Entrainment and scattering in microswimmer--colloid interactions

We use boundary element simulations to study the interaction of model microswimmers with a neutrally buoyant spherical particle. The ratio of the size of the particle to that of the swimmer is varied from $R^\mathrm{P} / R^\mathrm{S} \ll 1$, corresponding to swimmer--tracer scattering, to $R^\mathrm{P} / R^\mathrm{S} \gg 1$, approximately equivalent to the swimmer interacting with a fixed, flat surface. We find that details of the swimmer and particle trajectories vary for different swimmers. However, the overall characteristics of the scattering event fall into two regimes, depending on the relative magnitudes of the impact parameter, $\rho$, and the collision radius, $R^\mathrm{coll}=R^\mathrm{P} + R^\mathrm{S}$. The range of particle motion, defined as the maximum distance between two points on the trajectory, has only a weak dependence on the impact parameter when $\rho < R^\mathrm{coll}$ and decreases with the radius of the particle. In contrast, when $\rho>R^\mathrm{coll}$ the range decreases as a power law in $\rho$ and is insensitive to the size of the particle. We also demonstrate that large particles can cause swimmers to be deflected through large angles. In some instances, this swimmer deflection can lead to larger net displacements of the particle. Based on these results, we estimate the effective diffusivity of a particle in a dilute bath of swimmers and show that there is a non-monotonic dependence on particle radius. Similarly, we show that the effective diffusivity of a swimmer scattering in a suspension of particles varies non-monotonically with particle radius.Comment: 19 pages, 11 figures. Accepted in Physical Review Fluid

Selfduality of d=2 Reduction of Gravity Coupled to a Sigma-Model

Dimensional reduction in two dimensions of gravity in higher dimension, or more generally of d=3 gravity coupled to a sigma-model on a symmetric space, is known to possess an infinite number of symmetries. We show that such a bidimensional model can be embedded in a covariant way into a sigma-model on an infinite symmetric space, built on the semidirect product of an affine group by the Witt group. The finite theory is the solution of a covariant selfduality constraint on the infinite model. It has therefore the symmetries of the infinite symmetric space. (We give explicit transformations of the gauge algebra.) The usual physical fields are recovered in a triangular gauge, in which the equations take the form of the usual linear systems which exhibit the integrable structure of the models. Moreover, we derive the constraint equation for the conformal factor, which is associated to the central term of the affine group involved.Comment: 7 page

“I’m Not Dead Yet”: A Comparative Study of Indigenous Language Revitalization in the Isle of Man, Jersey and Guernsey.

At the outset of the twenty-first century, the survival of many minority and indigenous languages is threatened by globalization and the ubiquity of dominant languages such as English in the worlds of communication and commerce. In a number of cases, these negative trends are being resisted by grassroots activists and governments. Indeed, there are many examples of activists and governments working together in this manner to preserve and revitalize indigenous languages and cultures. Such coordinated efforts are vital to the success of language revitalization. This article compares the work of language activists and governments in three small island jurisdictions in the British Isles: the Isle of Man, Jersey and Guernsey. Comparison between these cases is greatly facilitated by similarities in their political, economic and demographic circumstances. The cases, however, reveal important differences in the way that activists and governments have responded to the challenges of language revitalization, as well as some interesting insights on the future prospects of the indigenous languages of these small island jurisdictions

An Open-Source Tool for Anisotropic Radiation Therapy Planning in Neuro-oncology Using DW-MRI Tractography.

