two drops of mercury on a flat surface are rolled towards each other. When they make contact with each other they fuse to form one drop. Why?​

1) It is due to large cohesive force acting between the molecules of mercury that the droplets of mercury when brought in contact pulled together to form a bigger drop in order to make potential energy minimum. The temperature of this bigger drop increases since the total surface area decreases.  

2) A spherical shape has the minimum surface area to volume ratio of all geometric forms. When two drops of a liquid are brought in contact, the cohesive forces between their molecules coalesce the drops into a single larger drop. This is because, the volume of the liquid remaining the same, the surface area of the resulting single drop is less than the combined surface area of the smaller drops. The resulting decrease in surface energy is released into the environment as heat.