Monday, November 11, 2013

Self-Assembly of Mesoscopic Materials to form Controlled and Continuous Patterns by Thermo-Optically Manipulated Laser Induced Microbubbles

Basudev Roy , Manish Arya , Preethi Thomas , Julius Jurgschat , K. Venkata Rao , Ayan Banerjee , Chilla Malla Reddy , and Soumyajit Roy
The formation of continuous patterns of nano-structured material using directed self assembly under external fields has generated considerable current research interest. We demonstrate for the first time such continuous patterning by inducing irreversible self-assembly leading to nucleation in mesocopic materials (inorganic, organic, and nano-particles) using a tightly focused laser beam in an optical tweezers apparatus. A dense aqueous dispersion or solution of the material which has high absorption at the laser wavelength is taken in a sample holder so that some material is adsorbed on the top surface. A hot spot is created on the top surface when the adsorbed material absorbs the high intensity at the focus of the laser beam (a sub-micron sized spot), due to which a water vapour bubble is formed. This causes self assembly of material around the bubble due to Gibbs-Marangoni convection and capillary flow after which the material eventually nucleates into a crystalline state. The bubble is ‘trapped’ at the hot spot due to the temperature gradient around it, and can be manipulated by thermal forces generated optically, so that the system may be described as a thermo-optic tweezers. We translate the trapped bubble using the microscope sample holder stage of the apparatus so that the nucleation site of the material is simultaneously translated generating continuous patterns. We have demonstrated the technique using exotic inorganic materials such as soft oxometalates, an organic material such as glycine, a fluorescent dye such as perylene, as well as with carbon nano-tubes. We have written patterns over lengths of nearly 1 mm at the rate of 1 Hz, with best resolution of about 4 μm. The technique has potential for a wide range of applications ranging from solution processed printable electronics to controlled catalysis.
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