Linear viscoelasticity of emulsions : I. The effect of an interfacial film on the dynamic viscosity of nondilute emulsions

The dynamic viscosity of nondilute monodisperse emulsions is calculated by using a cell model. Two possibilities for describing the mechanical properties of the interfacial film between the internal and the external phase are considered: (A) the film is assigned a two-dimensional linear viscoelastic behavior and (B) the film is treated as a shell with finite thickness containing a Newtonian liquid. The resulting expressions for the dynamic viscosity show that model B has two relaxation times and model A has at least two or more. If a Voigt-Kelvin model is used to describe the interfacial rheology, model A will also have just two relaxation times. The results obtained may be used to interpret measurements on emulsions in terms of microscopic parameters of these emulsions

Linear viscoelasticity of emulsions : II. Measurements of the linear viscoelastic behavior of emulsions in the kilohertz range

Linear viscoelasticity of emulsions in shear deformation in the kilohertz range is demonstrated experimentally. In order to avoid complications due to inertia effects, emulsions with small droplet sizes are studied. The preliminary measurements are interpreted as being the result of droplet deformations. If this interpretation is correct, measurements of the dynamic viscosity of emulsions may be used to obtain information about the mechanical properties of the interfacial layer between droplets and the continuous phase. In particular, the evaluation of the interfacial tension of emulsion droplets from bulk properties might be possible using this technique

Application of Dupont’s Dirty Dozen Framework to Commercial Aviation Maintenance Incidents

This study examined the 12 preconditions for maintenance errors commonly known as the Dirty Dozen and applied them to actual incident and accident data provided by a participating airline (PA). The data provided by the PA consisted of Maintenance Event Reports (MERs) (reactive), Maintenance Operations Safety Assessment (MOSA) reports (proactive), and the results of the 2017 Maintenance Climate Awareness Survey (MCAS) (subjective). The MER and MOSA reports were coded by aviation maintenance subject matter experts (SMEs) using the 12 Dirty Dozen categories as the coding scheme, while the MCAS responses were parsed according to the precondition category they best represented. An examination and qualitative analysis of these data sets as they related to the Dirty Dozen categories answered the following research questions: (1) How does the reactive data (MER) analysis compare to the proactive (MOSA) analysis in terms of the Dirty Dozen? Do they echo similar Dirty Dozen categories, or do they seem to reflect different aspects of the Dirty Dozen? (2) What other preconditions for maintenance error become apparent from the analyses? What do they have in common? How complete is the Dirty Dozen? (3) What insights can be gleaned from the subjective report data (MCAS) with regard to maintenance personnel’s perceptions of the organization’s safety culture? The results revealed not only the presence of each Dirty Dozen category to some degree, but also the difference in sensitivity of the MER (reactive) and MOSA (proactive) to the 12 Dirty Dozen categories. Recommendations for practice and future research are discussed

Topology and Sizes of HII Regions during Cosmic Reionization

We use the results of large-scale simulations of reionization to explore methods for characterizing the topology and sizes of HII regions during reionization. We use four independent methods for characterizing the sizes of ionized regions. Three of them give us a full size distribution: the friends-of-friends (FOF) method, the spherical average method (SPA) and the power spectrum (PS) of the ionized fraction. These latter three methods are complementary: While the FOF method captures the size distribution of the small scale H~II regions, which contribute only a small amount to the total ionization fraction, the spherical average method provides a smoothed measure for the average size of the H~II regions constituting the main contribution to the ionized fraction, and the power spectrum does the same while retaining more details on the size distribution. Our fourth method for characterizing the sizes of the H II regions is the average size which results if we divide the total volume of the H II regions by their total surface area, (i.e. 3V/A), computed in terms of the ratio of the corresponding Minkowski functionals of the ionized fraction field. To characterize the topology of the ionized regions, we calculate the evolution of the Euler Characteristic. We find that the evolution of the topology during the first half of reionization is consistent with inside-out reionization of a Gaussian density field. We use these techniques to investigate the dependence of size and topology on some basic source properties, such as the halo mass-to-light ratio, susceptibility of haloes to negative feedback from reionization, and the minimum halo mass for sources to form. We find that suppression of ionizing sources within ionized regions slows the growth of H~II regions, and also changes their size distribution. Additionally, the topology of simulations including suppression is more complex. (abridged

