Forces between elongated particles in a nematic colloid

Using molecular dynamics simulations we study the interactions between elongated colloidal particles (length to breath ratio ≫1) in a nematic host. The simulation results are compared to the results of a Landau–de Gennes elastic free energy. We find that depletion forces dominate for the sizes of the colloidal particles studied. The tangential component of the force, however, allows us to resolve the elastic contribution to the total interaction. We find that this contribution differs from the quadrupolar interaction predicted at large separations. The difference is due to the presence of nonlinear effects, namely, the change in the positions and structure of the defects and their annihilation at small separations

A Smooth Interface Method for Simulating Liquid Crystal Colloid Dispersions

A new method is presented for mesoscopic simulations of particle dispersions in liquid crystal solvents. It allows efficient first-principle simulations of the dispersions involving many particles with many-body interactions mediated by the solvents. Demonstrations have been performed for the aggregation of colloid dispersions in two-dimensional nematic and smectic-C* solvents neglecting hydrodynamic effects, which will be taken into account in the near future.Comment: 13 pages, 4 figure

Simulating Particle Dispersions in Nematic Liquid-Crystal Solvents

A new method is presented for mesoscopic simulations of particle dispersions in nematic liquid crystal solvents. It allows efficient first-principle simulations of the dispersions involving many particles with many-body interactions mediated by the solvents. A simple demonstration is shown for the aggregation process of a two dimentional dispersion.Comment: 5 pages, 5 figure

Defect structures and torque on an elongated colloidal particle immersed in a liquid crystal host

Combining molecular dynamics and Monte Carlo simulation we study defect structures around an elongated colloidal particle embedded in a nematic liquid crystal host. By studying nematic ordering near the particle and the disclination core region we are able to examine the defect core structure and the difference between two simulation techniques. In addition, we also study the torque on a particle tilted with respect to the director, and modification of this torque when the particle is close to the cell wall

Effective forces in colloidal mixtures: from depletion attraction to accumulation repulsion

Computer simulations and theory are used to systematically investigate how the effective force between two big colloidal spheres in a sea of small spheres depends on the basic (big-small and small-small) interactions. The latter are modeled as hard-core pair potentials with a Yukawa tail which can be both repulsive or attractive. For a repulsive small-small interaction, the effective force follows the trends as predicted by a mapping onto an effective non-additive hard-core mixture: both a depletion attraction and an accumulation repulsion caused by small spheres adsorbing onto the big ones can be obtained depending on the sign of the big-small interaction. For repulsive big-small interactions, the effect of adding a small-small attraction also follows the trends predicted by the mapping. But a more subtle ``repulsion through attraction'' effect arises when both big-small and small-small attractions occur: upon increasing the strength of the small-small interaction, the effective potential becomes more repulsive. We have further tested several theoretical methods against our computer simulations: The superposition approximation works best for an added big-small repulsion, and breaks down for a strong big-small attraction, while density functional theory is very accurate for any big-small interaction when the small particles are pure hard-spheres. The theoretical methods perform most poorly for small-small attractions.Comment: submitted to PRE; New version includes an important quantitative correction to several of the simulations. The main conclusions remain unchanged thoug

Assessing the effectiveness of the Ramsar Convention in preserving wintering waterbirds in the Mediterranean

lthough biological conservation is based on international agreements, its effectiveness depends on how countries implement such recommendations as effective conservation tools. The Ramsar Convention is the oldest international treaty for wetland and waterbird conservation, establishing the world's largest network of protected areas. However, since it does not constitute any binding measure, its effectiveness in protecting wintering waterbird populations at an international scale has been questioned. Here, we use long-term (1991–2012) count data to assess the effectiveness of the Ramsar Convention in the Mediterranean Basin. We compared abundance and temporal trends of 114 waterbird species between 251 Ramsar wetlands and 3486 non-Ramsar wetlands. We found that the Ramsar network is critical for wintering waterbirds, concentrating nearly half of all waterbirds counted in the Mediterranean Basin in only 7% of monitored wetlands. Waterbird trends followed a northwestsoutheast gradient, with a population decrease in the East. A significant and positive Ramsar effect on population trends was only found for the species of higher conservation concern in the Maghreb, particularly when a management plan was implemented. The Ramsar Convention was previously used on very important wetlands for waterbirds in Southern Europe, but is now an underused conservation tool. Our study suggests weaknesses in the use of Ramsar as an effective conservation tool in most of the Mediterranean Basin. However, the Ramsar Convention effectiveness to enhance waterbird populations in the Maghreb should encourage strengthening the Ramsar Convention. It should be done particularly in countries with limited environmental agreements and by systematic implementation of management plans. Conservation measures International conventions Protected areas Protection status Monitoring WetlandsacceptedVersio

