Curvature gradient driven assembly of trapped and reconfigurable structures

将微尺度粒子组织成定义良好的结构的能力是我们设计新的软的、可重构材料的能力的核心。通常，外部静电或磁场被用来引导粒子进入它们可以相互作用并形成结构的位置。这项工作研究的领域在过去没有得到广泛的重视或使用。流体界面上的颗粒会变形并增加其周围界面的面积。表面张力和面积增加的乘积是一个能量场，它取决于流体界面的曲率，因此粒子沿着曲率梯度移动。通过这个简单但显著的事实，接口本身的几何形状可以用来指导组装。在这里，对这些领域进行研究，以确定形成传统方法难以形成的结构的新方法，以制造新材料，并探索其性能。研究生和本科生在进行拟议研究的过程中接受培训，包括路易斯·斯托克斯少数民族参与联盟项目、推进工程女性项目和宾夕法尼亚大学MRSEC REU项目的学生。所发展的新知识将被纳入有关界面现象的研究生课程。技术摘要：本研究旨在建立微米和亚微米尺度颗粒定向组装的新策略，以远远超出通常的紧密包装组装。被困在流体界面上的粒子通过毛细作用沿界面曲率梯度相互作用和迁移。这些能量驱动与界面曲率场密切相关的复杂结构的形成，受粒子-粒子相互作用的影响。由于软物质具有固有的可变形性，因此这种相互作用是形成可重构、可调组件的自然途径。用光学显微镜观察不同类型的结构，用光刻定义的容器来塑造流体界面，用磁性探针和其他探针来干扰结构并指导它们的重新配置。研究了动力学捕获结构形成具有孔洞、致密区和取向结构的胶体单层膜，这些结构响应界面形状的变化。研究平衡结构，形成沿界面主轴排列的结构，并研究其在界面扰动下的重构。结构形成的动力学是通过光学显微镜观察颗粒的形状和大小，这些形状和大小是由它们激发的毛细相互作用的规模决定的，以及它们在结构响应扰动时形成具有相关各向异性的定向结构的能力。对于这两个极限，粒子位置/方向与界面曲率进行比较并与之相关。将观测结果与适当的预测进行比较，例如，基于捕获结构的Stokesian动力学模拟和平衡结构的Monte Carlo模拟。

Non-technical AbstratThe ability to organize microscale particles into well-defined structures lies at the heart of our ability to design new soft, reconfigurable materials. Often, external electrostatic or magnetic fields are used to guide particles into positions where they can interact and form structures. This work studies fields that have not been widely appreciated or used in the past. Particles on fluid interfaces deform and increase the area of the interface around them. The product of surface tension and this area increase is an energy field that depends on the curvature of the fluid interface, so particles move along curvature gradients. Through this simple but remarkable fact, the geometry of the interface itself can be used to direct assembly. Here, these fields are studied to identify new ways to form structures difficult to form by conventional means to make new materials whose properties are explored. Graduate and undergraduate students are trained in the course of performing the proposed research, including students in the Louis Stokes Alliance for Minority Participation program, the Advancing Women in Engineering Program and the UPENN MRSEC REU program. New knowledge developed will be incorporated in a graduate course on interfacial phenomena.Technical AbstractThis research seeks to establish new strategies for directed assembly of micron and sub-micron scale particles to go well beyond the usual close packed assemblies. Particle trapped at fluid interfaces interact and migrate along interface curvature gradients via capillarity. These energies drive formation of complex structures strongly correlated with the interface curvature field, influenced by particle-particle interactions. Since soft matter is inherently deformable, such interactions are a natural route to form reconfigurable, tunable assemblies. Different classes of structures are studied using optical microscopy to observe structures, lithographically defined vessels to mold fluid interfaces and magnetic and other probes to perturb the structures and to guide their reconfiguration. Kinetically trapped structures are studied to form colloidal monolayer membranes with voids, dense regions and oriented structures which respond to changes in interface shape. Equilibrated structures are studied to form structures aligned along principle axes of the interfaces and to study their reconfiguration upon interface perturbation. The (dynamics of) structure formation is observed by optical microscopy for particle shapes and sizes selected for the scale of capillary interactions that they excite and their ability to form oriented structures with associated anisotropies in the structural response to perturbation. For both limits, particle positions/ orientations are compared to and correlated with the interface curvature. Observations are compared to appropriate prediction based on, for example, Stokesian Dynamics simulations for trapped structures and Monte Carlo simulations for equilibrated structures.