Largest impact craters on Venus

High-resolution radar images from the Magellan spacecraft have allowed us to perform a detailed study on 25 large impact craters on Venus with diameters from 70 to 280 km. The dimension of these large craters is comparable with the characteristic thickness of the venusian lithosphere and the atmospheric scale height. Some physical parameters for the largest impact craters on Venus (LICV), such as depth, ring/diameter ratio, and range of ballistic ejecta deposits, have been obtained from the SAR images and the altimetry dataset produced by MIT. Data related to each of these parameters is discussed

Stiffness of the Crystal-Liquid Interface in a Hard-Sphere Colloidal System Measured from Capillary Fluctuations

Face-centered cubic single crystals of $σ=1.55 μm$ diameter hard-sphere silica colloidal particles were prepared by sedimentation onto (100) and (110) oriented templates. The crystals had a wide interface with the overlaying liquid that was parallel to the template. The location of the interface was determined by confocal microscopic location of the particles, followed by identification of the crystalline and liquid phases by a bond-orientation order parameter. Fluctuations in the height of the interface about its average position were recorded for several hundred configurations. The interfacial stiffness $γ̃$ was determined from the slope of the inverse squared Fourier components of the height profile vs the square of the wave number, according to the continuum capillary fluctuation method. The offset of the fit from the origin could quantitatively be accounted for by gravitational damping of the fluctuations. For the (100) interface, $γ̃ =(1.3±0.3)k_{B}T/σ^{2}$; for the (110) interface, $γ̃ =(1.0±0.2)k_{B}T/σ^{2}$. The interfacial stiffness of both interfaces was found to be isotropic in the plane. This is surprising for the (110), where crystallography predicts twofold symmetry. Sedimentation onto a (111) template yielded a randomly stacked hexagonal crystal with isotropic $γ̃ =0.66k_{B}T/σ^{2}$. This value, however, is less reliable than the two others due to imperfections in the crystal.Engineering and Applied SciencesPhysic

Speckle visibility spectroscopy and variable granular fluidization

We introduce a dynamic light scattering technique capable of resolving motion that changes systematically, and rapidly, with time. It is based on the visibility of a speckle pattern for a given exposure duration. Applying this to a vibrated layer of glass beads, we measure the granular temperature and its variation with phase in the oscillation cycle. We observe several transitions involving jammed states, where the grains are at rest during some portion of the cycle. We also observe a two-step decay of the temperature on approach to jamming.Comment: 4 pages, 4 figures, experimen

Low-emissivity impact craters on Venus

An analysis of 144 impact craters on Venus has shown that 11 of these have floors with average emissivities lower than 0.8. The remaining craters have emissivities between 0.8 and 0.9, independent of the specific backscatter cross section of the crater floors. These 144 impact craters were chosen from a possible 164 craters with diameters greater than 30 km as identified by researchers for 89 percent of the surface of Venus. We have only looked at craters below 6053.5 km altitude because a mineralogical change causes high reflectivity/low emissivity above the altitude. We have also excluded all craters with diameters smaller than 30 km because the emissivity footprint at periapsis is 16 x 24 km and becomes larger at the poles

Resolving long-range spatial correlations in jammed colloidal systems using photon correlation imaging

We introduce a new dynamic light scattering method, termed photon correlation imaging, which enables us to resolve the dynamics of soft matter in space and time. We demonstrate photon correlation imaging by investigating the slow dynamics of a quasi two-dimensional coarsening foam made of highly packed, deformable bubbles and a rigid gel network formed by dilute, attractive colloidal particles. We find the dynamics of both systems to be determined by intermittent rearrangement events. For the foam, the rearrangements extend over a few bubbles, but a small dynamical correlation is observed up to macroscopic length scales. For the gel, dynamical correlations extend up to the system size. These results indicate that dynamical correlations can be extremely long-ranged in jammed systems and point to the key role of mechanical properties in determining their nature.Comment: Published version (Phys. Rev. Lett. 102, 085702 (2009)) The Dynamical Activity Mapsprovided as Supplementary Online Material are also available on http://w3.lcvn.univ-montp2.fr/~lucacip/dam/movies.ht

Microrheology probes length scale dependent rheology

We exploit the power of microrheology to measure the viscoelasticity of entangled F-actin solutions at different length scales from 1 to 100 mu m over a wide frequency range. We compare the behavior of single probe-particle motion to that of the correlated motion of two particles. By varying the average length of the filaments, we identify fluctuations that dissipate diffusively over the filament length. These provide an important relaxation mechanism of the elasticity between 0.1 and 30 rad/sec

Determinants of communication between partners about STD symptoms: implications for partner referral in South Africa

