Magnetocapillary microrobots: hunting, harvesting and transporting objects at fluid interfaces

最近在对生物和人工微泳者的基本了解方面投入了大量的研究，这使得我们对生物游泳者的行为的了解有了显著的提高，如细菌或精子，或人工系统，例如基于胶体的人工系统，由于化学或热梯度可以积极推进。在这个项目中，我们的目标是对另一类人工游泳者的基本理解，这是最近由比利时列日的N.Vandewalle小组实验实现的。一组捕获在流体-流体界面的顺磁粒子可以被用来创造一种“磁毛细管游泳者”：由于在外加静磁场的情况下，由于引力使液体界面变形而产生的吸引力毛细管力和排斥磁偶极相互作用的竞争，出现了稳定的粒子集合。通过通过较小的线极化磁场来调制外部静态磁场，局部平衡变得扭曲，从而导致整个颗粒组件的定向运动。我们将结合模拟和分析处理来研究捕获在流体-流体界面上的顺磁粒子的组合。我们的格子Boltzmann模拟将基于现有的求解器，该求解器可以处理具有明确定义的表面张力的两种流体，以及悬浮粒子和磁相互作用。分析处理将基于由毛细、界面和磁力扩展的Najafi-Golestan三球模型。将特别关注颗粒的形状、大小和数量、微米和纳米范围之间问题的可扩展性以及磁场的设计对游泳者性能的影响。我们将探索游泳者在狩猎、收获和运输货物颗粒时的受控运动。这开启了磁性毛细血管游泳者作为微型机器人的潜在应用，例如，用于精确和可控的界面清洁或特定物体的运输和沉积。最后，我们将关注多个磁毛细管游泳者之间的相互作用：我们已经证明，游泳者在外部磁场的明确共振频率时达到最大速度。该频率的特定值不仅取决于流体，还取决于粒子属性。通过改变由不同游泳者组成的颗粒的大小，我们将能够创造出具有非常复杂的交互作用但精确可调的个体运动的微型机器人群体。

A tremendous amount of research was invested recently in the fundamental understanding of biological and artificial microswimmers which has led to a remarkable improvement of our knowledge about the behaviour of biological swimmers such as bacteria or sperm, or artificial systems based for example on colloids which can actively propel due to chemical or thermal gradients.In this project we aim at a fundamental understanding of an alternative class of artificial swimmers which was recently realised experimentally by the group of N. Vandewalle in Liege, Belgium. An assembly of paramagnetic particles trapped at a fluid-fluid interface can be used to create a "magnetocapillary swimmer'': Due to the competition of attractive capillary forces stemming from a deformation of the liquid interface by the gravitation force and repulsive magnetic dipolar interactions in case of an applied static magnetic field, a stable particle assembly emerges. By modulating the external static magnetic field via a smaller linearly-polarized magnetic field, the local equilibirum becomes distorted resulting in a directed motion of the entire particle assembly. We will combine simulations and analytical treatment to study assemblies of paramagnetic particles trapped at a fluid-fluid interface. Our lattice Boltzmann simulations will be based on an already existing solver that can handle two fluids with a well defined surface tension, as well as suspended particles and magnetic interactions. The analytical treatment will be based on the Najafi-Golestanian three-sphere model extended by capillary-, interface- and magnetic forces. A particular attention will be devoted to the influence of the particle shape, size and number, the scalability of the problem between micro- and nano- ranges as well as the design of the magnetic fields on the properties of swimmers. We will explore the controlled swimmer motion for hunting, harvesting and transporting of cargo particles. This opens potential applications of the magneticapillary swimmers as microrobots e.g. for the precise and controllable cleaning of interfaces or the transport and deposition of specific objects. At last we will focus on the interaction of multiple magnetocapillary swimmers: We have already shown that a swimmer reaches its maximum speed at a well defined resonance frequency of the external magentic field. The specific value of this frequency depends not only on the fluid- but also on the particle properties. By varying the size of the particles comprising different swimmers, we will be able to create swarms of microrobots with very complex interactions,but precisely tunable individual motion.