Rapid generation of highly uniform droplets using asymmetric microchannels fabricated on a single crystal silicon plate

A microfluidic device can be identified by the fact that it has one or more channels with at least one dimension less than 1 mm. The most common types of microfluidic devices are: (i) soft microfluidic devices fabricated in elastomeric materials such as PDMS by soft lithography [1], (ii) glass devices manufactured in quartz glass or glassy polymers such as PMMA by etching or micromachining [2], and (iii) microchannel (MC) array devices fabricated in silicon by photolithography and wet-etching or deep-reactive ion etching processing [3]. Microfluidic devices can be used for capillary electrophoresis [4], liquid-liquid extraction [5], immunoassays [6], cellomics [7], proteomics [8], DNA analysis [9], blood rheology measurements [10], microreactors [11], droplet formation [2], etc. The soft microfluidic devices such as T-junctions and flow focusing devices are suitable for rapid generation of monodispersed droplets with a coefficient of variation (CV) in a dripping regime of generally less than 3 %. Although the frequency of drop production can be as high as 7000 Hz, the overall productivity in terms of volume flow rate of the disperse phase is very low because the droplets are formed from a single channel. Silicon MC array devices are much more suitable for large-scale applications because the total number of microchannels on a chip can be hundreds of thousands. The aim of this work was to investigate the generation of uniform droplets at high production rates using novel asymmetric MC array microfabricated on a silicon plate [12]. Monodispersed emulsion droplets are much more favourable both in fundamental studies and practical applications. Emulsion appearance and rheology, stability against Oswald ripening and creaming, and the suitability of droplets as templates to the production of solid micro- and nano-particles are strongly influenced by their particle size distribution