There is evidence from histopathological studies that glioma tumor cells migrate preferentially along large white matter bundles. If the peritumoral white matter structures can be used to predict the likely trajectory of migrating tumor cells outside of the surgical margin, then this information could be used to inform the delineation of radiation therapy (RT) targets. In theory, an anisotropic expansion that takes large white matter bundle anatomy into account may maximize the chances of treating migrating cancer cells and minimize the amount of brain tissue exposed to high doses of ionizing radiation. Diffusion-weighted MRI (DW-MRI) can be used in combination with fiber tracking algorithms to model the trajectory of large white matter pathways using the direction and magnitude of water movement in tissue. The method presented here is a tool for translating a DW-MRI fiber tracking (tractography) dataset into a white matter path length (WMPL) map that assigns each voxel the shortest distance along a streamline back to a specified region of interest (ROI). We present an open-source WMPL tool, implemented in the package Diffusion Imaging in Python (DIPY), and code to convert the resulting WMPL map to anisotropic contours for RT in a commercial treatment planning system. This proof-of-concept lays the groundwork for future studies to evaluate the clinical value of incorporating tractography modeling into treatment planning

Evaluating Animation Parameters for Morphing Edge Drawings

Partial edge drawings (PED) of graphs avoid edge crossings by subdividing each edge into three parts and representing only its stubs, i.e., the parts incident to the end-nodes. The morphing edge drawing model (MED) extends the PED drawing style by animations that smoothly morph each edge between its representation as stubs and the one as a fully drawn segment while avoiding new crossings. Participants of a previous study on MED (Misue and Akasaka, GD19) reported eye straining caused by the animation. We conducted a user study to evaluate how this effect is influenced by varying animation speed and animation dynamic by considering an easing technique that is commonly used in web design. Our results provide indications that the easing technique may help users in executing topology-based tasks accurately. The participants also expressed appreciation for the easing and a preference for a slow animation speed.Comment: Appears in the Proceedings of the 31st International Symposium on Graph Drawing and Network Visualization (GD 2023

The topology of U-duality (sub-)groups

We discuss the topology of the symmetry groups appearing in compactified (super-)gravity, and discuss two applications. First, we demonstrate that for 3 dimensional sigma models on a symmetric space G/H with G non-compact and H the maximal compact subgroup of G, the possibility of oxidation to a higher dimensional theory can immediately be deduced from the topology of H. Second, by comparing the actual symmetry groups appearing in maximal supergravities with the subgroups of SL(32,R) and Spin(32), we argue that these groups cannot serve as a local symmetry group for M-theory in a formulation of de Wit-Nicolai type.Comment: 18 pages, LaTeX, 1 figure, 2 table

Communicating Criterion-Related Validity Using Expectancy Charts: A New Approach

Often, personnel selection practitioners present the results of their criterion-related validity studies to their senior leaders and other stakeholders when trying to either implement a new test or validate an existing test. It is sometimes challenging to present complex, statistical results to non-statistical audiences in a way that enables intuitive decision making. Therefore, practitioners often turn to expectancy charts to depict criterion-related validity. There are two main approaches for constructing expectancy charts (i.e., use of Taylor-Russell tables or splitting a raw dataset), both of which have considerable limitations. We propose a new approach for creating expectancy charts based on the bivariate-normal distribution. The new method overcomes the limitations inherent in the other two methods and offers a statistically sound and user-friendly approach for constructing expectancy charts

Hyperbolic billiards of pure D=4 supergravities

We compute the billiards that emerge in the Belinskii-Khalatnikov-Lifshitz (BKL) limit for all pure supergravities in D=4 spacetime dimensions, as well as for D=4, N=4 supergravities coupled to k (N=4) Maxwell supermultiplets. We find that just as for the cases N=0 and N=8 investigated previously, these billiards can be identified with the fundamental Weyl chambers of hyperbolic Kac-Moody algebras. Hence, the dynamics is chaotic in the BKL limit. A new feature arises, however, which is that the relevant Kac-Moody algebra can be the Lorentzian extension of a twisted affine Kac-Moody algebra, while the N=0 and N=8 cases are untwisted. This occurs for N=5, N=3 and N=2. An understanding of this property is provided by showing that the data relevant for determining the billiards are the restricted root system and the maximal split subalgebra of the finite-dimensional real symmetry algebra characterizing the toroidal reduction to D=3 spacetime dimensions. To summarize: split symmetry controls chaos.Comment: 21 page