Periodic Reporting for period 4 - DDD (Diffusive Droplet Dynamics in multicomponent fluid systems)
Periodo di rendicontazione: 2021-11-01 al 2022-10-31
Liquid-liquid extraction – the transfer of a solute from one solvent to another – is one of the core processes in chemical technology and analysis. The current challenge is to miniaturise the extraction process of the analyte and to optimize the extraction recovery and preconcentration factor. In lack of a priori calculations, this has traditionally been done by trial-and-error. However, to be able to control and optimize the extraction processes, it is crucial to quantitatively understand the diffusive droplet dynamics in multicomponent fluid systems. This is essential and urgently needed not only for modern liquid-liquid extraction processes for diagnostics and microanalysis, for droplet microfluidics, or in the paint & coating industry, but on larger scales also in the remediation industry, in chemical technology, or in food processing. These applications of droplets governed by diffusion include cases of immersed droplets in the bulk and on a surface, single and multicomponent droplets and solvents, growing or shrinking droplets, and cases with high droplet number density.
The objective of the project was to better understand these multiphase and multicomponent fluid systems with relevant diffusive droplet dynamics, filling the gap and coming to a quantitative understanding of diffusive droplet dynamics and thus to illuminate the fundamental fluid dynamics of diffusive processes of immersed (multicomponent) (surface) droplets on multiple scales.
The objective of the project was to better understand these multiphase and multicomponent fluid systems with relevant diffusive droplet dynamics, filling the gap and coming to a quantitative understanding of diffusive droplet dynamics and thus to illuminate the fundamental fluid dynamics of diffusive processes of immersed (multicomponent) (surface) droplets on multiple scales.
Such a better understanding could be achieved. We performed various key controlled experiments and numerical simulations for idealized setups, allowing for a one-to-one comparison between experiments and numerics/theory which in many case could be achieved. One highly of the project was that, with the help of an associated ERC-PoC grant, we could introduce a green, fast, one-step nanoextraction method for extraction and detection of target analytes from sub-milliliter dense suspensions using surface nanodroplets without toxic solvents and pre-removal of the solid contents. Another highlight was that we could apply our knowledge on the diffusive behavior of droplets to aerosols in the context of the COVID-19 pandemic and explain why the lifetime of aerosol droplets is two orders of magnitude larger as compared to the wrong models which had hitherto been used. But there are many more highlights. An particular appealing one is the description of the totally counterintuitive jumping behavior of an oil droplet in a stratified layer consisting of ethanol at the top of water at the bottom, which we could quantitatively measure, numerically simulate, and physically explain in a large parameter space.
We achieved several breakthroughs. The degree of mastering both experiments and numerical simulations on a level that a one-to-one comparison has become possible had not been expected a few years ago, but now is reality. Also on the numerical side for itself there had been several breakthroughs: That the evolution of a cloud of respiratory aerosols could be simulated in direct numerical simulations clearly had not been expected prior to the pandemic. This line of research may be the most surprising one coming out of this ERC Advanced Grant, as it could not have been foreseen six years back. It was only the ERC Advanced Grant that made it possible for me to immediately start with my research work on aerosols and spreading of the corona virus.
Another breakthrough is our work on the so-called Leidenfrost droplet, which also started off as sideline of the DDD project. Volatile drops deposited on a hot solid can levitate on a cushion of their own vapor, without contacting the surface. We proposed to understand the onset of this Leidenfrost effect through an analogy to nonequilibrium systems exhibiting a directed percolation phase transition. When performing impacts on superheated solids, we observed a regime of spatiotemporal intermittency in which localized wet patches coexist with dry regions on the substrate. We found a critical surface temperature, which marks the upper bound of a large range of temperatures in which levitation and contact coexist. In this range, with decreasing temperature, the equilibrium wet fraction increases continuously from zero to one. Also, the statistical properties of the spatiotemporally intermittent regime are in agreement with that of the directed percolation universality class. This analogy allowed us to redefine the Leidenfrost temperature and shed light on the physical mechanisms governing the transition to the Leidenfrost state. This is a totally different way of thinking of the Leidenfrost than it had been done for more than 250 years since the discovery of the effect. It really is a paradigmatic change. I
Another breakthrough is our work on the so-called Leidenfrost droplet, which also started off as sideline of the DDD project. Volatile drops deposited on a hot solid can levitate on a cushion of their own vapor, without contacting the surface. We proposed to understand the onset of this Leidenfrost effect through an analogy to nonequilibrium systems exhibiting a directed percolation phase transition. When performing impacts on superheated solids, we observed a regime of spatiotemporal intermittency in which localized wet patches coexist with dry regions on the substrate. We found a critical surface temperature, which marks the upper bound of a large range of temperatures in which levitation and contact coexist. In this range, with decreasing temperature, the equilibrium wet fraction increases continuously from zero to one. Also, the statistical properties of the spatiotemporally intermittent regime are in agreement with that of the directed percolation universality class. This analogy allowed us to redefine the Leidenfrost temperature and shed light on the physical mechanisms governing the transition to the Leidenfrost state. This is a totally different way of thinking of the Leidenfrost than it had been done for more than 250 years since the discovery of the effect. It really is a paradigmatic change. I