Wall Slip of Soft-Jammed Systems: A Generic Simple Shear Process.

From well-controlled long creep tests, we show that the residual apparent yield stress observed with soft-jammed systems along smooth surfaces is an artifact due to edge effects. By removing these effects, we can determine the stress solely associated with steady-state wall slip below the material yield stress. This stress is found to vary linearly with the slip velocity for a wide range of materials whatever the structure, the interaction types between the elements and with the wall, and the concentration. Thus, wall slip results from the laminar flow of some given free liquid volume remaining between the (rough) jammed structure formed by the elements and the smooth wall. This phenomenon may be described by the simple shear flow in a Newtonian liquid layer of uniform thickness. For various systems, this equivalent thickness varies in a narrow range (35±15 nm).

Foam clogging.

To what extent are aqueous foams prone to clogging? Foam permeability is measured as a function of particulate loading (trapped hydrophilic particles) under conditions where the particle to bubble size ratio is allowed to increase when the number of particles per bubble is fixed. In addition to experiments performed on the foam scale, we investigated experimentally and numerically the hydrodynamic resistance of a single foam node loaded with one particle. It is shown that, with respect to solid porous media, aqueous foams clog more efficiently due to two reasons: (i) the deformation of interfaces allows for larger particles to be incorporated within the interstitial network and (ii) the interfacial mobility contributes to lowering of the reduced permeability.

Recirculation model for liquid flow in foam channels.

Although extensively studied in the past, drainage of aqueous foams still offers major unaddressed issues. Among them, the behaviour of foam films during drainage has great significance as the thickness of the films is known to control the Ostwald ripening in foams, which in turn impacts liquid drainage. We propose a model relating the films' behavior to the liquid flow in foam channels. It is assumed that Marangoni-driven recirculation counterflows take place in the transitional region between the foam channel and the adjoining films, and the Gibbs elasticity is therefore introduced as a relevant parameter. The velocity of these counterflows is found to be proportional to the liquid velocity in the channel. The resulting channel permeability is determined and it is shown that Marangoni stresses do not contribute to rigidify the channel's surfaces, in strong contrast with the drainage of horizontal thin liquid films. New experimental data are provided and support the proposed model.

Ripening of a draining foam bubble.

A forced Ostwald ripening experiment is performed on a single foam bubble. The bubble size is followed as the system is wetted with a constant liquid flow rate delivered from one of the bubble Plateau borders. Obtained ripening velocities cannot be described with a model based on a constant film thickness assumption. Within these well-controlled experimental conditions, the film thickness is measured and found to depend on the imposed liquid flow rate. It is shown that the bubble growth rate is well predicted as the films thickness evolution is explicitly introduced in the ripening model. Finally, it is suggested that existing results for the coarsening of draining foams could be understood following the approach validated on the bubble scale.

Permeability of aqueous foams.

We perform forced-drainage experiments in aqueous foams and compare the results with data available in the literature. We show that all the data can be accurately compared together if the dimensionless permeability of the foam is plotted as a function of liquid fraction. Using this set of coordinates highlights the fact that a large part of the published experimental results corresponds to relatively wet foams (epsilon approximately 0.1). Yet, most of the foam drainage models are based on geometrical considerations only valid for dry foams. We therefore discuss the range of validity of the different models in the literature and their comparison to experimental data. We propose extensions of these models considering the geometry of foam in the relatively wet-foam limit. We eventually show that if the foam geometry is correctly described, forced drainage experiments can be understood using a unique parameter --the Boussinesq number.

Specific surface area model for foam permeability.

Liquid foams were recognized early to be porous materials, as liquid flowed between the gas bubbles. Drainage theories have been established, and foam permeability has been modeled from the microscopic description of the equivalent pores geometry, emphasizing similarities with their solid counterparts. But to what extent can the theoretical work devoted to the permeability of solid porous materials be useful to liquid foams? In this article, the applicability of the Carman-Kozeny model on foam is investigated. We performed measurements of the permeability of foams with nonmobile surfactants, and we show that, in introducing an equivalent specific surface area for the foam, the model accurately describes the experimental data over two orders of magnitude for the foam liquid fraction, without any additional parameters. Finally, it is shown that this model includes the previous permeability models derived for foams in the dry foams limit.

Node contribution to the permeability of liquid foams.

This paper deals with the drainage of liquid foams. The liquid velocity is known to be related to viscous dissipation occurring within the elements of the liquid network, i.e. the channels and the nodes. When compared together, available values for the hydrodynamic resistance of a foam node appear to span over more than one order of magnitude. To clarify this point, we propose an alternative experimental method to estimate the value of this parameter. In contrast to previous experimental work performed on the foam scale, the node resistance is not treated as a fitting parameter, but instead it is measured directly on the microscopic scale. The results allow a consistent range of values to emerge for this parameter.

