Novel Method to Simultaneously Adjust the Size and pH Value of Individual Microdroplets in Silicone Oil

In this paper, a strategy to modify each micro-droplet's volume and synchronously adjust its pH value as required based on the electrolysis reaction in silicone oil is demonstrated. A pair of platinum electrodes fixed onto the jaws of a vernier caliper was used to modify the micro-droplet's volume and adjust its pH value by simply adjusting the distance between the two electrodes. To get a micro-droplet with desirable volume and pH values, three models, the relationship between the droplet's volume and its diameter when the droplet was placed on a fluorinated ethylene propylene (FEP)-covered glass substrate, the relationship between the distance of the two electrodes and the size of the resulted micro-droplet, and the relationship between the pH value and the micro-droplet's consuming rate, were built through the least square method. In our experiments, a droplet (5% sodium chloride solution, 1.4 uL, pH = 7) could consume 98.9% of its initial volume and form a new droplet with a volume of 0.016 μL and pH of 12.2. In addition, to validate that this method is also suitable in the acid and alkaline solutions, 0.001 mol/L NaOH and H ₂ SO ₄ solutions were, respectively, operated using the same procedure. Both the volume and pH values could be controlled, which proved the potential application of our proposed method in analytical chemistry, precision engineering, and so on.