Mapping electrospray modes and droplet size distributions for chitosan solutions in unentangled and entangled concentration regimes

Chitosan is a natural polymer that exists as a polyelectrolyte in acidic aqueous solutions. The solution viscosity strongly depends on the polymer’s molecular weight and concentration in the solution, and the solution pH. Microparticle production by electrospraying is of significant interest in the drug delivery applications of this biocompatible polymer. We report herein a study aimed at empirical understanding of the influence of electrospray process parameters on the size distribution of microdroplets of chitosan solutions with different molecular weights and concentrations. How the nature of interchain interactions in the solution affects the electrospray mode, was studied for different applied voltages, $V$, over a wide range of solution flow rates, $Q$. Stable cone-jets were observed only for solutions in the unentangled regime. Unlike solutions of non-polymeric electrolytes of comparable viscosities, the chitosan droplet size distribution generally showed a strong dependence not only on $Q$ but also on $V$ and the solution viscosity. Carrying out the electrospraying process at a higher voltage in the stable cone-jet mode resulted in smaller and more narrowly dispersed droplets. For the droplets produced in this mode, We～Re^1.79, where $\mathit{We}$ and $\mathit{Re}$ are the droplet Weber and Reynolds numbers, respectively. The order of dependence of the Sauter mean droplet diameter on $Q$ was found to be 0.26. Diameters of droplets produced in (or close to) the precession mode exhibited a significantly weaker dependence on $Q$. For a given wt% concentration in solution, a polymer of lower molecular weight resulted in a lower droplet size polydispersity index.     

An analysis of measurement of solute segregation in Ni-base superalloys using X-ray spectroscopy

The extent of solute segregation in a multi-component Ni-base superalloy, IN713LC, is analysed using X-ray spectroscopy techniques. It is demonstrated in this study that such an analysis is strongly influenced by the volume averaging of γ and γ′ phases sampled by the electron beam; the γ′ having precipitated from the γ phase below the γ′ solvus temperature in the solid-state. It is found that the size of the precipitate (γ′) in relation to the interaction volume is crucial; volume averaging is dominant when the precipitate size (γ′) is comparable to the interaction volume. Accurate interpretation of the measurements is essential when they are used to infer the micro-segregation and solute partitioning during solidification. Implications of this analysis in the assessment of segregation during solidification in Ni-base superalloys are addressed.     

On a local predictor of shear instability and the initiation of local turbulence

The stability of a local laminar shear flow and its transition into turbulent flow is considered as a local phenomenon. This transition may remain local, in which case the flow field is partially laminar and partially turbulent, or it may spread and make the whole field turbulent. One of the applications of this analysis is the prediction of local heat-convection rates, which are enhanced by local turbulence. Another application is in heart-lung blood pumps, where excessive shear rates are detrimental to red blood cells.The analysis is Lagrangian, which concentrates on the stability of a fluid particle in maintaining its position in a laminar shear flow. This stability is shown to depend on the magnitude of a non-dimensional parameter, namely the local Reynolds numberRe _L =ha ²/v whereh is the local shear rate,a is the particle radius andv is the fluid's kinematic viscosity. It is shown that when, locally,Re _L > 530, the flow is, locally, unstable. The application of this criterion is simple and direct, and in certain cases it can be shown that the resulting unstable flow is indeed turbulent.Because the analysis relies on an experimental coefficient which has been obtained for a rigid sphere, rather than for a fluid particle, the criterion is introduced at this stage as a conjecture. Several examples are presented which demonstrate the criterion's ability to yield correct predictions for instability.     

