Polymeric filament thinning and breakup in microchannels

The effects of elasticity on filament thinning and breakup are investigated in microchannel cross flow. When a viscous solution is stretched by an external immiscible fluid, a low 100 ppm polymer concentration strongly affects the breakup process, compared to the Newtonian case. Qualitatively, polymeric filaments show much slower evolution, and their morphology features multiple connected drops. Measurements of filament thickness show two main temporal regimes: flow- and capillary-driven. At early times both polymeric and Newtonian fluids are flow-driven, and filament thinning is exponential. At later times, Newtonian filament thinning crosses over to a capillary-driven regime, in which the decay is algebraic. By contrast, the polymeric fluid first crosses over to a second type of flow-driven behavior, in which viscoelastic stresses inside the filament become important and the decay is again exponential. Finally, the polymeric filament becomes capillary-driven at late times with algebraic decay. We show that the exponential flow thinning behavior allows a novel measurement of the extensional viscosities of both Newtonian and polymeric fluids.Comment: 7 pages, 7 figure

Axiomatic geometric formulation of electromagnetism with only one axiom: the field equation for the bivector field F with an explanation of the Trouton-Noble experiment

In this paper we present an axiomatic, geometric, formulation of electromagnetism with only one axiom: the field equation for the Faraday bivector field F. This formulation with F field is a self-contained, complete and consistent formulation that dispenses with either electric and magnetic fields or the electromagnetic potentials. All physical quantities are defined without reference frames, the absolute quantities, i.e., they are geometric four dimensional (4D) quantities or, when some basis is introduced, every quantity is represented as a 4D coordinate-based geometric quantity comprising both components and a basis. The new observer independent expressions for the stress-energy vector T(n)(1-vector), the energy density U (scalar), the Poynting vector S and the momentum density g (1-vectors), the angular momentum density M (bivector) and the Lorentz force K (1-vector) are directly derived from the field equation for F. The local conservation laws are also directly derived from that field equation. The 1-vector Lagrangian with the F field as a 4D absolute quantity is presented; the interaction term is written in terms of F and not, as usual, in terms of A. It is shown that this geometric formulation is in a full agreement with the Trouton-Noble experiment.Comment: 32 pages, LaTex, this changed version will be published in Found. Phys. Let

Decoupling the effects of shear and extensional flows on the alignment of colloidal rods

Cellulose nanocrystals (CNC) can be considered as model colloidal rods and have practical applications in the formation of soft materials with tailored anisotropy. Here, we employ two contrasting microfluidic devices to quantitatively elucidate the role of shearing and extensional flows on the alignment of a dilute CNC dispersion. Characterization of the flow field by micro-particle image velocimetry is coupled to flow-induced birefringence analysis to quantify the deformation rate--alignment relationship. The deformation rate required for CNC alignment is 4$\times$ smaller in extension than in shear. Alignment in extension is independent of the deformation rate magnitude, but is either 0$^\circ$ or 90$^\circ$ to the flow, depending on its sign. In shear flow the colloidal rods orientate progressively towards 0$^\circ$ as the deformation rate magnitude increases. Our results decouple the effects of shearing and extensional kinematics at aligning colloidal rods, establishing coherent guidelines for the manufacture of structured soft materials

Microdevices for extensional rheometry of low viscosity elastic liquids : a review

Extensional flows and the underlying stability/instability mechanisms are of extreme relevance to the efficient operation of inkjet printing, coating processes and drug delivery systems, as well as for the generation of micro droplets. The development of an extensional rheometer to characterize the extensional properties of low viscosity fluids has therefore stimulated great interest of researchers, particularly in the last decade. Microfluidics has proven to be an extraordinary working platform and different configurations of potential extensional microrheometers have been proposed. In this review, we present an overview of several successful designs, together with a critical assessment of their capabilities and limitations

Outcomes of early language delay: II. Etiology of transient and persistent language difficulties

Genes are known to play an important role in causing specific language impairment, but it is unclear how far a similar etiology is implicated in transient language delay in early childhood. Two-year-old children with vocabulary scores below the 10th centile were selected from a cohort of over 2,800 same-sex twin pairs whose language was assessed by parental report at 2, 3, and 4 years of age. These children with early language delay (ELD) were divided into cases of transient and persistent language difficulties on the basis of outcome at 3 and 4 years. A DeFries-Fulker analysis (J. C. DeFries & D. W. Fulker, 1985) was used to compute group heritability (h²g) of 2-year vocabulary delay separately for those with transient and persistent difficulties. When 3-year and 4-year language attainments were used to categorize outcomes, h²g was similar and modest (.25 or less) for both transient and persistent difficulties. However, when persistent difficulties were defined according to whether parents expressed concern about language at 3 years or according to whether a professional had been consulted about language difficulties at 4 years, heritability was significantly higher. For 289 children with no professional involvement at 4 years, heritability of 2-year vocabulary delay was close to zero, whereas for 134 children with professional involvement, a significant h²g of .41 (SE= .127) was found. Early language delay appears largely environmental in origin for 2-year-olds whose parents do not go on to seek professional help

Mapping service activity: the example of childhood obesity schemes in England

Background: Childhood obesity is high on the policy agenda of wealthier nations, and many interventions have been developed to address it. This work describes an overview of schemes for obese and overweight children and young people in England, and the 'mapping' approach we used.Methods: Our search strategy, inclusion criteria and coding frame had to be suitable for describing a potentially large number of schemes within a short timeframe. Data were collected from key informants, scheme publicity and reports, and via a web-survey. To be included, schemes had to be based in England, follow a structured programme lasting at least two weeks, promote healthy weight, and be delivered exclusively to overweight and/or obese children and young people (age range 4-18). Data were entered into a coding frame recording similar information for each scheme, including any underpinning research evidence, evaluation or monitoring reports. Priority questions were identified in consultation with colleagues from the Department of Health and the Cross Government Obesity Unit.Results: Fifty-one schemes were identified. Some operated in multiple areas, and by using estimates of the number of schemes provided by multi-site scheme leads, we found that between 314 and 375 local programmes were running at any time. Uncertainty is largely due to the largest scheme provider undergoing rapid expansion at the time of the mapping exercise and therefore able to provide only an estimate of the number of programmes running. Many schemes were similar in their approach, had been recently established and were following NICE guidelines on interventions to promote healthy weight. Rigorous evaluation was rare.Conclusions: Our methods enabled us to produce a rapid overview of service activity across a wide geographic area and a range of organisations and sectors. In order to develop the evidence base for childhood obesity interventions, rigorous evaluation of these schemes is required. This overview can serve as a starting point for evaluations of interventions to address obesity. More generally, a rapid and systematic approach of this type is transferable to other types of service activity in health and social care, and may be a tool to inform public health planning