I will make a small introduction to microfluidics and then show three examples, studied at IPGG, where microfluidics plays a decisive role: photonic band gap systems, for which microfluidic solidified foams allow to reach interesting performances, microfluidic bubbles allowing to capture valuable information in medical ultrasound analysis, and microfluidic devices producing nanoparticles with size control. I will evoke potentially interesting perspectives for the field.

Durante el deshielo del 2015, las aguas del río Sagavanirktok (Alaska, USA) produjeron daños de magnitud a la única vía de comunicación terrestre, la ruta Dalton, entre Prudhoe Bay (un asentamiento industrial cercano al Ártico) y Fairbanks (la segunda ciudad de Alaska, ubicada en el interior del estado). Como consecuencia de este evento, el tránsito terrestre fue interrumpido por aproximadamente tres semanas. El departamento de Transporte de Alaska realizó un proyecto para levantar la cota de la ruta, utilizando material extraído del río adyacente y financió un estudio plurianual para estimar la carga de transporte de sedimentos del río. 

Para lograr ese objetivo, en el otoño de 2015, se excavaron siete trincheras de prueba dentro del canal principal, distribuidas en cuatro estaciones hidro-sedimentológicas (DSS1-DSS4) a lo largo de un tramo de aproximadamente 150 km del río Sagavanirktok para examinar la capacidad natural del transporte de sedimentos del río. Levantamientos topo-batimétricos fueron realizados con una frecuencia mensual en el verano durante el período 2015-2019. Erosión y sedimentación dentro (y fuera) de las trincheras fueron calculadas comparando levantamientos sucesivos. Utilizando pendientes del pelo de agua, diámetro de sedimentos característicos, e hidrogramas del río se calcularon las tensiones de corte para cada una de las estaciones, y el transporte de sedimentos fue estimado usando diferentes parámetros en las ecuaciones de transporte. Los resultados fueron comparados con los obtenidos a partir de la comparación de los relevamientos topo-batimétricos. Posteriormente, ecuaciones de transporte de sedimentos de fondo específicas para cada uno de los sitios de estudio (DSS1– DSS4) fueron desarrolladas. Además, la producción de sedimentos suspendidos en el río Sagavanirktok fue estimada en una de las estaciones (DSS2).

The resuspension of microparticles in ventilated duct is an issue that affects various sectors. It is particularly important during transient airflow periods like cases of fan start after a shutdown period, as particles can be released in the airflow and then contaminate or damage an industrial production, or represent a sanitary risk towards populations. The understanding of the mechanisms responsible for resuspension is thus an important issue.

In terms of airflow experienced by particles, it is obvious that a fan start necessarily means a transient temporal acceleration period before reaching steady state. Some studies have pointed out the significant influence of the transient acceleration period on the temporal evolution of the fraction of particles remaining on the duct wall (or detached) (Ibrahim et al. 2003, 2006). But the experimental protocols employed were not relevant to accurately depict the relationship between the instantaneous remaining fraction and the airflow characteristics where particles are located before being released, i.e. in the viscous sublayer.

This talk thus aims at presenting the experimental methodology that was developed to investigate the relationship between remaining fraction versus time curves to airflow characteristics, for airflow conditions representative of those used in air treatment systems. The experiments are conducted in a channel of rectangular cross sectional area. In order to focus on the phenomena responsible for the resuspension of particles by the airflow, a protocol was developed to generate initial monolayer deposits involving sparse spherical particles (in the 10 – 30 µm size range), in order to avoid particle-particle interactions (collisions…). The experimental protocol which was developed consists in following simultaneously the airflow velocity and / or friction velocity in the viscous sublayer and by counting particles remaining on the duct wall in a given Region Of Interest by an optical method. Velocity and friction velocity measurements are realized by hot wire and hot film anemometry respectively. The Eulerian monitoring of the deposit is conducted by making use of a CCD camera and then by processing the images thanks to a home made code (Cazes et al., 2023).

The temporal evolution of the velocity signal properties is analyzed, and reveals two stages during the transient period of acceleration from rest to steady state. It involves a first stage of acceleration without velocity fluctuation, and then a transition to the turbulent regime with the apparition of fluctuations. The temporal evolution of the remaining fraction shows that resuspension mostly occurs during the airflow acceleration, even though it persists at steady state. The influence of the airflow conditions (mean acceleration and velocity at steady state) and of the particle size on the remaining fraction versus time curves is investigated. It enables to point out the airflow properties which are responsible for the onset of resuspension.

Tsunami waves can be generated by earthquakes but also by landslides such as the event that occurred at Lago Cabrera (Chile) in 1965. These can be a threat to human activities along coastal areas as illustrated by the recent partial flank collapse of Anak Krakatau (Indonesia) in 2018. To improve our prediction of the tsunami waves that can be generated by subaerial landslides, we consider the collapse of a granular column into water in a quasi-two-dimensional setup and systematically investigate the influence of the initial geometry of the column and the water depth on the impulse wave generated.

