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Day 31: Water droplets

water droplets on the car window

I was heading for the car park at work. It had started to rain so I moved faster.

As I sat down in the driver’s seat I automatically reached for the seat belt. As I did this I noticed the water droplets sitting on the car’s windscreen. They looked like little glass beads. They were beautiful. I was fascinated by the different sizes of the droplets, the way they reflected the light, their different shapes, and the way they sat there. They didn’t break up and form an amorphous pool of liquid. As a science teacher in my past life you’d think I would have known the physics behind what I was seeing but I didn’t. Or more to the point, I couldn’t remember. This seemed to me to be an example of everyday magic.

The physics of bouncing water droplets underlies a wide range of industrial applications from crop spraying to ink-jet printing, and continues to fascinate after 200 years of research.

Whether standing in the shower, spilling the morning coffee or going to work in the rain, each day typically begins with water droplets splashing off a solid surface. In fact these phenomena are so common that they often go unnoticed. However, the basic physics that governs the dynamics of water droplets is extremely rich, and understanding these events in detail has important scientific and technological consequences.

In agriculture, for instance, the wax-like outer layer of a plant leaf produces a non-wetting interface that repels water and causes drops to bounce off the surface. As a result, the plant often retains less than half of an applied spray. This is both inefficient and hazardous, since the herbicides and pesticides that are destined for the plant can build up and eventually contaminate the soil and public water supplies. Finding a way to eliminate droplet rebound in such cases has both major economic and social benefits.

On the other hand, promoting droplet rebound so that all drops bounce off a surface can have many advantages. Imagine a car windscreen that can repel every raindrop in a downpour. It would make driving in the rain much safer. Perhaps we can learn from the natural ability of plants to repel droplets and apply the same strategy to car windows. Thus, one sees that preventing or enhancing drop rebound off a surface can have a significant impact on our daily lives.

Source: Bergeron, V. (2001). Water droplets make an impact. Physics World (May).

On any other day I would have got in my car, put on my seat belt, started the car, turned on the windscreen wipers, and driven away. I would have been totally unaware of what was right there in front of me. I wouldn’t even have cared if I had noticed anything different. The moment would have simply passed over me.

Today, I sat there spellbound. Water is such a wonder product. I must remember to stop complaining about the rain!

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