221 research outputs found
Colloidal crystal growth at externally imposed nucleation clusters
We study the conditions under which and how an imposed cluster of fixed
colloidal particles at prescribed positions triggers crystal nucleation from a
metastable colloidal fluid. Dynamical density functional theory of freezing and
Brownian dynamics simulations are applied to a two-dimensional colloidal system
with dipolar interactions. The externally imposed nucleation clusters involve
colloidal particles either on a rhombic lattice or along two linear arrays
separated by a gap. Crystal growth occurs after the peaks of the nucleation
cluster have first relaxed to a cutout of the stable bulk crystal.Comment: 4 pages, accepted for publication in Phys. Rev. Let
Phase-field crystal modelling of crystal nucleation, heteroepitaxy and patterning
We apply a simple dynamical density functional theory, the
phase-field-crystal (PFC) model, to describe homogeneous and heterogeneous
crystal nucleation in 2d monodisperse colloidal systems and crystal nucleation
in highly compressed Fe liquid. External periodic potentials are used to
approximate inert crystalline substrates in addressing heterogeneous
nucleation. In agreement with experiments in 2d colloids, the PFC model
predicts that in 2d supersaturated liquids, crystalline freezing starts with
homogeneous crystal nucleation without the occurrence of the hexatic phase. At
extreme supersaturations crystal nucleation happens after the appearance of an
amorphous precursor phase both in 2d and 3d. We demonstrate that contrary to
expectations based on the classical nucleation theory, corners are not
necessarily favourable places for crystal nucleation. Finally, we show that
adding external potential terms to the free energy, the PFC theory can be used
to model colloid patterning experiments.Comment: 21 pages, 16 figure
Density functional theory of freezing for soft interactions in two dimensions
A density functional theory of two-dimensional freezing is presented for a
soft interaction potential that scales as inverse cube of particle distance.
This repulsive potential between parallel, induced dipoles is realized for
paramagnetic colloids on an interface, which are additionally exposed to an
external magnetic field. An extended modified weighted density approximation
which includes correct triplet correlations in the liquid state is used. The
theoretical prediction of the freezing transition is in good agreement with
experimental and simulation data.Comment: 7 pages, 3 figures, submitted 200
Generation of defects and disorder from deeply quenching a liquid to form a solid
We show how deeply quenching a liquid to temperatures where it is linearly
unstable and the crystal is the equilibrium phase often produces crystalline
structures with defects and disorder. As the solid phase advances into the
liquid phase, the modulations in the density distribution created behind the
advancing solidification front do not necessarily have a wavelength that is the
same as the equilibrium crystal lattice spacing. This is because in a deep
enough quench the front propagation is governed by linear processes, but the
crystal lattice spacing is determined by nonlinear terms. The wavelength
mismatch can result in significant disorder behind the front that may or may
not persist in the latter stage dynamics. We support these observations by
presenting results from dynamical density functional theory calculations for
simple one- and two-component two-dimensional systems of soft core particles.Comment: 25 pages, 11 figure
Dynamics of a deformable self-propelled particle under external forcing
We investigate dynamics of a self-propelled deformable particle under
external field in two dimensions based on the model equations for the center of
mass and a tensor variable characterizing deformations. We consider two kinds
of external force. One is a gravitational-like force which enters additively in
the time-evolution equation for the center of mass. The other is an
electric-like force supposing that a dipole moment is induced in the particle.