Grain refining aluminium foundry alloys with commercial Al–B master alloys

Abstract

In Al–3B master alloy, a higher fraction of borides is of the AlB₂ variety, while in Al–8B alloy, the predominant species is the AlB₁₂ phase. AlB₁₂ is less stable than AlB₂ and is engaged in exchange reactions, leading to the formation of transition metal diborides that subsequently settle at the bottom of the melt. Hence, AlB₁₂ is involved more in precipitating transition elements than in refining the grain structure. With predominantly AlB₁₂ particles, Al–8B master alloys are better suited for the removal of transition metal impurities in the manufacture of aluminium conductors. Al–3B, on the other hand, is a better grain refiner as the majority of its borides are of the AlB₂ variety. Which of the two master alloys is used in grain refinement does not make a difference once the transition metal impurities have been precipitated. B is dedicated to grain refinement in an impurity free aluminium melt and produces exceptionally small equiaxed grains across the section of the samples.     

A review of the J and I integrals and their implications for crack growth resistance and toughness in ductile fracture

The application of the J and the I-integrals to ductile fracture are discussed. It is shown that, because of the finite size of the fracture process zone (FPZ), the initiation value of the J-integral is specimen dependent even if the plastic constraint conditions are constant. The paradox that the I-integral for steady state elasto-plastic crack growth is apparently zero is examined. It is shown that, if the FPZ at the crack tip is modelled, the I-integral is equal to the work performed in its fracture. Thus it is essential to model the fracture process zone in ductile fracture. The I-integral is then used to demonstrate that the breakdown in applicability of the J-integral to crack growth in ductile fracture is as much due to the inclusion in the J-integral of progressively more work performed in the plastic zone as it is to non-proportional deformation during unloading behind the crack tip. Thus J _R -curves combine the essential work of fracture performed in the FPZ with the plastic work performed outside of the FPZ. These two work terms scale differently and produce size and geometry dependence. It is suggested that the future direction of modelling in ductile fracture should be to include the FPZ. Strides have already been made in this direction.     

On the change in the modulus of elasticity with a decrease in the size of a nanocrystal

A model of nanocrystal in the form of a rectangular parallelepiped with a variable surface shape is used to show that, at high temperatures, modulus of elasticity B decreases with a decrease in size of the nanocrystal N, which is due to the increase in the surface pressure. However, at low temperatures, dependence B(N) is less pronounced and can even rise with a decrease in the nanocrystal size. This is because, at low temperatures, the increase in the surface pressure (which is larger than at high temperatures) leads to an increase in the modulus of elasticity for the entire nanocrystal. The more the nanocrystal shape deviates from the most energetically stable shape, the more pronounced the change in the dependence B(N) is.     

Effect of some metallic impurities on the density of localized states in a-Se80Te20 thin films

The present paper reports the d.c. conductivity measurements at high electric fields in vacuum-evaporated thin films of amorphous Se₈₀Te₂₀, Se₇₅Te₂₀Ge₅ and Se₇₅Te₂₀Sb₅ systems. Current-voltage (I-V) characteristics have been measured at various fixed temperatures. In all the samples, at low electric fields ohmic behavior is observed. However, at high electric fields (E∼10⁴ V/cm), non-ohmic behavior is observed. An analysis of the experimental data confirms the presence of space charge limited conduction (SCLC) in all the glassy materials studied in the present case. From the fitting of the data to the theory of SCLC, the density of defect states (DOS) near Fermi level is calculated. The role of the aforesaid impurities in a-Se₈₀Te₂₀ is found to be entirely different. In case of Sb, an increase in DOS is observed. However, a decrease is observed in case of Ge. The change in DOS on impurity incorporation is explained in terms of the change in structure of these glasses.     

Damage mechanics applied to concrete reinforced with amorphous cast iron fibers,concrete subjected to compression

The evolution of damage in compression, due to microcracking, is investigated in concrete reinforced by amorphous cast iron fibers and compared to damage in non reinforced concrete whose workability and strength were similar. As microcracking is anisotropic, two damage variables are defined, d₁ in the orientation of compression and d₂ in a perpendicular orientation. Damage values are calculated as a function of the loss of stiffness measured during loading-unloading cycles. Since the fiber arrangement is 2D (isotropic transverse), it is shown that damage is delayed in a reinforced concrete stressed in an orientation perpendicular to the fibers. On the other hand, due to fiber/concrete debonding, damage is greater when the compression is applied in an orientation parallel to the fibers. The damage behavior of the non reinforced concrete is intermediate.     

