Research

For the last fifteen years I have been studying the dynamics of two-phase systems by addressing the question of their control using temperature.

In the case of dense systems (a foam in our case), we have shown that it is possible to control the gravity drainage of the liquid by a temperature gradient.  This control allows us to envision the production of controlled cellular materials for various applications. One of these applications is to understand the role of liquid films on the acoustic absorption  observed in macro experiments.  For diluted systems (droplets), we show that integrating a heating resistor leads to the expansion of the material forming the cavity, propelling the droplets towards the place where the cavity is not deformed, i.e. towards the cold side. The thermocapillary effect, i.e. the flow generated by the surface tension gradient resulting from this temperature gradient, is negligible. 

 The observation of this thermomechanical mechanism has guided many of the projects.Indeed, once this mechanism was identified, it was necessary to understand to what extent we could control elementary operations on droplets (propulsion, sorting, breaking...), which we had shown essentially in a qualitative way.  On the other hand, understanding and modeling these elementary operations required additional studies. The first feature was to be able to predict the velocity of the propelled droplets, requiring first to know a reference situation, namely the velocity of a droplet in a flow without temperature gradient.  This simple question is actually very complex to answer because the dissipation mechanisms in the meniscus are not well known due to the presence of surfactants. This question has led to 3 theses. For both dilute and dense systems, we have shown that surfactants at the interfaces play a key role in the dynamics of these systems.It is in this context that I carried out a mobility in Rennes in the Soft Matter department of the IPR.  In addition to the dynamic effects that can be observed indirectly by measuring the thickness of the lubrication films, we asked ourselves the question of mass transport near an interface. Before studying a complex system, we wanted to look at a simple configuration: a solid surface, which means studying what is adsorbed by a sensor. The work was the subject of a thesis. By returning to the effect of a thermal stress on a droplet, we also wanted to understand the mechanisms that drive droplet relaxation and/or breakup when a local heating resistor (smaller than the size of the droplet) is turned on.  In particular, we have shown that the modeling of these systems requires to consider all the 3D flows around the deformed drop. 

Finally, this work integrating a heating resistor led to the idea of doing phase separation of binary liquids in microfluidic systems. Indeed, by locally heating a microfluidic cavity, the phase separation of a binary mixture is favored by the predominance of capillary mechanisms. Depending on the composition of the binary solution, three dynamic separation regimes are observed and then modeled. In this configuration, it is the combination of thermocapillary and thermomechanical effects that leads to phase separation.

PS: This introduction is not exhaustive, the other projects are also integrated, only those of the last 8 years appear in this introduction.