声悬浮条件下纳米颗粒胶体液滴蒸发自组装的SAXS原位研究

Evaporation of colloidal droplet is one of the most important approaches which can induce the self-assembly of colloidal particles. The project will combine the techniques of acoustic levitation and SAXS, and investigate the self-assembly of Au and Ag nanoparticles via the evaporation of acoustically levitated colloidal droplets containing these particles. The electromagnetic field will be introduced to the evaporation process to study how it influences the particle self-assembly. The shape evolution and evaporation dynamics of the droplets can be monitored using a CCD camera. The internal flow inside the droplet as well as the boundary layer of the acoustically levitation droplet will be studied via PIV technique. A model which described the heat and mass transfer of colloidal droplet under acoustic levitation will be developed based on the simulation and analysis of the sound field. The self-assembly of the nanoparticles and their evolution upon evaporation will be investigated via in-situ SAXS technique provided by Shanghai Synchrotron Radiation facility. Meanwhile, to clarify the effect of sound field and electromagnetic field on the phase transition in the drying droplet, the phase field method will be utilized to simulate the structure evolution of nanoparticle aggregates. In summary, the carrying out of this project will elucidate the coupled mechanisms of acoustic levitation and electromagnetic field on the evaporation dynamics and self-assembly of colloidal droplets, thereby clarify the relationship between sound field, electromagnetic field and self-assembled structures.These outcomes will be of great importance for the developing of novel controllable self-assembly technique based on the evaporation of acoustic levitated colloidal droplets.

胶体液滴的蒸发是诱导胶粒自组装的重要方式之一。本项目利用声悬浮与SAXS技术原位研究Au、Ag等纳米颗粒胶体液滴在无容器耦合电磁场条件下的蒸发自组装及其相行为。通过CCD实时观测和跟踪悬浮液滴的宏观形貌变化，阐明液滴的蒸发动力学和形貌演化规律；采用PIV技术研究声悬浮液滴的内部流动及其外部边界层，并结合声场的计算模拟，建立声悬浮液滴蒸发的传热传质理论模型；利用上海光源同步辐射小角X射线散射技术实时在线研究胶体液滴蒸发自组装的相结构演化，并利用透射电镜等对蒸发产物的最终结构进行分析表征；采用相场方法，模拟纳米颗粒自组装结构的演化过程，阐明自组装机制与胶粒性质、声场及电磁场的依赖关系。本项目的开展，有望揭示无容器条件下声场与电磁场对胶体液滴蒸发动力学及自组装相结构演化的耦合作用机制，阐明“声场-电磁场-自组装结构”之间的内在关联，为开发基于声悬浮技术的胶体液滴蒸发可调控自组装的新方法奠定基础。

液滴蒸发是驱动液滴内胶体颗粒自组装的重要方式。因而，胶体液滴的蒸发是研究相变物理和新材料制备的理想载体，逐渐成为软凝聚态物理领域的研究热点。然而，无容器条件和声场如何对胶体液滴蒸发及胶粒自组装产生影响，是该领域热切关心的问题。在此背景下，本项目研究了纳米颗粒在液滴表面的聚集及其对液滴形状的调控机制，探索了声悬浮条件下液滴的形状演变规律，研究了Au纳米颗粒在声悬浮液滴表面的蒸发自组装，进一步开展了纳米颗粒包覆液滴的应用研究。取得了以下主要结果：.1，研究了单层纳米颗粒在液滴表面的吸附，发现在外力作用下，纳米颗粒一旦吸附到液滴表面后便难以脱附，从而使得液滴表面呈现出复杂的应力状态，甚至可以出现界面堵塞态。利用纳米颗粒在气-液界面的堵塞，可以对液滴的形状进行精准控制，实现对液滴形状的设计。.2，利用透明液体弹珠实现了老鼠胚胎干细胞向心肌细胞的分化，实现了拟胚体在液滴微反应器中全过程的实时观测。液体弹珠表面颗粒层的界面弹性导致细胞与表面颗粒层之间的粘附作用显著减弱，从而导致拟胚体在微反应器中保持悬浮状态，实现了胚胎干细胞的“一锅式”悬浮培养和分化。.3，发现了多种液滴在声悬浮条件下的液滴-气泡转变现象，揭示了液膜的弯曲显著地加剧了液膜边缘的负压，阐明了液滴-气泡转变的力学根源。进一步揭示了液滴-气泡转变的声腔共振机制，声悬浮条件下弯曲液膜包围的空腔可看作一种与液体性质无关的声学谐振器。一旦弯曲的液膜腔体达到合适的体积，空腔即与声场发生共振，从而突然膨胀形成气泡。.4，以声悬浮液滴作为Au纳米颗粒组装的模板，实现了Au纳米颗粒在无容器条件下的自组装。实验发现，声悬浮条件抑制了常规液滴蒸发时的咖啡环效应，并利用声辐射力以及颗粒在液滴表面的堵塞，最终获得了具有双层结构、大面积、自支撑的有序纳米颗粒圆盘状自组装体。.上述结果为利用声悬浮技术研究胶体相变奠定了基础，为开发基于超声悬浮的纳米材料自组装新技术创造了条件。