Phase separation of a Ionic Liquid mixture by heating locally in a microfluidic system

In recents works, we have shown that heating locally (50 -100 µm) in a microfluidic system involves two mechanisms: i) locally the PDMS is deformed and leads to a thickness gradient which generates a local capillary pressure; ii) the temperature profile generated by a local resistance leads to surface tension gradients and thus to stresses along an interface between two immiscible phases, generating flows close to this interface.

We have shown that local heating in a microfluidic cavity can generate the phase separation of an ionic liquid of the LCST (low critical solution temperature) type into two separate phases: one rich in water, the other rich in ionic liquid. This system allows to consider an alternative to the recycling of these solutions which have many applications, e.g. in green chemistry. More precisely, we study the phase separation along the phase diagram of the solution composition. Three regimes are observed depending on the mass fraction of ionic liquid. These regimes display complex 3D flows combining the different idendified mechanisms that are rationalized. Interestingly, a thermocapillary-instability appear at intermediate ionic liquid mass fraction.