A Posteriori

Attempts to grapple with and elucidate empirical knowledge

Jump not to conclusions September 30, 2009

Filed under: General Physics — Rāhul @ 11:52
Tags: ,

Today’s post is on why it isn’t wise in science to jump to conclusions, at least not ones which we’re too hasty to take as facts. Yesterday, I was in the cleanroom, cleaning some wafer pieces for SIMS analysis. My samples were being cleaned at high temperature in a water bath, when I noticed something interesting. I whisked out my phone and got some pictures to illustrate my points.

I had 3 small beakers in the water bath, almost evenly spaced out, as shown in Fig: 1 below.

Beakers immersed in water bath with even spacing

Fig: 1 Beakers immersed in water bath at even spacing

After about 10 minutes, I noticed that beakers had migrated within the water bath to cluster together at one end, as shown below in Fig: 2.

Fig: 2 Beakers clustered together in the water bath

Fig: 2 Beakers clustered together in the water bath

The water was at about 80 degrees centigrade and the various stresses in it was creating many eddies and waves, but the question was why there was a net. effect towards clustering the beakers. I wondered if there was some reason why the forces on the beaker surface would vary in magnitude based on the amount of water beyond it. This might mean that the beakers will tend to cluster closer together as the greater force from the larger quantity of water outside the cluster than the force from inside the cluster would push them together. This would also explain why in Fig: 2, all three beakers didn’t move to the center of the water bath. There water-force proportionality argument works to keep the beakers closer to the edge of the water bath too.

I started exploring reasons why the quantity of water in the neighbourhood might affect the force on the beakers and cause clustering, but could not come up with any satisfactory answer. After all, the bombardment of high energy water molecules on the surface of the beaker (pressure) is just a function of the local temperature. Although temperature might vary radially in the water bath, that wouldn’t explain the behaviour. So, my hypothesis was proving hard to substantiate, even with the limited physics I employed. Then, I thought that maybe there is a gradient along the water bath towards the far side so that beakers move that way because gravity can overcome friction on the beakers when there is random agitation of water. To test this, I replaced the beakers as shown in Fig: 3 below.

Fig: 3 Beakers placed together on the near side

Fig: 3 Beakers placed together on the near side

If the reason for the 3 beakers in Figs 1 and 2 clustering on the far side was an effective force bringing them together, then, the beakers should now remain clustered. But, after a short while, the beakers moved as shown in Fig: 4 below.

Fig: 4 Beakers moving to the far side

Fig: 4 Beakers moving to the far side

The glass beakers moved over to the far side of the water bath while the Teflon beaker remains steady. We don’t know if this difference is related to the material or weight of the beakers or just due to their relative positions. But, from this observation, we can indeed reject any tendency to clusterise as the main reason why the beakers in Figs 1 and 2 moved over to the far side. There does seem to be a gravitational (or other) gradient favouring the far side of the water bath.  Even if there is a force favouring clustering, the gravitational gradient overcomes it, as seen in Figs 3 and 4. The Teflon beaker might not have moved to the far side in Fig: 4 because of a local energy minimum there. But, I am not certain because it was time for me to remove my samples from the water bath, and hence my experiments with the beakers had to cease!

This post just intends to illustrate why in Science we should remain ever-vigilant in seeking new information and never too dogmatic to re-evaluate the theories we might find interesting. My former theory sounded more fun to me, but further evidence pointed to something more prosaic. Prosaic, yet agreeing with evidence!

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