13,307 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
Consistent particle-based algorithm with a non-ideal equation of state
A thermodynamically consistent particle-based model for fluid dynamics with
continuous velocities and a non-ideal equation of state is presented. Excluded
volume interactions are modeled by means of biased stochastic multiparticle
collisions which depend on the local velocities and densities. Momentum and
energy are exactly conserved locally. The equation of state is derived and
compared to independent measurements of the pressure. Results for the kinematic
shear viscosity and self-diffusion constants are presented. A caging and
order/disorder transition is observed at high densities and large collision
frequency.Comment: 7 pages including 4 figure
Chemistry of butter and butter making I. A comparison of four methods for the analysis of butter with an explanation of a discrepancy found to exist in the fat determinations
1. A modification of the A. O. A. C. method for complete butter analysis is described. The method is considered applicable for use in control laboratories, in which eight or more samples are analyzed at a time.
2. This method, together with the Kohman and Mojonnier analyses for butter, was compared with the A. O. A. C. method as a standard. The modified official and the Kohman methods check closely with the A. O. A. C. method for all constituents of butter. With the 50 samples analyzed in duplicate the fat content was 0.22 percent lower by the Mojonnier method (abnormal values not included-see table I ) than by the A. O. A. C. method. About 10 percent of the Mojonnier analyses varied widely from the duplicates and from the A. O. A. C. method.
3. The variation between duplicates is attributed (a) to peculiarities in the emulsification of the fat in the extraction flasks, which caused incomplete extraction, and (b) to a blowing out of ether-fat solution around the stoppers when these were removed or to both.
4. Data are presented which show that the value by which normal Mojonnier fat determinations are lower than the A. O. A. C. method is equivalent to the fatty acids, which are not extracted in the Mojonnier procedure but which appear as fat with the A. O. A. C. analysis. It is further shown that this difference was larger, as the rancidity of the butter increased.
5. The modified official method is a rapid method and is considered sufficiently accurate for the analysis of good quality butter in control laboratory work. Likewise the A. O. A. C. method is considered to be accurate as an analytical standard for the analysis of good quality butter. The Kohman method as outlined is a rapid method sufficiently accurate for plant use. All three methods give values for fat which are too high by an appreciable amount for the analysis of rancid butter.
6. The Mojonnier and the la-gram extraction methods give a closer approximation of the true fat value of rancid butter than do the dry extraction methods (A. O. A. C., modified official and Kohman)
Rapid acid tests for cream
The Farrington rapid acid test has been employed for many years by the fluid milk industry for grading samples on the borderline of acidity set for their purchase. In performing the test, neutralizing solutions (containing phenolphthalein) used are of such strength that 1 dipper of solution added to 1 dipper (of the same size) of milk gives a pink mixture if the milk is 0.1 percent acid or less; 2 dippers of neutralizing solution grade at the 0.2 percent point and so on. By adjusting the neutralizing strength the grades corresponding to 1 and 2 dippers of neutralizing solution to 1 of milk can be made 0.2 and 0.4 percent, respectively, instead of 0.1 and 0.2 percent. This is the strength of the solution used for cream grading
Transmission of temporally correlated spike trains through synapses with short-term depression
Short-term synaptic depression, caused by depletion of releasable neurotransmitter, modulates the strength of neuronal connections in a history-dependent manner. Quantifying the statistics of synaptic transmission requires stochastic models that link probabilistic neurotransmitter release with presynaptic spike-train statistics. Common approaches are to model the presynaptic spike train as either regular or a memory-less Poisson process: few analytical results are available that describe depressing synapses when the afferent spike train has more complex, temporally correlated statistics such as bursts. Here we present a series of analytical results—from vesicle release-site occupancy statistics, via neurotransmitter release, to the post-synaptic voltage mean and variance—for depressing synapses driven by correlated presynaptic spike trains. The class of presynaptic drive considered is that fully characterised by the inter-spike-interval distribution and encompasses a broad range of models used for neuronal circuit and network analyses, such as integrate-and-fire models with a complete post-spike reset and receiving sufficiently short-time correlated drive. We further demonstrate that the derived post-synaptic voltage mean and variance allow for a simple and accurate approximation of the firing rate of the post-synaptic neuron, using the exponential integrate-and-fire model as an example. These results extend the level of biological detail included in models of synaptic transmission and will allow for the incorporation of more complex and physiologically relevant firing patterns into future studies of neuronal networks
Chemistry of butter and butter making IV. The relationships among the cream acidity, the churning loss and the churning time
1. The influence of acidity, developed in cream that had been pasteurized while sweet and subsequently ripened, was studied over a pH range (in the buttermilk) from 4.5 to 7.0.