Photoelastic strain measurement in GaP (100) wafers under external stresses

The sign of the absolute value of residual strain measured in LEC-grown GaP wafers with scanning infrared polariscope (SIRP) has been determined by measuring the residual strain in the round-shape wafer with and without compressive load to the diametric edges. Since the external stress induced by the compressive load is additive to the residual strain in wafer, the residual strain measured in the wafer with the compressive load is increased if the sign is negative while it is decreased if the sign is positive. The measurement of residual strain was successively performed in various LEC-grown GaP (100) wafers clamped their diametric edges in a specially-made vise, in which a movable jaw is pulled by a coil spring to slide parallel toward a fixed jaw with a compressive load. It is found from their results that the residual strain component of (S _r ? S _t), where S _r is the radial strain component and S _t is the tangential strain component defined in the cylindrical coordinate system matching to the round-shape wafer, is negative; that is, radially compressive in most wafers.     

Chloride penetration into concrete structures in the marine atmosphere zone – Relationship between deposition of chlorides on the wet candle and chlorides accumulated into concrete

The relationship between chlorides from marine aerosol and chlorides accumulated into concrete is discussed in this paper. The experimental programme comprised an environmental characterisation, with climatic and chloride deposition data, and a study of chloride penetration into concrete based on natural exposure of specimens in a marine atmosphere zone. Results show that salt concentration in marine aerosol strongly decreases in the first meters from the sea. Chlorides present in the atmosphere can be studied using the wet candle method and correlated with chlorides accumulated into concrete. This relationship can be represented by the equation $C_{\text{tot}}=C_0+k_{\text{d}}·\sqrt{D_{\text{ac}}}$, where k_d is a coefficient which depends on concrete and environmental characteristics, C_tot is the average total amount of chlorides accumulated into concrete, C₀ is the chloride content in concrete before exposure and D_ac is the accumulated dry deposition of chlorides.     

Mass-transport enhancement in regions bounded by rigid walls

The mass transport into a fluid bounded by stationary rigid walls in the limit of large Péclet number, Pe, is examined analytically. Two model systems are considered in detail: a stationary cavity and a model involving two concentric rotating cylinders. A macroscopic gradient is imposed between the top and bottom surfaces. It is demonstrated that mass transport into the fluid is enhanced owing to a recirculation zone which is connected to the solid boundary through a boundary layer of thickness O(Pe ^–1/3) in which cross-stream molecular diffusion is balanced by convection. The associated enhancement is large and scales as Pe ^1/3. Our asymptotic analysis is found to be in good agreement with numerical solutions of the full transport equation.     

Generating a Variable from the Tail of the Normal Distribution

The performance of three rules for dealing with outliers in small samples of size n from the normal distribution N(μ, σ²) is investigated when the primary objective of sampling is to obtain an accurate estimate of σ². It is assumed that at most one observation in the sample may be biased, arising from either N(μ + aσ, σ²) or N(μ, (1 + b) σ²). Performance of each rule is measured in terms of “Protection”, the fractional decrease in MSE obtained by using the rule when a biased observation actually is present in the sample. Numerical results have been obtained for n 5 ≤ 11 when μ is known and n = 3 when μ is unknown.     

Measurement of fibre waviness in industrial composite components

Fibre orientation is measured from polished sections of the unidirectional plies of two industrial CFRP components made by resin transfer moulding (RTM) or prepreg/vacuum consolidation. The image analysis technique described by Creighton et al. [Composites: Part A 2001; 32: 221–229] is used to determine the fibre orientation over typically 5 × 5 mm sections. The standard deviation in fibre orientation angle is in the range 0.6–1.8°, being smallest for in-plane waviness of the prepreg component. The length and width of the waviness region along and transverse to the fibres is characterised using autocorrelation. The length is in the range 1.1–4.4 mm, being significantly greater in the prepreg than in the RTM component. The width is in the range 0.37–1.30 mm and is broadly similar across the sample types. It is demonstrated that the image analysis method can also be applied to X-ray images, giving good agreement with results from the polished prepreg samples.     

