The conditions necessary for self-trapping of a quantum particle in a fluid are reviewed, using a mean-field model . A thermodynamic approach is used along with density functional methods. The possibility that self-trapping may nucleate a heterophase region in the fluid is discussed, emphasizing the equilibrium requirement of a pressure difference across a curved surface between coexisting phases. Numerical results for an excess electron in fluid neon are summarized. It is known that an

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... may self-trap in a fluid by distorting the local fluid structure. In this work, a description of the most probable state of the electron-fluid system is discussed. A thermodynamic approach, using a free energy model and density functional methods, is reviewed. The approach includes the possibility that self-trapping may nucleate a local heterophase region in the fluid. The electron-fluid interaction is described by energy-dependent atomic pseudopotentials arising from electron-atom interactions which are assumed to be non-overlapping. The fluid itself is described via local equations of state. The change in the most probable state from a delocalized electron to a selftrapped one has experimentally observable consequences.