Marie-Caroline Jullien's research interests focus on soft matter and droplet-based microfluidics, such as but not limited to, foams, emulsions, chaotic mixing, and 2D turbulence.

For more information on her projects, visit her website: (link)

Thermo-actuated migration in microsystems

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

in collaboration with Isabelle Cantat IPR-Rennes

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

A versatile technology for droplet-based microfluidics: thermomechanical actuation

Miralles et al., Soft Matter 2016

in collaboration with Isabelle Cantat (IPR-Rennes), E. Rio (LPS-Orsay)

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 accessible method to provide a useful tool for droplet microfluidics.

Lubrification film topography

Huerre et al., Phys. Rev. Lett., 2015.

Huerre et al., Lab. Chip, 2016.

in Collaboration with O. Theodoly and M..P. Valignat (LAI), I. Cantat (IPR) and A. Leshansky (Technion Univ.-Israel)

We study the motion of droplets in a confined, micrometric geometry, by focusing on the lubrication film between droplet and wall. When capillary forces dominate, the lubrication film thickness evolves non linearly with the capillary number due to viscous dissipation between meniscus and wall.  However, this film may become thin enough (tens of nanometers) that intermolecular forces come into play and affect classical scalings. We report the novel experimental characterization of two dynamical regimes as the capillary number increases:

 

(i) at low capillary numbers, the film thickness is constant and set by the disjoinging pressure, while

 

(ii) above a critical capillary number, the interface behavior is well described by a Bretherton-like viscous scenario.

 

At a high surfactant concentration, structural effects lead to the formation of patterns on the interface, which can be used to trace the interface velocity. Our experiments yield highly resolved topographies of the shape of the interface and allow us to bring new insights into droplet dynamics in microfluidics.