You should be able to do it with your lens. I used my Canon 55-250mm to do these at about 100mm.
The trick is to place something with a high contrast on the surface where you expect the drop to land. I used the multicoloured plastic straw I was using to release the drops. Use AF to get good focus on this point, once focus has been attained, let go of the shutter trigger without taking the shot and then switch the lens to manual focus.
I also closed the aperture down to about f11 give a bit of a deeper DOF.
These shots were taken at 100mm, f11, 1/200 with the flash in manual mode at 1/64th power. The flash was also set about 500mm away and to the left of the subject.
I hope that helps.