BackgroundSTDs as preventable diseases are a major public health problem in South Africa, both in terms of their effect on quality of life, their economic costs and the fact that STDs as co-factors drive the HIV epidemic. Their widespread occurrence and high prevalence rates are cause for concern. It is argued that the duration of infection increases the probability of harmful sequelae and STD transmission, including HIV, to others. The promotion of seeking health care for STD symptoms at an early stage and partner referral for STD treatment are important strategies in preventing STD transmission to others and re-infection of partners. The cost implications of contact tracing by healthcare workers has resulted in patients being encouraged to refer their partners for STD treatment. This has not always been effective, despite efforts to improve partner referral rates by improved “contact cards” (i.e. a card with a code representing the STD that the patient has been treated for to be given to sexual partners as a way to speed up treatment) and more accessible healthcare services. Other studies have found that the proportion of clients who present with contact cards at STD services ranged from about 2% to 39%, while the proportion of partners who were referred for treatment range from 16% to 30%. Mathews et al. argue that returning contact cards might not be a sensitive enough proxy indicator for partner referral rates.Partner referrals have been found to be seriously compromised by patients' causal explanations for STDs, as well as by the unequal power of the genders in sexual relationships, which impacts on the patients' ability to communicate about sexual matters. Patients often lack an understanding of the importance of referring their asymptomatic partners for treatment. Women's inability to discuss sexual issues due to their unequal status in sexual relationships might impact on partner referral behaviour. Men have been found to blame the STD on the “outside women” (sexual partners outside the primary relationship) and are therefore less likely to refer these partners. The conflict that could arise from informing a partner about an STD was viewed by men as a reason not to communicate about having a STD.While the ability to communicate about STDs with sexual partners is an essential prerequisite for referring them for medical treatment, little attention has been paid to understanding this process. This study is aimed at gaining some understanding of the determinants of communication between partners about STD symptoms. In this study, “talking with a partner about STD symptoms” before seeking medical treatment was viewed as an indication of the likelihood of future partner referral behaviour.Methods A randomly selected sample of 1 477 patients with STD symptoms was interviewed using a structured questionnaire. Logistic regression analysis was used to identify the determinants of talking to a partner about the present STD.ResultsIt was found that patients who had talked with their partner about their current STD symptoms were more likely to be female, be employed, have a tertiary level of education, have had only one sexual partner in the preceding six months, have used condoms, albeit inconsistently during the last six months, and to have thought about abstaining from sex while infected. Those who talked were also more likely to have good knowledge about the effects of STDs and the transmission of STDs in the absence of symptoms, had positive attitudes towards condoms and perceived social support for partner referral.ConclusionImproved partner referral through health education interventions needs to focus specifically on a subgroup of patients, e.g. men and the unemployed, and on the improvement of knowledge regarding the consequences of STDs and asymptomatic transmission. Social and partner support for partner referral and perceived self-efficacy in this regard should be encouraged and maintained. In the absence of skills and counselling services to manage the consequences of STD partner referral, this prevention strategy will remain vulnerable.For full text, click here:SA Fam Pract 2006;48(7):17-17

Drying of complex suspensions

We investigate the 3D structure and drying dynamics of complex mixtures of emulsion droplets and colloidal particles, using confocal microscopy. Air invades and rapidly collapses large emulsion droplets, forcing their contents into the surrounding porous particle pack at a rate proportional to the square of the droplet radius. By contrast, small droplets do not collapse, but remain intact and are merely deformed. A simple model coupling the Laplace pressure to Darcy's law correctly estimates both the threshold radius separating these two behaviors, and the rate of large-droplet evacuation. Finally, we use these systems to make novel hierarchical structures.Comment: 4 pages, 4 figure

Time Resolved Correlation measurements of temporally heterogeneous dynamics

Time Resolved Correlation (TRC) is a recently introduced light scattering technique that allows to detect and quantify dynamic heterogeneities. The technique is based on the analysis of the temporal evolution of the speckle pattern generated by the light scattered by a sample, which is quantified by $c\_I(t,\tau)$, the degree of correlation between speckle images recorded at time $t$ and $t+\tau$. Heterogeneous dynamics results in significant fluctuations of $c\_I(t,\tau)$ with time $t$. We describe how to optimize TRC measurements and how to detect and avoid possible artifacts. The statistical properties of the fluctuations of $c\_I$ are analyzed by studying their variance, probability distribution function, and time autocorrelation function. We show that these quantities are affected by a noise contribution due to the finite number $N$ of detected speckles. We propose and demonstrate a method to correct for the noise contribution, based on a $N\to \infty$ extrapolation scheme. Examples from both homogeneous and heterogeneous dynamics are provided. Connections with recent numerical and analytical works on heterogeneous glassy dynamics are briefly discussed.Comment: 19 pages, 15 figures. Submitted to PR

Coulomb-driven broken-symmetry states in doubly gated suspended bilayer graphene

The non-interacting energy spectrum of graphene and its bilayer counterpart consists of multiple degeneracies owing to the inherent spin, valley and layer symmetries. Interactions among charge carriers are expected to spontaneously break these symmetries, leading to gapped ordered states. In the quantum Hall regime these states are predicted to be ferromagnetic in nature whereby the system becomes spin polarized, layer polarized or both. In bilayer graphene, due to its parabolic dispersion, interaction-induced symmetry breaking is already expected at zero magnetic field. In this work, the underlying order of the various broken-symmetry states is investigated in bilayer graphene that is suspended between top and bottom gate electrodes. By controllably breaking the spin and sublattice symmetries we are able to deduce the order parameter of the various quantum Hall ferromagnetic states. At small carrier densities, we identify for the first time three distinct broken symmetry states, one of which is consistent with either spontaneously broken time-reversal symmetry or spontaneously broken rotational symmetry