In an exploratory experiment, we noticed the unusual behaviors of liquid droplets falling through layers of oil and water. A rectangular container was filled with an aqueous solution at the bottom and a layer of oil on top. A dropper was used to control the size of the droplet entering the liquid column. In addition, water was mixed with a small amount of Bromothymol Blue dye, a chemical indicator, to visualize the detailed flow processes. It was noticed that initially, the liquid droplet moved through the oil layer and was stopped at the oil/water interface, supported by surface tension and the buoyancy of the oil layer between the liquid droplet and the water below. As time passed, the support was weakened and the droplet would start to fall quickly through the water. Two types of droplets were used. The first case was a salt water solution with NaOH, and the second consisted of balsamic vinegar and NaOH. As soon as the salt water droplet touched the aqueous solution, it collapsed and sank as well as spread rapidly at the interface. The sinking motion eventually dragged the spreading fluid back to its center and then down. For the balsamic vinegar case, a trace amount of the droplet spread rapidly at the interface while the main portion of the droplet would first sink and then spontaneously explode. The difference in behavior is mainly due to the surface tension of the droplet in water. The underlying mechanisms of the droplet’s flow instability are from the effects of diffusion weakening the surface tension.
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