Aggregation of frictional particles due to capillary attraction

Capillary attraction between identical millimeter sized spheres floating at a liquid-air interface and the resulting aggregation is investigated at low Reynolds number. We show that the measured capillary forces between two spheres as a function of distance can be described by expressions obtained using the Nicolson approximation at low Bond numbers for far greater particle sizes than previously assumed. We find that viscous hydrodynamics interactions between the spheres needs to be included to describe the dynamics close to contact. We then consider the aggregates formed when a third sphere is added after the initial two spheres are already in contact. In this case, we find that linear superposition of capillary forces describes the observed approach qualitatively but not quantitatively. Further, we observe an angular dependence of the structure due to a rapid decrease of capillary force with distance of separation which has a tendency to align the particles before contact. When the three particles come in contact, they may preserve their shape or rearrange to form an equilateral triangle cluster - the lowest energy state - depending on the competition between attraction between particles and friction. Using these observations, we demonstrate that a linear particle chain can be built from frictional particles with capillary attraction.Comment: accepted for Physical Review

A numerical model of the VKS experiment

We present numerical simulations of the magnetic field generated by the flow of liquid sodium driven by two counter-rotating impellers (VKS experiment). Using a dynamo kinematic code in cylindrical geometry, it is shown that different magnetic modes can be generated depending on the flow configuration. While the time averaged axisymmetric mean flow generates an equatorial dipole, our simulations show that an axial field of either dipolar or quadrupolar symmetry can be generated by taking into account non-axisymmetric components of the flow. Moreover, we show that by breaking a symmetry of the flow, the magnetic field becomes oscillatory. This leads to reversals of the axial dipole polarity, involving a competition with the quadrupolar component.Comment: 6 pages, 5 figure

Chaotic magnetic field reversals in turbulent dynamos

We present direct numerical simulations of reversals of the magnetic field generated by swirling flows in a spherical domain. In agreement with a recent model, we observe that coupling dipolar and quadrupolar magnetic modes by an asymmetric forcing of the flow generates field reversals. In addition, we show that this mechanism strongly depends on the value of the magnetic Prandtl number.Comment: 4 pages, 5 figure

Transport of magnetic field by a turbulent flow of liquid sodium

We study the effect of a turbulent flow of liquid sodium generated in the von K\'arm\'an geometry, on the localized field of a magnet placed close to the frontier of the flow. We observe that the field can be transported by the flow on distances larger than its integral length scale. In the most turbulent configurations, the mean value of the field advected at large distance vanishes. However, the rms value of the fluctuations increases linearly with the magnetic Reynolds number. The advected field is strongly intermittent.Comment: 4 pages, 6 figure

Early postnatal home visits: a qualitative study of barriers and facilitators to achieving high coverage

BACKGROUND: Timely interventions in the postnatal period are important for reducing newborn mortality, and early home visits to provide postnatal care are recommended. There has been limited success in achieving timely visits, and a better understanding of the realities of programmes is needed if improvements are to be made. METHODS: We explored barriers and facilitators to timely postnatal visits through 20 qualitative interviews and 16 focus group discussions with families and Health Extension Workers in four Ethiopian sites. RESULTS: All sites reported some inaccessible areas that did not receive visits, but, Health Extension Workers in the sites with more difficult terrain were reported to make more visits that those in the more accessible areas. This suggests that information and work issues can be more important than moderate physical issues. The sites where visits were common had functioning mechanisms for alerting workers to a birth; these were not related to postnatal visits but to families informing Health Extension Workers of labour so they could call an ambulance. In the other sites, families did not know they should alert workers about a delivery, and other alert mechanisms were not functioning well. Competing activities reducing Health Extension Worker availability for visits, but in some areas workers were more organized in their division of their work and this facilitated visits. The main difference between the areas where visits were reported as common or uncommon was the general activity level of the Health Extension Worker. In the sites where workers were active and connected to the community visits occurred more often. CONCLUSIONS: If timely postnatal home visits are to occur, CHWs need realistic catchment areas that reflect their workload. Inaccessible areas may need their own CHW. Good notification systems are essential, families will notify CHWs if they have a clear reasons to do so, and more work is needed on how to ensure notification systems function. Work ethic was a clear influencer on whether home visits occur, studies to date have focused on understanding the motivation of CHWs as a group, more studies on understanding motivation at an individual level are needed

Susceptibility divergence, phase transition and multistability of a highly turbulent closed flow

Using time-series of stereoscopic particle image velocimetry data, we study the response of a turbulent von K\'{a}rm\'{a}n swirling flow to a continuous breaking of its forcing symmetry. Experiments are carried over a wide Reynolds number range, from laminar regime at $Re = 10^{2}$ to highly turbulent regime near $Re = 10^{6}$. We show that the flow symmetry can be quantitatively characterized by two scalars, the global angular momentum $I$ and the mixing layer altitude $z_s$, which are shown to be statistically equivalent. Furthermore, we report that the flow response to small forcing dissymetry is linear, with a slope depending on the Reynolds number: this response coefficient increases non monotonically from small to large Reynolds number and presents a divergence at a critical Reynolds number $Re_c = 40\,000 \pm 5\,000$. This divergence coincides with a change in the statistical properties of the instantaneous flow symmetry $I(t)$: its pdf changes from Gaussian to non-Gaussian with multiple maxima, revealing metastable non-symmetrical states. For symmetric forcing, a peak of fluctuations of $I(t)$ is also observed at $Re_c$: these fluctuations correspond to time-intermittencies between metastable states of the flow which, contrary to the very-long-time-averaged mean flow, spontaneously and dynamically break the system symmetry. We show that these observations can be interpreted in terms of divergence of the susceptibility to symmetry breaking, revealing the existence of a phase transition. An analogy with the ferromagnetic-paramagnetic transition in solid-state physics is presented and discussed.Comment: to appear in Journal of Statistical Mechanic