Dynamic control of the distribution of polystyrene suspended nanoparticles in evaporating droplets isudinvestigated using a 2.9 mm high power laser. Under laser radiation a droplet is locally heated and fluidudflows are induced that overcome the capillary flow, and thus a reversal of the coffee-stain effect isudobserved. Suspension particles are accumulated in a localised area, one order of magnitude smaller thanudthe original droplet size. By scanning the laser beam over the droplet, particles can be deposited in anudarbitrary pattern. This finding raises the possibility for direct laser writing of suspended particles throughuda liquid layer. Furthermore, a highly uniform coating is possible by manipulating the laser beam diameterudand exposure time. The effect is expected to be universally applicable to aqueous solutions independentudof solutes (either particles or molecules) and deposited substrates.
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