Using flash photography can be fun at times.... but messy.
- Position the flash/strobe close to the subject - this makes sure the subject is well lit, but the background remains in darkness.
- Use a high f-stop in order to get a deep depth of field so that the front and back of the milk splash both in focus
- Use the highest shutter speed available that syncs with your strobe, although no great issue if this is only 1/60th second
- The milk will appear frozen in place as the actual flash duration is typically about 1/1000th of a second.
- You will probably have to repeat multiple times to get the timing coordinated between the splash happening and operating your camera.