Calculation of the effect of inertia on the dynamic viscosity of dilute emulsions in a pure straining motion

The dynamic viscosity of a dilute emulsion is calculated for a pure straining motion. The emulsion consists of almost spherical drops of a Newtonian fluid immersed in another Newtonian fluid. The oscillating velocity field of the flow is derived from the Navier-Stokes equation, in which the linear inertia term is included, whereas the non-linear one is neglected. The dynamic viscosity is determined with the aid of a cell model. The results are calculated numerically and typical results are presented graphically. The influence of inertia on the dynamic viscosity appears to be appreciable. Special cases presented in the literature, frequency or density zero and rigid spheres, are confirmed

The Enigmatic HH 255

To gain insight into the nature of the peculiar Herbig-Haro object HH 255 (also called Burnham's nebula), we use previously published observations to derive information about the emission line fluxes as a function of position within HH 255 and compare them with the well-studied, and relatively well-behaved bow shock HH 1. There are some qualitative similarities in the H$\alpha$ and [O III] 5007 lines in both objects. However, in contrast to the expectation of the standard bow shock model, the fluxes of the [O I] 6300, [S II] 6731, and [N II] 6583 lines are essentially constant along the axis of the flow, while the electron density decreases, over a large distance within HH 255. We also explore the possibility that HH 255 represents the emission behind a standing or quasi-stationary shock. The shock faces upwind, and we suggest, using theoretical arguments, that it may be associated with the collimation of the southern outflow from T Tauri. Using a simplified magnetohydrodynamic simulation to illustrate the basic concept, we demonstrate that the existence of such a shock at the north edge of HH 255 could indeed explain its unusual kinematic and ionization properties. Whether or not such a shock can explain the detailed emission line stratification remains an open question.Comment: Accepted by PASP, 12 pages including 8 figure

Scaling relations between structure and rheology of ageing casein particle gels

Mellema, M. (Michel), Scaling relations between structure and rheology of ageing casein particle gels , PhD Thesis, Wageningen University, 150 + 10 pages, references by chapter, English and Dutch summaries (2000).The relation between (colloidal) interactions, structure and rheology of particle gels is discussed, especially the properties and the spontaneous ageing behavior of rennet-induced casein(ate) or skim milk gels.Methods involved were Brownian dynamics simulations, confocal microscopy, permeametry and rheometry (large- and small deformations). A categorization of relevant (fractal) scaling models and types of structural rearrangements (particularly those affecting the rheological properties) for particle gels has been made, and applied to experimental data of rennet-induced casein gels.Using Brownian dynamics simulations, the relation between colloidal particle interactions and gel structure was obtained. The simulation model used, included a repulsive barrier between the particles; the bonds formed were irreversible. The aggregation was delayed by a long-range repulsive barrier, and a high fractal dimensionality of the gels (2.4-2.5) resulted. This value was independent of the details of the interactions and volume fraction of particles in the range of 3-10 vol% particles. The simulation results agreed well with experimental results on rennet-induced aggregation and gelation in skim milk.The (fractal) structure of rennet-induced casein gels, as studied by confocal scanning laser microscopy and permeametry was monitored during aging, at various pH values (5.3-6.65) and temperatures (20-30 °C). At low pH, a gradual coarsening of the structure was observed; the size of the pores and compact structures increased. This was reflected in a decrease in apparent fractal dimensionality (from 2.4 to 1.7), and in an increase in pore size and lower cut-off length of the fractal regime (from 0.5 to 1.5μm).A scaling model was developed for the rheological behaviour of particle gels as a function of structure and particle volume fraction. The main structural parameters are the fractal dimensionality, the size of the compact building blocks and two parameters describing the number of deformable links in the strands and the dominating type of deformation of these links. Application to rheological data (storage modulus, maximum linear strain, yield stress) as a function of volume fraction (5-9 vol%) showed that rennet-induced casein gels contain straight, elastic strands.Application of the model to results on measurements of the storage modulus during ageing, showed that at low pH rearrangements induce a dramatic increase in compact building block size and early disappearence of the fractal structure. It is concluded that the main types of rearrangements in rennet-induced casein gels after gelation, are particle fusion and stretching and breaking of strands.Keywords : colloid, structure, rheology, casein, rennet, gel, fractal scaling, computer simulation, confocal microscopy.</p

