NSF-BSF: Interfacial freezing and shape transformations in surfactant/particle-co-stabilized emulsions

乳状液是由一种液体的液滴悬浮在另一种不相容的液体中制成的，例如悬浮在水中的油滴。由于表面张力，液滴呈球形。最近发现，对于60多种不同的化学组合，覆盖着被称为表面活性剂的分子的液滴表面可以经历冻结，这会导致液滴形状的急剧变化。这种界面冻结(IF)现象导致形成刻面液滴、薄片或棒状/毛发状乳状液滴。一方面，这些形状变化可能会导致工业过程中的重大复杂情况，如毛发状液滴堵塞管道。另一方面，这些形状变换为高级应用提供了一种合成具有复杂形状的粒子的强大方法。在许多工业和自然环境中，乳化液液滴的界面被超微粒和固体微粒的混合物覆盖，这种混合物被称为胶体。然而，表面吸附胶体对IF和液滴刻面现象的影响还没有被广泛研究，尽管这种胶体存在于许多现实世界的乳液中。这项拟议的工作旨在了解胶体对乳化液液滴在IF过程中形状转变的影响。深入了解乳状液的大小、浓度、形状和表面化学对乳状液形态变化的影响，将有助于在食品、石油和天然气、制药和化妆品工业中防止此类现象的有害影响，并导致新的合成技术来创造新的材料。液滴界面的组成、结构和弹性在工业和常见的化学和生物体系中决定乳状液的稳定性、包封性和可加工性。在许多乳液中，界面分子层可以经历冻结转变，从而极大地改变了乳液的性能。这种界面冻结(IF)转变极大地改变了乳化液液滴的形状，其大小跨越13个数量级的体积，以及60多种不同的油-表面活性剂组合。这种形状转变可能会导致形成多面液体物体、高纵横比小片或棒状/毛发状乳液滴，改变乳液的流动特性，并可能导致凝胶化，从而导致工艺复杂。液滴的工程形状变换也为合成具有独特功能的高度形状各向异性粒子提供了一种强大的方法。在许多工业和自然环境中，乳液的界面由颗粒和表面活性剂的混合物装饰，这些颗粒要么故意添加以稳定乳液，要么作为污染物存在。这个项目旨在了解表面吸附粒子和IF现象之间的相互作用，特别是各向同性粒子和Janus粒子对形状转换的影响。利用粒子跟踪和最近发展的微流控方法，本项目将研究润湿性质、颗粒大小、形状和界面浓度对IF驱动的液滴形状转变的影响。由于极低的界面张力，传统的各向同性胶体很容易从液滴界面排出，而Janus颗粒由于其固有的表面活性，将强烈吸附到这样的界面上，从而有可能控制形状转变。控制IF驱动的液滴自我成形的新方法将对石油和天然气、制药、食品、农业和化妆品行业产生潜在的变革性影响，这些行业的乳状液经常暴露在含有表面活性剂和颗粒的介质中。本科生和研究生将为当地高中教师和学生开发一个新的演示，说明界面曲率和结晶之间的相互作用。为了扩大来自代表性不足群体的学生的参与，不同背景的学生将通过接待波多黎各大学胡马考分校的学生以及与先进工程女性和路易丝-斯托克少数群体参与计划合作来招募。该奖项反映了NSF的法定使命，并通过使用基金会的智力优势和更广泛的影响审查标准进行评估，被认为值得支持。

Emulsions are made of droplets of one liquid suspended in another, immiscible, liquid such as oil droplets suspended in water. Liquid droplets adopt spherical shapes due to surface tension. It was recently discovered that, for over 60 different chemical combinations, the surface of droplets, which is covered with molecules called surfactants, can undergo freezing, which induces drastic changes in the shape of the droplets. This interfacial freezing (IF) phenomenon leads to the formation of faceted liquid droplets, thin platelets or rod/hair-like emulsion droplets. On one hand, these shape transformations may lead to major complications in industrial processes, such as pipe clogging by hair-like droplets. On the other hand, these shape transformations provide a powerful method of synthesizing particles with complex shapes for advanced applications. In numerous industrial and natural settings, the interfaces of emulsion droplets are covered by mixtures of surfantants and solid microparticles, known as colloids. However, the influence of surface-adsorbing colloids on the IF and droplet faceting phenomena has not been extensively investigated, although such colloids are present in many real-world emulsions. The proposed work aims to understand the effect of colloids on the shape transformation of emulsion droplets undergoing IF. Developing a deep understanding of the effect of particle size, concentration, shape and surface chemistry on the shape transformation of emulsions will enable strategies to prevent deleterious impacts of such phenomena in the food, oil and gas, pharmaceutical and cosmetic industries and lead to novel synthesis techniques to create new materials.The composition, structure and elasticity of the droplet interface play a crucial role in determining the stability, encapsulation capability and processability of emulsions in industry and in common chemical and biological systems. In many emulsions, the interfacial molecular layer can undergo a freezing transition, dramatically modifying the emulsion properties. This interfacial freezing (IF) transition drastically changes the shape of the emulsion droplets for sizes spanning 13 orders of magnitude in volume, and for over 60 different oil-surfactant combinations. Such shape transitions may lead to the formation of faceted liquid objects, high aspect ratio platelets or rod/hair-like emulsion droplets, changing the flow properties of the emulsions and potentially causing gelation leading to process complications. Engineering shape transformations of droplets also provides a powerful method of synthesizing highly shape-anisotropic particles with unique functionality. In numerous industrial and natural settings, the interfaces of the emulsions are decorated by mixtures of particles and surfactants, with the particles either added intentionally for emulsion stabilization, or being present as a contaminant. This project aims at understanding the interplay between the surface-adsorbed particles and the IF phenomena, with a particular focus on the effect of isotropic and Janus particles on the shape transformations. Using particle tracking and recently developed microfluidic methods, this project will investigate the effect of wetting properties, size, shape and interfacial concentration of particles on IF-driven shape transformations of droplets. While conventional isotropic colloids may be easily expelled from a droplet interface undergoing IF due to extremely low interfacial tension, Janus particles will strongly adsorb to such an interface due to their intrinsic surface activity, potentially enabling control over the shape transformation. New methods to control the IF-driven self-shaping of droplets will have a potentially transformative impact for oil and gas, pharmaceutical, food, agricultural and cosmetics industries, where emulsions are frequently exposed to surfactant- and particle-containing media. A new demonstration illustrating the interplay between the interfacial curvature and crystallization will be developed by undergraduate and graduate students for local high school teachers and students. To broaden participation of students from underrepresented groups, students with diverse backgrounds will be recruited by hosting students from the University of Puerto Rico-Humacao and by collaborating with Advancing Women in Engineering and Louise-Stoke Alliance for Minority Participation programs.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.