Description
Droplet microarrays provide a multitude of parallel compartments for (bio)chemical reactions and find various applications in fields such as high-throughput screening [1]. The ability to draw samples from these tiny reaction spaces is instrumental for monitoring assays or withdrawing liquid for further processing. We present a method for parallel sampling from droplet microarrays that is based on pulsed electric fields. The array is arranged between parallel capacitor plates to which a voltage is applied. As a result, droplets that are significantly smaller than the original droplets pinch off and are deposited on a surface facing the array substrate. In this way, a parallel contactless sample transfer from arrays with about one hundred droplets becomes possible. We consider aqueous droplets as well as droplets of dimethyl sulfoxide and quantify the mean and standard deviation of the transferred volume. In addition, we study the fluid mechanics of sample transfer using high-speed imaging. The high-speed images reveal a surprisingly complex sample transfer process. After switching on the electric field, a droplet from the array gets attracted to the upper surface, after which it bounces back. The sample transfer is finally achieved from an oscillating sessile droplet which gets in contact with the upper surface. We qualitatively explain this by electrohydrodynamic tip streaming through which charges are deposited that repel the sessile droplet.
[1] W. Feng, E. Ueda, and P. A. Levkin, Advanced Materials 30 (2018), 1706111