(2kf) Engineering of Transport Processes out of Equilibrium: Environmental Applications Driven By Fundamental Science

Teaching Interests: Fluid Mechanics, Thermodynamics, Heat Transfer, Mathematical Methods, Differential Equations, Dynamical Systems, Calculus.

Transport processes out of equilibrium are pervasive in both natural and engineered settings across different scales, from the water turbulence around a large cargo ship to the diffusion of nutrients through cell membranes. Engineering these complex phenomena is often a challenging task, requiring both a deep fundamental understanding of their physics and a considerable sophistication of experimental and mathematical techniques. Nevertheless, the successful application of transport phenomena often leads to impactful new technology for the benefit of society as a whole. My research, which I present below, is aimed at making an impact on some much-needed solutions to some environmental issues through the fundamental understanding of these complex processes.

The first line of research concerns surfactants. Any liquid, be it in a living organism, a machine, or the environment, contains at least trace amounts of these substances. A specific technological application in which surfactants play a pivotal role are superhydrophobic surfaces (SHSs): coatings able to retain a mattress of air bubbles when immersed in water, theoretically enabling a large reduction of friction between a solid object and its surrounding fluid. Potential application of these surfaces would enable large energy savings in technologies like maritime transportation or pumping. However, it has recently been shown that even trace amounts of surfactants can lead to large adverse forces on superhydrophobic surfaces, frequently negating any drag reduction whatsoever. My research has provided the first quantitative study of surfactant effects on SHSs, including a novel theory for laminar flows over surfactant-contaminated SHSs, detailed experimental and computational results in agreement with this model, and the identification of a key length scale, named the mobilization length, which is shown to be a crucial parameter to predict the drag reduction in practical applications. I have also worked on problems involving the spreading of surfactant on the surface of liquids, describing theoretically their self-similar spreading over a deep, viscous fluid. This model identifies different power laws for the spreading in time that hold universally, independently of the initial distribution of surfactant. I have also contributed, through experiments and theory, to the description of surfactant spreading through complicated maze-like networks of open channels. This description could have applications in medicine, where the transport of surfactant in lungs is a key process in the treatment of respiratory disorders.

My research is also focused on the study of diffusiophoresis, the spontaneous motion of particles in a fluid induced by a concentration gradient. This phenomenon has attracted increasing attention in recent years, with studies showing its relevance in processes ranging from fabric cleaning to particle delivery in porous materials. One of its particularly promising applications is water remediation, demonstrated using a chemical gradient perpendicular to a flow of water contaminated with solid micro-beads. The resulting particle migration concentrates the solids on the channel sides and, after the stream is split, results in membraneless water cleaning with an efficiency comparable to microfiltration. I have developed a model of this effect in the limit where the concentration of the chemical species and the particles is fully developed, meaning that the separation of solids is highest after the particle-free exclusion zone has reached its maximum thickness. The theory produces quantitative predictions for the filtration efficiency as a function of the chemistry and particle properties. These results are supplemented with experiments in microfluidic channels, showing a proof-of-concept of the improvement of filtration efficiency that this technique could have over traditional physical filtration.

I will also present results on the study of bacteria-driven bioremediation, an inexpensive method for the cleaning of contaminated soil based on the introduction of endogenous bacteria. These organisms are attracted to dissolved organic substances, and subsequently migrate towards and degrade the harmful chemicals trapped within a porous medium. It is currently unclear how the interplay between bacterial chemotaxis, contaminant dissolution, and soil structure at small scales affect the overall rate of pollutant degradation observed macroscopically. I have contributed to the development of new mathematical models that identify the distinct physical regimes leading to optimal bioremediation. This avenue of research is expected to result in improved methodologies, where it will be possible to tailor the introduced bacteria based on the specific type and distribution of contaminant, as well as the pore size and microstructure of the soil.

An additional line of research that I am involved in regards evaporation-driven chemical accumulation. In many technological applications, like the extraction of lithium from brines or the removal of heavy metals from natural waters, it is desirable to concentrate a given chemical that is present in the water source in a very diluted form. New techniques are focused on exploiting solar irradiation on floating porous substrates for this purpose, which results in the trapping of chemicals in the substrate and their subsequent accumulation as water evaporates. I have developed mathematical models that describe the interplay between the fluid flow induced by evaporation and the crystallization and chemical reactions of the concentrated substances. The quantification of these physical processes leads to predictions on how to optimize the chemical accumulation and improve the lifespan of these substrates, providing key steps towards the large-scale implementation of this technologies.