Fluidic Self-Assembly of Microstructures Using a Blade-Coating Technique: Influence of Volume of Water Droplets on Probability of Microstructure Placement

Fluidic self-assembly (FSA) is a promising technique for fabricating devices that are composed of large numbers of small electronic components. We have previously proposed a printing method that utilizes the FSA principle. In our method, by simply blade-coating first water and then a dispersion liquid of microstructures on a substrate, the microstructures are automatically placed on pre-patterned hydrophilic areas by means of water/solvent interfacial force. To improve the placement probability of microstructures on the intended hydrophilic areas, in the present study we investigate the influence of the volume of water droplets on the probability of microstructure placement. We prepared various sizes of hydrophilic patterns on a glass substrate to vary the volume of water droplets in hydrophilic areas, and placed square SiO2 plates, measuring 50 x 50 x 0.3 mu m(3), using FSA. The probability and accuracy of placement was evaluated using a high-speed microscope, and the results were interpreted using a simple model based on capture coefficients and the collision cross section of the water droplets. We verified that the model closely fitted the experimentally obtained probability of placement as observed using the high-speed microscope. We found that the capture coefficient increased with increasing area of the water droplet. These results indicate that the size of the hydrophilic area is one key to improving the probability and accuracy of our placement technique. (C) 2012 The Japan Society of Applied Physics