Today while drinking water, a drop of it accidentally felt on a page of my book, and I was thinking that, "Oh my god! The water will spread instantly, making the part of page wet". But, I observed, the drop remained as a drop, or to be precise, it didn't spread, it remained like a hemisphere over the page, and it's bottom was getting wet slowly. Also as wind was blowing tangentially over it, it was shaking just like a solid, thin ball. Actually I have observed it for long time, but never thought about it. My question is, what makes water, a liquid with weak intermolecular forces, which has no form of itself, can take up such a form? And also, why does it shake when there is wind blowing tangentially? Thanks. Looking for answers to clear my doubt.;)
There are three questions here:
Why didn't the water spread in the paper ?
What defines its shape ?
Why does it shake ?
Answer 1: when the drop approaches the surface, air trapped between them needs to escape. In some conditions (speed and size of drop, roughness of the paper, absorption properties of paper,... ), this is slow and consumes enough energy to use up all the momentum of the drop before a significant liquid-solid contact is established. The drop then sits there (possibly on a thin film of air above the substrate which will escape gradually under the pressure exerted by the drop due to its weight -- which is small,) hence the slow process.
Answer 2: the balance of surface tension, gravity, and pressure on the drop define the shape of the drop. If the drop is small enough, gravity can be neglected. Thus you have a spherical dome at the top (due to surface tension) and a flattened bottom (because air pressure is higher in the center, balancing the pressure in the drop).
Answer 3: Consider very short times compared to the time taken for absorption of the water . Everything is at equilibrium. The shearing wind will exert a transient force on the drop, which will elongate in some direction. When the wind force changes, the surface tension force tends to bring back the drop to the previous equilibrium, when this is reached the liquid has acquired some momentum and will thus go beyond that shape and shake until its viscosity dissipates away its kinetic energy.