Mach, Masks and garden sprinklers

Even if I wanted to, I cannot avoid wearing a mask – my wife is a dictator! For my good, I suppose.

No, that is where I got it wrong. While there could be some benefit to oneself because of wearing a mask, much of the personal benefit is indirect, and becomes operative only if all follow the rule. Then, to repeat what you may have heard a million times from thousands of sources, if you wear a mask, the possibility of droplets carrying the currently dreaded corona virus on a tour around the neighbourhood reduces, and you too benefit. However, the mask is only minimally effective in stopping ingress of the virus into your nostrils.

I have difficulty in understanding this one-way sign, for the virus.

There is another instance this one-way sign had confounded scientists. I thought perhaps we can take some hints from it. This instance has wonderful pedigree – asked first by the physicist-philosopher Ernst Mach in the late nineteenth century.

All of us know how garden sprinkler works. The photo below (accessed from the net) shows the dynamics of the sprinkler in a static photograph (longer exposure?).

We understand how the sprinkler rotates about the vertical axis thus watering the lawn from the middle of it. It all has to do with Newton’s Third Law – action-reaction pair. My readers are quite aware of the principles behind and I will not delve into it any deeper.

Mach did delve deeper and asked and found out in the course of a small experiment with an air pump and directional nozzles fixed to a shaft, that if the process is reversed, the devise did not rotate in the reverse direction – it stayed stationary, but airflow indeed reversed. This indeed is a puzzle, particularly as we understand Newton’s Third law.

In the early 1940s, this experiment got a lot of attention. We do not need to go into the history of it. In a much more rigorous experiment, the sprinkler was immersed in water a large carboy (the sealed container in which drinking water is delivered) and the intention was to suck water out and notice the direction of sprinkler rotation. The sprinkler did not rotate except for a small jerk when the suction started. It stayed more or less stationary afterwards.

Am I on a wild goose chase? I do not think so. The clue is in the directionality associated with the processes. When water is ejected out of the nozzle from a garden sprinkler, it has a specific direction, a narrow cone when idealized. On the other hand, when water is being sucked out by the pump, water particles from all around the nozzle congregate and each particle muscles its way in. It is all chaotic and random.

Now, take the case of a person sneezing. It has directionality and a tremendous force behind it. Each droplet tries to force its way out, and many pores in the mask get clogged – a screen is formed. So, the virus-carrying droplets are imprisoned.

In my own way of thinking, it is remiss on the part of people not to emphasize that after each sneeze or cough, the mask should be washed thoroughly – wherever you are – and start using a spare mask, or a handkerchief in multiple folds. If you allow the clog to evaporate away by not washing the sneezed-in / coughed-in mask, you are doing nothing but delaying the spread of the droplets.

I do not know whether I am right, but no one is going to stop me from sharing my process of analysis behind the one-way aspect of the mask under COVID-19.

You now tell me with a smirk, I have not talked about how the mask does not help you protect yourself in the same way. The simple answer, there is no directionality. Droplets hanging around your mask on the outside will eventually find their way into your nostrils and/or mouth. The air is much full of these. Unlike the water molecule entering the sprinkler after a struggle amongst themselves, the virus laden droplets just wait their time – they are so disciplined, much to our discomfort and danger.

By reducing the concentration of the virus in the air by wearing the mask, we help others and the others, doing likewise, help you.

It is all in the directionality.

The above coming from a non-physicist and a non-physician with wild anthropomorphizing tendencies deserves to be taken with a tablespoon full of salt.

Raghuram Ekambaram