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How liquid water in atmospheric clouds distributes over droplets of different sizes is important for the initiation of rainfall, and for the optical properties of the atmosphere. Droplet-number densities and droplet-size distributions change rapidly at turbulent cloud edges, where mixing of cloudy and dry air causes cloud dilution and droplet evaporation. The resulting droplet configurations are determined by how individual micrometer-sized droplets are exposed to dry air, by a turbulent flow whose length scales range from millimeters to hundreds of meters. A first part of this thesis is on mixing and evaporation at turbulent cloud edges. It is explained how several results from numerical simulations can be understood in a simple way by simplifying and non-dimensionalizing a widely used model. The simple understanding makes it possible to interpret empirical data, and to analyze the multiscale nature of mixing and evaporation in clouds. Two statistical models for mixing and evaporation are presented. The first reproduces the broadening of the droplet-size distribution observed in direct numerical simulations (DNS), but has an oversimplified supersaturation dynamics. This problem is partially resolved in the second model, which under some conditions reproduces the supersaturations of droplets in DNS quantitatively. A second part of the thesis is on the angular dynamics of small particles in flow. How particles spin and tumble is important for many different phenomena, including the intrinsic viscosity of particle suspensions, the settling of ice crystals in clouds, and the motion of plankton. It is predicted that particles that possess a rotation symmetry and a reflection symmetry in a plane that contains the axis of rotation symmetry spin and tumble like spheroids in Stokes flow. This prediction is verified in an experiment where the angular dynamics of triangular particles in a simple shear flow is observed. The angular dynamics of rotation-symmetric particles without the above-mentioned reflection symmetry is analyzed.

International audience; An exhaustive description of the dynamics under shear flow of a large number of red blood cells in dilute regime is proposed, which highlights and takes into account the dispersion in cell properties within a given blood sample. Physiological suspending fluid viscosity is considered, a configuration surprisingly seldom considered in experimental studies, as well as a more viscous fluid that is a reference in the literature. Stable and unstable flipping motions well described by Jeffery orbits or modified Jeffery orbits are identified, as well as transitions to and from tank-treading motion in the more viscous suspending fluid case. Hysteresis loops upon shear rate increase or decrease are highlighted for the transitions between unstable and stable orbits as well as for the transition between flipping and tank-treading. We identify which of the characteristic parameters of motion and of the transition thresholds depend on flow stress only or also on suspending fluid viscosity.

The orientational dynamics of weakly inertial axisymmetric particles in a steady flow is investigated. We derive an asymptotic equation of motion for the unit axial vector along the particle symmetry axis, valid for small Stokes number St, and for any axisymmetric particle in any steady linear viscous flow. This reduced dynamics is analysed in two ways, both pertain to the case of a simple shear flow. In this case inertia induces a coupling between precession and nutation. This coupling affects the dynamics of the particle, breaks the degeneracy of the Jeffery orbits, and creates two limiting periodic orbits. We calculate the leading-order Floquet exponents of the limiting periodic orbits and show analytically that prolate objects tend to a tumbling orbit, while oblate objects tend to a log-rolling orbit, in agreement with previous analytical and numerical results. Second, we analyse the role of the limiting orbits when rotational noise is present. We formulate the Fokker-Planck equation describing the orientational distribution of an axisymmetric particle, valid for small St and general Peclet number Pe. Numerical solutions of the Fokker-Planck equation, obtained by means of expansion in spherical harmonics, show that stationary orientational distributions are close to the inertia-free case when Pe St << 1, whereas they are determined by inertial effects, though small, when Pe >> 1/St >> 1., 20 pages, 5 figures