Design of experiments to study the impact of process parameters on droplet size and development of non-invasive imaging techniques in tablet coating

Atomisation of an aqueous solution for tablet film coating is a complex process with multiple factors determining droplet formation and properties. The importance of droplet size for an efficient process and a high quality final product has been noted in the literature, with smaller droplets reported to produce smoother, more homogenous coatings whilst simultaneously avoiding the risk of damage through over-wetting of the tablet core. In this work the effect of droplet size on tablet film coat characteristics was investigated using X-ray microcomputed tomography (XμCT) and confocal laser scanning microscopy (CLSM). A quality by design approach utilising design of experiments (DOE) was used to optimise the conditions necessary for production of droplets at a small (20 μm) and large (70 μm) droplet size. Droplet size distribution was measured using real-time laser diffraction and the volume median diameter taken as a response. DOE yielded information on the relationship three critical process parameters: pump rate, atomisation pressure and coating-polymer concentration, had upon droplet size. The model generated was robust, scoring highly for model fit (R2 = 0.977), predictability (Q2 = 0.837), validity and reproducibility. Modelling confirmed that all parameters had either a linear or quadratic effect on droplet size and revealed an interaction between pump rate and atomisation pressure. Fluidised bed coating of tablet cores was performed with either small or large droplets followed by CLSM and XμCT imaging. Addition of commonly used contrast materials to the coating solution improved visualisation of the coating by XμCT, showing the coat as a discrete section of the overall tablet. Imaging provided qualitative and quantitative evidence revealing that smaller droplets formed thinner, more uniform and less porous film coats

Time-Dependent Ventilation Flows Driven by Opposing Wind and Buoyancy

We consider transient flow in a box containing an isolated buoyancy source, ventilated by a windward high-level opening and a leeward low-level opening, so that prevailing wind acts to oppose buoyancy-driven flow. Hunt & Linden (J. Fluid Mech., vol. 527, 2005, p. 27) demonstrated that two stable steady states can exist above a critical wind strength: buoyancy-driven displacement ventilation with a two-layer stratification and wind-driven mixing ventilation with the whole interior contaminated by buoyant fluid. We present two time-dependent models for this system: a nonlinear ordinary differential equation (ODE) model following Kaye & Hunt (J. Fluid Mech., vol. 520, 2004, p. 135), assuming ‘perfect’ vertical mixing of fluid within each layer, and a partial differential equation model assuming zero vertical mixing, following Germeles (J. Fluid Mech., vol. 71, 1975, p. 601)

Mixing efficiency in high-aspect-ratio Rayleigh–Taylor experiments

High-Reynolds-number turbulence generated by Rayleigh–Taylor instability is known to be much more efficient at mixing density-stratified fluids than mixing driven by most other mechanisms. We demonstrate here that the final state of the instability in a high-aspect-ratio environment is uniform, corresponding to the maximum mixing efficiency possible. The efficiency of the mixing appears to be constant throughout the evolution of the flow, despite the flow changing from being inertially dominated with a high-Reynolds-number initial growth of the mixing zone, to a viscously dominated late-time decay. The initial growth, in which vertical transport takes the form of a turbulent diffusion, is characterized by a t2/5 power law, indicating that the diffusivity is not constant but rather decreases as the strength of the unstable density gradient driving the flow decreases. As the unstable stratification is reduced, inertia begins to play a lesser role with molecular viscosity taking over the controlling dynamics, but with vertical transport still dominated by parcels of fluid needing to pass around many vortexlike structures during the exponential decay toward the well-mixed final state

Mixing efficiency in high-aspect-ratio Rayleigh–Taylor experiments

High-Reynolds-number turbulence generated by Rayleigh–Taylor instability is known to be much more efficient at mixing density-stratified fluids than mixing driven by most other mechanisms. We demonstrate here that the final state of the instability in a high-aspect-ratio environment is uniform, corresponding to the maximum mixing efficiency possible. The efficiency of the mixing appears to be constant throughout the evolution of the flow, despite the flow changing from being inertially dominated with a high-Reynolds-number initial growth of the mixing zone, to a viscously dominated late-time decay. The initial growth, in which vertical transport takes the form of a turbulent diffusion, is characterized by a t2/5 power law, indicating that the diffusivity is not constant but rather decreases as the strength of the unstable density gradient driving the flow decreases. As the unstable stratification is reduced, inertia begins to play a lesser role with molecular viscosity taking over the controlling dynamics, but with vertical transport still dominated by parcels of fluid needing to pass around many vortexlike structures during the exponential decay toward the well-mixed final state

The ethical ambivalence of resistant violence: notes from postcolonial south Asia

In the face of mounting militarism in south Asia, this essay turns to anti-state, ‘liberatory’ movements in the region that employ violence to achieve their political aims. It explores some of the ethical quandaries that arise from the embrace of such violence, particularly for feminists for whom political violence and militarism is today a moot point. Feminist responses towards resistant political violence have, however, been less straightforward than towards the violence of the state, suggesting a more ambivalent ethical position towards the former than the latter. The nature of this ambivalence can be located in a postcolonial feminist ethics that is conceptually committed to the use of political violence in certain, albeit exceptional circumstances on the basis of the ethical ends that this violence (as opposed to other oppressive violence) serves. In opening up this ethical ambivalence – or the ethics of ambiguity, as Simone de Beauvoir says – to interrogation and reflection, I underscore the difficulties involved in ethically discriminating between forms of violence, especially when we consider the manner in which such distinctions rely on and reproduce gendered modes of power. This raises particular problems for current feminist appraisals of resistant political violence as an expression of women's empowerment and ‘agency’