Abstract
AbstractIt is demonstrated how aqueous droplets with volumes down to the sub-femtolitre range can be manipulated, including the withdrawal of minute samples from the droplets. The underlying principle is that of partial coalescence with a liquid reservoir in an applied electric field. Upon partial coalescence, a droplet merges with a reservoir and reappears with a smaller diameter. The droplets studied here perform a reciprocating motion between two reservoirs during which their volume gets reduced. Manipulation of droplets with diameters down to 400 nm is reported. A similarity relation is derived expressing the ratio of droplet diameters before and after partial coalescence as a function of the ratio between electric and interfacial-tension forces. The presented scheme allows the withdrawal of minute samples from small droplets and could prove helpful in various applications where droplets are used as tiny reaction spaces or when the goal is to tailor the size of individual droplets.
Highlights
Droplets with diameters of the order of 1 μm or below are of interest for a plethora of applications
The process reported here offers a solution to the liquid sampling problem and is controllable via the applied electric field. It could prove helpful in various applications where droplets are used as tiny reaction spaces or when the goal is to tailor the size of individual droplets
Different modes of droplet manipulation are possible with our microfluidic device
Summary
Droplets with diameters of the order of 1 μm or below are of interest for a plethora of applications. For low enough values of the electric conductivity inside the drop, i.e. low enough values of the salt concentration, the droplet that re-emerges after a droplet comes into contact with the aqueous reservoir has a reduced volume Based on this preliminary work, we have demonstrated a device with which it is possible to transfer dissolved species between a reservoir and a droplet in a well-controlled manner (Shojaeian & Hardt, 2020). In this device, a droplet reciprocates between two aqueous reservoirs, driven by a DC electric field. In context with synthetic cells that serve as a motivation for this work, this would mean that sampling operations become possible, analogous to the recently developed methods for extracting the contents of living cells (Actis, 2018)
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