Our experiments reveal three nonlinear wave regimes, depending on the Froude number Frf based on the ratio of the velocity of the advancing granular front at the interface and the velocity of gravity waves in shallow water. For large Frf , transient bores are generated, while for intermediate values of Frf quasi-symmetrical solitary-like waves are produced. Finally, nonlinear transition waves are observed at small Frf .

By modeling the spreading dynamics of the granular column we are able to develop a model coupling the grains dynamic and the wave generation process to predict the amplitude of the impulse wave generated in shallow water. The model allows us to estimate the maximum amplitude Am of the wave generated when the initial height H0 and width of the column L0, and the water depth h0 are known. We are also able to relate the final immersed volume of grains to the amplitude of the wave generated.

As a result, a tsunami wave generated by a landslide could be estimated from the knowledge of the pre-landslide geometry and bathymetry in a preventive approach, or after a geophysical event by estimating the final volume of immersed grains. The present modeling contributes to a better understanding of the rich hydrodynamics of the impulse waves generated by grains entering into water.

Wind-blown sand and dust aerosols belong to the most important agents of our planet’s climate, yet their physics is — despite almost a century of research — poorly understood.

Shifting desert sand is a driver of desertification, one of the biggest ecological challenges of our time and the cause of socio-economic and political instabilities in many countries of the world. Indeed, about one fifth of the Earth’s arid zones is covered by sand seas, but a data-based model based on climate change projection suggests potential expansion of the Earth’s desert cover area in the coming decades, with dramatic implications for multiple components of the Earth system.

Therefore, the accurate representation of Aeolian sand transport is essential for the development of reliable climate models. However, climate models employ semi-empirical functions for parameterizing Aeolian sand emission processes. Such parameterizations are typically based on observations available for specific soil conditions.

Several numerical models were developed to calculate the average trajectories of wind-blown sand grains in saltation—which consists of particles moving in nearly ballistic hops and ejecting new particles upon collision with the soil — and the concatenated splash function, i.e., the mass and momentum distribution of ejected particles after grain-bed collisions. However, the accurate parameterization of particle trajectories and splash processes relies on the modelling of inter-particle interaction processes, both in the Aeolian layer and within the bed, as well as of erodibility conditions associated with the broad range of natural soils.

The classical description of Aeolian transport is made using the so-called "splash function" which defines the distribution of energy and orientation of the ejected particles after one impactor at a given velocity and direction. Our study is first a complementary observation of one impact process and then an extension of this analysis by looking at the importance of the second impact before the full relaxation of the bead packing. An experimental setup was designed to model this new studies.

Soil bacterium Bradyrhizobium diazoefficiens is an N2-fixing symbiont of soybean that helps to improve grain quality. With two flagellar systems it swims in water-saturated pores but its motility, possibly important for root nodulation competitiveness, is still unclear. We design and fabricate microfluidic soil-on-chip (SOC), offering sustainable agriculture an innovative tool to directly visualize bacteria confined-behavior. Velocities and changes of direction were measured for two strains, wild-type and a mutant with only one flagellum. A reduction of speed and an increase in the proportion of 180° changes of directions is found for both strains in channels of decreasing microscopic cross section. Although the wild-type swims with higher speed in unconfined spaces, this advantage disappears in SOCs. Additionally, we use the motility parameters measured with a high statistic to model and simulate the B. diazoefficiens motion in SOC devices for long times and large scales, enabling further predictions of diffusion in soil.

Isolated bodies with widely different sizes, weights, and shapes may describe periodic paths when rising or falling under the effect of buoyancy in a fluid otherwise at rest. Familiar examples of such non-straight paths are provided by a paper card, or leaves, falling in air and bubbles rising in water. A better understanding of these complex paths is relevant to a large variety of applicative fields, such as mechanical engineering, aerodynamics, marine engineering, meteorology, sedimentology, and the biomechanics of plants and insect flight.

In a fluid initially at rest, non-rectilinear (e.g. periodic, chaotic,…) paths are known to be the result of the complex coupling between the flow generated by the body motion and the degrees of freedom in translation and rotation of the body. In the case of non-spherical bodies, the anisotropy of the body has also an influence on the fluid–body coupling, by impacting the flow structure as well as the response of the body to the hydrodynamic loads as soon as the symmetry axis of the body changes its orientation with respect to the body velocity.

After a general introduction, I will focus my presentation on the behavior of finite-length cylinders falling in a fluid at rest at moderate Reynolds numbers, and will discuss the results of a series of experiments covering different situations arising when the control parameters are varied (Archimedes number, elongation ratio of the cylinder, Cauchy number and density ratio of the cylinder relative to the fluid). Close enough to the transition from rectilinear to periodic motions, different types of periodic motion emerge in association with specific unsteady wakes, sharing specific symmetry properties. Scaling laws characterizing the body kinematics will be discussed for the fluttering regime, the most commonly observed of these regimes. In the case of sufficiently flexible cylinders, bending oscillations may also emerge as the result of the interplay between body deformability and wake dynamics. I will then turn to fluttering motions involving translational and rotational velocity fluctuations comparable in magnitude to the mean fall velocity experienced by the body. Thanks to the Kelvin-Kirchhoff equations, I will discuss the impact on this velocity of non-linear effects arising from the coupling of the fluctuating degrees of freedom of the body.