Water drops kept on different surface evaporate after a few minutes. How can I reduce the rate of evaporation of these drops? Can I add some additives which suppress evaporation?
Depends on what you allow for "additives" . If you're allowed to coat the surface (as is done when waxing a car), then you can change the surface energy, which changes the surface tension effects. A given volume of water will evaporate more slowly if it's on a waxed surface and forms a near-spherical shape as opposed to a dull surface where the water droplet forms a flat puddle. This is because the first case has considerably less surface area.
There are several ways to reduce the rate of evaporation of water. However, first it is important to understand the factors influencing the rate of evaporation:
1) Amount of humidity in the air 2) Speed of air flowing past the water droplets 3) Intramolecular bonding strength 4) Pressure
There are more such as surface area, density, etc. but since we are considering the same drop of same substance these can be ignored. (temperature is there too, obviously)
So what we can do: Make the air around the drops more humid. Put the drops in a closed environment, to reduce the speed of air flowing past it. For example, water droplets inside a room will evaporate slower than water droplets placed outside. There certainly are some additives to help increase the intramolecular bonding strength of a substance. Also, the substance the water droplets are placed on also makes a difference on the rate of evaporation. Decreasing your altitude will also decrease the rate of evaporation.