientemente el cultivo y aislamiento de Borrelia demora alrededor de 21 dias. El examen serologico mas frecuentemente realizado demora menos tiempo pero esta asociado con resultados falsos positivos relativamente altos. Sin embargo, nuestro nuevo medio solido reduce el tiempo de cultivo a 72 horas, lo que permite un rapido diagnostico de la enfermedad causada por Borrelia persica y Borrelia microti y el inicio temprano del tratamiento en estos pacientes

[New solid culture media for growing Borrelia persica and Borrelia microti]. / Nuevo medio sólido para el crecimiento de Borrelia persica y Borrelia microti.

A new solid means for the fast detection of Borrelia persica and Borrelia microti is described. Generally, culture and isolation of Borrelia takes about 21 days. The serological test, which is carried out more often, takes less time but it is associated with false positive reactions relatively high. However, our new solid means reduces the culture time to 72 hours, allowing to have a fast diagnosis of the disease caused by Borrelia persica and Borrelia microti, and to start the early treatment of these patients.

Adrenocorticotropin and corticosterone levels in pre-weanling spontaneously hypertensive rats.

Circulating plasma ACTH and corticosterone levels were measured in spontaneously hypertensive rats (SHR) and the corresponding, normotensive Wistar-Kyoto (WKY) strain at 10 and 20 days of age. In addition, stored levels of ACTH were measured in the pituitary glands of these animals. Circulating corticosterone levels were significantly lower, in both strains, at 10 days than at 20 days. Although the glucocorticoid was undetectable in WKY animals at 10 days, significant levels were observed in age-matched SHR. No difference in corticosterone concentrations was observed between the two strains at 20 days. Circulating ACTH levels did not reflect the values for circulating glucocorticoids. There were no significant differences in the levels of ACTH between strains or between age groups. Moreover, pituitary stores of ACTH between animals of different strains and ages were not found to be significantly different among any of the groups tested. These results demonstrate that there is a difference in circulating corticosterone levels between spontaneously hypertensive and Wistar-Kyoto rats at 10 days postnatally which is not evident just prior to weaning (20 days). This difference is not due to variations in stored or circulating ACTH. Indeed, ACTH levels are high at a time (10 days) when corticosterone is low - thus suggesting that the difference may reside within the responsiveness of the adrenal cortex.