This sounds very like a bubble jet printer cartridge type situation, the valve itself is passive, a sphincter or simply a hole so small that liquid can't flow at normal temperature/pressure. The pulses are delivered to a heating element which vaporizes the water and creates the pressure which actually release the compound. You need some pretty precise pulse delivery circuitry to control something like this on a small scale, the physical mechanism to hold the print head in position is actually far more complicated than the jets themselves.
You're looking at the pulse width, you'll never get the precision you need with on/off signals from a USB based device, you'll need a controller that can be set for a much finer pulse width resolution for any control. The controller only needs to generate the basic pulse though the data can come from USB.