Self-ratcheting Stokes drops driven by oblique vibrations

We develop and analyze a minimal hydrodynamic model in the overdamped limit to understand why a drop climbs a smooth homogeneous incline that is harmonically vibrated at an angle different from the substrate normal [Brunet, Eggers and Deegan, Phys. Rev. Lett. 99, 144501 (2007)]. We find that the vibration component orthogonal to the substrate induces a nonlinear (anharmonic) response in the drop shape. This results in an asymmetric response of the drop to the parallel vibration and, in consequence, in the observed net motion. Beside establishing the basic mechanism, we identify scaling laws valid in a broad frequency range and a flow reversal at high frequencies.Comment: 4 pages, 7 figure

Modelling of surfactant-driven front instabilities in spreading bacterial colonies

The spreading of bacterial colonies at solid-air interfaces is determined by the physico-chemical properties of the involved interfaces. The production of surfactant molecules by bacteria is a widespread strategy that allows the colony to efficiently expand over the substrate. On the one hand, surfactant molecules lower the surface tension of the colony, effectively increasing the wettability of the substrate, which facilitates spreading. On the other hand, gradients in the surface concentration of surfactant molecules result in Marangoni flows that drive spreading. These flows may cause an instability of the circular colony shape and the subsequent formation of fingers. In this work, we study the effect of bacterial surfactant production and substrate wettability on colony growth and shape within the framework of a hydrodynamic thin film model. We show that variations in the wettability and surfactant production are sufficient to reproduce four different types of colony growth, which have been described in the literature, namely, arrested and continuous spreading of circular colonies, slightly modulated front lines and the formation of pronounced fingers

From a thin film model for passive suspensions towards the description of osmotic biofilm spreading

Biofilms are ubiquitous macro-colonies of bacteria that develop at various interfaces (solid-liquid, solid-gas or liquid-gas). The formation of biofilms starts with the attachment of individual bacteria to an interface, where they proliferate and produce a slimy polymeric matrix - two processes that result in colony growth and spreading. Recent experiments on the growth of biofilms on agar substrates under air have shown that for certain bacterial strains, the production of the extracellular matrix and the resulting osmotic influx of nutrient-rich water from the agar into the biofilm are more crucial for the spreading behaviour of a biofilm than the motility of individual bacteria. We present a model which describes the biofilm evolution and the advancing biofilm edge for this spreading mechanism. The model is based on a gradient dynamics formulation for thin films of biologically passive liquid mixtures and suspensions, supplemented by bioactive processes which play a decisive role in the osmotic spreading of biofilms. It explicitly includes the wetting properties of the biofilm on the agar substrate via a disjoining pressure and can therefore give insight into the interplay between passive surface forces and bioactive growth processes

Field Theory of Branching and Annihilating Random Walks

We develop a systematic analytic approach to the problem of branching and annihilating random walks, equivalent to the diffusion-limited reaction processes 2A->0 and A->(m+1)A, where m>=1. Starting from the master equation, a field-theoretic representation of the problem is derived, and fluctuation effects are taken into account via diagrammatic and renormalization group methods. For d>2, the mean-field rate equation, which predicts an active phase as soon as the branching process is switched on, applies qualitatively for both even and odd m, but the behavior in lower dimensions is shown to be quite different for these two cases. For even m, and d~2, the active phase still appears immediately, but with non-trivial crossover exponents which we compute in an expansion in eps=2-d, and with logarithmic corrections in d=2. However, there exists a second critical dimension d_c'~4/3 below which a non-trivial inactive phase emerges, with asymptotic behavior characteristic of the pure annihilation process. This is confirmed by an exact calculation in d=1. The subsequent transition to the active phase, which represents a new non-trivial dynamic universality class, is then investigated within a truncated loop expansion. For odd m, we show that the fluctuations of the annihilation process are strong enough to create a non-trivial inactive phase for all d<=2. In this case, the transition to the active phase is in the directed percolation universality class.Comment: 39 pages, LaTex, 10 figures included as eps-files; submitted to J. Stat. Phys; slightly revised versio

Capacity constraints and irreversible investments: defending against collective dominance in UPM Kymmene/Norske Skog/Haindl.

Scrutiny of potential mergers by the European Commission often focuses on unilateral effects or single firm dominance. But some cases have involved concerns over coordinated effects: the concern that the merger could increase the likelihood of consumer harm through tacit collusion by the reduced number of firms in the industry (this is known as collective dominance). The economic and legal issues are far less certain in these cases and a particular challenge is how to bring empirical evidence to bear on the decision. In this chapter we examine a case in newsprint and magazine paper - UPM Kymmene/Norske Skog/Haindl . Here, coordinated effects were at the centre of the Commission’s concerns. We discuss how collusion theory and evidence were used to help clear the merger without remedies in the final Decision.

Gaia Science Operations Centre

Brief outline of Science Operations Centre activities for Gaia.Comment: 2 pages no figure

Interoperability and market foreclosure in the European Microsoft case.

In this paper we discuss some of the most important economic issues raised in European Commission vs. Microsoft (2004) concerning the market for work group servers. In our view, the most important economic issues relate to (a) foreclosure incentives and (b) innovation effects of the proposed remedy. We discuss the economic basis for the Commission’s claims that Microsoft had incentives to exclude rivals in the work group server market through degrading the interoperability of their server operating systems with Windows. We also examine the impact of compulsory disclosure of information on interoperability and argue that the effects on innovation are not unambiguously negative as Microsoft claim. We conclude with some general implications of the case for anti-trust enforcement in high innovation sectors.