In applications such as foods and consumer products, emulsions are often employed because of their remarkable ability to form a wide variety of materials ranging from low-viscosity fluids, to gels to highly elastic pastes. In some of these applications emulsions containing crystallizable oils are tempered and sheared to form bicontinuous gels by exploiting a phenomenon known as partial coalescence. Unlike coalescence in liquid droplets, partial coalescence occurs when the interfacial crystals of semi-crystalline droplets penetrate neighboring droplets. The mechanical strength of the crystalline network linking the droplets is capable of overcoming the Laplace pressure and maintains the integrity of the non-spherical aggregates. Despite prior investigations, partial coalescence remains a poorly understood convolution of two non-equilibrium and stochastic phenomena - nucleation and aggregation in emulsions. Emerging technologies such as topical drugs and phase change materials also rely on partial coalescence. Thus, fundamental understanding of partial coalescence in oil-in-water emulsions would broadly impact the technological development in these applications. Currently partial coalescence has confounded scientific understanding because of polydispersity in emulsions, shear and additives such as surfactants - all of which affect both the nucleation rates and the probability of partial coalescence. In addition, current methods such as X-ray diffraction and differential scanning calorimetry are top-down approaches and are inadequate to probe the stochastic nature of nucleation and partial coalescence at the level of individual droplets. To address these scientific challenges, the investigators deploy microfluidic technology and direct visualization methods to (1) Quantify nucleation kinetics in exceptionally monodisperse droplets and test the validity of current nucleation theories (2) Directly measure the kinetics of partial coalescence and test the applicability of kinetic theories of aggregation and (3) Directly quantify the probability of shear-induced coalescence in crystalline droplets.
The intellectual merit of this work is an integrated experimental effort combining microfluidics and microscopy to address a technological need to fundamentally understand nucleation and partial coalescence in emulsions. Novel aspects of the work include routing droplet traffic in a microfluidic network to control individual droplet parking in well-defined spots on a microfluidic device. Generating such large-scale single droplet arrays enables repeated crystallization-melting cycles to be performed to quantify the heterogeneity in the dynamics of nucleation. In addition, by manipulating the parking space available for droplets, the investigators will generate microfluidic doublets and hexagonally-packed monodisperse droplet arrays for direct visualization of the microscopic dynamics of partial coalescence. To probe shear-induced coalescence, we will generate two microfluidic trains of droplets and induce repeated head-on collisions between individual droplets. The ability to create such large statistical ensembles and perform measurements at the level of individual droplets is essential to discriminate the various mechanisms causing nucleation and will yield a never-before-available picture of the stochastic nature of nucleation and partial coalescence. Thus, this potentially transformative research moves beyond current top-down methods by introducing bottom-up approaches to investigate the non-equilibrium thermodynamics of nucleation and partial coalescence in emulsions.
This fundamental investigation of crystallization and partial coalescence in emulsions will broadly impact the technology and engineering in areas as diverse as foods, cosmetics, drug delivery and phase change materials. Furthermore, this study may catalyze the development of an entirely new class of droplet-based fluidic devices for rapid assessment of crystallization and emulsion stability. This work will also impact other engineering areas that rely on fundamental understanding of emulsion crystallization and stability such as oil recovery. The educational component of the project includes drawing graduate and undergraduate students to the visually striking microfluidics research and providing state-of-the-art training in microfluidics, emulsion science, non-equilibrium thermodynamics and microscopy. The PI will pursue outreach activities to high school students by developing a weeklong hands-on-activities and lectures on the theme "Bubbles on Chips"
Emulsions typically consist of two immiscible phases, where one phase present as droplets is surrounded by a continuous phase. Emulsions are widely used in foods and consumer products and often the droplets are present in a crystalline form. The major goal of this project is to fundamentally understand nucleation in emulsions where the droplet size is tightly controlled and the crystallization process can be directly observed in a large number of droplets under a microscope. This project led to the development of miniaturized devices where emulsion droplets of well-controlled size can be stored for prolonged duration and exposed to different temperature cycles. This microfluidic technology formed the basis of fundamental investigations into the nucleation behavior of oils and inorganic salts. We found that the onset of crystallization depends crucially on the size of droplets and was insensitive to the rate of cooling. We also demonstrated that the microfluidic devices can be used to rapidly determine at what concentration and temperature, salts dissolve or precipitate out of solution. Our methods alleviate the tediousness associated with generating crystallization and solubility data using conventional techniques. The techniques and understanding developed as part of this project can be used to better design and improve the performance of materials ranging from pharmaceuticals to foods to energetic materials. The methods developed could also be used to study natural phenomena such as biomineralization and ice nucleation in clouds. The project trained 2 female graduate students and several undergraduate students on microfabrication, microfluidics and microscopy. Educational training was also provided to the students in the areas of interfacial science, fluid dynamics and thermodynamics. An outreach activity titled ‘Plastic Chips and Microfluidics’ was conducted for middle school girls through the ‘Science – It’s a Girl Thing’ program. The activity focused on teaching micromolding methods and conducting experiments on laminar flows and droplets.
