Diphasic sytems under thermal control

This section lists the works that have associated the study of two-phase systems with thermal control. In order of presentation, the work on phase separation ofa binary mixture in a temperature gradient shows that several complex dynamic regimes are observed. The obtained regimes depend on the initial mass fraction of the components forming the binary solution. In each regime, a model has been developed to account for the 3D structure of the flows. In the applied systems section, we have shown that a droplet can be activated by the integration of heating resistors. The second section of this page shows that by using local resistors it is possible to make a microfluidic joystick allowing to generate most of the elementary functionalities: transport, storage, breakage. In a next section, the breakup and relaxation of droplets under local topographic variation (generated by locally heating a microchannel) are studied in more detail. Interestingly we show that thermal effects are negligible, the effects related to the deformation of the material dominate. We account for the complexity of the flows involved by the presence of menisci and gutters formed at the corners of the channels.

Phase separation of a binary mixture

PhD Thesis of Marc Pascual, in collaboration with Axelle Amon and Alexandre Vilquin

Phase separation of an ionic liquid mixture assisted by a temperature gradient

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

Thermocapillary instability of an ionic liquid-water mixture in temperature gradient

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

Droplet relaxation and break-up under local topographic variations

PhD of Margaux Kerdraon in collaboration with Benjamin Dollet and Josh McGraw

(see section below for the introduction to the thermomechanical effect)

Self-similar relaxation of confined microfluidic droplets

We report an experimental study concerning the capillary relaxation of a confined liquid droplet in a microscopic channel with a rectangular cross section. The confinement leads to a droplet that is extended along the direction normal to the cross section. These droplets, found in numerous microfluidic applications, are pinched into a peanutlike shape thanks to a localized, reversible deformation of the channel. Once the channel deformation is released, the droplet relaxes back to a pluglike shape. During this relaxation, the liquid contained in the central pocket drains towards the extremities of the droplet. Modeling such viscocapillary droplet relaxation requires considering the problem as 3D due to confinement. This 3D consideration yields a scaling model incorporating dominant dissipation within the droplet menisci. As such, the self-similar droplet dynamics is fully captured.

Kerdraon et al. Phys. Rev. Lett. 2019.

Droplet break-up under topological defect

Under construction

Thermomechanical actuation of droplets: a droplet joystick

Common projects of the PhD thesis of Bertrand SELVA, Vincent MIRALLES and Axel HUERRE

The principle of thermomechanical actuation

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

The droplet Joystick

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.

2D foam drainage control

PhD thesis of Vincent Miralles, in collaboration with Isabelle Cantat IPR-Rennes

Foam Drainage Control Using Thermocapillary Stress in a Two-Dimensional Microchamber

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

Investigating the role of a poorly soluble surfactant in a thermally driven 2D microfoam

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