“On Friday afternoon, May 15, it rained a bit and I scooped water from the ditch. I noticed that some raindrops floated on the water like pearls for seconds (see the photos). I immediately thought: this is something for the AW section. What causes this phenomenon?”
According to reader Kees E. who lives near Oudewater. His photos show ditch water on which droplets float that are so large that they became flattened and dented the water surface. From the water circles further along you can see that it was indeed raining when they were photographed and that the floating droplets hardly or not moved at all. The fact that they were drops and not bubbles is evident from their typical light refraction. It is important to note that the ‘gems’ for seconds floated on the water.
This is a belated confirmation of an observation that mathematics and physics teacher Gerard Schol made more than fifty years ago in Drachten. He was rowing, perhaps on the Geaster Djip, and noticed that the drops sliding from his oars regularly floated like ‘sparkling gems’, sometimes for as long as 15 seconds. Schol’s observation made sense the section ‘The Amateur Scientist’ by CL Stong Scientific American (August 1973). Schol showed how you could generate floating droplets yourself and showed that they were sensitive to electric fields, such as those from rubbed glass rods or balloons.
The drops are now known as ‘floating droplets’, or also ‘water boules’. The process they undergo is called ‘delayed coalescence’. Normal water droplets immediately merge with the water they land in.
The search terms mentioned lead to a sea of publications about the drops. Renowned scientists have had their say about it, in the nineteenth century you already had Lord Rayleigh (1879) and Osborne Reynolds (1881). Reynolds noted that the phenomenon only occurred on very clean water surfaces and assumed the presence of a layer of air between the droplet and the water surface (roughly like the Leidenfrost effect, see wiki). But a lot is still unclear. The Journal of Unsolved Questions tried in 2019 to come up with some concluding hypotheses, but Taiwanese researchers the use of fast cameras turned things upside down again in the same year.
Anyone who looks through the droplet literature gets the feeling that two types of droplets can be distinguished, although this is rarely done. There are those thick droplets such as in Oudewater and Drachten that hardly move and live quite a long time (several seconds) and there are small droplets that shoot away quickly and usually only exist for a fraction of a second. It doesn’t seem likely that they will survive in the same way.
The thick drops like in Oudewater hardly move.
Photos Kees E.
Allow soaking wet laundry to drip dry
Science is mainly interested in those small, fast droplets that do not visibly depress the water surface. These are precisely the droplets that the observant amateur often encounters; he sees them flashing by like silverfish on the floor of the shower cubicle or the bottom of the sink. Anyone who leaves soaking wet laundry to drip over a sink cannot miss them. The prerequisite is that on the receiving surfaces, thanks to a certain slopebut very little water stands, otherwise bubbles and bubbles will form and distort the image. The lovers of slow coffee see the floating droplets appear when the coffee filter continues to drip at the end of the brewing process. The same droplets would appear on hot tea into which cold milk is poured, but this could not be confirmed. Did it have to be cream?
For the research The ‘floating droplets’ allow drops to fall from a minimal height into reservoirs within which the water is only a few millimeters high. The fate of the drops is monitored with all kinds of optical equipment. Experiments are conducted with varying drop heights, with temperature differences between drops and reservoir, with air pressure, humidity, surface tension and so on. Silicone oil or hydraulic fluid is also used instead of water. The observed ‘residence time’, the duration of floating, is always an important measure. Do you vibrate the receiving reservoir, as Jearl Walker of ‘The Amateur Scientist’ did (Scientific AmericanJune 1978), the droplets will float indefinitely.
The air layer assumed by Reynolds turns out to indeed be there and you could say that the chances of survival of the droplets depend on the extent to which they manage to acquire and retain that air layer upon impact. Fast cameras (more than 1,000 frames per second) have shown that the tiny droplets are even more dynamic than they already appear. They bounce across the surface of the water and when they eventually merge with the reservoir water, it happens in a strange stepped process. Violent currents exist within the droplets.
It is unclear what the situation is with the larger drops that fell from a greater height in Oudewater and Drachten. There wasn’t much to find about it. Whether the presence of a clean water surface is such a compelling requirement, as Reynolds thought, remains to be seen. The modern insight is that water surfaces actually never ‘clean’ are, but that most surfaces are somewhat less dirty than the areas that ‘slicksare mentioned. These are the intriguing silent places that become especially visible in soft rain or wind because they suppress the small water ripples (capillary waves). That’s the job of the biosurfactantssurfactants of biological origin such as excretion and digestion products of algae. Do floating droplets form just as easily inside the slicks as outside them? This must now be investigated quickly. And then interest may shift to the slicks themselves, because surprisingly little is known about them. For example, do the slicks respond to wind, as Minnaert thought, or to deep currents? More later.