Collaborative Research: Impact of a colloidal suspension droplet: Suspension flows at extreme shear rates

非技术摘要：撞击液滴不仅是一种引人注目的现象，也是研究材料性能的理想试验平台。 这是由于撞击液滴扩散的速度很高（高达10 m/s）;在普通流体中已经仔细研究了这种扩散过程，但很少有人研究这种撞击行为在复杂流体中是如何改变的。 复杂流体，如番茄酱、油漆和oobleck（玉米淀粉水溶液），其粘度（流动阻力）取决于所施加的应力。 因此，这些流体根据它们所经受的应力而流动得相当不同。 这个研究项目探讨了这种依赖于压力的粘度如何改变跌落冲击行为。 复杂流体无处不在，深入了解它们在冲击过程中的行为对于食品和药物加工、增材制造和印刷/涂层等行业至关重要。除了PI Drivel和Co-PI Cheng之间的研究协同作用外，合作还为扩大学生培训提供了一个独特的机会：已经实施了人员交流，因此参与该项目的学生每年在他们的非家乡机构度过两周。这种人员交流为学员提供了与更大的科学家网络建立联系的机会，以及另一个学科和机构的经验。此外，研究团队利用令人惊叹和引人注目的跌落冲击现象来设计演示，以教育更广泛的公众了解复杂流体的迷人特性。技术摘要：液滴冲击是流体力学和软材料中的经典问题，几十年来一直被广泛研究。相比之下，关注非牛顿流体液滴，特别是悬浮液滴的影响的研究很少，尽管这些过程在食品和药物加工，增材制造和印刷/涂层等行业中至关重要。该项目通过对影响胶体悬浮液滴进行广泛研究来解决这一知识差距，提供对悬浮液滴冲击动力学的全面了解，并在牛顿和非牛顿液滴冲击过程之间进行直接比较。这种系统的研究改变控制参数，如悬浮体体积分数，颗粒形状，冲击能量，和目标尺寸。 利用先进的成像技术以及测量冲击力和应力分布的新工具，该团队旨在全面表征撞击悬浮液滴的运动学和动力学。悬滴冲击提供了一个理想的模型系统，以探测复杂和极端条件下的悬浮动力学：一个蔓延的液滴产生的剪切速率至少比那些通过标准流变仪可获得的高一个数量级。此外，在跌落冲击中的孤立的空气-流体界面施加了在其他情况下很少见到的独特的开放边界。因此，该项目提供了极端条件下悬架动力学的关键缺失信息，补充目前的理解悬浮液流在传统的散装流变测试。该司的材料研究（DMR）赠款支持研究，研究影响胶体悬浮液液滴之间的牛顿和非牛顿下降的影响过程与资金从凝聚态物理（CMP）该奖项反映了NSF的法定使命，并通过使用基金会的知识价值和更广泛的影响审查标准进行评估，被认为值得支持。

Non-technical Abstract:An impacting liquid drop is not only an eye-catching phenomenon, it is also an ideal testbed for studying material properties. This is due to the high speed at which an impacting drop spreads out (up to 10 m/s); this spreading process has been carefully studied in ordinary fluids, but little work has been done to understand how this impact behavior is modified in a complex fluid. Complex fluids such as ketchup, paint, and oobleck (cornstarch-in-water) have the property that their viscosity (resistance to flow) depends on the applied stress. Thus, these fluids flow quite differently depending on the stress they experience. This research project explores how this stress-dependent viscosity alters drop-impact behavior. Complex fluids are ubiquitous, and a deeper understanding of their behavior in impact processes is essential to industries such as food and drug processing, additive manufacturing, and printing/coating. In addition to the research synergy between PI Driscoll and Co-PI Cheng, the collaboration allows a unique opportunity for expanding students’ training: a personnel exchange has been implemented, so that the students involved in the project spend two weeks each year at their non-home institution. This personnel exchange provides trainees opportunities to connect with a larger network of scientists, as well as the experience of another discipline and institution. Additionally, the research team leverages the stunning and eye-catching phenomena of drop impact to design demos to educate the broader public about the fascinating properties of complex fluids. Technical Abstract:Liquid drop impact is a classic problem in fluid mechanics and soft materials and has been studied extensively for many decades. In comparison, studies focusing on the impact of non-Newtonian fluid droplets, particularly suspension droplets, have been few and far between, even though such processes are essential in industries such as food and drug processing, additive manufacturing, and printing/coating. This project addresses this knowledge gap by conducting an extensive study of impacting colloidal suspension droplets, providing a full understanding of the dynamics of suspension-drop impact and drawing a direct comparison between Newtonian and non-Newtonian drop impact processes. This systematic study varies control parameters such as suspension volume fraction, particle shape, impact energy, and target size. Using both advanced imaging techniques, as well as new tools to measure impact forces and stress distributions, the team aims to fully characterize both the kinematics and dynamics of impacting suspension droplets. Suspension-drop impact provides an ideal model system to probe suspension dynamics under complex and extreme conditions: a spreading droplet generates shear rates at least an order of magnitude higher than those that are accessible via standard rheometry. In addition, the isolated air-fluid interface in drop impact imposes a unique open boundary rarely seen in other circumstances. Thus, this project provides crucial missing information on suspension dynamics under extreme conditions, complementary to the current understanding of suspension flows obtained in conventional bulk rheological tests.This Division of Materials Research (DMR) grant supports research to study impacting colloidal suspension droplets between Newtonian and non-Newtonian drop impact processes with funding from the Condensed Matter Physics (CMP) Program in DMR of the Mathematical and Physical Sciences (MPS) Directorate.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.