Investigating the effect of a monovalent ion on the droplet’s size and distribution in a surfactant-free droplet generation microfluidic chip

Abstract

In droplet-based microfluidic reactors (microreactors), the droplet size, shape, and distribution are essential characteristics for optimum mass transfer and reaction performances. The phase’s apparent physical properties forming the droplets in microreactors control the droplet’s generation rate, size, and shape. Modifying the interfacial properties of the continuous and dispersed phases by surfactants is a common practice to control the droplets’ size, but, in many cases, it affects the reaction environment. Surfactant-free droplet generation (emulsions formation) is a new promising approach to minimize the effects of long-chained additives, like polymers and surfactants, on the liquids’ chemical and physical properties involved in the reaction emulsions creation. The effect of monovalent ions generated from NaNO₃ salt on the droplets’ size and distribution will be investigated in a micro-flow system in the present work. A microfluidic chip was designed and fabricated for this work using the direct-writing soft-lithography method. The continuous phase was chosen to be Octanoic acid, while the dispersed phase was NaNO₃ aqueous solutions with salt concentrations ranged between 0.0 and 1.0 M. The droplets’ size and distribution were determined by visualizing the movement of the droplets using a high-speed camera mounted on a high-precision electronic microscope. The experimental results showed a dramatic reduction in the dispersed phase droplet size when monovalent ions are introduced. The ion concentration effect was not linear in all the investigated concentrations where the droplet size reduces by increasing the ion concentration from 0.2 to 0.6 M. Further increase in the ion concentration showed a reverse effect where the droplet size started to increase. It is believed that this non-linear effect of the ion concentration is directly related to the double-layer thickness that may be reduced by increasing the ion concentration beyond 0.6 M.