Geometry of wave propagation on active deformable surfaces

Fundamental biological and biomimetic processes, from tissue morphogenesis to soft robotics, rely on the propagation of chemical and mechanical surface waves to signal and coordinate active force generation. The complex interplay between surface geometry and contraction wave dynamics remains poorly understood, but will be essential for the future design of chemically-driven soft robots and active materials. Here, we couple prototypical chemical wave and reaction-diffusion models to non-Euclidean shell mechanics to identify and characterize generic features of chemo-mechanical wave propagation on active deformable surfaces. Our theoretical framework is validated against recent data from contractile wave measurements on ascidian and starfish oocytes, producing good quantitative agreement in both cases. The theory is then applied to illustrate how geometry and preexisting discrete symmetries can be utilized to focus active elastic surface waves. We highlight the practical potential of chemo-mechanical coupling by demonstrating spontaneous wave-induced locomotion of elastic shells of various geometries. Altogether, our results show how geometry, elasticity and chemical signaling can be harnessed to construct dynamically adaptable, autonomously moving mechanical surface wave guides.Comment: text changes abstract and intro, new results on self-propelled elastic shells added; 5 pages, 3 figures; videos available on reques

Competency development of southern African housing officers

The Report on the Ministerial Committee for the Review of the Provision of Student Housing at South African Universities (Department of Higher Education and Training, 2011) has provided a comprehensive review of residences across several housing functional areas. In one of the residence management and administration recommendations it stated, “The professionalisation of housing staff is an urgent priority” (p. 141). This coupled with the report’s estimated “current residence bed shortage of approximately 195 815 beds […] with a cost of overcoming this shortage over a period of ten years is estimated at R82.4 billion” (pp. xvii–xviii) will mean the hiring and training of hundreds of housing professional staff to meet not only the demand of the additional residence beds but the training of current housing staff. In 2010 The Association of College and University Housing Officers – International Southern Africa Chapter (ACUHO-I SAC) initiated a Student Housing Training Institute (SHTI) first held in 2011 to meet the demands for professionalising housing staff. The SHTI was organised using a competency development model first used to develop the Association of College and University Housing Officers –International (ACUHO-I) James C. Grimm National Housing Training Institute (NHTI) held in the US.Keywords: competency, development, housing officers, higher education, professionalisatio

Competency development of southern African housing officers

The Report on the Ministerial Committee for the Review of the Provision of StudentHousing at South African Universities (Higher Education and Training, 2011) hasprovided a comprehensive review of residences across several housing functional areas. Inone of the residence management and administration recommendations it stated, “Theprofessionalisation of housing staff is an urgent priority” (p. 141). This coupled with thereport’s estimated “current residence bed shortage of approximately 195 815 beds … witha cost of overcoming this shortage over a period of ten years is estimated at R82.4 billion”(pp. xvii–xviii) will mean the hiring and training of hundreds of housing professional staffto meet not only the demand of the additional residence beds but the training of currenthousing staff. In 2010 The Association of College and University Housing Officers –International Southern Africa Chapter (ACUHO-I SAC) initiated a Student HousingTraining Institute (SHTI) first held in 2011 to meet the demands for professionalisinghousing staff. The SHTI was organised using a competency development model first usedto develop the Association of College and University Housing Officers – International(ACUHO-I) James C. Grimm National Housing Training Institute (NHTI) held in the US

Low Reynolds number hydrodynamics of asymmetric, oscillating dumbbell pairs

Active dumbbell suspensions constitute one of the simplest model system for collective swimming at low Reynolds number. Generalizing recent work, we derive and analyze stroke-averaged equations of motion that capture the effective hydrodynamic far-field interaction between two oscillating, asymmetric dumbbells in three space dimensions. Time-averaged equations of motion, as those presented in this paper, not only yield a considerable speed-up in numerical simulations, they may also serve as a starting point when deriving continuum equations for the macroscopic dynamics of multi-swimmer suspensions. The specific model discussed here appears to be particularly useful in this context, since it allows one to investigate how the collective macroscopic behavior is affected by changes in the microscopic symmetry of individual swimmers.Comment: 10 pages, to appear in EPJ Special Topic

Intraocular Nematodiasis in a Llama (\u3cem\u3eLama glama\u3c/em\u3e)

