Amoeboid motion in confined geometry.

Many eukaryotic cells undergo frequent shape changes (described as amoeboid motion) that enable them to move forward. We investigate the effect of confinement on a minimal model of amoeboid swimmer. A complex picture emerges: (i) The swimmer's nature (i.e., either pusher or puller) can be modified by confinement, thus suggesting that this is not an intrinsic property of the swimmer. This swimming nature transition stems from intricate internal degrees of freedom of membrane deformation. (ii) The swimming speed might increase with increasing confinement before decreasing again for stronger confinements. (iii) A straight amoeoboid swimmer's trajectory in the channel can become unstable, and ample lateral excursions of the swimmer prevail. This happens for both pusher- and puller-type swimmers. For weak confinement, these excursions are symmetric, while they become asymmetric at stronger confinement, whereby the swimmer is located closer to one of the two walls. In this study, we combine numerical and theoretical analyses.

Rheology of particulate suspensions in a Poiseuille flow.

Particulate dense suspensions behave as complex fluids. They do not lend themselves easily to analytical solution. We propose an analytical model to mimic this problem. Namely, we consider arrays of long parallel plates which represent a simplification of arrays of chains of spherical particles. This simplified model can be solved analytically. The effect of effective rotation of the spherical particles is taken into account by attributing different velocities on each side of the plate that mimics the fact that particles are subject to shear. This work is an extension of a previous study where particle rotation was disregarded. The flow rate, the dissipation and the apparent viscosity are studied as a function of the underlying structure. For a single plate placed out of the flow center, the viscosity is lower when rotation is taken into account. For two plates, the minimal viscosity corresponds to the situation where the particles are as close as possible to the center and arranged symmetrically with respect to the center. We compute the rheological properties for arbitrary plate positions, and exploit them for a periodic arrangement. For N plates, and in a confined geometry, the viscosity is about twice as small as compared to the situation where rotation is ignored. We have conducted a numerical study of a suspension of spherical particles, and linear chains of spherical particles. The numerical study is in good qualitative and semiquantitative agreement with the analytical theory considering long plates. This agreement highlights the fact that our analytical model captures the essential features of a real suspension. The numerical study is based on a fluid dynamic particle method where the particles are represented by a scalar field having high viscosity inside.

Undulated blistering during thin film delamination

We study the delamination of a compressed thin film from a solid substrate with continuum elasticity theory. Our model enables one to describe the advance of an undulated blister. It is shown that an elastic tension between the inner fold and the boundary of a blister is at the origin of the undulations. In addition, the essential experimental observations and recent simulations are reproduced very well: (i) Above a strain threshold, straight blister growth is unstable and starts to undulate. (ii) It is found that the period of the undulations scales with epsilon(*-1/2), where epsilon(*) is the isotropic compressive strain of the film. (iii) Similar periodic corrections to this power law scaling are recovered and are found to be associated with a growth instability.

Fractal growth of epitaxial surface clusters with elastic interaction.

The fractal growth of clusters adsorbed on crystal surfaces has been studied by Monte Carlo simulations. Elastic interactions between the atoms through the substrate have been included. Attractive and repulsive interaction potentials 1/r(3) have been used, including a varying cutoff for the range of interaction. As an important result we find that there exists a crossover radius beyond which the fractal dimension of the cluster corresponds to the fractal dimension of conventional two-dimensional diffusion limited aggregation. The crossover radius itself and the properties of the cluster inside that radius depend sensitively on the details of the interaction. The results have been analyzed by a scaling theory. Furthermore, we have implemented a multigrid scheme which allows for very efficient simulation of a large number of mobile atoms with long-range interaction on the surface.