Philippe Brunet

The growth of cultures and formation of mucilage blooms in reaction to salt stress of cyanobacterial cultures are investigated with a focus on the influence of pH. In non-buffered medium, cultures show their pH increasing from 6.5 just after inoculation, up to 11 during the exponential phase. We record the time-evolution of concentration and pH, with different initial OD$_{\text{0}}$. In a second set of experiments, we extract the doubling time of the unbuffered cultures in comparison with those inoculated in pH-buffered BG11 media at four different pH from 6.3 to 10.5 : in the most acid media, all cultures die or grow very slowly. At pH = 10.5, we obtain the fastest growth for all four strains, allowing to qualify these cyanobacteria as being alkaliphiles, though for all strains with comparable initial OD$_{\text{0}}$, the doubling time is shorter for unbuffered cultures. Following a previous study \cite{Dervaux2015}), we finally investigate the influence of pH on mucilage formation and biomass uplift induced by salt stress, involving EPS floculation by cations. Our results show that operating in buffered media significantly influences the mucilage formation, though the observed regimes cannot be simply correlated to the pH value.

The wetting dynamics is often considered in the frame of stationary conditions, i.e. when the contact-line advances or recedes at constant velocity. The purpose of most of these studies is to determine a relationship between the contact-line velocity and the shape of the free-surface in the vicinity of the line, the latter being often quantified by a dynamical contact-angle θd, which is velocity-dependent. The present review aims to consider the dynamics of wetting in unsteady cases, which have been so far less investigated, where inertia is susceptible to play a significant, if not major, role. This includes the first stages of spreading after the liquid has contacted a substrate, drops sliding along low-friction surfaces, the spreading or receding of liquids with a stick-slip dynamics, the fast and/or oscillating rising of menisci in capillaries and the response of drops or menisci to periodic mechanical forcing. This latter case is particularly remarkable in the fact that the relationship between instantaneous velocity VCL and dynamical contactangle θd shows a complex, multivalued dependence. After having presented some of the most significant experimental and theoretical studies, we present the first results on an original setup where a liquid bridge of height h is sheared by the periodic motion of its lower basal substrate. By varying the frequency such that one encompasses a significant span of ratio δ/h (δ being the thickness of the viscous boundary layer), we show that this aforementioned complex behavior appears if δ/h is below a critical value δ * , while if δ/h > δ * one recovers a relation between velocity and contact-angle compatible with the classical Cox-Voinov hydrodynamic framework. We finally conclude on the remaining questions and challenges in unsteady wetting.

The model eukaryotic microalgae Chlamydomonas reinhardtii is well known for its ability to generate bioconvection flows that are associated to intricate concentration patterns. Recently, it was demonstrated that the propensity of these algae to move toward a light source – a phenomenon termed phototaxis – can be exploited to locally concentrate micro-organisms and induce (photo)-bioconvection in algal suspensions by inducing a localised excess of density. In the present study we show experimentally that a cell population in a thin liquid layer self-organises in the presence of a heterogeneous light field and displays remarkable symmetry-breaking instabilities that are ruled by both the width of the light beam and the photo-bioconvection Rayleigh number. Beside circular stable states, fingers, dendrites and wave instabilities are reported, quantified and classified in a general phase diagram. Next, we use lubrication theory to develop an asymptotic model for bioconvection in a thin liquid layer, that includes the influences of both gyrotaxis and phototaxis. We obtain a single nonlinear anisotropic diffusion–drift equation describing the spatiotemporal evolution of the depth-averaged algal population. Analytical and numerical solutions are presented and show a very good agreement with the experimental results. In particular, we show that the dendrite instability arises as a result of a subtle coupling between the nonlinearity of the phototactic response, the gyrotactic effect and the self-induced bioconvective flows. Such complex flow fields might find applications in photo-bioreactors, through the efficient stirring of the harvested biomass.