The lateral and dorsal neurons of Drosophila melanogaster: new insights about their morphology and function.

This chapter summarizes our present knowledge about the master clock of the fruit fly at the neuronal level. The clock is organized in distinct groups of interconnected pacemaker neurons with different functions. All of these neurons appear to communicate with one another in order to produce the species-specific activity rhythm, which is organized in morning (M) and evening (E) activity bouts. These two activity components are differentially influenced by distinct groups of pacemaker neurons reminiscent of the Pittendrigh-Daan dual oscillator model. In the original work (Grima et al. 2004; Stoleru et al. 2004), the ventrolateral (LN(v)) and dorsolateral (LN(d)) plus some dorsal groups (DN) of clock neurons have been defined as M and E cells, respectively. We further specify that the clock neurons belong to the M and E oscillators and define a more complex picture of the Drosophila brain clock.

Speeding to a stop: the finite-time singularity of a spinning disk.

The final stages of a coin spinning on a flat surface have recently been proposed [H.K. Moffatt, Nature (London) 404, 833 (2000)] as an example of a finite-time singularity, wherein the precession rate of the symmetry axis of the coin diverges as it comes to a stop. We report measurements by high-speed video imaging of the rolling motion of disks and rings on a variety of surfaces. We find that the precession rate, Omega, diverges as a power law in time: Omega(t) proportional, variant (t-t(o))(-1/n), where t(o) is the instant the motion ceases. The exponent n varies between 2.7 and 3.2 under different experimental conditions. The value of n, as well as the systematic dependence of precession rate on coefficients of friction, establishes that the primary mechanism of energy dissipation is rolling friction rather than air drag, as previously suggested.

Defining the role of Drosophila lateral neurons in the control of circadian rhythms in motor activity and eclosion by targeted genetic ablation and PERIOD protein overexpression.

The ventral lateral neurons (LNvs) of the Drosophila brain that express the period (per) and pigment dispersing factor (pdf) genes play a major role in the control of circadian activity rhythms. A new P-gal4 enhancer trap line is described that is mostly expressed in the LNvs This P-gal4 line was used to ablate the LNvs by using the pro-apoptosis gene bax, to stop PER protein oscillations by overexpressing per and to block synaptic transmission with the tetanus toxin light chain (TeTxLC). Genetic ablation of these clock cells leads to the loss of robust 24-h activity rhythms and reveals a phase advance in light-dark conditions as well as a weak short-period rhythm in constant darkness. This behavioural phenotype is similar to that described for disconnected1 (disco1) mutants, in which we show that the majority of the individuals have a reduced number of dorsally projecting lateral neurons which, however, fail to express PER. In both LNv-ablated and disco1 flies, PER cycles in the so-called dorsal neurons (DNs) of the superior protocerebrum, suggesting that the weak short-period rhythm could stem from these PDF-negative cells. The overexpression of per in LNs suppresses PER protein oscillations and leads to the disruption of both activity and eclosion rhythms, indicating that PER cycling in these cells is required for both of these rhythmic behaviours. Interestingly, flies overexpressing PER in the LNs do not show any weak short-period rhythms, although PER cycles in at least a fraction of the DNs, suggesting a dominant role of the LNs on the behavioural rhythms. Expression of TeTxLC in the LNvs does not impair activity rhythms, which indicates that the PDF-expressing neurons do not use synaptobrevin-dependent transmission to control these rhythms.

Velocity fluctuations in a homogeneous 2D granular gas in steady state

We have measured the spectrum of velocity fluctuations in a granular system confined to a vertical plane and driven into a homogeneous, steady state by strong vertical vibration. The distribution of horizontal velocities is not Maxwell-Boltzmann and is given by P(v) = Cexp[-beta(|v|/sigma)(alpha)] where alpha = 1.55+/-0.1 at all frequencies and amplitudes investigated, and also for varying boundary conditions. The deviation from Maxwell-Boltzmann statistics occurs in the absence of spatial clustering and does not result from an inhomogeneous average over regions of varying local density. Surprisingly, P(v) has the same shape over a wide range of densities.

Genetic and developmental models for the neural control of breathing in vertebrates.

