A novel approach for drop-on-demand and particle encapsulation based on liquid bridge breakup

•We suggested a simple method to generate picoliter droplets out of a single microliter drop.•We investigated the effects of fluidic properties on the generated droplet size.•This is a method to dispense particle-encapsulated picoliter droplet-on-demand, hand-powered.

A simple and robust way is devised to generate picoliter droplets out of a single microliter drop for the use of generating monodisperse droplets in droplet-based microfluidics. A single aqueous drop is placed between two hydrophilic substrates and then immersed in silicone oil, to form a liquid bridge. Then one substrate is moved away with a predefined velocity. As the distance between two glass plates increases, the liquid bridge breaks up and smaller droplets or satellites are formed. A picoliter-droplet was successfully dispensed on demand repeatedly for 100 times within 2% relative standard deviation in size under velocity-controlled system. It is found that, for the case of fixed inner and outer fluids, the droplets of nearly the same size are generated over several orders of the moving velocity. Also, the size of the maximum satellite droplet increases with the increase of the mother drop size and inner fluid viscosity, and it decreases with the increase of the outer fluid viscosity. Its feasibility of cell and particle encapsulation has been confirmed by capturing a single Arabidopsis thaliana protoplast as well as polystyrene microparticles successfully using this method without complex control. Based on these results, a simple forcep was designed to dispense ultrasmall droplets and its functionality was confirmed to be similar with the velocity controlled case without external devices. This simple method is expected be used to divide a small amount of bio sample on-demand into several smaller droplets for further analyses.

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