1. Why Do Water Beads Try to Become Round?
After rain, water on leaves or spiderwebs does not spread out flatly; it forms round beads. Basic science explains that water molecules attract each other, and because this force does not point outward from the surface of a sphere, water tries to become spherical [1]. This property is called surface tension. Liquids try to make their surface area as small as possible [2]. For the same volume, the shape with the smallest surface area is a sphere. Therefore, water becomes round as much as the place it is on allows.
Here is a mystery: the water beads on spider threads are not one large bead, but a row of small beads. What happens to water that is attached in a long, thin shape like a thread?
2. Why Does Long Water on a Thread Split into Droplets?
Long columns of water or thin films of water on a thread cannot keep their shape. To reduce surface area, certain parts bulge out, and the water splits into droplets. This phenomenon is called the Plateau–Rayleigh instability, named after the people who discovered it [2]. A 2015 paper lists the sparkling water beads on spiderwebs as a common example of this instability. Becoming droplets reduces the liquid’s surface area, and the energy stored on the surface decreases accordingly [3].
However, not all wobbles in a thin liquid grow. In a cylindrical liquid, only wobbles where the wave length is longer than the cylinder’s circumference grow [2]. Short wobbles actually increase the surface area, so they disappear. So, among the growing wobbles, which length is the most noticeable? That length determines the spacing of the water beads.
3. What Decides the Spacing of Water Beads on a Thread?
What determines the spacing of the water beads? In its glass-fiber and polystyrene-film model, the paper shows that for a liquid film on a thread, the wavelength of the fastest-growing wobble is 2π√2 times the combined radius of the thread and the film. This is approximately 8.9 times [3]. In this model, the wavelength of the "fastest-growing wobble" is proportional to the radius [3].
For example, if the combined radius of the thread and film is 0.01 mm, the calculated spacing is about 0.09 mm. This is a value calculated from the formula, not a measurement from an actual web. There are two points to note. First, the experiments in this paper were done with polystyrene film on glass fibers, not measurements of spider thread. Second, the number "about 9 times" comes from the formula for a film on a thread. For a liquid column with no support, the wavelength of the fastest-growing disturbance is expressed by a different formula as about 9.02 times the radius [2]. The numbers are close, but the formulas are not the same. Therefore, "lined up at almost equal intervals" is a rough guide. The model suggests that the combined radius of the thread and water is a guide for the spacing.
We now know that water beads line up naturally. But are the sticky beads that spiders make on their threads also made by the same mechanism?
4. How Does Humidity Change the Sticky Beads on Garden Spider Webs?
The sticky beads on the horizontal threads of the garden spider species (*Argiope trifasciata*) are reported to be formed when liquid on the thread splits into evenly spaced beads due to the Plateau–Rayleigh instability [4]. The logic of water beads lining up seems to apply to the sticky beads spiders make themselves. This liquid in the sticky beads contains small substances that attract water, such as salts and small peptides. In experiments where relative humidity was raised from 10% to 90%, the volume of the beads increased by about 100%, meaning it doubled [6]. The numbers differ by species.
When the beads take in water, the stickiness of the liquid inside changes. In one species, adhesion force was reported to change by almost 10 times depending on humidity [7]. A spiderweb on a humid morning may have different properties than one on a dry day. So far, we have discussed "water or liquid on a thread becoming beads." But are there threads that actively change shape to gather water?
5. How Do the Wet Capture Threads in the 2010 Spider Study Form Knots?
The capture threads of the spider in the 2010 study change shape when wet. According to research published in 2010, spindle-shaped knots made of randomly oriented nanofibers rearrange into a periodic structure separated by joints of aligned nanofibers [5]. Between the knots and the joints, there is a gradient in surface energy and a difference in pressure inside the water droplets (Laplace pressure). The research explains that these two factors work together to gather water droplets from the joints to the knots, allowing water to collect continuously [5].
Small water droplets formed by condensation of water vapor in the air move along the smooth joints to the knots as they grow. When they merge with larger droplets at the knots, the joints become empty, so new droplets can be gathered again [8]. This spider uses threads of fine fibers without using mucus. This is different from the horizontal threads of garden spiders, which have sticky balls [8]. Even for water beads on the same spiderweb, there are cases where they split naturally on the thread and cases where the thread changes shape to gather them.
6. How Can You Observe Water Beads on Webs After Rain?
If you find a spiderweb on a morning after rain, observe the line of water beads from a distance. Are the intervals even? Where are the large and small beads? Are there dry threads? Do not touch the web. If you cannot find a web, you might see how water beads line up by stretching a thin thread at home (such as string or sewing thread) and spraying water on it. Do this with an adult in a place where water will not touch machines or electricity. By comparing how the arrangement changes with the thickness of the thread and the amount of water, you can observe how the beads arrange themselves.
If you want to know more about this mechanism, the 2015 paper by Haefner et al. [3] and the Japanese summary of the 2010 paper by Zheng et al. [5] are good entry points.