Observational Study of the Multistructured Planetary Nebula NGC 7354

We present an observational study of the planetary nebula (PN) NGC 7354 consisting of narrowband Halpha and [NII]6584 imaging as well as low- and high-dispersion long-slit spectroscopy and VLA-D radio continuum. According to our imaging and spectroscopic data, NGC 7354 has four main structures: a quite round outer shell and an elliptical inner shell, a collection of low-excitation bright knots roughly concentrated on the equatorial region of the nebula, and two symmetrical jet-like features, not aligned either with the shells' axes, or with each other. We have obtained physical parameters like electron temperature and electron density as well as ionic and elemental abundances for these different structures. Electron temperature and electron density slightly vary throughout the nebula. The local extinction coefficient c_Hbeta shows an increasing gradient from south to north and a decreasing gradient from east to west consistent with the number of equatorial bright knots present in each direction. Abundance values show slight internal variations but most of them are within the estimated uncertainties. In general, abundance values are in good agreement with the ones expected for PNe. Radio continuum data are consistent with optically thin thermal emission. We have used the interactive three-dimensional modeling tool SHAPE to reproduce the observed morphokinematic structures in NGC 7354 with different geometrical components. Our SHAPE model is in very good agreement with our imaging and spectroscopic observations. Finally, after modeling NGC 7354 with SHAPE, we suggest a possible scenario for the formation of the nebula.Comment: Accepted for publication in AJ, 12 pages, 8 figure

Magnetic Collimation in PNe

Recent studies have focused on the the role of initially weak toroidal magnetic fields embedded in a stellar wind as the agent for collimation in planetary nebulae. In these models the wind is assumed to be permeated by a helical magnetic field in which the poloidal component falls off faster than the toroidal component. The collimation only occurs after the wind is shocked at large distances from the stellar source. In this paper we re-examine assumptions built into this ``Magnetized Wind Blown Bubble'' (MWBB) model. We show that a self-consistent study of the model leads to a large parameter regime where the wind is self-collimated before the shock wave is encountered. We also explore the relation between winds in the MWBB model and those which are produced via magneto-centrifugal processes. We conclude that a more detailed examination of the role of self-collimation is needed in the context of PNe studies

Stellar Outflows Driven by Magnetized Wide-Angle Winds

We present two-dimensional, cylindrically symmetric simulations of hydrodynamic and magnetohydrodynamic (MHD) wide-angle winds interacting with a collapsing environment. These simulations have direct relevance to young stellar objects (YSOs). The results may also be of use in the study of collimated outflows from proto-planetary and planetary nebulae. We study a range of wind configurations consistent with asymptotic MHD wind collimation. The degree of collimation is parameterized by the ratio of the wind density at the pole to that of the equator. We find that a toroidal magnetic field can have a significant influence on the resulting outflow, giving rise to a very dense, jet-like flow in the post-shock region. The properties of the flow in this region are similar to the asymptotic state of a collimated MHD wind. We conclude that wide-angle MHD winds are quite likely capable of driving molecular outflows. Due to difficulty in treating MHD winds ab-initio in simulations we choose magnetic field strengths in the wind consistent slow magnetic rotators. While MHD launched winds will be in the fast rotator regime we discuss how our results, which rely on toroidal pinch effects, will hold for stronger field strengths