Experiment of The Month
Non-Coalescing Coffee Drops
The forces that hold a falling drop of water in a spherical shape can be strong enough that the drop fails to merge immediately with another drop of water. The effect can be even seen when a falling drop meets the top surface of a pool of water.
While keeping vigil at his coffee pot, Dr. Cooney noticed that this "non-coalescence" was remarkably visible for drops rebounding from the coffee dripping from above into the half-full pot. Senior Arzie Yeater took up the challenge of making careful observations and of understanding the phenomenon. The figure at the below shows two frames of his high-speed video for one non-coalescing drop, in motion from left to right.
![Splash Splash](splash.jpg)
Mr. Yeater used our high speed (500 frames per second) camera to follow the evolution of non-coalescing drops. He also refined the experiment, manufacturing his own drops rather than using a coffee pot. Interestingly, however, he still needed a weak coffee solution to get the drops to form reliably.
In the picture (below), he drops coffee from a straw (used like a pipette) while recording with the camera seen in the lower part of the photo. The adjustable hotplate maintained a constant temperature for the water. Mr. Yeater compiled data for the quality of non-coalescing drops as a function of the temperature of the water. He found that the optimum temperature was about 50C.
![Yeater Yeater](yeater.jpg)
He measured quality by the "yield," the fraction of drops from the straw that resulted in a non-coalescing drop, formed on the rebound. The figure shows how the non-coalescing drop is formed.
If the water is deep enough, the falling heavy drop does not produce the famous "crown." Instead, it produces a column of water shooting up at the center of the fall location. When that column falls down, its top often pinches off, leaving a small drop to fall by itself. Drops like this are the non-coalescing drops.
The figure shows the yield that Mr. Yeater measured, plotted versus water temperature. The graph at the right clearly shows a peak between 50C and 65C. A yield of .5 means that half the time he released a drop, he created a non-coalescing drop.
He has proposed two mechanisms that keep the drop from merging with the pool of water below it: A pressure gradient from water evaporating up from the pool, and surface tension which makes it energetically favorable for a small drop to remain intact.
Surface tension increases at lower temperatures, and the evaporation increases at higher temperatures. In this model, drops form most favorably when both effects join together.
![Data Data](yieldgraph.jpg)
Mr. Yeater's movies are online and may be downloaded from the list below.
- Double
- Short
- Shorter
- Even Shorter
- Good One
- LightCoffee
- Fuzzy
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Contact Information
Contact Number: 717-871-4297
Email: physics@millersville.edu