Kevin Roger

How do intermolecular interactions give rise to colloidal structures that govern the macroscopic behavior of a multiphase process?

My research objective is to study the structuring of colloidal systems driven along a path out of equilibrium—that is, when one or several system parameters are varied over time. The conceptual goal is to develop a categorization of the major types of out-of-equilibrium structuring and the levers that give rise to them. The practical goal is to control the relationship between fabrication/structuration/property/function by tuning the interactions between species and the morphology of the out-of-equilibrium path. I am specifically interested in three major families of processes: drying, emulsification, and precipitation.

Drying

I approach the drying of complex systems through the lens of thermodynamic non-ideality and its coupling with mass transport, with the following major advances:

• Design of a controlled drying device. Conceptually, this device makes it possible to impose constant boundary conditions, and the flow is unidirectional. Practically, it is simple and inexpensive to fabricate, compatible with a wide range of characterization techniques, and requires very small sample volumes. Since it never reaches a steady state, this device enables the generation of very large concentration gradients, allowing high-resolution spatial characterization.

• Study of drying dynamics for a diversity of colloidal systems. The identification of a diffusive scaling law across a wide range of systems (amphiphile or polymer solutions, soft particle dispersions) is recognized as the signature of drying in systems dominated by intermolecular interactions in the concentrated state.

• Measurement of the mutual diffusion coefficient of binary mixtures. The coexistence of a scaling law and controlled constant boundary conditions enables complete mathematical modeling that transforms an experimental observable—the concentration gradient—into a measurement of the mutual diffusion coefficient of binary mixtures.

• An evaporation mechanism insensitive to humidity. In a wide range of systems, I observed a mechanism through which water evaporation is almost independent of air humidity. This mechanism relies on a subtle compensation between the change in driving force (humidity) and the change in transport pathway (permeability). Encountered across many systems, including biological ones, this mechanism is a signature of evaporation in complex systems governed by molecular-scale processes.

• A method for the assembly and characterization of biomembranes. The device makes it possible to concentrate a dilute system in a controlled way, probe a gradient with excellent resolution, and rescale this gradient to a relevant thickness. It has thus been used to reconstitute a pulmonary surfactant film from its elementary components (lipids, proteins).

• A holistic study of airborne virus transmission. The problem of airborne transmission is fundamentally a drying problem of a complex fluid, since expelled droplets are aqueous solutions of proteins (and salts). Drying primarily determines the two characteristic times of the problem: the suspension time—how long aerosol droplets remain airborne—and the infectivity time—how long viruses remain contagious inside droplets.

• Multicomponent diffusion in evaporating soft matter. Multicomponent systems exhibit qualitatively new transport and structuring behaviors that cannot be inferred from the study of idealized binary systems. In particular, the couplings between diffusion, non-ideal thermodynamics, and structural evolution—including the presence of hydration gradients—can drive the segregation of solutes within mixtures. These couplings give rise to emergent, robust, and generic phenomenologies, observed both in model systems and in complex biological fluids.

Nanoprecipitation

I focus on understanding and developing robust processes for nanoprecipitation techniques aimed at producing organic or inorganic nanoparticles, with the following main points:

• Design of a continuous-flow synthesis device with sequential additions. A fast-mixing device enabling quantitative and controlled production, compatible with industrial scale-up, has been developed and fabricated. This device performs the mixing step in less than 1 ms, which is necessary to decouple mixing from precipitation and thereby achieve control over the synthesis. For the first time worldwide, the combination of multiple mixing units has made it possible to perform syntheses in which the composition can be modified within 1 ms, opening numerous new synthesis pathways. This makes it possible, for instance, to decouple nucleation from growth.

• Understanding the synthesis mechanisms of anisotropic nanoparticles. The development of tools to probe syntheses ex situ and the control of synthesis pathways enabled us to understand how anisotropic nanoparticles such as platelets can be formed by decoupling the role of each reactant.

• Overcoming the concentration limits of the Ouzo effect. Solvent shifting, or the Ouzo effect, can produce nano-objects but is restricted to very low concentration ranges. We have elucidated the origin of this limitation and proposed a method to overcome it using fast mixers. We are also developing thermodynamic tools to better predict phase diagrams.

• Controlling the morphology of silica gels in hydrometallurgy. In several hydrometallurgical processes, silica precipitates during extraction and clogs filters. We worked on controlling precipitation pathways to adjust the mesostructure and filterability of silica gels.

• Novel synthesis of lipid nanocarriers. I am interested in designing new pathways to form lipid nanocarriers encapsulating polynucleotides, based on micro and millifluidic tools, small-angle scattering techniques and a clear understanding of the underlying physical chemistry.
  
  

Emulsification

My research has focused on emulsions (macro- and nanoemulsions) stabilized by amphiphiles or polymers/proteins, with a focus on formation and metastability mechanisms, requiring a local description and the understanding of molecular interactions and interfacial structure. 

• Towards a rational formulation of parenteral emulsions. Parenteral emulsions are essential for feeding certain patients, and their formulation is highly constrained in terms of composition and droplet metastability. A minor change in composition can lead to catastrophic failure. We proposed a framework that explains the link between composition and emulsion metastability, paving the way for a rational formulation strategy.

• Better use of gum arabic. Gum arabic is a natural, digestible compound well-suited for stabilizing emulsions, but historically requires very large quantities. We uncovered the stabilization mechanism, enabling a ten-fold reduction in the amount required while maintaining metastability.

• Nanoemulsification near phase inversion. I showed that low-energy nanoemulsification near phase inversion proceeds through the disruption of bicontinuous self-assembled structures rather than through phase inversion itself. By analyzing the role of the surfactant preferred curvature and the path followed in parameter space, this study established that nanoemulsions form only when a specific non-equilibrium access state is crossed. This clarified a long-standing confusion in the field and provided a unifying framework to rationalize and control nanoemulsification pathways based on non-equilibrium self-assembly.

• Contact ripening in emulsions. I identified contact ripening as a previously overlooked out-of-equilibrium coarsening mechanism in emulsions. Unlike Ostwald ripening or coalescence, this mechanism relies on molecular exchange during transient droplet–droplet contacts and can dominate even when the classical pathways are suppressed. This work revealed that emulsion aging can be governed by collective interfacial processes controlled by collision frequency and interfacial fluctuations, reshaping the understanding of emulsion metastability in dense systems.