I am interested to know what topological properties are possessed by non-smooth functions. I suspect this is well known, but I can't find what I am looking for from google.

Is there a topological "definition" of non-smooth functions (something elegant like the open set def of continuous functions) ?

My motivation involves the Naiver-Stokes millennium problem.

My intuition tells me that the highest derivative of a non-smooth function should be a non-differentiable continuous function, and furthermore that the non-differentiability is reflected in some form of scaling "symmetry", actually I suspect there is a broken scaling symmetry (i.e. the non-smooth solutions of N-S violate the scaling symmetry of the N-S equations, while still maintaining some sort of correlation across length scales)

The basic idea would be to show that the broken scale symmetry of the solutions of N-S have a topological property which forces them to be non-smooth. I'm certain this approach has massive issue's that need to be resolved, and that it has been thought of before since it's kind of obvious. Does anyone know about any results in this direction?