Let us briefly summarize the basic picture of the QCD vacuum that has emerged from the instanton liquid model. The main point is that the gauge fields are very inhomogeneous. Strong, polarized fields are concentrated in small regions of space-time. Quark fields, on the other hand, cannot be localized inside instantons. In order to have a finite tunneling probability, quarks have to be exchanged between instantons and anti-instantons. This difference leads to significant differences between gluonic and fermionic correlation functions. Gluonic correlators are short-range and the mass scale for glueballs is significantly larger than that for mesons. In addition to that, the fact that the gluonic fields are strongly polarized leads to large spin splittings in both glueballs and light mesons.
From this 1997 review.
Basically, the strong force is concentrated in globs within the vacuum of about 0.35 femtometers (fm) that interact with each other and take up a bit less than 2% of the space-time in a hadron. Quarks hop from one of these globs to the next in quantum tunnelling interactions (i.e. they can move from point A to point B in a field even though they lack the energy to cross field potentials that exist between those two points) at predictable rates.
This model remains relevant and accurate in predicting a wide variety of experimental and lattice QCD results such as the conditions under which quark-gluon plasmas form, and provide a way to mediate between raw QCD equations for interactions between individual quarks and gluons and the phenomenological models that fit the experimental results.
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