This report describes a unique case of presumed migration of Parelaphastrongylus tenuis through the spinal cord into the eye of a llama where it survived and matured within the ocular environment. Blindness of the eye was most likely attributable to migration of the parasite through the central nervous tissue. Résumé Infestation par les nématodes intraoculaire chez un lama (Lama glama). Ce rapport décrit un cas unique de migration présumée de Parelaphastrongylus tenuis dans la colonne vertébrale jusque dans l’œil d’un lama où il a survécu et est parvenu à maturité dans l’environnement oculaire. La cécité de l’œil a été le plus probablement attribuable à la migration du parasite dans les tissus du système nerveux central. (Traduit par Isabelle Vallières

Thermal equilibrium and statistical thermometers in special relativity

There is an intense debate in the recent literature about the correct generalization of Maxwell's velocity distribution in special relativity. The most frequently discussed candidate distributions include the Juettner function as well as modifications thereof. Here, we report results from fully relativistic one-dimensional (1D) molecular dynamics (MD) simulations that resolve the ambiguity. The numerical evidence unequivocally favors the Juettner distribution. Moreover, our simulations illustrate that the concept of 'thermal equilibrium' extends naturally to special relativity only if a many-particle system is spatially confined. They make evident that 'temperature' can be statistically defined and measured in an observer frame independent way.Comment: version accepted for publication (5 pages), part of the introduction modified, new figures, additional reference

Relativistic diffusion processes and random walk models

The nonrelativistic standard model for a continuous, one-parameter diffusion process in position space is the Wiener process. As well-known, the Gaussian transition probability density function (PDF) of this process is in conflict with special relativity, as it permits particles to propagate faster than the speed of light. A frequently considered alternative is provided by the telegraph equation, whose solutions avoid superluminal propagation speeds but suffer from singular (non-continuous) diffusion fronts on the light cone, which are unlikely to exist for massive particles. It is therefore advisable to explore other alternatives as well. In this paper, a generalized Wiener process is proposed that is continuous, avoids superluminal propagation, and reduces to the standard Wiener process in the non-relativistic limit. The corresponding relativistic diffusion propagator is obtained directly from the nonrelativistic Wiener propagator, by rewriting the latter in terms of an integral over actions. The resulting relativistic process is non-Markovian, in accordance with the known fact that nontrivial continuous, relativistic Markov processes in position space cannot exist. Hence, the proposed process defines a consistent relativistic diffusion model for massive particles and provides a viable alternative to the solutions of the telegraph equation.Comment: v3: final, shortened version to appear in Phys. Rev.

Noisy swimming at low Reynolds numbers

Small organisms (e.g., bacteria) and artificial microswimmers move due to a combination of active swimming and passive Brownian motion. Considering a simplified linear three-sphere swimmer, we study how the swimmer size regulates the interplay between self-driven and diffusive behavior at low Reynolds number. Starting from the Kirkwood-Smoluchowski equation and its corresponding Langevin equation, we derive formulas for the orientation correlation time, the mean velocity and the mean square displacement in three space dimensions. The validity of the analytical results is illustrated through numerical simulations. Tuning the swimmer parameters to values that are typical of bacteria, we find three characteristic regimes: (i) Brownian motion at small times, (ii) quasi-ballistic behavior at intermediate time scales, and (iii) quasi-diffusive behavior at large times due to noise-induced orientation flipping. Our analytical results can be useful for a better quantitative understanding of optimal foraging strategies in bacterial systems, and they can help to construct more efficient artificial microswimmers in fluctuating fluids.Comment: minor changes/additions in the text, references added/updated, to appear in Phys. Rev.

Extracting the spectral function of the cuprates by a full two-dimensional analysis: Angle-resolved photoemission spectra of Bi2Sr2CuO6

Recently, angle-resolved photoemission spectroscopy (ARPES) has revealed a dispersion anomaly at high binding energy near 0.3-0.5eV in various families of the high-temperature superconductors. For further studies of this anomaly we present a new two-dimensional fitting-scheme and apply it to high-statistics ARPES data of the strongly-overdoped Bi2Sr2CuO6 cuprate superconductor. The procedure allows us to extract theself-energy in an extended energy and momentum range. It is found that the spectral function of Bi2Sr2CuO6 can be parameterized using a small set of tight-binding parameters and a weakly-momentum-dependent self-energy up to 0.7 eV in binding energy and over the entire first Brillouin zone. Moreover the analysis gives an estimate of the momentum dependence of the matrix element, a quantity, which is often neglected in ARPES analyses.Comment: 8 pages, 5 figure