This force is added to the equation for the deformation tensor. It is shown
that a rich variety of dynamics appears by changing the strength of the forces
and the migration velocity of self-propelled particle
Association between COVID-19 lockdown measures and the incidence of iatrogenic versus spontaneous very preterm births in the Netherlands:a retrospective study
Background: The COVID-19 pandemic led to regional or nationwide lockdowns as part of risk mitigation measurements in many countries worldwide. Recent studies suggest an unexpected and unprecedented decrease in preterm births during the initial COVID-19 lockdowns in the first half of 2020. The objective of the current study was to assess the effects of the two months of the initial national COVID-19 lockdown period on the incidence of very and extremely preterm birth in the Netherlands, stratified by either spontaneous or iatrogenic onset of delivery, in both singleton and multiple pregnancies. Methods: Retrospective cohort study using data from all 10 perinatal centers in the Netherlands on very and extremely preterm births during the initial COVID-19 lockdown from March 15 to May 15, 2020. Incidences of very and extremely preterm birth were calculated using an estimate of the total number of births in the Netherlands in this period. As reference, we used data from the corresponding calendar period in 2015–2018 from the national perinatal registry (Perined). We differentiated between spontaneous versus iatrogenic onset of delivery and between singleton versus multiple pregnancies. Results: The incidence of total preterm birth < 32 weeks in singleton pregnancies was 6.1‰ in the study period in 2020 versus 6.5‰ in the corresponding period in 2015–2018. The decrease in preterm births in singletons was solely due to a significant decrease in iatrogenic preterm births, both < 32 weeks (OR 0.71; 95%CI 0.53 to 0.95) and < 28 weeks (OR 0.53; 95%CI 0.29 to 0.97). For multiple pregnancies, an increase in preterm births < 28 weeks was observed (OR 2.43; 95%CI 1.35 to 4.39). Conclusion: This study shows a decrease in iatrogenic preterm births during the initial COVID-19-related lockdown in the Netherlands in singletons. Future studies should focus on the mechanism of action of lockdown measures and reduction of preterm birth and the effects of perinatal outcome
Seeing blue: negotiating the politics of Avatar media activism
This thesis examines how the Hollywood blockbuster Avatar (2009) has been taken-up in media activism directed towards Indigenous struggles against imperialism. It assumes the importance of locating this phenomenon within the discursive and material regimes that mediate, enable, and constrain it. I therefore offer a contextualised analysis of the film and media relating to its appropriation, which focuses on the representational practices and structural mechanisms that inform the production, circulation, and reception of the texts. This approach emphasises the tensions and contradictions that underpin activists’ relationship to the media they mobilise. Such contradictions are particularly apparent in relation to the politics of race that shape Avatar, the Indigenous activism that references it, and the media regimes that make this possible. The very forces that marginalise Indigenous voices empower auteur James Cameron to speak on their behalf and to be heard. Activists must also negotiate the tension between co-opting media spectacle and being commercialised as spectacle. However, refusing a simple critique of the representations activists deploy as media spectacles, I argue for a model that foregrounds the alliances that they seek to engender. Drawing on the work of feminist scholars Oliver (2001) and Deslandes (2010), I signal a theoretical approach that focuses on how the mediated spectator relates to such representations and insists on the spectator’s responsibility to respond. Acknowledging that the tensions that animate Avatar media activism can be both constrictive and creative, this project seeks a model that maximises the potential for the latter. It thus resists the paralysis of activism that can come with critiquing how we fight for the world we imagine
Glancing angle metal evaporation synthesis of catalytic swimming Janus colloids with well defined angular velocity
The ability to control the degree of spin, or rotational velocity, for catalytic swimming devices opens up the potential to access well defined spiralling trajectories, enhance cargo binding rate, and realise theoretically proposed behaviour such as chiral diffusion. Here we assess the potential to impart a well-defined spin to individual catalytic Janus swimmers by using glancing angle metal evaporation onto a colloidal crystal to break the symmetry of the catalytic patch due to shadowing by neighbouring colloids. Using this approach we demonstrate a well-defined relationship between the glancing angle and the ratio of rotational to translational velocity. This allows batches of colloids with well-defined spin rates in the range 0.25 to 2.5 Hz to be produced. With reference to the shape and thickness variations across the catalytically active shapes, and their propulsion mechanism we discuss the factors that can lead to the observed variations in rotational propulsion
A circle swimmer at low Reynolds number
Swimming in circles occurs in a variety of situations at low Reynolds number.
Here we propose a simple model for a swimmer that undergoes circular motion,
generalising the model of a linear swimmer proposed by Najafi and Golestanian
(Phys. Rev. E 69, 062901 (2004)). Our model consists of three solid spheres
arranged in a triangular configuration, joined by two links of time-dependent
length. For small strokes, we discuss the motion of the swimmer as a function
of the separation angle between its links. We find that swimmers describe
either clockwise or anticlockwise circular motion depending on the tilting
angle in a non-trivial manner. The symmetry of the swimmer leads to a
quadrupolar decay of the far flow field. We discuss the potential extensions
and experimental realisation of our model.Comment: 9 pages, 9 Figure
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