My research projects are mainly related to colloid science problems that can be investigated with optical tweewers, and nucleation:
Optical tweezers are a great tool in microrheology to both perturb locally a system and to measure its response. In optical tweezers, laser beams are focalized on particles whose refractive index differ from the fluid, thus resulting in a force on the particles. We use optical tweezers to study colloidal solutions. Baths of colloidal particles are widely used as models of atomic systems to study phase transitions and nucleation, and the local and controlled changes that optical tweezers can apply onto a system provides a powerful tool to study microscopic phenomena occuring in colloidal baths.
In the first movie below, the sample is a bath of fluorescent PMMA colloids that are density- and index-matched with the organic solvent that surrounds them. A polymer has also been dissolved to introduce a depletion interaction. The system is perturbed by adding bigger colloids that are not index-matched. These can be trapped and heated with optical tweezers. By controlling the laser power we can create a local aggregation of the PMMA colloids around the trapped particles because of thermophoresis. The second movie shows what happens when the sample starts being moved at a constant speed with the colloid still trapped. At this large velocity the shell of a few layers of small colloids that forms the aggregate is wiped by the flow.
We expect that such aggregates could be used to create locally controlled phase transitions to study phenomena such as nucleation.
The main motivation in this project is to understand here the hydrodynamic synchronization of cilia and flagella. Motile cilia and flagella are thin biological filaments that are present at the surface of some cells. They bend in a cyclic pattern to generate a flow of the fluid that surrounds them. It is known that assemblies of cilia coordinate their beats and show some degree of synchrony. This work investigates the role of the hydrodynamic interaction on the coordination properties of such oscillators. The knowledge gained in this field is relevant to the development of artificial swimmers as well as the diagnosis of diseases that involve the loss of motility of the active cilia.
Optical tweezers can help! We model an oscillating cilium by a colloid driven by optical tweezers in a cyclic pattern. This model retains most of the physics of cilia (far field hydrodynamic interactions at low Re number and presence of thermal fluctuation). Since all the complex dynamics of bending of the cilia is coarse-grained into spherical active colloids, it is possible to identify the key parameters that control synchronization and the experiments can be complemented by Brownian Dynamics simulations and analytical calculations.
Using such systems, we have been able to show that hydrodynamic coupling very often leads to strong synchronization. For two oscillators, we have studied how the type of the drive or the shape of the driving potentials change the state and strength of synchronization. For systems of more than two oscillators, complex dynamic phase patterns are seen, and we have started to characterize the emergence of metachronal waves. A summary of our results can be found in this review article.
Until recently, all the experiments were performed in a Newtonian fluid. The biological fluids in which cilia move are however often vicoelastic which is the object of current investigations.
This work is being done in the group of Frédéric Caupin.
Cavitation is the process of creating, from a metastable liquid, a stable gas phase that will grow indefinitely. This happens for example when the pressure of a liquid is lowered below a certain value. Good estimates of nucleation thresholds are required by many applications or to prevent disasters (filling/emptying of nanopores, climate models, spill accidents...).
Nucleation is usually described by classical nucleation theory (CNT), but it often leads to inaccurate nucleation thresholds. CNT relies on the calculation of the work required to create a nucleus of a given size. This work is highly dependent on the surface tension associated to the liquid-gas interface that forms the nucleus. At the nanoscale, the interface is however not very well defined, and surface tension is therefore not well defined either. In this project, I was measuring cavitation thresholds in order to test models based on CNT, but with radius-dependent formulas for the surface tension. We have shown that the most common correction to the surface tension (Tolman equation) does not describe properly nucleation data from experiments.
Experimentally, we lower the pressure of a liquid to negative values with a piezo-electric transducer (acoustic cavitation) and pressures are measured by monitoring changes in the refractive index of the liquid (hence in the density) with an optical fiber that allows to measure the reflective coefficient at the glass/liquid interface of the fiber. The discrepancy between the nucleation thresholds from the experiments and from the CNT predictions gives an insight into the shape of the R-dependent surface tension.