Rebound

Can you guess what this is?

 

Today I was walking by a pool, noticed rain drops on the surface, and thought it might be fun to photograph them to document the rain showers on 9 July 2015.

 

When I checked the pictures to see if the lighting looked okay, I noticed something interesting:  I had caught some rebounding droplets on film!

The first image above is a clear example.  I looked through more pictures, and found several other examples of this phenomenon.

Most of the droplets represented complete rebounds — i.e., the droplets were free of the surface.  How many rebounding droplets can you spot in the image below?

 

I think there are four rebounding droplets — three that are very easy to see, and a fourth very small droplet near the top edge of the picture. 

Later I tried to learn about drop impact dynamics.  It's fascinating to think about the complexity of this interaction.  When a droplet hits the surface, four basic results are possible: splashing, spreading, receding, or bouncing.  The factors that influence the results include things like the properties of the droplet (e.g., its size and density), the surface tension of the liquid that the droplet is hitting, and the impact velocity.

Sometimes a column of water is sent up from the impact zone — and sometimes a droplet forms at the top of the column (see below).  The column of water is called a "Worthington jet."  If the droplet remains attached to the jet, it's termed a "partial rebound."  If the droplet is sent upward and breaks free from the jet, it's called a "complete rebound."

(Below, I'm not sure if the droplet broke free after I took the picture, but I'm guessing it did.)

 

It was so nice to see some rain, especially in July during a drought.  And then there was the additional bonus of a foray into drop impact dynamics.  Perhaps these photos will also inspire you to explore and wonder about the fate of rain drops!

 

P.S.  Some of the facts above are from Yarin, A.L.  2006.  Drop impact dynamics: Splashing, spreading, receding, bouncing...Annu. Rev. Fluid Mech. 38: 159-192.