Particle Manipulation Based on Optically Controlled Free Surface Hydrodynamics
Particle Manipulation Based on Optically Controlled Free Surface Hydrodynamics
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DOI：
10.1002/anie.201302111
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发表时间：
2013-07-08
影响因子：
16.6
通讯作者：
Biesalski, Markus
中科院分区：
文献类型：
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作者：
Varanakkottu, Subramanyan Namboodiri;George, Sajan Daniel;Biesalski, Markus
在光照射下发生可逆结构变化的光开关分子有许多应用，从生命科学到界面流体力学。[2,3]一些需要提到的是在固体表面上产生可逆的润湿性梯度[2]或在液-液界面上产生界面张力梯度[3]，以输送毫米大小的液滴。本文的目的是证明光开关分子也可以操纵微米大小的粒子。常用的粒子操作技术有光学捕获、[4]介电电泳、[5]光电镊子、[6]或等离子体镊子所有这些方案都依赖于所谓的梯度力，在瑞利模式中，梯度力是粒子直径的三次方因此，捕集力随粒子直径的减小而迅速减小。本文提出了一种基于光诱导马兰戈尼流的光学方法，用于捕获和操纵吸附在气液界面上的微米级颗粒。捕获机制的流体动力学性质意味着力与颗粒直径成比例，而不是它的三次方。该方法依赖于吸附在界面上的光响应表面活性剂，这些表面活性剂可以在两种异构体状态之间可逆地切换。实验证明，与传统的光镊相比，利用光诱导马兰戈尼流，微球可以用更低的光强度来操纵。产生光诱导流型的原理如图1a所示。光响应表面活性剂允许使用光束局部控制液体的表面张力。在本研究中使用的表面活性剂是基于众所周知的偶氮苯基序，并存在两种异构体状态，一个反式和顺式。波长为325 nm的光诱导反式偶氮苯变为顺式偶氮苯，而波长为442nm的光诱导反式偶氮苯变为顺式偶氮苯。为了创建具有光切换表面张力的空气-水界面，使用了一种缩写为C4AzoOC4E2的分子，当其顺式异构体的浓度增加时，其表面张力会更高
Photoswitchable molecules that undergo reversible structural change upon light irradiation have numerous applications, ranging from life science [1] to interfacial fluid mechanics.[2, 3] A few to mention are the creation of a reversible wettability gradient over a solid surface [2] or an interfacial tension gradient at a liquid–liquid interface [3] to transport millimetersized droplets. The objective of this article is to demonstrate that photoswitchable molecules also enable the manipulation of micron-sized particles. Commonly employed particle manipulations techniques are, for example optical trapping,[4] dielectrophoresis,[5] optoelectronic tweezers,[6] or plasmonic tweezers.[7] All these schemes rely on the so-called gradient force that scales as the third power of the particle diameter in the Rayleigh regime.[8] For this reason the trapping force rapidly diminishes with decreasing particle diameter. Here we present an optical method for the trapping and manipulation of micron-sized particles adsorbed at a gas–liquid interface based on optically induced Marangoni flow. The hydrodynamic nature of the trapping mechanism implies a force scaling with the particle diameter instead of its third power. The method relies on photoresponsive surfactants adsorbed to the interface that can be reversibly switched between two isomeric states. It is experimentally demonstrated that, using optically induced Marangoni flow, microspheres can be manipulated using lower light intensities compared to conventional optical tweezers.The principle of creating photoinduced flow patterns is shown in Figure1a. Photoresponsive surfactants allow to locally control the surface tension of a liquid using a light beam. The surfactants used in the present study are based on the well-known azobenzene-motif, and exist in two isomeric states, a trans and a cis state. Light of 325 nm wavelength induces a conformational change from trans-to cis-azobenzene, light of 442nm the reverse. To create an air–water interface with photoswitchable surface tension, a molecule abbreviated as C4AzoOC4E2 was used which exhibits a higher surface tension if the concentration of the cis isomers at the