Fluid dynamicist here who works on the breakup of larger water jets. How the jet breaks up depends on the design of the system. (I assume you mean break up, not

Fluid dynamicist here who works on the breakup of larger water jets. How the jet breaks up depends on the design of the system. (I assume you mean break up, not diffuse. Diffusion is a different physical process which occurs independent of breakup.) A low speeds (laminar flow), Rayleigh's analysis shows that a simple liquid jet breaks up due to surface tension effects. https://en.wikipedia.org/wiki/Plateau%E2%80%93Rayleigh_instability https://en.wikipedia.org/wiki/Plateau%E2%80%93Rayleigh_insta... At higher speeds (e.g., transitional or turbulent flows), the mechanisms change. For your typical spray bottle (without a swirl chamber) I'd say turbulence is the primary cause of breakup. Aerodynamic influence won't factor in at typical air densities and cavitation isn't happening as it probably would break the nozzle. Here's a conference paper I wrote on turbulent breakup theory, which hopefully is readable for anyone with an understanding of probability theory and basic turbulence theory: https://engrxiv.org/35u7g/ https://engrxiv.org/35u7g/ (RMS velocity is just the standard deviation of the velocity. Turbulent kinetic energy is an energy formed from the RMS velocity. Kolmogorov's 5/3 theory is used a bit, but you can just view it as power law distribution. Etc.) I also have some small spray bottles with swirl chambers, which makes the bulk fluid spin, destabilizing the interface, leading to droplet formation. The spray angle for these is large compared against the simple pressure atomizer described previously.