Kevin ROGER

I focus my research on understanding how complex colloidal systems structure themselves along non-equilibrium pathways. I investigate three main processes: emulsification, drying and precipitation, using a large diversity of systems at the crossroads of engineering, chemistry and biology.

Process 1: Drying complex colloidal systems

Water evaporation from a complex fluids offers an interesting but challenging phenomenology that relates to the build-up of concentration gradients as drying proceeds. This also generate gradients in water activity and in mesostructure from the air/liquid interface towards the fluid bulk. I specifically investigate colloidal systems that are well hydrated in bulk such as surfactant solutions, lipid dispersions, hydrophilic polymer solutions, microgels, or more recently saliva in the context of COVID-19 airborne transmission.

The key idea that has been unveiled in the past 5 years is the compensation between a change in evaporation driving force, the air relative humidity, and the evaporation pathway, water permeability throughout the concentration gradient. This compensation is very generic as it rests simply on the idea that water activity will drastically decrease with decreasing water content in the vicinity of the air/liquid interface, where the system is driest, together with a collapse of the structure’s permeability to water. A striking resulting signature is that the air relative humidity plays only a minor role in the evaporation of such complex fluids, which completely shifts the paradigm of what is known so far from fluid mechanics description of hard-sphere colloidal drying.

Process 2:  From self-emulsification to industrial formulations

I have investigated in details the so-called phase inversion emulsification processes, and showed that phase inversion was not at all involved in any part of these processes. Instead, I evidenced the key role of bicontinuous structures that existed in the vicinity of Phase inversion (temperature or composition). Disrupting these structures lead to the formation of nanoemulsions with well-defined and controlled sizes.

More recently I have rather shifted my attention to two industrial systems, lipid stabilized emulsions for parenteral nutrition and gum Arabic stabilized emulsions for beverages. Gum Arabic is a hydrocolloid used since the dawn of civilization in various applications but its behavior at interfaces was not well-described.  We have systematically studied interfacial coverage and structuration to unravel the exceptional stabilization mechanism gum Arabic can provide to emulsions. This leads to a more rational design of industrial formulations.

Process 3:  Nanoprecipitation

I have firstly been interested in unveiling the mechanisms at play during solvent-shifting nanoprecipitation, also known as the Ouzo effect. With in situ measurements, we monitored the formation of polymer particles and evidenced a self-focusing behavior that could be described in a Smoluchowski framework. I have then turned to other nanoprecipitation methods involving redox reactions to produce metal nanoparticles, using a unique millifluidic approach that combines sequential multi-step additions at the millisecond time-resolution. This opens a new part of the parameters space to design and understand nanostructures.

I have also investigated industrial problems of silica precipitation for filtration or hydrometallurgy, in which adapted non-equilibrium pathways were proposed to modify mesostructures and thus filtration properties.

Research supervision

Present:

– 2 PhD student: Nouha El Amri and Antoine Violas (co-supervised with Olivier Masbernat)

– 2 post-doc : Tania Merhi and Omer Atasi

Alumni:

– PhD students: Marina Atgié, Christian Manfoumbi, Brice Courtois, Airama Albisa Novo

– Post-docs : Léo Garcia and Jenny Andersson

Foreign collaborators: Emma Sparr (Lund university), Jesus Perez-Gil (Madrid university), Jérôme Crassous (Aachen University)