Light induced manipulation of isotropic and anisotropic liquids absorbed on photosensitive surfaces

我们建议的总体目标是研究由各向同性或各向异性（液晶）液体组成的液滴在光可切换聚合物衬底上的行为，其润湿特性可以响应适当的光学刺激而局部，动态和可逆地改变。从长远来看，我们将创建一个适应性强的平台，在这个平台上可以建立对吸附液滴的合并、分裂、混合或输送等基本操作。光敏衬底由含偶氮苯的聚合物组成。只需要轻微的照射就能引起地形和表面能的显著变化。众所周知，当含偶氮苯的聚合物薄膜以光学干涉模式照射时，薄膜的形貌发生剧烈变化，形成表面起伏光栅：在最简单的情况下，形貌模仿强度分布并变形成波状正弦形式，其振幅可与薄膜的总厚度相当。这个过程发生在玻璃状态下，没有软化。我们之前已经证明，这些光栅可以出现或消失，甚至“启动”，也就是说，通过利用两个干涉光束中重复的偏振变化，使其像涟漪一样在水中移动。在目前的项目中，我们将引入两个新的方面来研究具有可切换润湿特性的功能化表面。首先，被吸附的液滴将被有限的、不同润湿性的非均质模式所操纵；这些更改是完全可逆的，使系统可重新配置。其次，将明确考虑润湿性发生变化的动力学。我们的系统有两个固有的时间尺度：表面能几乎是瞬间变化的，但纹理和地形的不断变化将需要更长的时间。后这些方面可能会显著影响如何定义方便的辐照方案，以实现对液滴的快速操作，此外还受到其他条件的影响，例如偶氮苯分子的性质、聚合物和聚合物变形的程度。

The general goal of our proposal is to study the behavior of droplets, consisting of isotropic or anisotropic (liquid crystalline) liquids on photo-switchable polymer substrates, the wetting properties of which can be changed locally, dynamically and reversibly in response to suitable optical stimuli. In the long run, we shall create an adaptable platform on which elementary operations on adsorbed liquid droplets such as merging, splitting, mixing or transport can be established. The photosensitive substrate consists of azobenzene containing polymers. Only gentle irradiation is needed to trigger significant changes in topography and surface energy. It is well known that when azobenzene containing polymer films are irradiated with optical interference patterns the film topography changes drastically to form surface relief gratings: in the simplest case, the topography mimics the intensity distribution and deforms into a wave-like, sinusoidal manner, the amplitude of which may be comparable to the total film thickness. This process takes places in the glassy state without softening. We have previously shown that these gratings can be made to appear or disappear or even “set in motion”, that is, be made to move like ripples across water by exploiting repetitive polarization changes in the two interfering beams. With the current project, we shall introduce two new aspects to the study of functionalized surfaces with switchable wetting properties. First, adsorbed liquid droplets will be manipulated by finite, heterogeneous patterns of varying wettability; these changes are completely reversible rendering the system reconfigurable. Second, the dynamics by which changes in wettability occur will be considered explicitly. Inherent to our systems there are two time scales: surface energies are expected to change almost instantaneously, but the evolving changes in texture and topography will take significantly longer. These latter aspects may significantly influence how a convenient irradiation protocol may be defined to achieve swift operations on a droplet, additionally impacted by other conditions, such as nature of azobenzene molecule, polymer and the extent of polymer deformation.