Microrheology of colloidal dispersions: Shape matters

We consider a “probe” particle translating at constant velocity through an otherwise quiescent dispersion of colloidal “bath” particles, as a model for particle-tracking microrheology experiments in the active (nonlinear) regime. The probe is a body of revolution with major and minor semiaxes a and b, respectively, and the bath particles are spheres of radii b. The probe's shape is such that when its major or minor axis is the axis of revolution the excluded-volume, or contact, surface between the probe and a bath particle is a prolate or oblate spheroid, respectively. The moving probe drives the microstructure of the dispersion out of equilibrium; counteracting this is the Brownian diffusion of the bath particles. For a prolate or oblate probe translating along its symmetry axis, we calculate the nonequilibrium microstructure to first order in the volume fraction of bath particles and over the entire range of the Péclet number (Pe), neglecting hydrodynamic interactions. Here, Pe is defined as the non-dimensional velocity of the probe. The microstructure is employed to calculate the average external force on the probe, from which one can infer a “microviscosity” of the dispersion via Stokes drag law. The microviscosity is computed as a function of the aspect ratio of the probe, â=a/b, thereby delineating the role of the probe's shape. For a prolate probe, regardless of the value of â, the microviscosity monotonically decreases, or “velocity thins,” from a Newtonian plateau at small Pe until a second Newtonian plateau is reached as Pe-->[infinity]. After appropriate scaling, we demonstrate this behavior to be in agreement with microrheology studies using spherical probes [Squires and Brady, “A simple paradigm for active and nonlinear microrheology,” Phys. Fluids 17(7), 073101 (2005)] and conventional (macro-)rheological investigations [Bergenholtz et al., “The non-Newtonian rheology of dilute colloidal suspensions,” J. Fluid. Mech. 456, 239–275 (2002)]. For an oblate probe, the microviscosity again transitions between two Newtonian plateaus: for â3.52 the microviscosity at small Pe is less than at large Pe, which suggests it “velocity thickens” as Pe is increased. This anomalous velocity thickening—due entirely to the probe shape—highlights the care needed when designing microrheology experiments with non-spherical probes

Damping of vertical coronal loop kink oscillations through wave tunneling

The decay rate of vertical kink waves in a curved flux tube is modeled numerically. The full MHD equations are solved for a curved equilibrium flux tube in an arcade geometry and the decay of ψ, the integral over the flux tube of the modulus of the velocity perpendicular to the local magnetic field, is measured. These simulations are 2D and are thus restricted to kink oscillations in the loop plane. The decay rate is found to increase with increasing wavelength, increasing β and decreasing density contrast ratio. The wave tunneling effect is shown to be a possible mechanism for the high decay rate of the recent observed kink oscillation reported by Wang & Solanki (2004)

Professionalising the IT industry: towards the creation of a professional association.

