Microfluidics to probe partial coalescence in emulsions containing interfacial crystals

微流体探测含有界面晶体的乳液中的部分聚结

• 批准号: 0967172
• 负责人: Siva Vanapalli
• 金额: $ 27万
• 依托单位: Texas Tech University
• 依托单位国家: 美国
• 项目类别: Continuing Grant
• 财政年份: 2010
• 资助国家: 美国
• 起止时间: 2010-05-15 至 2014-04-30
• 项目状态: 已结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=0967172&HistoricalAwards=false
• 关键词: Microfluidics; probe; partial; coalescence; emulsions

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"

0967172Vanapalli 在食品和消费品等应用中，经常使用乳液，因为它们具有形成各种材料（从低粘度流体到凝胶到高弹性糊状物）的卓越能力。在其中一些应用中，通过利用称为部分聚结的现象，对含有可结晶油的乳液进行调温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。连接液滴的晶体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。尽管之前进行了研究，但部分聚结仍然是两种非平衡和随机现象（乳液中的成核和聚集）的卷积，人们对此知之甚少。外用药物和相变材料等新兴技术也依赖于部分聚结。因此，对水包油乳液中部分聚结的基本理解将广泛影响这些应用的技术发展。目前，由于乳液、剪切力和表面活性剂等添加剂的多分散性，部分聚结已经混淆了科学理解——所有这些都会影响成核速率和部分聚结的概率。此外，当前的方法（例如 X 射线衍射和差示扫描量热法）是自上而下的方法，不足以探测单个液滴水平上成核和部分聚结的随机性质。为了应对这些科学挑战，研究人员采用微流控技术和直接可视化方法来（1）量化异常单分散液滴中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动力学的适用性聚集理论；(3) 直接量化结晶液滴中剪切诱导聚结的概率。这项工作的智力优点是结合微流体和显微镜的综合实验工作，以满足从根本上理解乳液中成核和部分聚结的技术需求。这项工作的新颖之处包括在微流体网络中路由液滴流量，以控制微流体设备上明确定义的点上的单个液滴停放。生成如此大规模的单液滴阵列可以重复进行结晶-熔化循环，以量化成核动力学的异质性。此外，通过操纵可用于液滴的停车空间，研究人员将生成微流体双体和六边形堆积的单分散液滴阵列，以直接可视化部分聚结的微观动力学。为了探测剪切引起的聚结，我们将生成两个微流体液滴序列，并诱导各个液滴之间重复的正面碰撞。创建如此大的统计集合并在单个液滴水平上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聚结的随机性质的图像。因此，这项潜在的变革性研究超越了当前的自上而下的方法，引入自下而上的方法来研究乳液中成核和部分聚结的非平衡热力学。这项对乳液中结晶和部分聚结的基础研究将广泛影响食品、化妆品、药物输送和相变材料等多个领域的技术和工程。此外，这项研究可能会促进开发一种全新的基于液滴的流体装置，用于快速评估结晶和乳液稳定性。这项工作还将影响其他依赖乳液结晶和稳定性基础知识的工程领域，例如石油采收率。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、乳液科学、非平衡热力学和显微镜方面最先进的培训。 PI 将通过开展为期一周的实践活动和主题为“芯片上的泡泡”的讲座，向高中生开展外展活动