This is a fun sort of experiment for a rainy day. First, what you're seeing here:
A sidelit shot of a milk drop about to strike a small blue saucer that has about a quarter inch of milk already in it.
Path to success:
* Get the flash off-camera, either using a synch cord or wireless setup.**
* Use a medicine dropper or similar device to regulate drop flow, and anchor the dropper to something solid. For these shots, I duct taped the dropper to a kitchen cabinet, with the dropper's tip hanging past the bottom of the cabinet, allowing quick 're-fueling.'
* Focus is critical, and each drop disappears in an instant. So place a toothpick or other thin object in the exact spot where your drops land. Focus on your marker and then remove. Check your focus accuracy after shooting the first dozen.
* Learn to anticipate the drops, and you'll likely need to fire the shutter earlier than what might seem appropriate. That's one good reason to position the dropper at least 3 feet above the saucer. This will aid your timing, buying you time to respond to each falling drop.
* Shoot lots! It's the joy of digital!

**While wireless sounds grand, you may become frustrated by the delay inherent in wireless camera-flash communications. That's why I always use a (dedicated) synch cord for flash synch.