Since time immemorial human beings have observed the curious phenomena of non-coalescence of drops. This happens when a drop of a liquid comes in contact with a liquid surface and does not merge (coalesce) with the liquid surface right away. Rather the drop may remain as if floating on the liquid surface for periods of time ranging from seconds to milliseconds before finally merging with it. In the video below, I used a straw to pick up a volume of my coffee and gently add drops onto the surface of the coffee. The non-coalescence effect is observed in the drops to various extents, and it can be seen clearly in the part of the video slowed down to 240 frames per second.
Although this phenomenon has been investigated by several scientists spanning a time period of more than 100 years, we still don’t know for certain how it happens. The non-coalescence of drops depends on many variables including the nature of the liquid in the drop and the surface upon which it lands, the chemicals dissolved in them, the temperature gradient between the drop and the liquid, the charge of the drops, and the air pressure.
A current hypothesis is that those areas of the drop or liquid surface in contact with the air phase (interfacial) have a molecular organization that is different from the areas away from the air phase (the bulk phase). Thus the drop and the surface upon which it lands do not tend to mix right away when placed in contact with each other. However, as time goes by, the interfacial layer of the drop and the liquid surface tends to dissipate at the point of contact between them (which is no longer exposed to air), and after it has sufficiently thinned, the water drop coalesces with the liquid surface.
As I have explained before, water molecules due to their atomic makeup have one end with a partial negative charge (where the oxygen atom is) and another end with a partial positive charge (where the hydrogen atoms are). This gives rise to a phenomenon called surface tension where water molecules stick to each other (positive to negative) and to surfaces. This effect can be seen in the video below when I poured milk into my coffee before breakfast. The milk, which is more than 90% water, stuck to the side of the glass, and even thought I was tilting the glass more than 80 degrees, not a single drop of milk fell outside!
In case you are wondering, as in my previous Science Before Breakfast video, I had scrambled eggs with bacon and home fries for breakfast but no blueberry toast this time.
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Just before eating my breakfast at a local restaurant, I fancied testing some scientific principles related to pressure and temperature differentials and buoyancy. First, I inserted my straw into the hot coffee and blocked the top of the straw with my finger. In this way I was able to remove the straw with a column of hot coffee inside it leaving the bottom open. Some people claim that this occurs because the gravity pulling down on the column of coffee produces a vacuum inside the straw, and the vacuum sucks the coffee into the straw preventing it from flowing out. This is not true. Like I have explained before, vacuums don’t suck. The pull of gravity tends to create a low pressure area inside the straw, and the push of the atmospheric pressure against the bottom of the column of coffee is enough to counter gravity and keep it inside the straw.
I then proceeded to place the tip of the straw with the coffee inside the cold milk. The hot coffee is less dense than the cold milk. Under the influence of a gravitational field, liquids that are less dense will float on top of liquids that are denser. You can see in the video that, even though the coffee is in direct contact with the milk at the bottom of the straw, the column of coffee inside the straw remains by and large unperturbed (with the exception of some coffee at the coffee-milk interface mixing with the milk due to equilibration of the temperatures of the liquids). Then I did the opposite. I placed a column of milk inside the straw in contact with the coffee. Because the hot coffee is less dense than the milk it starts flowing up the straw almost immediately, while the cold milk that is denser flows in the opposite direction.
After this I ate my breakfast: scrambled eggs with bacon and home fries and blueberry toast (with no butter).