Giant saltation on Mars

Saltation, the motion of sand grains in a sequence of ballistic trajectories close to the ground, is a major factor for surface erosion, dune formation, and triggering of dust storms on Mars. Although this mode of sand transport has been matter of research for decades through both simulations and wind tunnel experiments under Earth and Mars conditions, it has not been possible to provide accurate measurements of particle trajectories in fully developed turbulent flow. Here we calculate the motion of saltating grains by directly solving the turbulent wind field and its interaction with the particles. Our calculations show that the minimal wind velocity required to sustain saltation on Mars may be surprisingly lower than the aerodynamic minimal threshold measurable in wind tunnels. Indeed, Mars grains saltate in 100 times higher and longer trajectories and reach 5-10 times higher velocities than Earth grains do. On the basis of our results, we arrive at general expressions that can be applied to calculate the length and height of saltation trajectories and the flux of grains in saltation under various physical conditions, when the wind velocity is close to the minimal threshold for saltation.     

Geometry-induced protein pattern formation

Protein patterns are known to adapt to cell shape and serve as spatial templates that choreograph downstream processes like cell polarity or cell division. However, how can pattern-forming proteins sense and respond to the geometry of a cell, and what mechanistic principles underlie pattern formation? Current models invoke mechanisms based on dynamic instabilities arising from nonlinear interactions between proteins but neglect the influence of the spatial geometry itself. Here, we show that patterns can emerge as a direct result of adaptation to cell geometry, in the absence of dynamical instability. We present a generic reaction module that allows protein densities robustly to adapt to the symmetry of the spatial geometry. The key component is an NTPase protein that cycles between nucleotide-dependent membrane-bound and cytosolic states. For elongated cells, we find that the protein dynamics generically leads to a bipolar pattern, which vanishes as the geometry becomes spherically symmetrical. We show that such a reaction module facilitates universal adaptation to cell geometry by sensing the local ratio of membrane area to cytosolic volume. This sensing mechanism is controlled by the membrane affinities of the different states. We apply the theory to explain AtMinD bipolar patterns in Δ EcMinDE Escherichia coli. Due to its generic nature, the mechanism could also serve as a hitherto-unrecognized spatial template in many other bacterial systems. Moreover, the robustness of the mechanism enables self-organized optimization of protein patterns by evolutionary processes. Finally, the proposed module can be used to establish geometry-sensitive protein gradients in synthetic biological systems.Protein patterns serve to initiate and guide important cellular processes. A classic example is the early patterning of the Drosophila embryo along its anterior–posterior axis (1). Here, maternal morphogen gradients initiate a complex patterning process that subsequently directs cell differentiation. However, protein patterns play a regulatory role even at the single-cell level. For example, they determine cell polarity and the position of the division plane. In the yeast Saccharomyces cerevisiae, the GTPase Cdc42 regulates cell polarization, which in turn determines the position of a new growth zone or bud site. This pattern-forming process is driven by the interaction between a set of different proteins that cycle between the plasma membrane and the cytoplasm (2, 3). In the rod-shaped bacterium Escherichia coli, Min proteins accumulate at the ends of the cell to inhibit the binding of the division proteins (4, 5). Here, the main player in the pattern-forming process is the ATPase MinD. It attaches to the membrane in its ATP-bound state and recruits MinE and further MinD-ATP from the cytosol (6). Cycling of proteins between membrane and cytosol is mediated by the action of MinE, which stimulates the intrinsic ATPase activity of MinD and thereby initiates its detachment. The ensuing oscillatory pattern directs the division machinery to midcell, enabling proper cell division in two viable daughter cells.In all of these processes, regulatory proteins establish chemical gradients or patterns that reflect aspects of cell shape. However, how is gradient or pattern formation achieved in the absence of an external template? Many possible mechanisms have been proposed and they are by no means fully classified yet (7, 8). Establishing a pattern involves definition of preferred accumulation points and requires that the symmetry of