Abstract
A method to manipulate and control droplets on a surface is presented. The method is based on inducing electric dipoles inside the droplets using a homogeneous external electric field. It is shown that the repulsive dipole force efficiently suppresses the coalescence of droplets moving on a liquid-infused surface (LIS). Using a combination of experiments, numerical computations and semi-analytical models, the dependence of the repulsion force on the droplet volumes, the distance between the droplets and the electric field strength is revealed. The method allows to suppress coalescence in complex multi-droplet flows and is real-time adaptive. When the electric field strength exceeds a critical value, tip streaming from the droplets sets in. Based on that, it becomes possible to withdraw minute samples from an array of droplets in a parallel process.
Highlights
A method to manipulate and control droplets on a surface is presented
While in droplet microfluidics it is desirable to have a base configuration ensuring that two approaching droplets do not merge, at the same time it is desirable to induce the coalescence of specific droplets in a controlled manner
The coalescence event can be prevented by applying a homogeneous electric field normal to the liquid-infused surface (LIS) which is realized by applying a voltage between the LIS and an indium tin oxide (ITO)-coated glass cover
Summary
A method to manipulate and control droplets on a surface is presented. The method is based on inducing electric dipoles inside the droplets using a homogeneous external electric field. The key is to suppress undesired droplet coalescence, which is usually done by adding surfactants to one of the two immiscible phases[7] These surfactants can have a number of undesired effects. When droplets are used as reaction spaces for the synthesis of specific materials, surfactants introduce contaminations that compromise the purity of the reaction products For these reasons, efforts were made to establish surfactant-free versions of these processes. Tailor-made droplet manipulation based on surface acoustic waves is another method to induce coalescence[25]. It has been studied how coalescence can be promoted by tuning the surfactant concentration at the liquid–liquid interface[26,27]
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