Ionic liquids have remarkable properties and are commonly harnessed for green chemistry, lubrication, and energy applications. In this paper, we study a thermoresponsive ionic liquid (IL) solution which has the property of phase separating above a critical temperature, an interesting feature for the recovery of the IL-rich phase. For this purpose, we generate a temperature gradient in a microfluidic cavity where the confinement strengthens wetting effects and enhances the demixing. We show that the phase separation is performed by the joint effects of sedimentation and thermocapillary actuation giving rise to a threedimensional flow structure, which is quantitatively captured by our model. Altogether those mechanisms lead to the accumulation of the wetting phase near the heating source. We believe this work will find applications in the recycling of ionic liquids.

Pascual et al. Phys. Rev. Fluids, 2021. a

We study a thermoresponsive Ionic Liquid (IL) solution which has the property of phase separating above a critical temperature, an interesting feature for the recovery of the IL-rich phase. For this purpose, we generate a temperature gradient in a microfluidic cavity where the confinement strengthens wetting effects and enhances the demixing. In this experimental configuration, we report the separation patterns along the phase diagram of the binary mixture composition. Three separation dynamics regime are identified that may display complex three-dimensional flows. In spite of this complexity, we rationalize all the observed regimes. Only two regimes lead to a complete spatial separation of the two phases. Interestingly, one is reminiscent of a Marangoni instability in radial geometry, even at confinement below 100 μm. We believe this work will find applications in the recycling of ionic liquids.

Pascual et al. Phys. Rev. Fluids, 2021. b

We perform studies of pancake-like shaped bubbles submitted to a temperature gradient in a micrometric height Hele-Shaw cell. We show that under the experimental conditions, usually found in microfluidic devices, the temperature-induced dilation of the cavity overcomes the thermocapillary convection due to surface tension variation, effectively driving the bubble toward

the cold side of the cavity. The bubble velocity is experimentally characterized as a function of the bubble radius, the temperature gradient, and the initial Hele-Shaw cell thickness. We propose a theoretical prediction of the bubble velocity, based on the analytical resolution of the hydrodynamical problem. The equations set closure is ensured by the pressure value near the bubble and by the dissipation in the moving meniscus.

Selva et al., Lab Chip 2010

Selva et al., Physics of Fluids, 2011

We report on a versatile technique for microfluidic droplet manipulation that proves effective at every step: from droplet generation to propulsion to sorting, rearrangement or break-up. Non-wetting droplets are thermomechanically actuated in a microfluidic chip using local heating resistors. Controlled temperature variation induces local dilation of the PDMS wall above the resistor, which drives the droplet away from the hot (i.e. constricted) region (B. Selva, I. Cantat and M.-C. Jullien, Phys. Fluids, 2011, 23, 052002). Adapted placing and actuation of such resistors thus allow us to push forward, stop, store and release, or even break up droplets, at the price of low electric power consumption (<150 mW). We believe this technically accessiblemethod to provide a useful tool for droplet microfluidics.

V. Miralles et al., Lab Chip 2015.

We investigate the drainage of a 2D microfoam in a vertical Hele-Shaw cell, and show that the Marangoni stress at the air-water interface generated by a constant temperature gradient applied in situ can be tuned to control the drainage. The temperature gradient is applied in such a way that thermocapillarity and gravity have an antagonistic effect. We characterize the drainage over time by measuring the liquid volume fraction in the cell and find that thermocapillarity can overcome the effect of gravity, effectively draining the foam towards the top of the cell, or exactly compensate it, maintaining the liquid fraction at its initial value over at least 60 s.We quantify these results by solving the mass balance in the cell, and provide insight into the interplay between gravity, thermocapillarity, and capillary pressure governing the drainage dynamics.

The figure aside represents the time evolution of the liquid fraction for varying the applied temperature gradient. At a critical temperature gradient, the gravitational drainage is stopped.

Figure : Liquid fraction for different temperature gradients.

Miralles et al., Phys. Rev. Lett. 2014

The foam drainage dynamics is known to be strongly affected by the nature of the surfactants stabilising the liquid/gas interface. In the present work, we consider a 2D microfoam stabilized by both soluble (sodium dodecylsulfate) and poorly soluble (dodecanol) surfactants. The drainage dynamics is driven by a thermocapillary Marangoni stress at the liquid/gas interface and the presence of dodecanol at the interface induces interface stress acting against the applied thermocapillary stress, which slows down the drainage dynamics. We define a damping parameter that we measure as a function of the geometrical characteristics of the foam. We compare it with predictions based on the interface rheological properties of the solution.

Figure: Damping factor r as a function of the DOH bulk concentration

Miralles et al., Soft Matter 2016