Understanding and manipulating coalescence in dense emulsions

Coatings and paints play a significant role in daily life; they prolong the lifetime of materials by offering protection against, for example, corrosion, weathering or fouling, and literally add color to our lives. Due to their widespread use, their environmental consequences have become focus of increasingly strict regulations and public awareness. There has been a strong effort to replace traditional solvent-based coatings with waterborne coatings to reduce or eliminate the volatile organic compounds (VOC) that traditionally formed the main component of paints. A pronounced shift from solvent-based to water-based systems has already taken place for decorative (consumer) coatings. However, for more demanding applications in industry, the replacement of solvent-based paints with greener waterborne formulations still has a long way to go, due to their lower performance in terms of both mechanical, durability and aesthetic aspects. The development of waterborne coatings with the same or better performance than solvent-borne systems is thus an important step towards the further vanishing of VOC-rich coatings. Ultimately, the final aimis to replace all solvent-borne coatings with VOC-free paint formulations. Waterborne paints form a very promising candidate, yet several key aspects of their properties during storage, handling and during their lifetime as a coating, remain poorly understood. Waterborne coatings are complex multiphase systems, containing a wide variety of dissolved and dispersed components in the common aqueous continuous phase. During the drying of the paint, after application, this complex mixture must undergo a phase inversion to achieve a homogeneous film of the resinfrom its initial dispersed state. While this state governs the structure, and thus final properties of the coating film, its complexity precludes a deep understanding to date. This is due to the complexity of the drying and phase inversion process, which is governed by a seemingly immense number of chemical and physical parameters. We therefore adopted a simplification approach, minimizing the number of parameters to obtain a first-pass insight into the phase inversion process. We started by directly visualizing how coalescence occurs in a drying 2D emulsion film, both on the single-particle scale, with confocal microscopy, and by macroscopic imaging. Based on these observations, we built a hydrodynamic model that explains some of the key governing parameters in the film formation process. Furthermore, we explored the possibilities to manipulate phase inversion and coalescence, by developing new thermoresponsive surfactants. These new strategies allow us to obtain new insights into this complex problem. Understanding coalescence in dense emulsions The first part of this thesis focusses on understanding how coalescence and phase inversion occurs in a drying emulsion film, through direct quantitative imaging. Our observations at different length scales are unified in a hydrodynamic model to arrive at a microscopic understanding of this complex macroscopic phenomenon. In Chapter 2 we observed two distinctmodes of phase inversion in surfactant-stabilized o/wemulsions exposed to aunidirectional drying stress. Coalescence occurs either through a nucleation-and-growth mechanism, where coalesced pockets form and grow randomly throughout the sample, or through a coalescence front that propagates into the sample from the drying end. The way in which coalescence occurs is determined by a balance between the established pressure profile across the film and the local critical disjoining pressure in the emulsion. For very stable emulsions, narrow plateau borders can develop, leading to steep pressure gradients; the actual pressure only exceeds the critical pressure in a narrow zone around the drying front and front coalescence results. The opposite occurs for unstable emulsions; only shallow pressure profiles develop before coalescence commences throughout the bulk of the sample. Moreover, we find that surfactant concentration plays a significant role through its effect on the critical disjoining pressure atwhich coalescence occurs. This, to our knowledge, is the first observation and explanation of different modes of coalescence dynamics in dense emulsion films. In chapter 3 we present a hydrodynamic model for the water flow in a jammed emulsion, subjected to a unidirectional drying stress. Water flows through the Plateau borders towards the drying end, driven by gradients in the capillary pressure. Our model predicts the pressure gradients that arise, and allows us to explain the different modes of coalescence observed experimentally in chapter 2. From these results, we estimate the boundaries (critical pressure and evaporation rate) between bulk and front coalescence. We explore the parameter space of our hydrodynamic model, to further investigate the key factors involved in film formation. We show that, those two distinct coalescence behaviors can be obtained within the same model by varying the critical disjoining pressure. Furthermore, we get a ‘coalescence modes phase diagram’ to show where and how the coalescence transit from one to other. Manipulating coalescence in dense emulsions In Chapter 4 we showthe successful synthesis of well-defined thermoresponsive surfactants through Atom Transfer Radical Polymerisation (ATRP) using a alkyl-functional initiator. These surfactantscan be used to stabilise emulsions for over four months at room temperature, below the collapse transition of the hydrophilic block of the surfactant, yet can be triggered to break the emulsion within minutes when the sample is heated to above 40 °C. This on-demand coalescence is mediated by desorption of the surfactants from parts of the surface, as evidenced by surface tension measurements and direct microscopic observations of the droplets surface. Our results suggest that these well-defined thermoresponsive surfactants form an interesting platform to study droplet coalescence and triggered phase inversion in emulsion systems. Moreover, the ability to break a very stable emulsion on demand has industrial relevance for several applications, such as in film formation of waterborne emulsion paints and the recovery of products during emulsion-based extraction and reaction processes. In Chapter 5 we reported on a new approach to study coalescence in dense thermoresponsive emulsions using a microfluidic-based microcentrifugation method in which a constant external field can be applied. We have shown that both thermodynamic and kinetic properties can be measured through automated image analysis, and that the temperature-responsivity of the surfactants can be used to trigger different modes of coalescence on demand. These results form further proof that our conclusions in Chapters 3&4 regarding the nature of the transition from front to bulk coalescence are valid; also here we observe that changing the critical disjoining pressure, through changing the temperature, can induce a spontaneous switch in coalescence mode. This new approach forms a stepping stone for further investigations into the governing mechanisms that dominate phase inversion and film formation. Using the knowledge and methods developed in this thesis, new avenues for studying film formation have been opened. Our work focussed on highly idealised emulsions, real coating systems exhibit some complicating factors, such as viscoelasticity of the latex droplets, and even chemical reactions between different species of droplets, interactions with several surface active species and solid pigment particles. Moreover, the length scales in real paints are a few orders of magnitude smaller, requiring the development of new methodologies suitable for these length scales. These topics will be subject for future study, and are required to fully understand and control the properties of water-based coatings.