Parallel active mixing microdroplet array

Abstract

In the micro- and nano-fluidics field, the mixing of droplets is a difficult challenge. This function is usually achieved by mixing two or more continuous flows via their injection in the same micro-channel. This strategy may be further improved by the active mixing introduced by an additional energy source plugged into the system to create flow instabilities. For example, ultrasonic mixers using stationary wave patterns or surface acoustic waves (SAW) were developed in order to decrease the mixing time and to improve the homogeneity of continuous-flow mixtures. Some micro-devices using microchanels even permit to generate micro-drops of reagents and to coalesce them in a carrier continuous phase. An alternative approach to continuous microfluidic systems is the manipulation of discrete droplets. The electrowetting-based linear-array droplet mixer, for example, proves that microdroplets can be transported, merged and actively mixed using an electrostatic field. Acoustic field can also be used for that purpose, several examples have been presented using, for the most part, high frequency vibrations such as SAW devices. In this paper, we introduce a parallel microdroplet mixing strategy based on local acoustic field generation using low frequency vibrations. An active acoustic mixer array was designed and fabricated. This microfluidic device permits the creation in parallel of an active mixture in a matrix of 25 microdroplets that are localized on a surface. It was designed to avoid contamination between droplets while they are excited. Experiments showing the independent and active mixture of droplets will be presented