The main aim of the project is to develop a set of guidelines that will enable the current IT profession to evolve into one that can take its place along side the established professions in society today. At this stage of the research we will use a working definition of a profession as ‘a vocation in which a professed knowledge of some department of learning or science is used in its application to the affairs of others or in the practice of an art founded upon it’ (Oxford English Dictionary). This definition is endorsed by Carr-Saunders & Wilson (1964) who go on to further explain that ‘special competence acquired as a result of prolonged and specialised training, is the chief distinguishing characteristic of the professions’ and that ‘a profession can only be said to exist when there are bonds between practitioners, and these bonds can take but one shape – that of formal association’. By understanding the core elements of what constitutes a profession and learning from the workings of other professions, I will identify the differences and unique elements of the IT profession and what lessons can be learnt to help enhance the professional status of IT. The impact of the IT industry and the relationship with other professions will be taken into account in defining the professional model for IT. The main beneficiaries of the research will be those organisation and institutions interested in growing and cultivating the IT profession. The beneficiaries of the research will be: • Computer Societies There are many IT organisations working to represent the interest of IT professionals in each country. This research will help identify the differences between them and the elements of a professional body. It may well be that these organisations are the vehicle through which improvements can be made towards creating an IT profession. • IT Industry Companies (e.g. IBM) which have a vested interest in attracting, recruiting, motivating and retaining IT staff. It is also of key interest to them that their staff are well qualified, professional and works in an ethical manner. The increased attractiveness of IT as a career will benefit them while the improved standing of the profession within the society will help grow their business • Academic Institutions Academic Institutions play a key role in the development of the next generation of IT professionals and hence will benefit from any recommendations from this project that will improve the level of professionalism. These recommendations may be applicable to the designers of future IT educational material, structure and content. • The State and Regulatory Bodies All professionals operate within the boundaries conferred on them by the state and regulatory bodies. Throughout this research recommendations may be identified that suggest changes to the relationship between the State and IT Professional organisations. • IT Professionals Many of the changes required to improve the IT Profession come from the IT Professionals themselves. Its members must want to make the changes and be willing to take action to make the changes happen. How individual professionals act will have a big impact on the image of the profession as a whole. While developing the research proposal, I have had informal conversations with a number of stakeholders: • Colleagues and members of the IT profession. The general consensus was that the IT profession had grown very fast over the last 20 years and is probably now entering a maturing phase. There was a feeling that the timing is right now in defining the future shape of the profession. • IT recruitment. The sharp decline in the number of IT students selecting IT as university courses is a concern for most societies. The apparent contradiction of skills shortages and lack of interest in IT needs to be addressed. It was felt that the unstructured nature of the profession coupled with poor image problems contributes to the decline of interest in IT. A strong professional body for IT would help address this. • IBM Management Like the rest of the IT industry, IBM is facing the same challenges of recruiting, retaining and motivating IT staff. This project is in support of IBM’s interest and other initiatives in this area. • Council of European Professional Informatics Societies (CEPIS) CEPIS is an umbrella organisation representing Computer Societies in the majority of European counties. Following discussions with senior officers of this organisation they confirmed that they had identified the area of professionalism within the IT industry as one of their top focus areas. They were excited about my area of research and offered their endorsement when contacting the various Computer Societies. In general, there was consensus regarding the aims of the project and its timing from the main interested stakeholders of this research

A new resistance function for two rigid spheres in a uniform compressible low-Reynolds-number flow

The pressure moment of a rigid particle is defined as the trace of the first moment of the surface stress acting on the particle. We calculate the pressure moments of two unequal rigid spheres immersed in a uniform compressible linear flow, using twin multipole expansions and lubrication theory. Following the practice established in previous studies on two-body hydrodynamic interactions at low Reynolds number, the results are expressed in terms of a new (stresslet) resistance function

On the bulk viscosity of suspensions

The bulk viscosity of a suspension relates the deviation of the trace of the macroscopic or averaged stress from its equilibrium value to the average rate of expansion. For a suspension the equilibrium macroscopic stress is the sum of the fluid pressure and the osmotic pressure of the suspended particles. An average rate of expansion drives the suspension microstructure out of equilibrium and is resisted by the thermal motion of the particles. Expressions are given to compute the bulk viscosity for all concentrations and all expansion rates and shown to be completely analogous to the well-known formulae for the deviatoric macroscopic stress, which are used, for example, to compute the shear viscosity. The effect of rigid spherical particles on the bulk viscosity is determined to second order in volume fraction and to leading order in the Péclet number, which is defined as the expansion rate made dimensionless with the Brownian time scale. A repulsive hard-sphere-like interparticle force reduces the hydrodynamic interactions between particles and decreases the bulk viscosity

Mapping systemic risk: critical degree and failures distribution in financial networks

The 2008 financial crisis illustrated the need for a thorough, functional understanding of systemic risk in strongly interconnected financial structures. Dynamic processes on complex networks being intrinsically difficult, most recent studies of this problem have relied on numerical simulations. Here we report analytical results in a network model of interbank lending based on directly relevant financial parameters, such as interest rates and leverage ratios. Using a mean-field approach, we obtain a closed-form formula for the "critical degree", viz. the number of creditors per bank below which an individual shock can propagate throughout the network. We relate the failures distribution (probability that a single shock induces $F$ failures) to the degree distribution (probability that a bank has $k$ creditors), showing in particular that the former is fat-tailed whenever the latter is. Our criterion for the onset of contagion turns out to be isomorphic to the condition for cooperation to evolve on graphs and social networks, as recently formulated in evolutionary game theory. This remarkable connection supports recent calls for a methodological rapprochement between finance and ecology.Comment: 19 pages, 4 figure

Data analysis of continuous gravitational wave: Fourier transform-I

We present the Fourier Transform of a continuous gravitational wave. We have analysed the data set for one day observation time and our analysis is applicable for arbitrary location of detector and source. We have taken into account the effects arising due to rotational as well as orbital motions of the earth.Comment: Accepted in MNRAS, 22 pages, 9 figure