Your implicit expectation that it is possible to uniformly typeset the
closure of any piece of formula results from mistaking TeX for a
markup system while it is a typesetting system. Often enough, you can
ignore the distinction between these two concepts, which is made
especially blurry by LaTeX. But when it comes to typesetting maths, it
is not possible to assume that both concepts are the same (see how
MathML distinguishes presentation and semantics).
To solve your problem, add near the definition of \cellFluid a new
definition for a macro \cellFluidBar typesetting the closure of
\cellFluid.
Alternatively you can look for alternative notations for the closure,
such as $\mathop{\mathrm{Adh}}_X Y_0$, that are cumbersome but smooth
that edge between markup and typesetting.
If you enjoy programming TeX, you can devise a \closure macro taking
a unique argument and typestting its closure in a generic way but
supports a dictionary of exceptions. Aside from fun, there is probably
very little benefits over the first, easy and straightforward,
solution.
\bar\cellSolidwill only put the bar on the Y, not the Y with subscript. – ChrisS Oct 09 '13 at 08:39$\overline{\cellSolid}$produce the desired result? – jub0bs Oct 09 '13 at 09:21\overlineextends over the index as well. And even on its own, it's much wider than\bar, so it looks wrong. – user1350992 Oct 09 '13 at 09:23\overlinefor closures,\barbeing too short. If a set is calledY_0I would want the line to go over the whole symbol pair, to distinguish from a "0opertaion" on the closure. Thus I think Jubobs suggestion is the correct one. – Andrew Swann Oct 09 '13 at 09:27