the homogeneous state is broken. In Bacillus subtilis, there is good evidence suggesting that DivIVA recognizes negative membrane curvature directly by a mechanism that is intrinsic to this cell division protein (7, 9). In contrast, enrichment of MinD at the cell poles in E. coli is an emergent property of the collective dynamics of several proteins. As shown in refs. 10–16, the nonlinear dynamics of the Min system leads to a polar pattern, which oscillates along the long axis and is clearly constrained by cell geometry. A clear disadvantage of such self-organized symmetry breaking through a dynamical instability is that the kinetic parameters must be fine-tuned to allow the establishment of a stable polar pattern.Here, we show that cell geometry itself can enforce a broken symmetry under generic conditions without any need for fine-tuning. We introduce a class of geometry-sensing protein systems whose only stable state is a spatial pattern that is maintained by energy consumption through an ATPase or GTPase (NTPase). The proposed mechanism is based on a generic property of diffusion: The probability that a protein diffusing through the cytosol will strike (and attach) to the membrane scales with the area of membrane accessible to it. Thus, close to the poles of a rod-shaped cell, most of the trajectories available lead to the membrane. Close to midcell, where the membrane is almost flat, about one-half of the possible paths lead away from the membrane. However, on its own, this mechanism only produces transient patterns on the membrane, as the system approaches a stable, uniform equilibrium in finite time (17). Moreover, patterns only emerge from specific initial conditions. In this paper, we ask, how can this generic property of diffusion be complemented by a minimal set of biomolecular processes to robustly maintain patterns? We show that the NTPase activity of a single protein that cycles between membrane and cytosol is sufficient to achieve this goal. Our analysis shows that an inhomogeneous density profile is established on the membrane in the generic case where the affinities of NTP- and NDP-bound forms differ. Moreover, these membrane-bound patterns are amplified if the proteins are able to bind cooperatively to the membrane (e.g., due to dimerization). This mechanism is highly robust because the stable, uniform equilibrium is simply replaced by a unique, stable patterned state. In particular, the mechanism involves no dynamical instability and requires no fine-tuning of parameters.Experimental support for the proposed mechanism comes from E. coli mutants in which both EcMinD and EcMinE were replaced by chloroplastic AtMinD (MinD homolog from Arabidopsis thaliana) (18). With this single ATPase (19) the system establishes a bipolar pattern along the long axis, rescuing the ΔMinDE mutant from cell division pathologies. Mutation studies of the Walker-A binding module show that AtMinD (unlike EcMinD) can form dimers on the membrane even in its ADP-bound form (19–21), suggesting that both forms can cooperatively bind to the membrane. Our study shows that such cooperativity leads to a bipolar pattern along the long axis of the cell, as observed. Furthermore, we suggest that, due to its generic nature, the binding module might also play an essential role in other bacterial pattern-forming systems.     

Kinetics of colony formation by BFU-E grown under different culture conditions in vitro

Colony formation by erythroid burst-forming units (BFU-E) involves a variable number of cell divisions before individual 'subcolonies' begin to appear. Consequently the numbers of subcolonies vary amongst individual bursts. If this observation is interpreted as a reflection of a stochastic process, the number of subcolonies in each individual burst represents the number of divisions by the BFU-E prior to commitment to terminal differentiation. This provides a means for quantitating the probability of erythroid differentiation (pD) and the probability of renewal (1 − pD). In order to determine whether these kinetics of burst formation can be influenced by exogenous factors we used three commercially available media designed for the growth of BFU-E. We found that subcolony numbers per burst ranged from one to 64 and that the cumulative distributions of subcolonies per burst followed a logarithmic curve ( r   > 0.90). Differences were observed in the distribution of subcolonies per burst when BFU-E were grown in different media (  P  = 0.03; Kruskall-Wallis test). The probability of immediate terminal differentiation (i.e. committment to form a subcolony) was 0.25 for two of the media and 0.7 for the third. The corresponding renewal probabilities were 0.75 and 0.3. These data indicate that the proliferation kinetics of BFU-E are susceptible to regulation by exogenous factors.     