The present paper reviews some of the possible mechanisms that may link gene function in the brainstem and breathing patterns in vertebrates. On one hand, adaptation and acclimatisation of mature breathing to environmental constraints such as hypoxia, involves complex regulation of the gene expression in precise cardiorespiratory sites of the brainstem. On the other hand, targeted inactivation of different genes suggests that postnatal respiratory variables at rest depend on genes controlling the prenatal development of the brainstem. During embryogenesis, neurotrophins (gdnf, bdnf) regulate the survival of specific cellular populations composing the respiratory neuronal network. The expression of developmental genes such as Hox and Krox-20 initiates hindbrain segmentation, the earliest sign of regionalisation in the brainstem. As shown in the chick embryo, segmental specifications allow the establishment of an active embryonic rhythmic network and later insertion of specific neuronal circuits increasing the primordial rhythm frequency to near mature values.

Effects of circadian mutations and LD periodicity on the life span of Drosophila melanogaster.

The pervasive occurrence of circadian clocks throughout the living world underlines their adaptive value. Nonetheless, there is surprisingly little evidence for a negative impact, on any animal species, of a constant discrepancy between the environmental and endogenous periods. Male Drosophila melanogaster per mutants with altered circadian periods were compared to the wild type in two different LD schedules. Life span was used as a global index of physiological adaptation. The life span of the mutants was significantly reduced by up to 15% for the flies whose period differs most from that of the wild type. A reduction was observed even when flies were kept in an LD schedule fitting a mutant period. The LD schedule made no significant difference on its own, but the authors found evidence for an interaction between genotype and LD schedule in determining life span. These results are consistent with the importance of the circadian clock in maintaining internal temporal order independent of environmental cycles. Nonetheless, a large difference between the environmental and endogenous periods has a measurable impact.

A new gene encoding a putative transcription factor regulated by the Drosophila circadian clock.

Circadian rhythms of locomotor activity and eclosion in Drosophila depend upon the reciprocal autoregulation of the period (per) and timeless (tim) genes. As part of this regulatory loop, per and tim mRNA levels oscillate in a circadian fashion. Other cycling transcripts may participate in this central pacemaker mechanism or represent outputs of the clock. In this paper, we report the isolation of Crg-1, a new circadianly regulated gene. Like per and tim transcript levels, Crg-1 transcript levels oscillate with a 24 h period in light:dark (LD) conditions, with a maximal abundance at the beginning of the night. These oscillations persist in complete darkness and depend upon per and tim proteins. The putative CRG-1 proteins show some sequence similarity with the DNA-binding domain of the HNF3/fork head family of transcription factors. In the adult head, in situ hybridization analysis reveals that per and Crg-1 have similar expression patterns in the eyes and optic lobes.

Analysis of period circadian expression in the Drosophila head by in situ hybridization.

The expression of the period (per) gene of Drosophila melanogaster has been studied by in situ hybridization in the adult's head, where it is required for the fly to exhibit behavioral circadian rhythms. We have used non-radioactive in situ hybridization to obtain a high sensitivity and specificity on head sections, with single cell resolution. Consistent with previous per protein- or per reporter gene-expression, per-expressing cells were detected in the optic lobes and the central brain, as well as in the head sensory organs: eyes, ocelli, maxillary palps and proboscis. In the brain and the eyes, circadian fluctuations of the per mRNA abundance were observed in different per expressing cells.

Members of a family of Drosophila putative odorant-binding proteins are expressed in different subsets of olfactory hairs.

A polymerase chain reaction-based method was used to generate a Drosophila melanogaster antennal cDNA library from which head cDNAs were subtracted. We identified five cDNAs that code for antennal proteins containing six cysteines in a conserved pattern shared with known moth antennal proteins, including pheromone-binding proteins. Another cDNA codes for a protein related to vertebrate brain proteins that bind hydrophobic ligands. In all, we describe seven antennal proteins which contain potential signal peptides, suggesting that, like pheromone-binding proteins, they may be secreted in the lumen of olfactory hairs. The expression patterns of these putative odorant-binding proteins define at least four different subsets of olfactory hairs and suggest that the Drosophila olfactory apparatus is functionally segregated.

Isolation of sequences from Xp22.3 and deletion mapping using sex chromosome rearrangements from human X-Y interchange sex reversals.

A repeated DNA element (STIR) interspersed in Xp22.3 and on the Y chromosome has been used as a tag to isolate seven single-copy probes from the human sex chromosomes. The seven probes detect X-specific loci located in Xp22.3. Using a panel of X-chromosomal deletions from X-Y interchange sex reversals (XX males and XY females), these X-specific loci and some additional ones were mapped to four contiguous intervals of Xp22.3, proximal to the pseudoautosomal region and distal to STS. The construction of this deletion map of the terminal part of the human X chromosome can serve as a starting point for a long-range physical map of Xp22.3 and for a more accurate mapping of genetic diseases located in Xp22.3.