WHY DOES A CORK almost ALWAYS FLOAT TO THE SIDE OF A GLASS INSTEAD OF THE CENTER?

Question

Place the cork into a glass of water.

Answer 1

This is a complex phenomenon.

If a cork starts out on the surface in the middle of the glass (and not from the bottom so that water from all sides rushes in to fill the void it leaves as it floats upwards), there would be no net volume-displacement force to pull it to a side of the glass.

However, assume there is no wind blowing, if either the water in the glass is not of the same temperature as that of the room air, or the relative humidity is not about 100% so to cause cooling evaporation, convection flows would occur.

If the air is cooler than the water body, the water touching a side of the glass will get cooler and will sink whereas the water in the center will rise. This will cause an outwards radial flow on the top surface which pushes the cork to a side of the glass as you observed.

However, if room air temperature is hotter than the water body, the water on the surface would flow radially inwards. Same net inward radial flow would occur if the room air and the water body are of the same temperature but humidity is less than 100% and evaporation causes cooling on the surface—in this case the evaporation in the center should be greater than that near a side of the glass because of less falling back of the newly departing vapor molecules.

As the cork flows touching a side of the glass, its capillary and wetting effects and the wetting effect of the glass surface will form an escalated water surface. This surface will be of a specific shape so that its surface tension keeps the internal energy to the lowest possible level. And this lower energy causes a potential barrier to park the cork to the side of the glass.

Answer 2

When a submerged cork is released from the bottom of a glass of water, as it shoots up, it will cause the water to flow with it, thereby creating a donut shaped turbulence or circulation. That means, when the cork reaches the surface, the current of this donut-shaped circulation will carry it away from the center and towards the side of the glass. The effect is very small, but a cork is very light, and thus susceptible to even this tiny effect.

Addendum: In the case of the cork simply being put in the center of the surface, read the other good answers for this one.

Answer 3

the convex meniscus's answer is the best.

to make the cork float in the center, fill the glass completely to the top, then add just a little more water till the Glass is almost over flowing. the cork will go to the center.

Answer 4

Water forms a concave miniscus so the level at the edge of the glass is slightly higher than in the center. The cork floats to the highest point at the edge.

You can see the same effect if you sprinkle pepper in a glass of water or put cinammon in your coffee and let it sit.

Answer 5

I would also add in that due to the adhesion of the water along the side of the cork with the side of the glass would keep it there...