116 research outputs found
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
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 smaller in extension
than in shear. Alignment in extension is independent of the deformation rate
magnitude, but is either 0 or 90 to the flow, depending on its
sign. In shear flow the colloidal rods orientate progressively towards
0 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
Model for coiling and meandering instability of viscous threads
A numerical model is presented to describe both the transient and
steady-state dynamics of viscous threads falling onto a plane. The steady-state
coiling frequency w is calculated as a function of fall height H. In the case
of weak gravity, w ~ H^{-1} and w ~ H are obtained for lower and higher fall
heights respectively. When the effect of gravity is significant, the relation w
~ H^2 is observed. These results agree with the scaling laws previously
predicted. The critical Reynolds number for coil-uncoil transition is
discussed. When the gravity is weak, the transition occurs with hysteresis
effects. If the plane moves horizontally at a constant speed, a variety of
meandering oscillation modes can be observed experimentally. The present model
also can describe this phenomenon. The numerically obtained state diagram for
the meandering modes qualitatively agrees with the experiment.Comment: 12 pages, 10 figure
Trouton-Noble paradox revisited
An apparent paradox is obtained in all previous treatments of the Trouton-Noble experiment; there is a three-dimensional torque in an inertial frame S in which a thin parallel-plate capacitor is moving, but there is no 3D torque in S', the rest frame of the capacitor. In this paper instead of using 3D quantities and their ``apparent'' transformations we deal with 4D geometric quantities their Lorentz transformations and equations with them. We introduce a new decomposition of the torque N (bivector) into 1-vectors N_{s} and N_{t}. It is shown that in the frame of ``fiducial'' observers, in which the observers who measure N_{s} and N_{t} are at rest, and in the standard basis, only the spatial components N_{s}^{i} and N_{t}^{i} remain, which can be associated with components of two 3D torques. In such treatment with 4D geometric quantities the mentioned paradox does not appear. The presented explanation is in a complete agreement with the principle of relativity and with the Trouton-Noble experiment without the introduction of any additional torque
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
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
Parents and Teachers Make Different Contributions to a Shared Perspective on Hyperactive–Impulsive and Inattentive Symptoms: A Multivariate Analysis of Parent and Teacher Ratings on the Symptom Domains of ADHD
Attention deficit hyperactivity disorder (ADHD) is characterised by developmentally inappropriate and impairing levels of inattentive and hyperactive–impulsive behaviours. We aimed to investigate the differential effects of parent and teacher ratings on inattention and hyperactivity–impulsivity and the extent of genetic overlap between the two behavioural dimensions. Multivariate structural equation modelling was performed on DSM-IV based ADHD ratings by parents and teachers collected on a general population sample of 672 twin pairs, at ages 7–10 years. This study is the first to simultaneously use parent and teacher ratings in twin modelling to examine the effects of different raters on the two behavioural dimensions of ADHD. The findings indicated that hyperactivity–impulsivity and inattention load on to separate latent factors that represent a common behavioural view for both parents and teachers, although there are additional aspects to the observations of these behaviours that are unique to each type of rater. The findings further indicate some shared aetiology for hyperactivity–impulsivity and inattention as measured by both parent and teacher ratings, in agreement with previous findings on the aetiology of the two symptom dimensions of ADHD
Midlife managerial experience is linked to late life hippocampal morphology and function
An active cognitive lifestyle has been suggested to have a protective role in the long-term maintenance of cognition. Amongst healthy older adults, more managerial or supervisory experiences in midlife are linked to a slower hippocampal atrophy rate in late life. Yet whether similar links exist in individuals with Mild Cognitive Impairment (MCI) is not known, nor whether these differences have any functional implications. 68 volunteers from the Sydney SMART Trial, diagnosed with non-amnestic MCI, were divided into high and low managerial experience (HME/LME) during their working life. All participants underwent neuropsychological testing, structural and resting-state functional MRI. Group comparisons were performed on hippocampal volume, morphology, hippocampal seed-based functional connectivity, memory and executive function and self-ratings of memory proficiency. HME was linked to better memory function (p = 0.024), mediated by larger hippocampal volume (p = 0.025). More specifically, deformation analysis found HME had relatively more volume in the CA1 sub-region of the hippocampus (p < 0.05). Paradoxically, this group rated their memory proficiency worse (p = 0.004), a result correlated with diminished functional connectivity between the right hippocampus and right prefrontal cortex (p < 0.001). Finally, hierarchical regression modelling substantiated this double dissociation
The Genetic Association Between ADHD Symptoms and Reading Difficulties: The Role of Inattentiveness and IQ
Previous studies have documented the primarily genetic aetiology for the stronger phenotypic covariance between reading disability and ADHD inattention symptoms, compared to hyperactivity-impulsivity symptoms. In this study, we examined to what extent this covariation could be attributed to “generalist genes” shared with general cognitive ability or to “specialist” genes which may specifically underlie processes linking inattention symptoms and reading difficulties. We used multivariate structural equation modeling on IQ, parent and teacher ADHD ratings and parent ratings on reading difficulties from a general population sample of 1312 twins aged 7.9–10.9 years. The covariance between reading difficulties and ADHD inattention symptoms was largely driven by genetic (45%) and child-specific environment (21%) factors not shared with IQ and hyperactivity-impulsivity; only 11% of the covariance was due to genetic effects common with IQ. Aetiological influences shared among all phenotypes explained 47% of the variance in reading difficulties. The current study, using a general population sample, extends previous findings by showing, first, that the shared genetic variability between reading difficulties and ADHD inattention symptoms is largely independent from genes contributing to general cognitive ability and, second, that child-specific environment factors, independent from IQ, also contribute to the covariation between reading difficulties and inattention symptoms
- …