5,182 research outputs found
Characterising shear-induced dynamics in flowing complex fluids using differential dynamic microscopy
Microscopic dynamics reveal the origin of the bulk rheological response in
complex fluids. In model systems particle motion can be tracked, but for
industrially relevant samples this is often impossible. Here we adapt
differential dynamic microscopy (DDM) to study flowing highly-concentrated
samples without particle resolution. By combining an investigation of
oscillatory flow, using a novel "echo-DDM" analysis, and steady shear, through
flow-DDM, we characterise the yielding of a silicone oil emulsion on both the
microscopic and bulk level. Through measuring the rate of shear-induced droplet
rearrangements and the flow velocity, the transition from a solid-like to
liquid-like state is shown to occur in two steps: with droplet mobilisation
marking the limit of linear visco-elasticity, followed by the development of
shear localisation and macroscopic yielding. Using this suite of techniques,
such insight could be developed for a wide variety of challenging complex
fluids.Comment: 11 pages, 8 figure
Particle sizing for flowing colloidal suspensions using flow-differential dynamic microscopy
Particle size is a key variable in understanding the behaviour of the
particulate products that underpin much of our modern lives. Typically obtained
from suspensions at rest, measuring the particle size under flowing conditions
would enable advances for in-line testing during manufacture and
high-throughput testing during development. However, samples are often turbid,
multiply scattering light and preventing the direct use of common sizing
techniques. Differential dynamic microscopy (DDM) is a powerful technique for
analysing video microscopy of such samples, measuring diffusion and hence
particle size without the need to resolve individual particles while free of
substantial user input. However, when applying DDM to a flowing sample,
diffusive dynamics are rapidly dominated by flow effects, preventing particle
sizing. Here, we develop "flow-DDM", a novel analysis scheme that combines
optimised imaging conditions, a drift-velocity correction and modelling of the
impact of flow. Flow-DDM allows a decoupling of flow from diffusive motion that
facilitates successful particle size measurements at flow speeds an order of
magnitude higher than for DDM. We demonstrate the generality of the technique
by applying flow-DDM to two separate microscopy methods and flow geometries.Comment: 9 pages, 8 figure
Exposing a dynamical signature of the freezing transition through the sound propagation gap
The conventional view of freezing holds that nuclei of the crystal phase form in the metastable fluid through purely stochastic thermal density fluctuations. The possibility of a change in the character of the fluctuations as the freezing point is traversed is beyond the scope of this perspective. Here we show that this perspective may be incomplete by examination of the time autocorrelation function of the longitudinal current, computed by molecular dynamics for the hard-sphere fluid around its freezing point. In the spatial window where sound is overdamped, we identify a change in the long-time decay of the correlation function at the known freezing points of monodisperse and moderately polydisperse systems. The fact that these findings agree with previous experimental studies of colloidal systems in which particle are subject to diffusive dynamics, suggests that the dynamical signature we identify with the freezing transition is a consequence of packing effects alone
Encapsulated bacteria deform lipid vesicles into flagellated swimmers
We study a synthetic system of motile Escherichia coli bacteria encapsulated inside giant lipid vesicles. Forces exerted by the bacteria on the inner side of the membrane are sufficient to extrude membrane tubes filled with one or several bacteria. We show that a physical coupling between the membrane tube and the flagella of the enclosed cells transforms the tube into an effective helical flagellum propelling the vesicle. We develop a simple theoretical model to estimate the propulsive force from the speed of the vesicles and demonstrate the good efficiency of this coupling mechanism. Together, these results point to design principles for conferring motility to synthetic cells
Run-to-tumble variability controls the surface residence times of E. coli bacteria
Motile bacteria are known to accumulate at surfaces, eventually leading to
changes in bacterial motility and bio-film formation. We use a novel
two-colour, three-dimensional Lagrangian tracking technique, to follow
simultaneously the body and the flagella of a wild-type . We observe long surface residence times and surface escape
corresponding mostly to immediately antecedent tumbling. A motility model
accounting for a large behavioural variability in run-time duration, reproduces
all experimental findings and gives new insights into surface trapping
efficiency
Designing the EMBeRS summer school: Connecting stakeholders in learning, teaching and research
© 2017 Asia-Pacific Society for Computers in Education. All rights reserved. In this paper, we describe our research investigating design, teaching and learning aspects of the EMBeRS Summer School. In 2016, thirteen graduate Environmental Science students participated in a ten-day Summer School to learn about interdisciplinary approaches to researching socio-environmental systems. Using the Employing Model-Based Reasoning in Socio-Environmental Synthesis (EMBeRS) approach, students learned about wicked problems, team composition, systems thinking and modelling, stakeholder management, and communication. They applied this approach to their own research, as well as to a case study, in order to, ultimately, further the EMBeRS approach in their own institutions. Learning sciences researchers, environmental science instructors and learners collaborated in design, teaching, and learning during the 2016 Summer School in order to co-create and co-configure the tasks, social arrangements, and tools for learning, teaching and design. This paper identifies four examples of connections between the stakeholders (researchers, instructors and learners), the tools that facilitated the connection, and the implications for learning, teaching and design
Validation of large-volume batch solar reactors for the treatment of rainwater in field trials in sub-Saharan Africa
The efficiency of two large-volume batch solar reactors [Prototype I (140 L) and II (88 L)] in treating rainwater on-site in a local informal settlement and farming community was assessed. Untreated [Tank 1 and Tank 2-(First-flush)] and treated (Prototype I and II) tank water samples were routinely collected from each site and all the measured physico-chemical parameters (e.g. pH and turbidity, amongst others), anions (e.g. sulphate and chloride, amongst others) and cations (e.g. iron and lead, amongst others) were within national and international drinking water guidelines limits. Culture-based analysis indicated that Escherichia coli, total and faecal coliforms, enterococci and heterotrophic bacteria counts exceeded drinking water guideline limits in 61%, 100%, 45%, 24% and 100% of the untreated tank water samples collected from both sites. However, an 8 hour solar exposure treatment for both solar reactors was sufficient to reduce these indicator organisms to within national and international drinking water standards, with the exception of the heterotrophic bacteria which exceeded the drinking water standard limit in 43% of the samples treated with the Prototype I reactor (1 log reduction). Molecular viability analysis subsequently indicated that mean overall reductions of 75% and 74% were obtained for the analysed indicator organisms (E. coli and enterococci spp.) and opportunistic pathogens (Klebsiella spp., Legionella spp., Pseudomonas spp., Salmonella spp. and Cryptosporidium spp. oocysts) in the Prototype I and II solar reactors, respectively. The large-volume batch solar reactor prototypes could thus effectively provide four (88 L Prototype II) to seven (144 L Prototype I) people on a daily basis with the basic water requirement for human activities (20 L). Additionally, a generic Water Safety Plan was developed to aid practitioners in identifying risks and implement remedial actions in this type of installation in order to ensure the safety of the treated water
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