From the Cover: The outbreak of cooperation among success-driven individuals under noisy conditions

According to Thomas Hobbes' Leviathan [1651; 2008 (Touchstone, New York), English Ed], “the life of man [is] solitary, poor, nasty, brutish, and short,” and it would need powerful social institutions to establish social order. In reality, however, social cooperation can also arise spontaneously, based on local interactions rather than centralized control. The self-organization of cooperative behavior is particularly puzzling for social dilemmas related to sharing natural resources or creating common goods. Such situations are often described by the prisoner's dilemma. Here, we report the sudden outbreak of predominant cooperation in a noisy world dominated by selfishness and defection, when individuals imitate superior strategies and show success-driven migration. In our model, individuals are unrelated, and do not inherit behavioral traits. They defect or cooperate selfishly when the opportunity arises, and they do not know how often they will interact or have interacted with someone else. Moreover, our individuals have no reputation mechanism to form friendship networks, nor do they have the option of voluntary interaction or costly punishment. Therefore, the outbreak of prevailing cooperation, when directed motion is integrated in a game-theoretical model, is remarkable, particularly when random strategy mutations and random relocations challenge the formation and survival of cooperative clusters. Our results suggest that mobility is significant for the evolution of social order, and essential for its stabilization and maintenance.     

Evidence for the role of organics in aerosol particle formation under atmospheric conditions

New particle formation in the atmosphere is an important parameter in governing the radiative forcing of atmospheric aerosols. However, detailed nucleation mechanisms remain ambiguous, as laboratory data have so far not been successful in explaining atmospheric nucleation. We investigated the formation of new particles in a smog chamber simulating the photochemical formation of H₂SO₄ and organic condensable species. Nucleation occurs at H₂SO₄ concentrations similar to those found in the ambient atmosphere during nucleation events. The measured particle formation rates are proportional to the product of the concentrations of H₂SO₄ and an organic molecule. This suggests that only one H₂SO₄ molecule and one organic molecule are involved in the rate-limiting step of the observed nucleation process. Parameterizing this process in a global aerosol model results in substantially better agreement with ambient observations compared to control runs.     

From the Cover: Cortical instability drives periodic supracellular actin pattern formation in epithelial tubes

An essential question of morphogenesis is how patterns arise without preexisting positional information, as inspired by Turing. In the past few years, cytoskeletal flows in the cell cortex have been identified as a key mechanism of molecular patterning at the subcellular level. Theoretical and in vitro studies have suggested that biological polymers such as actomyosin gels have the property to self-organize, but the applicability of this concept in an in vivo setting remains unclear. Here, we report that the regular spacing pattern of supracellular actin rings in the Drosophila tracheal tubule is governed by a self-organizing principle. We propose a simple biophysical model where pattern formation arises from the interplay of myosin contractility and actin turnover. We validate the hypotheses of the model using photobleaching experiments and report that the formation of actin rings is contractility dependent. Moreover, genetic and pharmacological perturbations of the physical properties of the actomyosin gel modify the spacing of the pattern, as the model predicted. In addition, our model posited a role of cortical friction in stabilizing the spacing pattern of actin rings. Consistently, genetic depletion of apical extracellular matrix caused strikingly dynamic movements of actin rings, mirroring our model prediction of a transition from steady to chaotic actin patterns at low cortical friction. Our results therefore demonstrate quantitatively that a hydrodynamical instability of the actin cortex can trigger regular pattern formation and drive morphogenesis in an in vivo setting.Self-organization is one of the principal mechanisms of biological pattern formation at the molecular, cellular, and tissue scale. Although the pioneering work of Turing (1) has suggested reaction–diffusion as a generic route toward pattern generation (2), a concrete biomolecular or mechanical understanding of how this might occur in vivo remains elusive, except in a few specific cases (3–5). For instance, Kondo and coworkers (6) demonstrated that pigment patterning on the skin of the Pomocanthus imperator can be understood quantitatively from the simple attraction–repulsion kinetics of two cell types.At the cellular level, active structures, such as the cytoskeleton, are generically expected to display a large variety of structures from a theoretical perspective (7–12), many of which have been reproduced in elegant in vitro studies (13–15). In the case of actomyosin gels, the contractile stresses arising from molecular motors have been shown to create large actin flows that can reorganize the cortex (16, 17). Because actin filaments and motors are “self-advected,” or transported, by their own flow (18), there is a self-reinforcing loop in gel density, capable of creating patterns. Nevertheless, most theoretical studies do not consider the cross-effects of polymerization and diffusion, which resist pattern formation. Interestingly, in the past years, several groups have reported in vivo examples of actin patterns: mammalian axons (19), Caenorhabditis elegans embryo (20), and Drosophila trachea (21) are all cellular cylinders that display a regular array of concentric actin rings on their cortex.In this article, we study the example of ring formation in the Drosophila trachea and propose a generic mechanism for stable actin pattern formation, arising from the interplay of actin turnover and myosin activity. The model makes clear predictions, which we test through fly genetics and drug experiments.     