An analysis of creep crack propagation on the basis of the plastic singular stress field

In order to gain an insight into creep crack propagation problems a mechanics approach is presented in this paper on the basis of the plastic singular stress field near a crack tip under the Mode III and I steady-state creep conditions, in combination with a generalized creep damage hypothesis. Closed form equations which predict rates of creep crack propagation are obtained analytically. According to this analysis, it is found that $\text{b}{}_1\text{(n + 1)}$ is the index which characterizes the effect of damage accumulation in front of a crack, where n is the creep exponent and b₁ is the exponent involved in the creep damage hypothesis used in this analysis. The tendency of the predicted crack propagation behaviors is consistent with experimental data available in the literature.     

The structure of solvent crazes in polystyrene

The structure of crazes grown in polystyrene (PS) immersed inn-heptane and methanol at room temperature has been determined using refractive index measurements, transmission electron microscopy, and fractographic analysis of craze fracture surfaces.n-heptane crazes in thin films exhibit a low number density of thick, load-bearing fibrils whereas methanol crazes consist of a highly interconnected network of fibrils not unlike the craze structure found in crazes grown in air. A row of large voids at the centre line of the craze which is typical of the structure observed in air crazes is not found, however, in either methanol orn-heptane crazes, indicating a different growth mechanism for the solvent crazes. The craze structure found in thin films is in agreement with the void contents determined from refractive index measurements and with the results from scanning electron microscopy of fracture surfaces of bulk crazes grown with methanol andn-heptane. Glass transition temperature measurements of equilibrium swollen PS films giveT _g=91° C for methanol andT _g=6° C forn-heptane. The results suggest that the structure of crazes grown with slowly diffusing crazing agents (methanol andn-heptane) is strongly dependent on whether the growth temperature is above or belowT _g, the glass transition temperature of the plasticized region just ahead of, and in, the craze. IfT _g is below the growth temperature, weak crazes are formed with a large void content. During the growth, thin fibrils break by viscous flow leaving only a small number of load-bearing fibrils. The stress in the neighbourhood of the growing craze is strongly relieved favouring propagation of a single craze. IfT _g is above the growth te'mperature, strong crazes are formed with the fibrils strain-hardened during the growth process. There is hardly any change in stress next to the craze and therefore multiple crazing (craze bundles) is favoured over the propagation of a single craze.     

Power-series solution for the two-dimensional inviscid flow with a vortex and multiple cylinders

The problem of a point vortex and N fixed cylinders in a two-dimensional inviscid fluid is studied and an analytical-numerical solution in the form of an infinite power series for the velocity field is obtained using complex analysis. The velocity distribution for the case of two cylinders is compared with the existing results of the problem of a vortex in an annular region which is conformally mapped onto the exterior of two cylinders. Limiting cases of N cylinders and the vortex, being far away from each other are studied. In these cases, “the dipole approximation” or “the point-island approximation” is derived, and its region of validity is established by numerical tests. The velocity distribution for a geometry of four cylinders placed at the vertices of a square and a vortex is presented. The problem of vortex motion with N cylinders addressed in the paper attracted attention recently owing to its importance in many applications. However, existing solutions using Abelian function theory are sophisticated and the theory is not one of the standard techniques used by applied mathematicians and engineers. Moreover, in the N ≥ 3 cylinder problem, the infinite product involved in the presentation of the Schottky–Klein prime function must also be truncated. So, the approach used in the paper is simple and an alternative to existing methods. This is the main motivation for this study.