293 research outputs found
High-speed video microscopy in optical tweezers
Optical tweezers have become an invaluable tool for measuring and
exerting forces in the pico-Newton regime. Force measurements have
in the past concentrated on using only one trapped particle as a
probe, partly due to the difficulties in tracking more than one par-
ticle at high enough frame rate. Recent advances in video camera
technology allow the collection of images at several kHz. However,
there has been little use of high-speed cameras in optical tweezers,
partly due to data management problems and affordability. This the-
sis presents seven experiments carried out during my PhD involving
the use of several different high-speed cameras.
Chapter 3 presents the use of a CMOS high-speed camera with in-
tegrated particle tracking built by Durham Smart Imaging. The
camera was used in a Shack-Hartmann sensor setup to determine
rapidly and non-ambiguously the sign and magnitude of the orbital
angular momentum of a helically-phased beam light beam, as an
alternative to interferometric techniques. Chapter 4 presents a di-
rect comparison of a CCD high-speed video camera with a quadrant
photodiode to track particle position. Particle tracking was possible
at high enough accuracy and bandwidth to allow convenient trap
calibration by thermal analysis. Chapter 5 reports an investigation
of the resulting change in trap stiffness during the update of trap
positions in holographic optical tweezers. Chapter 6 presents the re-
sults from using a high-speed camera to successfully track multiple
particles in a microfluidic channel to measure the viscosity at sev-
eral points simultaneously. The last three chapters investigate the
hydrodynamic interactions between trapped particles under different
conditions and comparisons were made with theory
Past production constrains current energy demands: persistent scaling in global energy consumption and implications for climate change mitigation
Climate change has become intertwined with the global economy. Here, we
describe the importance of inertia to continued growth in energy consumption.
Drawing from thermodynamic arguments, and using 38 years of available
statistics between 1980 to 2017, we find a persistent time-independent scaling
between the historical time integral of world inflation-adjusted economic
production , or , and
current rates of world primary energy consumption , such that
Gigawatts per trillion 2010 US dollars.
This empirical result implies that population expansion is a symptom rather
than a cause of the current exponential rise in and carbon dioxide
emissions , and that it is past innovation of economic production efficiency
that has been the primary driver of growth, at predicted rates
that agree well with data. Options for stabilizing are then limited to
rapid decarbonization of through sustained implementation of over
one Gigawatt of renewable or nuclear power capacity per day. Alternatively,
assuming continued reliance on fossil fuels, civilization could shift to a
steady-state economy that devotes economic production exclusively to
maintenance rather than expansion. If this were instituted immediately,
continual energy consumption would still be required, so atmospheric carbon
dioxide concentrations would not balance natural sinks until concentrations
exceeded 500 ppmv, and double pre-industrial levels if the steady-state was
attained by 2030
Cosmogenic 3HE Measurements Provide Insight into Lithologic controls on Bedrock Channel Incision:Examples from the South African Interior
Resistant bedrock outcrops can exert control on river long profile adjustment, upstream transmission of base level fall, and valley development, particularly in post-orogenic settings. To examine how variation in lithologic resistance impacts landscape development in the post-orogenic eastern South African interior, cosmogenic 3He in pyroxene from Karoo dolerite was measured in samples from valleys of the Klip and Mooi Rivers and the Schoonspruit. The denudation rates measured from cosmogenic 3He in the Klip and Mooi Rivers and Schoonspruit are widely variable, with channel bed denudation rates ranging from 14-255 m/Myr and valley side and top denudation rates ranging from 11-50 m/Myr. Various processes of channel bed erosion occurring at grain to block scales (abrasion, plucking, subaerial weathering) result in the widely ranging channel bed incision rates. In this setting, river incision rates are restricted by moderate unit stream powers (~20 to >50 W/m2) and by limited sediment supply, resulting in a lack of abrasive tools. In many dolerite valleys, channel bed incision is commonly slow enough for local base levels to remain essentially stable for extended periods of time (>10 kyr). These results suggest that in the post-orogenic eastern South African interior, resistant dolerite in channel long profiles can result in spatially variable rates of channel bed incision, with implications for the patterns and tempo of wider landscape dynamics
Commentary on 'What is the point: will screening mammography save my life?' by Keen and Keen
Commentary on Keen and Keen 'What is the point: will screening mammography save my life?' BMC Medical Informatics and Decision Making, 200
Independent polarisation control of multiple optical traps
We present a system which uses a single spatial light modulator to control the spin angular momentum of multiple optical traps. These traps may be independently controlled both in terms of spatial location and in terms of their spin angular momentum content. The system relies on a spatial light modulator used in a "split-screen" configuration to generate beams of orthogonal polarisation states which are subsequently combined at a polarising beam splitter. Defining the phase difference between the beams with the spatial light modulator enables control of the polarisation state of the light. We demonstrate the functionality of the system by controlling the rotation and orientation of birefringent vaterite crystals within holographic optical tweezers
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