Environment and Rural Affairs Monitoring & Modelling Programme - ERAMMP Report-60: ERAMMP Integrated Modelling Platform (IMP) Land Use Scenarios

Six scenarios consisting of changes in farm-gate prices (T1 to T6) have been applied to the ERAMMP Integrated Modelling Platform (IMP) to simulate impacts on land use change, biodiversity and ecosystem services (carbon, water quality and air quality). The scenarios were based on discussions held between stakeholders in the Evidence and Scenario subgroup (Roundtable Wales and Brexit1) and Welsh Government (WG) policy officials. These discussions took place in late 2020 before the arrangements for the UK leaving the EU were agreed, therefore are based on broad assumptions around the detail of the trade agreement with the EU as well as other third countries including Australia, New Zeland and USA. It is important to note that the outputs of these discussions which were used as inputs into the ERAMMP IMP may therefore not accurately reflect the outcomes achieved within the finalised trade agreements. The T1 scenario assumes no EU trade deal and trade liberalisation, with no tariffs applied to imported products and T2 an EU trade deal with no change to the trade arrangements with third countries. These two scenarios used the changes to farm-gate prices modelled by FAPRI2. The assumptions used in the T3 to T6 scenarios were based on expert opinion from the stakeholder group, and include impacts on farm-gate prices which potentially could have resulted from different combinations of trade deals with New Zealand, Australia and USA. Scenarios which include “no EU deal” options (T1 and T4) are no longer relevant. In no way whatsoever do T1, T3, T4, T5 and T6 represent a WG position; our understanding of the nature and impact of new and emerging trade deals has evolved significantly and the WG Trade Policy Team lead in this area. The objective of this work was to gain an early understanding of how changes in farm-gate prices potentially resulting from trading relationships may influence land use and subsequently effect entry into the Sustainable Farming Scheme. We note that many other factors are also likely to influence Welsh farmgate prices, such as (but not limited to), currency exchange rates, energy prices and extreme weather events in other parts of the world. This report provides an overview of the land use implications of all these scenarios, but focuses on the T2 scenario, which represents an EU Trade Deal. This T2 scenario is being used as the counterfactual scenario against which the costs and benefits of the land use implications of the proposed Sustainable Farming Scheme will be assessed in the Regulatory Impact Assessment for the proposed Agricultural Bill. This includes the estimated environmental outcomes of the EU Trade Deal scenario and, where the ERAMMP IMP has attached monetary valuations to these, the value of these outcomes to society. In the Cost Benefit Analysis, these monetary values will inform the overall estimated Net Present Value (NPV) of this business-as-usual counterfactual. The IMP involves many assumptions and these need to be borne in mind when interpreting and using its outcomes. By necessity, all models are a simplification of the real situation, but can still provide very useful insights if applied for a specific purpose and with caution. The collaborative and iterative consortium-based approach to co-designing the IMP has meant that Welsh Government and IMP teams have clear, open channels of communication for asking questions. This ensures that the modelling represents government aspirations as well as possible and the limits of the approach are well understood. IMP outputs for the T2 scenario show that some simulated full-time farms (>1 FTE labour) come under economic pressure (7%) and are simulated to be unable to produce a sufficient Farm Business Income to be economically viable. For these farm types, no options to transition to a more alternative profitable farm type are available and they are assumed to leave full-time agriculture. A greater number of farms transition to dairying resulting in a 75% increase in the number of dairy farms. This is associated with large increases in the number of dairy cattle (73%) and reductions in sheep (-34%). A general intensification of grassland systems is simulated resulting from the farm type transitions, with a 66% increase in temporary grasslands and a 21% decrease in permanent grasslands. Overall, these changes in agriculture and land use are simulated to lead to mixed, but predominantly negative, effects on biodiversity, increases in GHG emissions and deterioration in air and water quality. The T2 scenario predicts the least change in agriculture out of the six scenarios. T1 simulates the greatest impacts on agriculture due to significant farm-gate price reductions across dairy, beef and sheep systems, with a large number of full-time farms leaving agriculture. This leads to large increases in woodland area and generally positive effects on biodiversity and ecosystem services. T3 and T4 also simulate large impacts on agriculture. These are associated with significant farm transitions to dairy (due to increases in milk prices and significant decreases in beef and lamb prices) resulting in larger increases in GHG emissions and greater declines in air and water quality, compared to the T2 scenario. The T5 and T6 scenarios fall between these extremes, with T6 projecting the second greatest impacts on agriculture (after T1) in terms of farms under pressure. These simulated changes in agriculture are associated with net benefits for air and water quality, but net costs for GHG emissions; although these costs are lower than for scenarios T3-T5