Why jets entrain mass? 
This very beautiful picture shows a jet of fuel in the combustion chamber of a diesel engine. Does anydody could explain me what is the physical reason that the jet entrains air? Why jets and plumes entrain mass from the environment? 
Edit. I believe it has to do with the conservation of momentum and i always wait for the happy day when people will stop mentioning the Bernoulli equation.
 A: By friction along the jet then diffusion of the celerity. 
A: Your many questions shows remarkable effort on questioning. You seem to note the point.

Does anydody could explain me what is the physical reason that the jet entrains air? 

"Diffusion" is now given as an answer. But If that would be true, a lot of air would be diffused also on the diesel stored in the tank. But there this doesn't happend in 10 years. This video on 14:00->
"Celerity" is nothing else than saying it happends fast. But why should it happen fast?
I bolded "air" on your question. You are seeking the solution for Turbulence. I mostly avoid linking this paper of mine here, as it's not Mainstream Physics; Mainstream Physics can't explain turbulence. I would also like to link you these videos of mine. As it seems that you are able to ask you through this, up to the solution.
Turbulence is fluid Split in parts. Surfaces inside a fluid. Cracks which aloud the air to be Entrained. These cracks are caused by the "friction" this part of the first answer is correct. I rather use word collision. Look the Prince Rubert drops video in my playlist. And you sure find a certain celerity.
