1,720,967 research outputs found
Professor Sir Charles Frank (1911-1998): historical perspectives on the development of liquid crystal continuum theory
No full textImmortality is granted to those few scientists who give their names to phenomena or constants. In liquid crystal science, one of the best known of these is Sir Charles Frank, who died on 5 April 1998. Sir Charles was much loved and respected by his scientific peers and successors, and those of us who were privileged to know him can still hear his characteristically English incisive tones as he analysed some problem in theoretical physics
Soap, Science and Flat-Screen TVs: A History of Liquid Crystals
No full textThe terms 'liquid crystal' or 'liquid crystal display' (LCD) are well-known in the context of flat-screen televisions, but the properties and history of liquid crystals are little understood. This book tells the story of liquid crystals, from their controversial discovery at the end of the nineteenth century, to their eventual acceptance as another state of matter to rank alongside gases, liquids and solids. As their story unfolds, the scientists involved and their works are put into illuminating broader socio-political contexts. In recent years, liquid crystals have had a major impact on the display industry, culminating in the now widely available flat-screen televisions; this development is described in detail over three chapters, and the basic science behind it is explained in simple terms accessible to a general reader. New applications of liquid crystals in materials, bio-systems, medicine and technology are also explained
Population models of sperm-dependent parthenogenesis
No full textOrganisms that reproduce by sperm-dependent parthenogenesis are asexual clones that require sperm of a sexual host to initiate egg production, without the genome of the sperm contributing genetic information to the zygote. Although sperm-dependent parthenogenesis has some of the disadvantages of sex (requiring a mate) without the counterbalancing advantages (mixing of parental genotypes), it appears amongst a wide variety of species. We develop initial models for the density dependent dynamics of animal populations with sperm-dependent parthenogenesis (pseudogamy or gynogenesis), based on the known biology of the common Enchytraeid worm Lumbricillus lineatus. Its sperm-dependent parthenogenetic populations are reproductive parasites of the hermaphrodite sexual form. Our logistic models reveal two alternative requirements for coexistence at density-dependent equilibria: (i) If the two forms differ in competitive ability, the form with the lower intrinsic birth rate must be compensated by a more than proportionately lower competitive impact from the other, relative to intraspecific competition. (ii) If the two forms differ in their intrinsic capacity to exploit resources, the sperm-dependent parthenogen must be superior in this respect and must have a lower intrinsic birth rate. In general for crowded environments we expect a sperm-dependent parthenogen to compete strongly for limiting resources with the sexual sibling species. Its competitive impact is likely to be weakened by its genetic uniformity, however, and this may suffice to cancel any advantage of higher intrinsic growth rate obtained from reproductive investment only in egg production. We discuss likely thresholds of coexistence for other sperm-dependent parthenogens. The fish Poeciliopsis monacha-lucida likewise obtains an intrinsic growth advantage from reduced investment in male gametes, and so its persistence is likely to depend on it being a poor competitor. The planarian Schmidtea polychroa obtains no such intrinsic benefit because it produces fertile sperm, and its persistence may depend on superior resource exploitation
Long-range dispersal, stochasticity and the broken accelerating wave of advance
Full text linkRare long distance dispersal events are thought to have a disproportionate impact on the spread of invasive species. Modelling using integrodifference equations suggests that, when long distance contacts are represented by a fat-tailed dispersal kernel, an accelerating wave of advance can ensue. Invasions spreading in this manner could have particularly dramatic effects. Recently, various authors have suggested that demographic stochasticity disrupts wave acceleration. Integrodifference models have been widely used in movement ecology, and as such a clearer understanding of stochastic effects is needed. Here, we present a stochastic non-linear one-dimensional lattice model in which demographic stochasticity and the dispersal regime can be systematically varied. Extensive simulations show that stochasticity has a profound effect on model behaviour, and usually breaks acceleration for fat-tailed kernels. Exceptions are seen for some power law kernels, with $\bet
Pulling polymers on energetically disordered surfaces: molecular dynamics tests of linear and non-linear response
Full text linkWe present equilibrium and non-equilibrium molecular dynamics simulations of flexible polymer chains absorbed on heterogeneous surfaces. The surfaces are flat but energetically disordered, as they consist of a random mixture of weakly and more strongly absorbing sites (94% and 6%, respectively). For comparison, we have also simulated the two corresponding homogeneous surfaces. We find that this apparently weak energetic disorder can produce very significant changes of the chain statistics (expansion of the radii of gyration), equilibrium dynamics (reduction of the diffusion coefficients) and non equilibrium response to a horizontal pulling force. On the disordered surfaces, the polymer-surface effective friction coefficient (ratio of pulling force and drift velocity) becomes strongly force-dependent,as the dominant mode of motion changes from localized stick-slip events to smooth and continuous sliding. This is strongly reminiscent of the Schallamach model of rubber friction and the Maier-Goritz picture of the Payne effect in filled elastomers
Parameterised Spin Model For FePt
No full textDataset supporting: Waters, J. M. et. al. (2019). Resolving Anomalies in the Critical Exponents of FePt Using Finite-Size Scaling in Magnetic Fields. Physical Review Applied. DOI:10.1103/PhysRevApplied.11.024028
Interaction strengths between Fe atoms in a parameterised spin model of FePt in the L10 crystal structure.
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Modelling Paleoindian dispersals
No full textIt is reasonable to expect that the global dispersal of modern humans was influenced by habitat variation in space and time; but many simulation models average such variation into a single, homogeneous surface across which the dispersal process is modelled. We present a demographic simulation model in which rates of spatial range expansion can be modified by local habitat values. The broad-scale vegetation cover of North America during the late last glacial is reconstructed and mapped at thousand-year intervals, 13,000-10,000 radiocarbon years BP. Results of the simulation of human dispersal into North America during the late last glacial are presented; output appears to match observed variation in occupancy of habitats during this period (as assessed from discard rates of diagnostic artefacts), if we assume that intrinsic population growth rates were fairly high and that local population densities varied as a function of environmental carrying capacity. Finally, a number of issues are raised relating to present limitations and possible future extensions of the simulation model
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