LAMT

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Light-actuated Marangoni Tweezers

 

 

 

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Control over matter using light is a fascinating subject. Light is a powerful tool for the manipulation of particles on the microscale. Over the last 40 years, there has been considerable interest in optical manipulation of individual particles, which enables the study of biological motors to the interaction between colloidal particles. Commonly employed optical techniques for particle manipulation are based on optical tweezers, relying on forces due to gradients in the electromagnetic field strength. In the Rayleigh regime these forces scale as the particle volume and therefore rapidly diminish with decreasing particle diameter. For this reason, the trapping and manipulation of nanoscale objects with light remains a challenge.

In that context, we propose a novel technique for the optical manipulation of particles adsorbed at a gas-liquid interface based on optically induced Marangoni flow. The method relies on photoswitchable surfactants that can be reversibly switched between two isomeric states, cis and trans. The surface tension of liquid covered with cis molecules is higher than that of a surface covered with trans molecules. When a water surface covered with the transform of the surfactant is locally illuminated with 325 nm light, the surface tension in the illuminated region increases. The gradient in surface tension induces a Marangoni flow directed radially inward in a coordinate frame centred at the focal spot, as depicted in the figure below. The left part of the figure shows a schematic of the physical principle, while the right part shows a visualization of the resulting flow based on particle streakline velocimetry. It has been experimentally demonstrated that the optically induced Marangoni flow can be utilized to trap and manipulate microparticles adsorbed to the interface at lower light intensity than with conventional optical tweezers. The favourable force scaling with the particle diameter makes this a promising principle for the manipulation of nanoscale objects.

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Related publications:

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[1] S. N. Varanakkottu, S. D. George, T. Baier, S. Hardt, M. Ewald and M. Biesalski (2013), Particle manipulation based on optically controlled free surface hydrodynamics, Angew. Chem. Int. Ed., 52(28), 7291-7295. Full paper

[2] S. N. Varanakkottu, S. D. George, T. Baier, S. Hardt, M. Ewald and M. Biesalski (2013), Manipulation of microparticles and biological cells using light-induced Marangoni flow, 

MicroTAS 2013, 748-750. Full paper

[3] S. N. Varanakkottu (2013), Light-Induced Microfluidic Transport Phenomena, PhD thesis, TU Darmstadt.

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Evaporative optical marangoni flow

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