Light-driven diffusioosmosis: from active manipulation over self-propulsion to collective behaviour of small particles at solid liquid interfaces

我们提出了一种新的机制来诱导自推进，并操纵在固液界面上捕获的具有不同成分（例如，双面型或多孔型）的微米级颗粒的集合。这一过程的物理起源与我们的两个小组S. Santer（实验，波茨坦）和O. Vinogradova（理论，亚琛）最近提出的所谓的光驱动扩散有关。用聚焦光照射含偶氮苯表面活性剂溶液时，在固/液界面处形成局部水动力流动。相应的水动力足以迅速清除被困在界面上的粒子。在初步实验中，我们可以验证当胶体变成Janus粒子时，它们可以在含偶氮苯表面活性剂的溶液中在紫外光的全局/均匀照射下启动自推进。此外，我们发现多孔但未修饰的颗粒的聚集体在紫外线照射下表现出明显的排斥倾向，形成单峰颗粒间分离的模式。然而，这些现象背后的确切物理化学机制仍不清楚。在我们的项目中，我们将把理论和实验结合起来，以了解在不同的环境条件下（光照波长、盐和表面活性剂浓度），粒子表面的特性（Janus修饰或孔隙度）与固体基质的特性（图案、非均质性和各向异性，例如ζ电位）是如何影响粒子整体的粒子个体运动和集体运动的。在我们的项目中开发的方法，我们希望建立光驱动流体动力学作为一个有用的和通用的工具来研究自推进粒子和聚集的集体运动。

We propose a novel mechanism to induce self-propulsion and to manipulate ensembles of micron-sized particles with varying composition (e.g., Janus-type, or porous) trapped at solid-liquid interfaces. The physical origin of the process is related to so-called light driven diffusioosmosis introduced recently by our two groups S. Santer (experiment, Potsdam) and O. Vinogradova (theory, Aachen). During irradiation of a solution of azobenzene containing surfactants with focused light, there is a formation of local hydrodynamic flow at a solid/liquid interface. The corresponding hydrodynamic forces are sufficient to swiftly clean the illuminated area from particles trapped at the interface. In preliminary experiments we could verify that when the colloids are turned into Janus particles, their self-propulsion can be initiated in the solution of azobenzene containing surfactant under global/homogeneous illumination with UV light. Additionally, we have found that aggregates of porous but unmodified particles show a pronounced tendency to repel under UV illumination forming a pattern with a unimodal inter-particle separation. The precise physico-chemical mechanisms underlying these phenomena remain, however, unclear. In our project we shall combine theory and experiment in order to understand how the properties of particle surfaces (Janus modification or porosity) in combination with those of the solid substrate (patterning, heterogeneity and anisotropy of, e.g, the zeta-potential) under varying environmental conditions (illumination wave length, salt and surfactant concentration) affect the particle individual motion and collective motion of particle ensembles. With the methodology to be developed in our project we want to establish light-driven hydrodynamics as a useful and versatile tool for investigating collective motion of self-propelled particles and aggregation.