Water molecules have strong cohesive and adhesive properties. Water (H2O) molecules are highly cohesive to one another because of their magnetic properties. The oxygen molecules of H2O are negatively charged and the hydrogen molecules are positively charged making them attractive to one another thus making individual H2O molecules stick to one another. Because water molecules like to stick together, raindrops like to follow the previous paths of other drips when falling down glass surfaces. The cohesive property of water is important for a lot of things including many functions in our bodies & movement up and down plant stems.

I think the surface tension or stickyness of the water molecule is lesser in water. Therefore, it tends to spread out and stick with path of water that have a trace on the surface. Thus, following the same path flows.How about trying experiments with liquid mercury drops?!