The goal of the proposed work is to develop experimental methods for controlling the presence of adsorbed species at mobile (flowing) interfaces. This work is guided by the framework of a simplified theoretical model that has been developed for interface breakup in the presence of surfactants. The intellectual merit of this work is the development of tools to tackle problems involving coupled surfactant dynamics and fluid flow. We will develop these tools through detailed experimental analysis and modeling of a specific example: surfactant-mediated tipstreaming for formation of submicron droplets. These studies will help elucidate the critical mechanisms controlling a broader set of interfacial flow applications, enabling greater control over key processes. This framework will enable the engineering of mobile interfaces, and we will use this capability to develop novel methods for quantifying the kinetics of ad/desorption of more complex molecules (i.e, biomolecules, macromolecules and colloids) and tackle a specific application, quantification of the kinetics of denaturation of proteins at interfaces. The results of this work will have broad technological impact because of the importance of surfactant-laden multiphase flows in technologies such as emulsification, spraying and coatings and in emerging applications such as ink-jet printing of new materials or two-phase flows in microfluidic devices, which have been exploited to synthesize monodisperse droplets, bubbles, colloidal particles, and other novel colloidal assemblies. The interfacial and microfluidic flows characterized by this work will have broad educational impact by providing highly visual images and fluid dynamics examples that can feed into the major outreach, undergraduate research, and course development activities of the two PIs.
The goal of this work was to develop experimental methods for controlling the presence of adsorbed species at mobile (flowing) interfaces, guided by transport and interfacial mechanical modeling. Over the course of this grant period, we have accomplished these goals and achieved the desired outcome of enabling the engineering of mobile interfaces. In the process, we have also developed novel methods for quantifying the kinetics of ad/desorption of surfactants and particles. The problem of coupled ad/desorption of surface active agents and flow is a problem of competing timescales. Knowledge has largely advanced experimentally, with relatively little accompanying analysis to inform experiments. Fluid dynamical analysis with single-phase liquids is robust, as is analysis involving surfactant-free fluid interfaces. Analysis involving complex interfaces in which ad/desorption kinetics and convection compete is much less reported, even though a number of studies involve the use of surface active molecules for stabilizing droplets and bubbles and aiding in their breakup. With the support of this grant, we focused on one particular phenomenon – droplet breakup – in which the presence of surfactants plays a critical role. Through this specific example we developed detailed experimental and theoretical models for the interplay of surfactant mass transport with flow. Thus, the intellectual merit of this work is the elucidation of critical mechanisms controlling a broader set of interfacial flow applications, enabling greater control over key processes. The specific outcomes of the work conducted with the support of this grant are four-fold: (1) A simplified model and scaling analysis has been developed that accurately predicts the viable conditions for tipstreaming of micron and submicron scale droplets for arbitrary surfactant-oil-water mixtures. (2) A control scheme has been implemented for the tipstreaming process to eliminate unwanted large droplets. (3) Guidelines have been developed for the control of the droplet sizes as a function of flow and surfactant conditions, and the simplified model supports these. Tipstreaming is thus a viable method for continuous production of micron and submicron scale droplets, and our work has elucidated physicochemical mechanisms underlying the phenomenon and built upon those to develop design criteria for optimizing the process. Finally, (4) New interfacial characterization methods were also developed to quantify surfactant parameters needed to validate the model. These methods are independently valuable for broader characterization of transport and mechanics of complex fluid interfaces. The results of this work will have broad technological impact because of the importance of surfactant-laden multiphase flows in technologies such as emulsification, spraying and coatings and in emerging applications such as ink-jet printing of new materials or two-phase flows in microfluidic devices, which have been exploited to synthesize monodisperse droplets, bubbles, colloidal particles, and other novel colloidal assemblies. Natural phenomena are also strongly influenced by surfactants. For example, surface contamination enhances expulsion of jets from wave crests and air entrainment in breaking waves, and has a negative impact on water-walking behavior of insects, which have inspired novel robotic applications. The interfacial and microfluidic flows characterized by this work will have broad educational impact by providing highly visual images and fluid dynamics examples that have been incorporated into the major outreach, undergraduate research, and course development activities of the two PIs.