2. Three series of creams, viz., 20, 30 and 37.5 percent fat, were investigated.
3. The losses (calculated as percentage of the total fat) for all three series varied little in the pH range 7.0 to 5.5. In this region the least variation was encountered with 30 percent cream; such tendency in loss changes as was exhibited by 30 percent cream was toward a decreasing loss with decreasing pH, while the 37.5 percent cream losses tended to pass through a. minimum at pH 6.3 to 6.4. From pH 5.5 to pH 4.8 or 4.9 the losses rose to maxima (at 4.8 to 4.9) with 20 and 37.5 percent cream; a slight rise with no definite maximum at pH 4.8 to 4.9 occurred with 30 percent cream. With all three creams a marked change of function in the curves (loss vs. pH of buttermilk) occurred at pH 4.8 to 4.9; the loss dropped sharply and in practically linear fashion from that point to pH 4.5.
4. The above facts (especially the maximum at pH 4.8 to 4.9) were interpreted as indicating that casein plays an important role in the protection of the fat globules in cream, if the churning loss is taken as a measure of protective action.
5. The churning loss data correlated very well with electro-kinetic potentials of the fat globules, determined by Sommer and North and re-presented here.
6. Churning times show closer correlation with pH of buttermilk the lower the fat test of the cream. Other factors such as change in protein to fat ratio, increased viscosity, greater ease of whipping, lower specific gravity, etc., may be involved in affecting the churning times of the richer creams.
7. Churning time data in this and the third bulletin of this series indicate that, if the fat and serum in cream are in proper physical state and chemical equilibrium, no hard and fast rule can be drawn that long or short churning times must be associated with high losses.
8. Data show that the fat test of the buttermilk in low fat (18 to 20 percent), highly ripened creams (pH 4.5 to 4.6) is considerably lower than those for high fat (30 to 37.5 percent), sweet cream (pH 6.5). Calculated as the percentage of the total fat churned, however, the low fat, highly ripened cream losses are approximately equivalent to those for 30 percent sweet cream and are slightly higher than those for 37.5 percent sweet cream. This shows that the American, Australian and New Zealand churning losses compare very favorably with those obtained in Denmark, Germany and Holland.
9. Based on the data presented and others from the literature it was hypothecated that the protective action at the fat globule interface was caused by two types of protective materials- one labile and one non-labile. The latter is closely associated with the fat, presumably on the fat side of the interface, and consists of a protein-phospholipin complex. The former is oriented from the water side of the interface and is composed of all the surface tension lowering constituents of the serum. Of the serum constituents casein probably plays the most important protective role as indicated by certain dairy phenomena.
10. If the validity of the hypothesis presented is assumed, the following explanation of the churning process seems logical: Utilization of the labile protective materials, to stabilize foam interfaces, decreases their concentration at the fat-serum interface. When the labile to non-labile protective material ratio is sufficiently small that the fat globules are in an unstable state, they merge and lose their identity. This merger weakens the forces at the force centers of the fat globules to such an extent that the non-labile materials are released from the fat globule surfaces and are incorporated in the buttermilk, while the fat unites to form butter
Thermodiffusion in model nanofluids by molecular dynamics simulations
In this work, a new algorithm is proposed to compute single particle
(infinite dilution) thermodiffusion using Non-Equilibrium Molecular Dynamics
simulations through the estimation of the thermophoretic force that applies on
a solute particle. This scheme is shown to provide consistent results for
simple Lennard-Jones fluids and for model nanofluids (spherical non-metallic
nanoparticles + Lennard-Jones fluid) where it appears that thermodiffusion
amplitude, as well as thermal conductivity, decrease with nanoparticles
concentration. Then, in nanofluids in the liquid state, by changing the nature
of the nanoparticle (size, mass and internal stiffness) and of the solvent
(quality and viscosity) various trends are exhibited. In all cases the single
particle thermodiffusion is positive, i.e. the nanoparticle tends to migrate
toward the cold area. The single particle thermal diffusion 2 coefficient is
shown to be independent of the size of the nanoparticle (diameter of 0.8 to 4
nm), whereas it increases with the quality of the solvent and is inversely
proportional to the viscosity of the fluid. In addition, this coefficient is
shown to be independent of the mass of the nanoparticle and to increase with
the stiffness of the nanoparticle internal bonds. Besides, for these
configurations, the mass diffusion coefficient behavior appears to be
consistent with a Stokes-Einstein like law
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