Microfluidics to probe partial coalescence in emulsions containing interfacial crystals

在食品和消费品等应用中，乳液经常被使用，因为它们能够形成从低粘度流体到凝胶再到高弹性浆料的各种材料。在其中一些应用中，含有结晶油的乳状液通过利用一种称为部分聚结的现象被调和和剪切，以形成双连续凝胶。与液滴中的聚结不同，当半结晶液滴的界面晶体穿透相邻液滴时，会发生部分聚结。连接液滴的结晶网络的机械强度能够克服拉普拉斯压力，并保持非球形聚集体的完整性。尽管已有研究，但部分聚结仍然是两种非平衡和随机现象--乳状液中的成核和聚集--的卷积，人们对此知之甚少。局部药物和相变材料等新兴技术也依赖于部分结合。因此，对水包油乳状液中部分聚结的基本了解将广泛影响这些应用领域的技术发展。目前，由于乳状液、剪切和表面活性剂等添加剂中的多分散性，部分聚结已经扰乱了科学认识--所有这些都会影响成核速率和部分聚结的可能性。此外，目前的方法如X射线衍射法和差示扫描量热法都是自上而下的方法，不足以在单个液滴的水平上探索成核和部分聚集的随机性质。为了应对这些科学挑战，研究人员采用微流体技术和直接可视化方法来(1)量化异常单分散液滴的成核动力学并测试当前成核理论的有效性(2)直接测量部分聚结的动力学并测试聚集动力学理论的适用性，以及(3)直接量化晶体液滴中剪切诱导聚结的可能性。这项工作的学术价值是一项综合的实验努力，结合了微流体和显微镜，以满足从根本上了解乳状液中的成核和部分聚结的技术需要。这项工作的新方面包括在微流控网络中路由液滴流量，以控制单个液滴停放在微流控设备上明确定义的点上。产生这种大规模的单液滴阵列使重复的结晶-熔化循环得以进行，以量化成核动力学中的异质性。此外，通过操纵可供液滴停放的空间，研究人员将生成微流控双液滴和六角填充的单分散液滴阵列，用于直接可视化部分聚结的微观动力学。为了探索剪切诱导的聚结，我们将产生两个液滴的微流体序列，并在单个液滴之间引起反复的迎头碰撞。创建如此大的统计集合并在单个液滴一级进行测量的能力对于区分导致成核的各种机制至关重要，并将产生前所未有的成核和部分结合的随机性质的图景。因此，这项潜在的变革性研究超越了目前的自上而下的方法，引入了自下而上的方法来研究乳状液中成核和部分聚结的非平衡热力学。这项关于乳液结晶和部分聚结的基础性研究将广泛影响食品、化妆品、药物输送和相变材料等领域的技术和工程。此外，这项研究可能会促进一类全新的基于液滴的流体设备的开发，用于快速评估结晶和乳状液稳定性。这项工作还将影响其他依赖于对乳化液结晶和稳定性的基本了解的工程领域，如石油回收。该项目的教育部分包括吸引研究生和本科生参与视觉上引人注目的微流体研究，并提供微流体、乳状液科学、非平衡热力学和显微镜方面的最先进培训。该协会将通过开展为期一周的实践活动和以“薯条上的泡泡”为主题的讲座，向高中生开展外展活动。

0967172VanapalliIn 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"