Dissolution, and Vaporization of Superheated Droplets, and Capsules
Saeed Saleem is a PhD student in the Department of Physics of Fluids. (Co)Promotors are prof.dr. M. Versluis and dr. G.P.R. Lajoinie from the Faculty of Science & Technology.
Intumescent coatings form a family of specialized paints used in modern architecture for fire protection. When exposed to heat, they greatly expand through bubble generation, i.e., intumesce, and subsequently act as a thermal barrier that delays structural collapse. Current intumescence technology relies on melamine as a chemical source of gas for bubble generation (blowing agent). However, melamine's carcinogenic nature and its tendency to create uncontrolled bubbles limit the coating's effectiveness and impacts both people’s health and the environment. It is therefore on the upcoming list of restricted substances. Fire protection technologies are thus in dire need of a new paradigm for bubble generation. Furthermore, a novel concept for bubble generation may prove invaluable to improve the coating's mechanical resistance and insulation properties, provided that it also allows control over the intumescence process. In this thesis, we explore the possibility of using physical means rather than chemical reactions to generate bubbles in coatings. More specifically, we aim at designing liquid precursors and exploiting controlled vaporization as a source of intumescence.
In the introduction, we demonstrate the intumescence of a coating, which we use to outline the problem: intumescence in a coating is a multistep process that encompasses precursor design and fabrication, controlled vaporization, and complex viscoelastic interactions upon embedding within a polymeric resin. At the heart of it all, however, lays the problem of understanding the vaporization of a single precursor.
The second chapter investigates the vaporization of a water droplet in an organic liquid when this droplet is subjected to a slow temperature ramp as is the case during the growth of a fire. We introduce a model utilizing a Rayleigh-Plesset-type equation to capture bubble dynamics, which includes heat transfer through the convection-diffusion equation and subsequent phase change. Our results show that the mechanism driving vaporization, specifically inertia or thermal diffusion, varies depending on the size of the droplet and the degree of superheat.
In the third chapter, we conduct experiments on the vaporization of isolated water droplets immersed in oil and subjected to a gradual temperature increase. We observe a significant increase in the solubility of water in the oil phase as the droplets approach their boiling point. We quantify dissolution using a semi-empirical form of the Epstein-Plesset equation and show that applying a temperature ramp leads to the existence of a minimum droplet size below which vaporization cannot occur. Furthermore, we show that droplet vaporization is, in practice, limited by the retraction of the liquid film around the bubble. The model developed in the second chapter is adapted to account for these essential and asymmetric fluid dynamic processes.
In the fourth chapter, we coat droplets with a composite shell consisting of polylactic acid and nanoparticles. These coated droplets are exposed to a temperature ramp. We show that the shells partly shield the droplets against dissolution until the shell itself reaches its melting point. We develop a theoretical model that accounts for the temperature-dependent saturation concentration, which provides deeper insight into the dissolution process and allows for evaluating key properties of the liquids pertaining to droplet dissolution and stability. Furthermore, we show that, if nucleation is achieved, vapor bubble dynamics are similar to those of uncoated droplets.
In the fifth chapter, we develop a microfluidic system to create monodisperse, water-filled microcapsules. This process utilizes a 3D-printed chip whose 3D geometry is capable of generating both single and double emulsions, and of handling fluids with a broad range of wetting properties. To demonstrate its potential, we produce droplets and double emulsions consisting of curable epoxy resin with melamine, which we solidify post-production into particles and capsules. We quantify the operating range of the chip, as well as the size distributions and properties of the particles and capsules produced.
Finally, in the conclusion, we discuss the relevance of these findings to intumescent coatings as well as the promising future directions that this research could take.
