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

摘要：冲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台。这是由于冲击落点扩散的速度很高（高达10米/秒）；这种扩散过程已经在普通流体中得到了仔细的研究，但在复杂流体中如何改变这种冲击行为方面所做的工作却很少。复杂的液体，如番茄酱、油漆和欧不立（水里的玉米淀粉），它们的粘度（抗流动）取决于施加的应力。因此，这些流体的流动方式取决于它们所受到的压力。这个研究项目探讨了这种依赖于应力的粘度是如何改变水滴撞击行为的。复杂流体无处不在，深入了解其在冲击过程中的行为对于食品和药品加工、增材制造和印刷/涂料等行业至关重要。除了Driscoll主任和Cheng副主任之间的研究协同作用外，此次合作还为扩大学生的培训提供了一个独特的机会：双方实施了人员交流，以便参与该项目的学生每年在他们的非家乡机构度过两周。这种人员交流为受训者提供了与更大的科学家网络联系的机会，并获得了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水滴撞击现象来设计演示，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领域的经典问题，几十年来得到了广泛的研究。相比之下，关注非牛顿流体液滴（特别是悬浮液滴）影响的研究很少，尽管这些过程在食品和药品加工、增材制造和印刷/涂料等行业中至关重要。该项目通过对胶体悬浮液液滴的撞击进行广泛的研究，解决了这一知识差距，提供了对悬浮液-液滴撞击动力学的充分理解，并对牛顿和非牛顿液滴撞击过程进行了直接比较。该系统研究了不同的控制参数，如悬浮体积分数、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，该团队的目标是全面表征撞击悬浮液滴的运动学和动力学。在复杂和极端条件下，悬液-液滴碰撞为探测悬液动力学提供了理想的模型系统：扩散的液滴产生的剪切速率至少比通过标准流变法获得的剪切速率高一个数量级。此外，在液滴碰撞中，孤立的气液界面形成了一个独特的开放边界，这在其他情况下是罕见的。因此，该项目提供了极端条件下悬浮动力学的关键缺失信息，补充了目前对传统体流变试验中获得的悬浮流动的理解。该材料研究部（DMR）资助研究牛顿和非牛顿液滴撞击过程之间的胶体悬浮液滴的影响，资金来自数学和物理科学（MPS）理事会DMR的凝聚态物理（CMP）计划。该奖项反映了美国国家科学基金会的法定使命，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.