Tolerance of Perovskite Solar Cells under Proton and Electron Irradiation

In this work, radiation experiments and simulations were carried out on perovskite solar cells (PSCs). The experimental results show that the PSCs in this work were robust to proton irradiation but more sensitive to electron irradiation, which is different from the results of previous studies. Simulations based on the Monte Carlo method show that the energy loss at the interface was much higher than that in the material bulk, and the interface was more sensitive to electron incidents.     

Kinetic studies on the formation of nitrosamines I

Summary The kinetics of nitrosation of dimethylamine (DMA) in aqueous perchloric acid solution haves been studied using a differential spectrophotometric technique. The rate law is Initial rate=e[DMA]₀ [nitrite] ₀ ² [H⁺]/(f+[H⁺])² where [DMA]₀ and [nitrite]₀ represent initial stoichiometric concentrations. At 310.0 K and =2.0 M, e=(2.2±0.2)×10^–5 M^–1 s^–1 and f=(1.28±0.02) ×10^–3M. The associated activation energy is 56±3kJ mol^–1. A clear inhibition of the nitrosation rate by ionic strength has been observed in which only the kinetic parameter (f) has an effective change. It is concluded that under the experimental conditions of this work only the dinitrogen trioxid is the effective carrier for the nitrosation.     

The formation of tubular structures by endothelial cells is under the control of fibrinolysis and mechanical factors

This study highlights the importance of several factors involved in the formation of capillary-like structure formation (CLS) using Human Umbilical Vein Endothelial Cells (HUVEC) and Bovine Retinal Endothelial Cells (BREC) cultured on fibrin gels. The fibrin concentration inducing (CLS) was 0.5 mg/ml for HUVEC and 8 mg/ml for BREC. The high fibrin concentration required for the latter cells appeared necessary to counterbalance the extensive fibrinolysis of the gel by the BREC. Fibrin degradation products measured in the culture media showed that fibrin degradation was mandatory but not sufficient for CLS formation. Fibrin degradation acted in concert with the mechanical, concentration dependent properties of the gels to induce CLS. For example, HUVEC did not form CLS on a rigid fibrin of 8 mg/ml in spite of fibrinolysis. As cell reorganisation occurred, the fibrin was disrupted (HUVEC) or pleated (BREC) giving indirect proof of the development of mechanical forces. During CLS formation, an increasing amount of latent TGFβ1 was measured in the medium (1000–1700 pg/ml). The active form of TGFβ1 was not, however, detected and the addition of anti-TGF-β1 antibody to the medium did not influence the formation of the CLS network. Yet, added activated TGF-β1 led to the formation of less organised structures, that were completely abolished by the concomitant addition of the same anti-TGF-β1 antibody. Thus, it is likely that TGF-β1 secreted by the endothelial cells remained in its latent form. In conclusion, a balance between the mechanical properties of fibrin and the fibrinolytic activity of each cell type may regulate CLS formation in our models. We think that the high fibrinolitic activity of the BREC may represent a defense mechanism to protect the retina against thrombosis-induced damage in vivo. This revised version was published online in June 2006 with corrections to the Cover Date.     

