Segregation in Networks

International audienc

Size segregation in a granular bore

We investigate the effect of particle-size segregation in an upslope propagating granular bore. A bidisperse mixture of particles, initially normally graded, flows down an inclined chute and impacts with a closed end. This impact causes the formation of a shock in flow thickness, known as a granular bore, to travel upslope, leaving behind a thick deposit. This deposit imprints the local segregated state featuring both pure and mixed regions of particles as a function of downstream position. The particle-size distribution through the depth is characterized by a thin purely small-particle layer at the base, a significant linear transition region, and a thick constant mixed-particle layer below the surface, in contrast to previously observed S-shaped steady-state concentration profiles. The experimental observations agree with recent progress that upward and downward segregation of large and small particles respectively is asymmetric. We incorporate the three-layer, experimentally observed, size-distribution profile into a depth-averaged segregation model to modify it accordingly. Numerical solutions of this model are able to match our experimental results and therefore motivate the use of a more general particle-size distribution profile

The probabilistic nature of dune collisions in 2D

Dunes are bedforms of different size and shape, appearing throughout aeolian, subaqueous and extraterrestrial environments. Collisions between dunes drive dune field evolution, and are a direct result of interacting dunes of different heights, travelling at different speeds. We perform 2D cellular automaton simulations of collisions between dune pairs migrating in a steady flow. Modelled collisions can result in either ejection, where dunes exchange mass before separating, or downstream- or upstream-dominant coalescence (merging of dunes). For each of these three elementary types of interaction, we identify the mass exchange mechanism and the distinctive intermediate morphologies. Surprisingly, we show that the collision outcome depends probabilistically on the initial dune area ratio r and can be described by a narrow sigmoidal function centred on r=1/2. Finally, we compare our simulations with laboratory experiments of dune collisions, finding good agreement concerning the intermediate morphology and the collision outcome. Our results can motivate further observational or experimental studies that validate our probabilistic collision predictions and fully determine the controls on the coalescence–ejection transition.</p

Wake Induced Long Range Repulsion of Aqueous Dunes.

Sand dunes rarely occur in isolation, but usually form vast dune fields. The large scale dynamics of these fields is hitherto poorly understood, not least due to the lack of longtime observations. Theoretical models usually abstract dunes in a field as self-propelled autonomous agents, exchanging mass, either remotely or as a consequence of collisions. In contrast to the spirit of these models, here we present experimental evidence that aqueous dunes interact over large distances without the necessity of exchanging mass. Interactions are mediated by turbulent structures forming in the wake of a dune, and lead to dune-dune repulsion, which can prevent collisions. We conjecture that a similar mechanism may be present in wind driven dunes, potentially explaining the observed robust stability of dune fields in different environments

High-resolution radar measurements of snow avalanches

Two snow avalanches that occurred in the winter 2010-2011 at Vallée de la Sionne, Switzerland, are studied using a new phased array FMCW radar system with unprecedented spatial resolution. The 5.3 GHz radar penetrates through the powder cloud and reflects off the underlying denser core. Data are recorded at 50 Hz and have a range resolution better than 1 m over the entire avalanche track. We are able to demonstrate good agreement between the radar results and existing measurement systems that record at particular points on the avalanche track. The radar data reveal a wealth of structure in the avalanche and allow the tracking of individual fronts and surges down the slope for the first time. Key Points Validation between our radar results and existing point measurement systems High-resolution radar allows tracking of fronts and surges from start to finish Velocity linked with topography may be used to measure rheology of snow ©2013. American Geophysical Union. All Rights Reserved

Sedimentary structure of large sand dunes: examples from Dumont and Eureka dunes, California

The current research presents ground penetrating radar images up to 30 m in depth of two large desert dunes in California, USA— a barchanoid ridge in the Dumont field and a linear dune in the Eureka expanse. The radar images show a complicated structure of internal layering with ascending cross-strata, cross-bedding and bounding surfaces cutting through layers. Additional research using seismic refraction surveys and sand sampling refine the image of the subsurface (<5 m) structure. The sedimentary structure of the dune shows a strong internal layering with a cemented structure that may immobilize and influence migration of dune expanses. The subsurface features of the sand dune fields in the Mojave Desert provide evidence of dune building, wind regime and precipitation history

UV-induced ligand exchange in MHC class I protein crystals

High-throughput structure determination of protein−ligand complexes is central in drug development and structural proteomics. To facilitate such high-throughput structure determination we designed an induced replacement strategy. Crystals of a protein complex bound to a photosensitive ligand are exposed to UV light, inducing the departure of the bound ligand, allowing a new ligand to soak in. We exemplify the approach for a class of protein complexes that is especially recalcitrant to high-throughput strategies: the MHC class I proteins. We developed a UV-sensitive, “conditional”, peptide ligand whose UV-induced cleavage in the crystals leads to the exchange of the low-affinity lytic fragments for full-length peptides introduced in the crystallant solution. This “in crystallo” exchange is monitored by the loss of seleno-methionine anomalous diffraction signal of the conditional peptide compared to the signal of labeled MHC β2m subunit. This method has the potential to facilitate high-throughput crystallography in various protein families

Looking inside an avalanche using a novel radar system

Snow avalanches are a significant natural hazard in alpine regions and their flow dynamics have similarities to pyroclastic flows and other geological mass movements. However, the potential for artificial release and the temporary nature of their deposits makes them somewhat easier to study. This article explains recent developments in radar technology for imaging these flows. These new data mean that, for the first time, we are seeing the whole flow averaged over spatial scales that are dynamically relevant. This provides an opportunity to properly test existing models for the dynamics used in risk applications and to gain knowledge of the flow physics, which will guide the next generation of model formulations

Class I major histocompatibility complexes loaded by a periodate trigger

Class I major histocompatibility complexes (MHCs) present peptide ligands on the cell surface for recognition by appropriate cytotoxic T cells. The unstable nature of unliganded MHC necessitates the production of recombinant class I complexes through in vitro refolding reactions in the presence of an added excess of peptides. This strategy is not amenable to high-throughput production of vast collections of class I complexes. To address this issue, we recently designed photocaged MHC ligands that can be cleaved by a UV light trigger in the MHC bound state under conditions that do not affect the integrity of the MHC structure. The results obtained with photocaged MHC ligands demonstrate that conditional MHC ligands can form a generally applicable concept for the creation of defined peptide−MHCs. However, the use of UV exposure to mediate ligand exchange is unsuited for a number of applications, due to the lack of UV penetration through cell culture systems and due to the transfer of heat upon UV irradiation, which can induce evaporation. To overcome these limitations, here, we provide proof-of-concept for the generation of defined peptide−MHCs by chemical trigger-induced ligand exchange. The crystal structure of the MHC with the novel chemosensitive ligand showcases that the ligand occupies the expected binding site, in a conformation where the hydroxyl groups should be reactive to periodate. We proceed to validate this technology by producing peptide−MHCs that can be used for T cell detection. The methodology that we describe here should allow loading of MHCs with defined peptides in cell culture devices, thereby permitting antigen-specific T cell expansion and purification for cell therapy. In addition, this technology will be useful to develop miniaturized assay systems for performing high-throughput screens for natural and unnatural MHC ligands