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Writing with lasers: a new technique of controlled lithography using thermooptically manipulated microbubbles

We have developed a new technique of optical micro-patterning using micro bubbles based on light induced self- assembly of materials using thermo-optic tweezers. Presently, we use a liquid matter- Soft Oxo-Metalate (SOMs) that have high absorption near the wavelength of the tweezers laser at 1064 nm. An aqueous dispersion of the sample solution is taken in a glass sample chamber and introduced into the translation stage of our tweezers set-up (inverted microscope) where a highly focused laser beam is aimed at SOM particles adsorbed on the top surface of the sample chamber. The high absorptivity of SOMs ensures the creation of a local `hot-spot' which leads to the nucleation of a micro-bubble in this region. Thus, a large local surface-tension gradient is introduced in the vicinity of the micro-bubble due to the temperature gradient produced at the two ends of the bubble, which leads to a Marangoni type convective ow around the bubble. This ow causes material to be self-assembled at the base of the bubble. As the translation stage is moved, the `hot-spot' moves simultaneously, and due to the resulting ow dynamics, the microbubble is also translated thus causing continuous accumulation of the SOMs around it. Simultaneously, due to the sudden thermal shock generated when the `hot-spot' is moved away from the self-assembled SOMs, they undergo a phase transition from soft (liquid) to hard (crystalline) state, resulting in the formation of a stable permanent pattern of choice on the glass substrate. This technique can have diverse applications with materials other than SOMs including carbon nano tubes, organic dyes, catalysts and conducting polymers, etc, being co-deposited from aqueous dispersions of the particular material with the SOMs. The patterns thus formed have been used for various applications including the development of catalytic micro-chips, and solution processed printable micro-circuits.