Localization Properties of a Quasiperiodic Ladder under Physical Gain and Loss: Tuning of Critical Points,Mixed-Phase Zone and Mobility Edge

We explore the localization properties of a double-stranded ladder within a tight-binding framework where the site energies of different lattice sites are distributed in the cosine form following the Aubry–André–Harper (AAH) model. An imaginary site energy, which can be positive or negative, referred to as physical gain or loss, is included in each of these lattice sites which makes the system a non-Hermitian (NH) one. Depending on the distribution of imaginary site energies, we obtain balanced and imbalanced NH ladders of different types, and for all these cases, we critically investigate localization phenomena. Each ladder can be decoupled into two effective one-dimensional (1D) chains which exhibit two distinct critical points of transition from metallic to insulating (MI) phase. Because of the existence of two distinct critical points, a mixed-phase (MP) zone emerges which yields the possibility of getting a mobility edge (ME). The conducting behaviors of different energy eigenstates are investigated in terms of inverse participation ratio (IPR). The critical points and thus the MP window can be selectively controlled by tuning the strength of the imaginary site energies which brings a new insight into the localization aspect. A brief discussion on phase transition considering a multi-stranded ladder was also given as a general case, to make the present communication a self-contained one. Our theoretical analysis can be utilized to investigate the localization phenomena in different kinds of simple and complex quasicrystals in the presence of physical gain and/or loss.     

Assessment of Pozzolanic Activity of Ground Scoria Rocks under Low- and High-Pressure (Autoclave) Steam Curing

Two sources of natural scoria rocks were procured and ground for use in concrete as natural pozzolans (NP1 and NP2). The evaluation of their pozzolanic reactivity is carried out using different techniques and approaches. The primary goal of employing these techniques is to monitor the amount of portlandite (CH=Ca(OH)₂) consumed during steam curing at low or high pressure. The pozzolanicity of NP powders is determined either directly by monitoring CH variation or indirectly by compressive strength and microstructure development. Autoclave curing is known to stimulate the pozzolanicity of the inert siliceous and aluminosiliceous materials under its high-pressure steam conditions. Both steam-curing conditions were applied in this investigation. In this study, X-ray diffraction, scanning electron microscope, thermogravimetric, Fourier transform infrared, and isothermal analyzers were used. It is concluded that the nature and types of minerals in SR determine their pozzolanic reactivity as either low-pressure steam-reactive or high-pressure steam-reactive cementitious materials. Due to the nature of their silicate structures, notably single-chain or 3D-framework structures, plagioclase feldspars (albite-anorthite) minerals are high-pressure steam-reactive minerals, whereas pyroxene (enstatite and diopside) minerals are low-pressure steam-reactive minerals. Using high-pressure steam curing, varied replacement levels of up to 60% were achieved in NP1, with a consistent strength activity index (SAI) of 99%, while an SAI of 79% was obtained with NP2. During low-pressure steam curing, NP1 and NP2 consumed around 72 and 80% of portlandite, respectively, demonstrating their relative pozzolanic reactivity. When compared to the control concrete mix, the strength activity indices of NP1, NP2, and class F fly ash in their normal concrete mixes reached 74.3, 82, and 73.7%, respectively, after 56 days of normal curing conditions.